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Complexion: A New Concept for Kinetic Engineering in Materials Science
Abstract
Interfaces and the movement of atoms within an interface play a crucial role in determining the processing and properties of virtually all materials. However, the nature of interfaces in solids is highly complex and it has been an ongoing challenge to link material performance with the internal interface structure and related atomic transport mechanisms. Interface complexions offer a missing link to help solve this universal problem. We have theoretically predicted the existence of multiple interface complexions by thermodynamics, but the present work represents the most comprehensive characterization and proof of their existence in a real material system. An interface complexion can be considered as a separate phase, which can be made to transform into different complexions (phases) with vastly different properties by chemistry and heat treatment, thereby enabling the engineering control of material properties on a level not previously realizable. As such, complexions offer a solution to outstanding fundamental scientific mysteries, such as the origin of abnormal grain growth in inorganic materials, a problem which leading researchers in the field have struggled to explain for the past 50 years. It is also described how interface complexions will likely have widespread impact across all branches of material science and related disciplines.
... complexions, which can significantly influence the electrical, thermal, optical, and magnetic properties of ZnO [3,4,6,9,12,14,16,21,26,27]. In their pioneering work, Harmer and coworkers introduced the concept of GB complexions and reported that the GB complexion transition takes place as the concentration of dopant varies at the GB [28,29]. Hence, researchers try and optimize the dopant concentration to design favorable GB complexions. ...
... Therefore, a transition from the single-layer to the multilayer GB complexion can be observed at this concentration. The GB complexion transition should be accompanied by a discontinuity in the observable thermodynamic quantities [28][29][30]. As noted earlier, a change in the slope of the segregation energy curve is also observed at the same concentration in Fig. 3, which can be attributed to the GB complexion transition observed in the atomic structures of the segregated GBs. ...
... GBs (Fig. 6b,c), of the majority of dopants are distributed between − 3 to + 3 Å width about the GB, i.e. within the first atomic layer from the GB. We can call it the first segregation layer, which is analogous to complexion I and II in the Harmer's GB complexions [29]. At low concentration, almost all the dopants segregate to the GB within the first segregation layer. ...
... Depending on thermodynamic conditions and processing history, GBs can adopt multiple configurations with distinct boundary properties, such as diffusivity, mobility and cohesive strength 1,4,16 . Increasing evidence suggests that, under thermal or mechanical stimuli, different GB configurations may undergo phase-like transitions inducing abrupt changes in microstructural evolution (for example, abnormal grain growth 3,17 and stable nanocrystalline alloys 2,5 ) and materials properties (for example, liquid metal embrittlement 18 ). Because most functional and engineering materials are polycrystalline solids, an in-depth understanding of GB configurations and their transitions is thus essential for further optimization of material properties 1,3,4,18,19 . ...
... Increasing evidence suggests that, under thermal or mechanical stimuli, different GB configurations may undergo phase-like transitions inducing abrupt changes in microstructural evolution (for example, abnormal grain growth 3,17 and stable nanocrystalline alloys 2,5 ) and materials properties (for example, liquid metal embrittlement 18 ). Because most functional and engineering materials are polycrystalline solids, an in-depth understanding of GB configurations and their transitions is thus essential for further optimization of material properties 1,3,4,18,19 . For this purpose, detailed information about GB configurations and transition mechanisms at the atomic scale is of paramount importance. ...
Grain boundaries (GBs), with their diversity in both structure and structural transitions, play an essential role in tailoring the properties of polycrystalline materials1–5. As a unique GB subset, {112} incoherent twin boundaries (ITBs) are ubiquitous in nanotwinned, face-centred cubic materials6–9. Although multiple ITB configurations and transitions have been reported7,10, their transition mechanisms and impacts on mechanical properties remain largely unexplored, especially in regard to covalent materials. Here we report atomic observations of six ITB configurations and structural transitions in diamond at room temperature, showing a dislocation-mediated mechanism different from metallic systems11,12. The dominant ITBs are asymmetric and less mobile, contributing strongly to continuous hardening in nanotwinned diamond¹³. The potential driving forces of ITB activities are discussed. Our findings shed new light on GB behaviour in diamond and covalent materials, pointing to a new strategy for development of high-performance, nanotwinned materials.
... Zhou et al. discussed the effect of the ligand environment of Er 3+ on the temperature sensing properties in Y 2 O 3 : Er 3+ ceramics [8], which indicated the possibility of tailoring the fluorescence thermometric properties by regulating the local environment of Er 3+ . In the past decades, the grain boundary complexions chemically induced by certain additives (CaO, SiO 2 , etc.) in Al 2 O 3 ceramics have come into focus [20][21][22]. This equilibrium state of matter at crystalline interfaces is chemically and structurally distinct from any bulk phases (e.g., crystalline and/or amorphous) [23]. ...
... Evidently, there was no uneven distribution of matrix elements, while the Er 3+ ions had a distinct accumulation along the grain boundary. This structure is notably similar to the grain boundary "complexion" in Al 2 O 3 ceramic [22]. ...
Fluorescence thermometry is considered one of the most perspective temperature sensing techniques. However, few materials reported could combine a wide operating temperature range with stable and high sensing sensitivity. Herein, the fluorescence thermometry based on MgAlON:0.01 at.% Er 3+ transparent ceramic was successfully proposed using the idea of local structural engineering, which distributed the Er 3+ uniformly into the grain boundary of MgAlON transparent ceramic. It is demonstrated that the special local environment of Er 3+ led to different fluorescence responses from those in crystalline or amorphous structures. Instead of the classic Boltzmann exponential relationship, a highly linear downshift fluorescence intensity ratio of thermally coupled energy levels of Er 3+ with temperature was exhibited. A novel and constant absolute sensitivity (0.0156 K − 1) was obtained within the tested temperature range (300-573 K). This work reveals the significance of the local structure of the activators in the design of high-performance fluorescence temperature sensors. In recent years, fluorescence thermometry has been well proven to satisfy the requirements of advanced temperature sensors, such as electrical contactless, fast response, high sensitivity, wide measurement range and strong environmental adaptability [1,2]. Among them, the fluorescence intensity ratio (FIR) technique of dual-emitting lanthanide ions (Ln 3+) has attracted much attention due to its immunity to spectral loss and fluctuations in excitation power density [3]. As known, the temperature-induced repopulation of electrons on thermally coupled energy levels (TCELs) is one of the most important basic principles for the FIR technique. A classic example for applying TCELs involves the upconversion fluorescence of Er 3+ /Tm 3+ /Ho 3+ [4-6]. However, the temperature-dependent FIR of TCELs usually follows Boltzmann's exponential relationship [7-12]. An exponential function expressed FIR leads to a fluctuating thermometric absolute sensitivity (S a) within the measurement range since the absolute sensitivity is defined as the rate at which the FIR changes for a certain change in temperature [13]. What's more, to ensure that the upper level can be thermally populated, the slight energy separation (200-2000 cm − 1) between TCELs is necessary. It results in an upper limit of absolute sensitivity (usually S a < 0.012 K − 1) [8]. On the other hand, considering the thermal effect generated by laser, a downshift luminescence detector without laser excitation is sometimes preferred [14]. In addition, the temperature range of fluo-rescence thermometers can be widened by tuning the downshift fluo-rescence intensities of inter-and intra-configurational Ln 3+ transitions, e.g., Pr 3+-based fluorescence thermometry [15]. Attributing to the abundant energy levels of Er 3+ , numerous works have exploited its upconversion fluorescence of the TCELs for temperature sensing [7-12], but research related to its temperature-dependent downshift fluores-cence is limited so far. The temperature sensors reported are based on various host materials , including nanoparticles, glasses, ceramics, etc. [7-12]. Among them, transparent ceramic is one of the suitable host materials. High transmittance of ceramics ensures that activator ions inside the bulk can be excited and participate in the luminescence process [14]. The thermal and chemical stability of transparent ceramics makes the temperature measurement repeatability better than that of powders and quantum dots [16]. In our previous studies, the novel optical quality MgAlON transparent ceramics with cubic spinel type structure were fabricated [17,18], in which wide ultraviolet (UV) transmission is conducive to the effective excitation of Er 3+ in bulk ceramics by UV light.
... More generally, GB premelting belongs to ''GB surface phase transitions''. The latter describe the transitions between GB phases or complexions [17,18], i.e. the special atomic structures that develop at GBs owing to the breaking of the crystal translational symmetry. In this respect, GB premelting is also involved in liquid metal embrittlement, that is often associated with a cascade of GB phase transitions [19]. ...
... We reiterate that, as shown in [8], the complexions adopted by the GBs may vary (sub-monolayer adsorption, inter-granular film, wetting film...) with their position in the sample, and the associated variation of their mobility may also influence the grain structure. In particular, abnormal grain growth is also observed in samples where the dopant concentration is spatially homogeneous (in comparison the dopant is here localized close to the surface of the polycrystal in contact with the bulk liquid), with isolated grains growing much faster than their immediate neighbors (see for example Ref. [18]). These abnormal grains may exhibit a large aspect ratio, and this anisotropic growth is attributed to the mobility anisotropy of the GBs, inherently linked to their atomic structure. ...
... Solute segregation can lead to structural transitions between different types of grain boundary complexions, or interfacial phases in thermodynamic equilibrium with their abutting grains [1,2]. Nanoscale film grain boundary complexions have a stable, finite thickness and can be structurally ordered or disordered depending on grain boundary crystallography, temperature, and chemistry [3,4]. ...
... For example, amorphous complexions can be separated into two structurally distinct regions: (1) the complexion interior with a fully amorphous structure and (2) the amorphous-crystalline transition regions (ACTRs), which connect the interior with neighboring crystalline grains [18]. ...
Amorphous grain boundary complexions act as toughening features within a microstructure because they can absorb dislocations more efficiently than traditional grain boundaries. This toughening effect should be a strong function of the local internal structure of the complexion, which has recently been shown to be determined by grain boundary crystallography. To test this hypothesis, molecular dynamics are used here to simulate dislocation absorption and damage nucleation for complexions with different distributions of structural short-range order. The complexion with a more disordered structure at the dislocation absorption site is actually found to fail earlier, as damage tolerance requires delocalized deformation and the operation of shear-transformation zones through the complexion thickness. The more damage tolerant complexion accommodates plastic strain efficiently within the entire complexion, providing a key mechanistic insight into the toughening effect of amorphous complexions as well as a pathway to design nanocrystalline alloys with improved mechanical properties.
... GB segregation by dopant elements during high temperature annealing can promote formation of amorphous intergranular thin film (AIF), i.e., amorphous GB complexion, as demonstrated in a variety of binary and ternary metallic alloys such as Ni-W, Ni-Mo, Ni-Zr, Cu-Zr, Cu-Zr-Hf and Al-(Mg, Fe, Ni)-Y [83][84][85][86][87][88]. The formation of AIF is compositionally selective under the thermodynamic and kinetic constraints, i.e., encouraging the segregation of dopants to interfaces and lowering the formation energy for a glassy structure [82,[89][90][91]. Correspondingly, Schuler and Rupert [85] proposed that dopant elements with a positive enthalpy of segregation and negative enthalpy of mixing promote the formation of amorphous GB complexions. ...
... More importantly, the amorphous GB complexion is usually formed under high-temperature annealing. interfaces and lowering the formation energy for a glassy structure [82,[89][90][91]. Correspondingly, Schuler and Rupert [85] proposed that dopant elements with a positive enthalpy of segregation and negative enthalpy of mixing promote the formation of amorphous GB complexions. ...
Crystalline metals generally exhibit good deformability but low strength and poor irradiation tolerance. Amorphous materials in general display poor deformability but high strength and good irradiation tolerance. Interestingly, refining characteristic size can enhance the flow strength of crystalline metals and the deformability of amorphous materials. Thus, crystalline–amorphous nanostructures can exhibit an enhanced strength and an improved plastic flow stability. In addition, high-density interfaces can trap radiation-induced defects and accommodate free volume fluctuation. In this article, we review crystalline–amorphous nanocomposites with characteristic microstructures including nanolaminates, core–shell microstructures, and crystalline/amorphous-based dual-phase nanocomposites. The focus is put on synthesis of characteristic microstructures, deformation behaviors, and multiscale materials modelling.
... While physical properties of a GB (e.g. the amount of segregated dopant atoms at the GB, or the GB formation energy) typically change continuously with thermodynamic parameters such as temperature and dopant concentration, it is also possible for discontinuous changes to occur at certain special thermodynamic parameters [73]. Such discontinuities are akin to first-order phase transitions commonly observed in bulk systems; they are a result of first-order transitions between GB complexions -where the term 'complexion' has become standard to describe an interfacial analogue of a bulk phase whose structure is distinct from any phases found in the bulk [73][74][75][76][77]. With this in mind, the discontinuities mentioned above in the amount of Zr segregation versus in the 3(211) and 9(221) suggest the existence of firstorder complexion transitions in these GBs. ...
... The atomic structure of grain boundaries can be complex and depends on parameters such as composition, crystal geometry, and temperature. Grain size is generally used to predict bulk material characteristics, including yield strength and fracture resilience [1][2][3], the emission of dislocations in metals [4,5], and the stability of nanocrystalline metals [6][7][8]. GBs in polycrystalline metal alloys serve as sites for solute segregation, creating concentration gradients which can impact alloy properties [9][10][11][12][13][14][15], such as GB mobility and grain growth stabilization in metals [16,17]. ...
... In metallic materials, grain boundaries (GBs) are common planar defects and exhibit distinct crystallographical and compositional characteristics from the bulk [1]. Several processes that can alter the properties of the interfaces, dramatically affect the behavior of polycrystalline materials such as change in GB diffusivity due to structural transitions at the GB [2], abnormal grain growth due to the presence of two different GB structures (with different mobility) [3], and embrittlement of Cu GBs caused by Bi segregation [4]. Furthermore, each GB is distinguished by its unique thermodynamic excess properties that are controlled by the local atomic structure and chemistry of a GB [5][6][7][8][9][10]. ...
... For example, Bi segregation to Ni interfaces substantially reduces grain boundary cohesion due to the formation of ordered superstructures [13]. More broadly, grain boundary transitions are associated with macroscopic behaviors such as activated sintering [18], liquid-metal embrittlement [3], and abnormal grain growth [19], emphasizing the need for a comprehensive understanding of transition behavior in engineering materials. ...
Grain boundary structural transitions can lead to significant changes in the properties and performance of materials. In multi-principal element alloys, understanding these transitions becomes complex due to phenomena such as local chemical ordering and multi-component segregation. Using atomistic simulations, we explore a metastable-to-equilibrium grain boundary structural transition in NbMoTaW. The transition, characterized by structural disordering and reduced free volume, shows high sensitivity to its local chemical environment. Most notably, the transition temperature range of the alloy is more than twice that of a pure metal. Differences in composition between coexisting metastable and equilibrium structures highlight the change in local site availability due to structural relaxation. Further examination of grain boundaries with fixed chemical states at varying temperatures reveals that the amount of segregation significantly influences the onset temperature yet has minimal effect on the transition width. These insights underscore the profound effect of chemical complexity and ordering on grain boundary transitions in complex concentrated alloys, marking a meaningful advancement in our understanding of grain boundary behavior at the atomic level.
... To be clear, a "phase-and-defect diagram" as defined here refers to a diagram that contains information about a system containing both the phases defined by bulk equilibrium thermodynamics as well as potential defects that are not rigorously considered phases. Defects can experience phase-like behavior, as commonly observed for grain boundary complexions [38,[45][46][47][48][49] however, the full system containing these defects will be referred to here as a "defect state" to ensure clarity. ...
... Thus a bimodal grain size distribution was observed for the S1350 ceramic. The phenomenon of bimodal grain growth could be associated with the change in the grain boundary energies with the rise in temperature [36,37]. The bimodal grain size distribution was also reflected in the high span ( ...
Translucent, monoclinic-free, novel calcia-stabilised zirconia ceramics were developed utilising conventional (pressureless) sintering. The 10 mol% CaO-doped zirconia (10CaSZ) ceramic depicted a maximum transmittance of 81% at 600 nm for a 0.6 mm thick pellet while for 3 mol% CaO-doped zirconia (3CaSZ) phase-pure tetragonal system, the value was recorded as 59% at the same wavelength and thickness. The ultrafine (~ 3 nm) nanoparticles sintered via two-step sintering process yielded full densification (> 99%) at temperatures as low as 1150 °C. The average grain size of sintered 3CaSZ ceramic was 85 nm. For 10CaSZ ceramic, it increased from 170 nm to 0.76 μm with elevated sintering temperatures. The increase of sintering temperature ensured different microstructure from ultrafine grains, a bimodal grain size distribution, and ultimately leading to a grain-locking morphology. The influence of microstructure and constituent phases (tetragonal and cubic) on total forward transmittance in the visible range was analysed and explained.
... Regarding the RSed LAT631 alloys, the increasing strain noticeably promotes Al solutes segregated around the Sn-rich precipitates. Firstly, the segregation of elements with low solubility is expected in the strong deformation process, which helps stabilize the microstructures [35]. Xiao et al. [36] indicated that Ag solutes preferred to segregate at the region of dislocation entanglements and grain boundary to reduce the local elastic distortions in the cold-rolled Mg-Ag alloy. ...
A Mg-6Li-3Al-1Sn (LAT631) alloy with a bimodal-grained structure was prepared via hot extrusion, followed by room temperature rotary swaging (RTRS). The as-extruded LAT631 alloy undergoes dynamical recrystallization, and massive twins are introduced during RTRS. Especially, Al segregation is generated with massive Al solutes surrounding Sn-rich particles via RTRS. Introducing twins and Al-rich segregation are critical factors for enhancing strength, and the increased proportion of DRXed grains is beneficial to improving the elongation in RSed alloy. This research demonstrates that strategic control of recrystallization and segregation by rotary swaging provides an innovative approach to optimizing the mechanical properties of Mg-Li alloys.
... Solid-solid interfaces are widely present in materials and have a significant impact on their physical and chemical properties [1][2][3] . They are crucial scientific questions in multiple important fields. ...
The atomic structures of solid-solid interfaces in materials are of fundamental importance for understanding the physical properties of interfacial materials, which is, however, difficult to determine both in experimental and theoretical approaches. New theoretical methodologies utilizing various global optimization algorithms and machine learning (ML) potentials have emerged in recent years, offering a promising approach to unraveling interfacial structures. In this review, we give a concise overview of state-of-the-art techniques employed in the studies of interfacial structures, e.g., ML-assisted phenomenological theory for the global search of interface structure (ML-interface). We also present a few applications of these methodologies.
... The cause of non-normal grain growth, in particular AGG, has long been studied and several mechanisms and models have been proposed. They include the second phase particle (or pore) drag mechanism [9][10][11], the solute drag mechanism [12][13][14], the liquid film and specific complexion (boundary type) enhancement mechanism [15][16][17][18][19], the boundary energy and mobility anisotropy mechanism [20][21][22][23][24][25], and the mixed control mechanism of boundary migration [6,8,26]. These mechanisms can provide explanations for the deviation of grain growth behavior from normal. ...
This paper suggests future research directions in grain growth and related subjects. A discussion on the available mechanisms of grain growth has led to an emphasis on the mixed control mechanism of boundary migration and grain growth. The mixed control mechanism of boundary migration and grain growth is briefly described and some possible future directions for grain growth research are presented. Studies on the suggested research subjects seem to be relevant for the enhancement of our basic understanding of grain growth with respect to boundary structure and its application to practical systems.
... These different structures can be treated as interface phases, which can only exist in contact with the abutting crystallites, and can be treated using a thermodynamic framework [58][59][60][61][62][63]. They are called complexions [64][65][66][67] or GB phases [68]. Complexion transitions, then, are analogous to bulk phase transitions: The GB structure, composition, and properties change discontinuously at critical values of thermodynamic parameters such as temperature, pressure, and chemical potential [62][63][64][66][67][68]. ...
The migration of grain boundaries leads to grain growth in polycrystals and is one mechanism of grain-boundary-mediated plasticity, especially in metallic nanocrystals. This migration is due to the movement of dislocation-like defects, called disconnections, which couple to externally applied shear stresses. Here, we investigate a $\Sigma$19b symmetric tilt grain boundary without pre-existing defects using atomistic computer simulations with classical potentials. This specific grain boundary exhibits two different atomic structures with different microscopic degrees of freedom (complexions), called ``domino'' and ``pearl'' complexion. We show that the grain boundary migration is affected by both the formation energy of a disconnection dipole and the Peierls-like barrier required to move the disconnections. For the pearl complexion, the latter is much higher, leading to a high stress required for grain boundary migration at low temperatures. However, in absolute values, the Peierls barrier is low and can be overcome by thermal energy even at room temperature. Since the domino complexion has higher disconnection formation energies, it is more resistant to migration at room temperature and above.
... Although the present study focuses on premelting initiated inside a crystal, it is worthwhile to compare our observations with previous results regarding grain-boundary complexions, which encompass various transitions of faceting, order−disorder, wetting, premelting, and adsorption at grain boundaries as a general concept [46][47][48] . For a direct comparison, we carried out additional STEM analyses on more than 50-grain boundaries in a polycrystalline sample annealed at 1400°C. ...
Since two major criteria for melting were proposed by Lindemann and Born in the early 1900s, many simulations and observations have been carried out to elucidate the premelting phenomena largely at the crystal surfaces and grain boundaries below the bulk melting point. Although dislocations and clusters of vacancies and interstitials were predicted as possible origins to trigger the melting, experimental direct observations demonstrating the correlation of premelting with lattice defects inside a crystal remain elusive. Using atomic-column-resolved imaging with scanning transmission electron microscopy in polycrystalline BaCeO3, here we clarify the initiation of melting at two-dimensional faults inside the crystals below the melting temperature. In particular, melting in a layer-by-layer manner rather than random nucleation at the early stage was identified as a notable finding. Emphasizing the value of direct atomistic observation, our study suggests that lattice defects inside crystals should not be overlooked as preferential nucleation sites for phase transformation including melting.
... In Fig. 11 (b) and Fig. (c), the crack deflection and crack bridging can be clearly observed in the crack propagation path, respectively. Crack deflection and crack bridging can absorb crack propagation energy, which can effectively inhibit crack propagation and improve the fracture toughness of the sample [34]. Fig. 11 (c) shows that the sample fracture mode is mainly transgranular fracture, with less intergranular fracture, indicating good bond strength between the bonding phase and the granular interface. ...
Grain boundaries play an important role in governing the mechanical and physical properties of materials. Classical grain boundaries in crystalline materials typically have a thickness of 1–2 atomic layers. Thick grain boundaries are referred to as grain boundaries of interfacial phases in-between crystalline grains with a typical thickness of 1–10 nm. This unique interface endows metallic materials with remarkable mechanical behaviors and thermal stability. In this review, we summarize the deformation mechanisms in nanocrystalline metals with thick grain boundaries, focusing on strength enhancement and strength–ductility synergy. The thermal stability of nanocrystalline metals with thick grain boundaries is also discussed. We finally highlight the potential contribution of design strategies to the development of metallic materials with thick grain boundaries and potential approaches to further understanding the underlying mechanisms behind deformation and thermal stability.
Grain boundary (GB) segregation models are derived for multi-principal element alloys (MPEAs) and high-entropy alloys (HEAs). Differing from classical models where one component is taken as a solvent and others are considered solutes, these models are referenced to the bulk composition to enable improved treatments of MPEAs and HEAs with no principal components. An ideal solution model is first formulated and solved to obtain analytical expressions that predict GB segregation and GB energy in MPEAs and HEAs. A regular solution model is further derived. The GB composition calculated using the simple analytical expression derived in this study and data from the Materials Project agrees well with a prior atomistic simulation for NbMoTaW. The simplicity of the derived analytical expressions makes them useful for not only conveniently predicting GB segregation trends in HEAs but also analyzing nascent interfacial phenomena in compositionally complex GBs. As an application example, the models are used to further derive a set of equations to elucidate an emergent concept of high-entropy grain boundaries.
As a prototypical photocatalyst, TiO 2 has been extensively studied. An interesting yet puzzling experimental fact was that P25—a mixture of anatase and rutile TiO 2 —outperforms the individual phases; the origin of this mysterious fact, however, remains elusive. Employing rigorous first-principles calculations, here we uncover a metastable intermediate structure (MIS), which is formed due to confinement at the anatase/rutile interface. The MIS has a high conduction-band minimum level and thus substantially enhances the overpotential of the hydrogen evolution reaction. Also, the corresponding band alignment at the interface leads to efficient separation of electrons and holes. The interfacial confinement additionally creates a wide distribution of the band gap in the vicinity of the interface, which in turn improves optical absorption. These factors all contribute to the enhanced photocatalytic efficiency in P25. Our insights provide a rationale to the puzzling superior photocatalytic performance of P25 and enable a strategy to achieve highly efficient photocatalysis via interface engineering.
In this study, atomistic simulation technique is utilized to examine the impact of grain boundary (GB) character
(disorientation angle), structure (boundary-free volume) and energy (boundary excess energy) on site-specific Nb
solute segregation behaviour and structural transition in nanocrystalline (NC) Ni. Further, the role of site-specific
solute segregation on deformation response in NC Ni is also studied. Towards this, hybrid Monte Carlo/molecular
dynamics simulations is performed to obtain the equilibrium structure in a Ni-Nb binary alloy at 500 K and 1200
K. The spectral nature of GB atoms is explored in NC Ni, and found that the GBs have a skew-normal distribution
for excess atomic volume and excess atomic energy. Furthermore, comprehensive segregation energy calculations
are performed for each GB site, revealing the preferential affinity of Nb towards GB sites in NC Ni. The study
also examined interfacial solute excess for each boundary as a function of GB energy and Voronoi volume. The
findings indicate that the GB energy decreased as the solute excess increased, whereas the Voronoi volume
demonstrated an increment in response to the solute excess. Moreover, simulations conducted at 1200 K reveal
that not all GBs within the microstructure are transformed to amorphous complexions; rather some GBs remain
ordered. The simulated tensile tests conducted at 500 K and 1200 K reveal that the strengthening in Ni-Nb
enhances with the increase in dopant concentration. Additionally, the simulated shear test conducted at 1200
K shows that the deformation in Ni-5Nb occurs more uniformly than pure NC Ni due to formation of thicker
amorphous GBs in the former one.
Fracture toughness is a basic property of ceramic materials, and it is strongly influenced by their microstructures. However, the influences of grain‐scale microstructural factors, including grain fracture energy, boundary fracture energy, and grain size of the microstructure, on the fracture toughness remain unclear. To investigate this, a dual‐scale finite element method was developed. Models were established to assess the microstructural mechanical properties, with an average grain size of 0.1–100 μm, and different grain boundary fracture energies and grain fracture energies. Based on the microstructural mechanical properties, a macroscale three‐point bending model, combined with the extended finite element method was used to evaluate the fracture toughness. The results indicated that fracture toughness increased with the decrease in the grain size when the grain size was below 5 μm. Conversely, when the grain size was greater than 5 μm, the fracture toughness remained relatively constant. Additionally, the grain boundary fracture energy played a more important role for the fracture toughness than the grain fracture energy. Finally, a formula that quantitatively describes the relationship between the microstructure and fracture toughness was obtained. This work provides a valuable reference for evaluating the performances of structural ceramics.
A novel electromagnetic shocking treatment (EST) was put forward to selectively embellish interface microstructure of solid alloy and improve its performance in our previous work. However, considering the nonlinear response characteristic of phase evolution under EST, the effect of precipitates evolution on precipitate and microhardness distribution of Al7075-T73 alloy is worth of attention and still unknown. Herein, microhardness distribution test and TEM characterization of precipitates for Al7075-T73 alloy are carried out to explore the influence mechanism of EST on the precipitate and microhardness distribution. Compared with the received sample, the microhardness distribution turns more homogeneous, while the average microhardness remains basically constant for EST sample. Besides, for EST sample, grain boundary precipitates (GBPs) dissolve and coarsen obviously, while there also exists that the distribution of matrix intragranular precipitates remains basically constant with that of the received sample. Meanwhile, stripy grain boundaries (GBs) can be observed more commonly and GB wetting occurs for EST sample. It indicates that EST promotes nonlinear interface wetting and results in nonlinear phase transition, inducing significant GBPs transformation and then gradient phase transition of precipitates within grain size scale. The resultant gradient phase distribution combined with nonlinear interface bridging promotes a more homogeneous microhardness distribution without losing the mechanical properties of aluminum alloy. This work provides a new understanding about the effect of EST on the microstructure evolution process of solid alloy and is beneficial to the optimization of EST process to improve the service performance of alloys and even finished parts.
Graphical abstract
Although single-band emission upconversion fluorescence, which is particularly important in sensors, biomedical and imaging applications, has been extensively investigated in nanoparticles using energy transfer engineering, it is still difficult to be achieved in transparent ceramics. Herein, the MgAlON: Er3+/Yb3+ transparent ceramics with functionalized grain boundaries were successfully fabricated. Lanthanide ions were investigated to highly enrich at the MgAlON grain boundaries, resulting in novel fluorescent properties. Upon 980 nm laser excitation, the sample emitted temperature and excitation power-independent single-band red upconverted fluorescence with a red-to-green fluorescence intensity ratio of ~25. The new mechanism of Yb3+-Yb3+ cooperative fluorescence-assisted energy transfer, which is responsible for the single-band feature, was elucidated. Moreover, the upconverted fluorescence chromaticity can be modified by the excitation wavelength, relying on the existence of the cooperative fluorescence level and its related energy transfer mechanism. This work reveals the potential of grain boundary functionalization of transparent ceramics in energy transfer engineering.
This work examines the impact of sintering thermal history on kinetics of grain growth for 1264 ppm (0.1 wt.%) MgO‐doped alumina. Results suggest that sintering with a high heating rate in the order of 10 ³ °C⋅min ⁻¹ retards grain growth rate. By contrast, a slow heating rate of 5°C⋅min ⁻¹ resulted in an increase in grain growth constant by a factor of 4. The segregation of Mg to grain boundaries, favored by short sintering duration, is considered to be one of the possible factors contributing to the retarded grain growth rate for the rapidly heated compacts. Furthermore, it is proposed that the simultaneous combination of retarded grain boundary migration rate and limited surface‐diffusion‐driven grain coarsening, promoted by rapid heating, enables pressure‐less fabrication of translucent alumina without a need for vacuum or hydrogen atmosphere. In fact, it is demonstrated that weakly translucent alumina can be produced at relatively low temperatures of 1475 – 1550°C by sintering with a high heating rate (herein, 10 ³ °C⋅min ⁻¹ ) in ambient atmosphere.
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The grain boundary (GB) microstructure influences and is influenced by the development of residual stresses during synthesis of polycrystalline thin films. Recent studies have shown that the frustration between the preferred growth direction and rotations of abutting crystals to local cusps in GB energies leads to internal stresses localized within nanoscopic surface layers around the valleys and ridges that form at emergent boundaries (eGBs). Using a combination of continuum frameworks, numerical analyses, and all-atom simulations of bicrystal 〈111〉 copper films, we show that eGBs tune their surface morphology and rotation extent in response to external strains. Compression favors rotation to and growth of low-energy GB phases (complexions) at eGB valleys while tension favors the transitions at eGB ridges, a reflection of the stress-induced mass efflux/influx that changes the energetic balance between interfacial and deformation energies. Molecular dynamics simulations of strained and growing bicrystal films reveal that the eGB phase transition is coupled to island formation at the surface triple junctions, providing a direct link between eGB phases and surface step flow. The interplay between eGB structure, morphology, and mechanics emerges as a crucial ingredient for predictive understanding of stress and morphological evolution during film growth, with broad implications for multifunctional response of polycrystalline surfaces in a diverse range of surface phenomena such as surface-mediated deformation, interfacial embrittlement, thermal grooving, stress corrosion, surface catalysis, and topological conduction.
The role of anisotropic grain boundary energy in grain growth is investigated using textured microstructures that contain a high proportion of special grain boundaries. Textured and untextured Ca‐doped alumina was prepared by slip casting inside and outside a high magnetic field, respectively. At 1600°C, the textured microstructure exhibits faster growth than the untextured microstructure and its population of low‐angle boundaries increases. Atomic force microscopy (AFM) is employed to measure the geometry of thermal grooves to assess the relative grain boundary energy of these systems before and after growth. In the textured microstructure, the grain boundary energy distribution narrows and shifts to a lower average energy. Conversely, the energy distribution broadens for the untextured microstructure as it grows and exhibits abnormal grain growth. Further analysis of the boundary networks neighboring abnormal grains reveals an energy incentive that facilitates their growth. These results suggest that coarsening is not the only dominant grain growth mechanism and that the system can lower its energy effectively by replacing high energy boundaries with those of low energy. The faster growth of lower energy boundaries suggests that isotropic simulations do not adequately account for anisotropic grain growth mechanisms or anisotropic mobility.
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Nanoscale multilayers offer a convenient way to determine interdiffusion coefficients at low temperatures. However, knowledge regarding the impact of the microstructure on measurements is limited. In the present study, we measure the interdiffusion coefficient in the face-centered cubic (fcc) solid solution of the Ni–Cr system at 440 °C using multilayers composed of alternating layers of pure Ni and Ni78Cr22 (at.%), with a nominal wavelength of 4.5 nm. Three techniques were used to characterize the evolution of the multilayers with annealing time: atom probe tomography (APT), energy-dispersive X-ray spectroscopy (STEM-EDX) and X-ray reflectivity (XRR). Each technique allowed to determine an interdiffusion coefficient. The results evidence that the interdiffusion coefficient is dependent from the technique used to measure it. The primary cause is a very complex microstructure resulting from the elaboration method used to obtain the fine concentration modulation. The analysis of atom probe tomography volumes reveals a high density of columnar grain boundaries (GB) with extended chemical widths. The segregation at GB was measured and a model was derived, allowing to dissociate the contribution of lattice interdiffusion between layers from that of diffusion along and perpendicularly to GBs. The present results could serve as guides for future diffusion investigations involving multilayers.
Stabilization of grain structure is important for nanocrystalline alloys, and grain boundary segregation is a common approach to restrict coarsening. Doping can alter grain boundary structure, with high temperature states such as amorphous complexions being particularly promising for stabilization. Dopant enrichment at grain boundaries may also result in precipitate formation, giving rise to dopant partitioning between these two types of features. The present study elucidates the effect of dopant choice on the retention of amorphous complexions and the stabilization of grain size due to various forms of interfacial segregation in three binary nanocrystalline Al-rich systems, Al-Mg, Al-Ni, and Al-Y as investigated in detail using transmission electron microscopy. Amorphous complexions were retained in Al-Y even for very slow cooling conditions, suggesting that Y is the most efficient complexion stabilizer. Moreover, this system exhibited the highest number density of nanorod precipitates, reinforcing a recently observed correlation between amorphous complexions and grain boundary precipitation events. The dopant concentration at the grain boundaries in Al-Y is lower than in the other two systems, although enrichment compared to the matrix is similar, while secondary segregation to nanorod precipitate edges is much stronger in Al-Y than in Al-Mg and Al-Ni. Y is generally observed to be an efficient doping additive, as it stabilizes amorphous features and nanorod precipitates, and leaves very few atoms trapped in the matrix. As a result, all grains in Al-Y remained nanosized whereas abnormal grain growth occurred in the Al-Mg and Al-Ni alloys. The present study demonstrates nanocrystalline stability via simple alloy formulations and fewer dopant elements, which further encourage the usage of bulk nanostructured materials.
The growing trend towards engineering interfacial complexion (or phase) transitions has been seen in the grain boundary and solid surface systems.Meanwhile, little attention has been paid to the chemically heterogeneous solid/liquid interfaces. In this work, novel in-plane multi-interfacial states coexist within the Cu(111)/Pb(l) interface at a temperature just above the Pb freezing point is uncovered using atomistic simulations.Four monolayer interfacial states, i.e., two CuPb alloy liquids and two pre-freezing Pb solids, are observed coexisting within two interfacial layers sandwiched between the bulk solid Cu and bulk liquid Pb. Through computing the spatial variations of various properties along the direction normal to the in-plane solid-liquid boundary lines for both interfacial layers, a rich and varied picture depicting the inhomogeneity and anisotropy in the mechanical, thermodynamical, and dynamical properties is presented. The bulk values extracted from the in-plane profiles suggest that each interfacial state examined has distinct equilibrium values from each other and significantly deviates from those of the bulk solid and liquid phases, and indicate that the complexion (or phase) diagrams for the Cu(111)/Pb(l) interface bears a resemblance to that of the eutectic binary alloy systems, instead of the monotectic phase diagram for the bulk CuPb alloy. The reported data could support the development of interfacial complexion (or phase) diagrams and interfacial phase rules and provide a new guide for regulating heterogeneous nucleation and wetting processes.
If variety is the spice of life, then abnormal grain growth (AGG) may be the materials processing equivalent of sriracha sauce. Abnormally growing grains can be prismatic, dendritic, or practically any shape in between. When they grow at least an order of magnitude larger than their neighbors in the matrix—a state we call extreme AGG—we can examine the abnormal/matrix interface for clues to the underlying mechanism. Simulating AGG for various formulations of the grain boundary (GB) equation of motion, we show that anisotropies in GB mobility and energy leave a characteristic fingerprint in the abnormal/matrix boundary. Except in the case of prismatic growth, the morphological signature of most reported instances of AGG is consistent with a certain degree of GB mobility variability. Open questions remain, however, concerning the mechanism by which the corresponding growth advantage is established and maintained as the GBs of abnormal grains advance through the matrix.
Expected final online publication date for the Annual Review of Materials Research, Volume 53 is July 2023. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
The surface of ice exhibits the swath of phase-transition phenomena common to all materials and as such it acts as an ideal test bed of both theory and experiment. It is readily available, transparent, optically birefringent, and probing it in the laboratory does not require cryogenics or ultrahigh vacuum apparatus. Systematic study reveals the range of critical phenomena, equilibrium and nonequilibrium phase-transitions, and, most relevant to this review, premelting, that are traditionally studied in more simply bound solids. While this makes investigation of ice as a material appealing from the perspective of the physicist, its ubiquity and importance in the natural environment also make ice compelling to a broad range of disciplines in the Earth and planetary sciences. In this review we describe the physics of the premelting of ice and its relationship with the behavior of other materials more familiar to the condensed-matter community. A number of the many tendrils of the basic phenomena as they play out on land, in the oceans, and throughout the atmosphere and biosphere are developed.
The effects of 20 oxide dopants on the microstructure development of Mn-Zn ferrite during sintering were investigated by diffusion-couple-like experiments. Nine oxides enhanced grain growth in ferrite; the effects of TiO2 dopant were studied in detail. The TiO2-induced exaggerated grain growth shows a parabolic time dependence, as was also true for the diffusion of Ti into ferrite as shown by X-ray spectrometry. For samples sintered at a given temperature but for different times, the Ti concentrations at the growth fronts of the exaggerated grains are identical. Exaggerated grain growth kinetics and Ti diffusivities have similar activation energies. Furthermore, the mobility of the exaggerated grains is reduced to that of the matrix grains after the TiO2 layer is removed, showing that the exaggerated grain growth is sustained by Ti diffusion into ferrite. Probable mechanisms for TiO2-promoted exaggerated grain growth are proposed. Some self-consistent arguments are presented for the observed effects of other dopants. Grain boundary mobilities of undoped Mn-Zn ferrite were measured and are in good agreement with those reported in the literature.
Sintering metal particle (Cu or Ni) reinforced alumina composites, doped with CaO and SiO2 additives, resulted in metal particles found as occlusions within alumina grains, and located at alumina grain boundaries and triple junctions. In composites sintered without additives, only isolated cases of metal particle occlusion were found. In the doped samples, intergranular films were observed at alumina grain boundaries and metal-alumina interfaces, for both types of particles. The occluded particles were found to have a limited size-range of 250 +/- 20 nm, while particles at alumina grain boundaries had an average size of similar to1.4 +/- 0.3 mum. It is suggested that the higher driving force for grain growth during the initial stages of sintering, together with an increase in grain boundary mobility due to the intergranular films, results in increased particle-grain boundary detachment during the initial stages of sintering.
Abnormal grain growth without strong anisotropy or faceting of the grains has been observed in high-purity yttria-doped alumina specimens, often starting at the surface and spreading right through the bulk at higher sintering temperatures. This appears to occur because of an interaction between Si contamination from sintering and the yttria doping; no such effect is seen for undoped samples. Similar microstructures were observed after deliberate Y/Si codoping. Analytical STEM showed that some grain boundaries bordering on large grains contained more Si than Y. HRTEM and diffuse dark-field imaging revealed thin (0.5–0.9 nm) disordered layers at some boundaries bordering large grains. It appears that Si impurities are accumulating at some boundaries and together with the Y inducing a grain boundary structural transformation that accounts for the dramatically increased mobility of these boundaries.
Recently models of faceted crystal growth and of grain boundaries were proposed based on the gradient system with nondifferentiable energy. In this article, we study their most basic forms given by the equations u
t=(u
x/|u
x|)
x
and u
t=(1/a)(au
x/|u
x|)
x
, where both of the related energies include a |u
x| term of power one which is nondifferentiable at u
x=0. The first equation is spatially homogeneous, while the second one is spatially inhomogeneous when a depends on x. These equations naturally express nonlocal interactions through their singular diffusivities (infinitely large diffusion constant), which make the profiles of the solutions completely flat. The mathematical basis for justifying and analyzing these equations is explained, and theoretical and numerical approaches show how the solutions of the equations evolve.
The grain growth and densification have been investigated in very high-purity α-alumina doped with varying amounts of yttrium (0 to 3000 wt ppm of yttria) and sintered in air at 1450, 1550 and 1650 °C. Yttrium doping inhibited densification and coarsening at 1450 °C, but had very little effect at 1550 °C and no effect at 1650 °C. The change in densification behaviour is suggested to be related to the transition with increasing temperature from grain boundary diffusion to lattice diffusion controlled densification. The coarsening rate increases faster with temperature than the densification rate. This was correlated with a higher measured activation energy for grain growth than for the diffusion processes, which control the densification.
We present a two-dimensional phase field model of grain boundary statics and dynamics. We begin with a brief description and physical motivation of the crystalline phase field model. The description is followed by characterization and analysis of several microstructural implications: the grain boundary energy as a function of misorientation, the liquid–grain–grain triple junction behavior, the wetting condition for a grain boundary and stabilized widths of intercalating phases at these boundaries, and evolution of a polycrystalline microstructure by solidification and impingement, followed by both grain boundary migration and grain rotation. Simulations that demonstrate these implications are presented, with a description of the numerical methods that were used to obtain them.
Alumina specimens with various amounts of CaO and SiO2 (1:2 ratio) were prepared, and their abnormal grain growth (AGG) kinetics were investigated. A plot of the area fraction covered by abnormal grains versus log (sintering time) had a sigmoidal shape with an apparent incubation period before the onset of AGG. The overall kinetics of AGG was similar to that of a phase transformation controlled by nucleation and growth. The incubation time and the end point of AGG were strongly dependent on the amount of liquid-forming additives. Correspondingly, the final microstructure was affected by the liquid content: a large grain size and a high aspect ratio at low liquid content and a small grain size and a low aspect ratio at high liquid content.
Distribution of TiO2 dopants and SiO2 impurities in the bimodal microstructure of Al2O3 with anisotropic and equiaxed grains is systematically analyzed using analytical electron microscopy (AEM). The TiO2-doped ceramic materials were hot-pressed at 1500°C in a reducing environment. Different amounts of Ti solutes in the anisotropic or equiaxed grains were observed after removal of the contamination signal stemming from Ti on the surface. SiO2 and TiO2 exhibit a selective segregation behavior. The boundary between the equiaxed grains is segregated mainly by TiO2 but the boundary at the (0001) basal plane of anisotropic grains is covered with a thin amorphous film made up of mostly SiO2. Precipitation of Al2TiO5 occurs at high TiO2 doping levels. A bimodal microstructure develops in three stages, characterized successively by segregation, solution, and precipitation. The preferential adsorption of SiO2 to the (0001) basal plane initiates the anisotropic grain growth, starting at low TiO 2 doping level. At higher TiO2 doping level, bi-level Ti solution occurs, either as a result of equilibration between segregants and solutes, or incorporated as transient Ti solutes in the anisotropic grains due to fast-moving fronts. Further doping starts Al2TiO5 precipitation, which may result in de-wetting of the basal boundary, possibly due to a change of interface energy. The correlation and competition between segregation, solution, and precipitation characterize and dictate the evolution of microstructure, as monitored by the aspect ratio of anisotropic grains.
Single crystals of Al2O3 were reproducibly grown from an MgO-doped polycrystalline precursor. The single crystals were grown through controlled abnormal grain growth at temperatures between 1670 degrees and 1945 degrees C. It was observed that CaO impurities segregated to the boundary between the single crystal and the polycrystalline region, and formed a wetting intergranular film. This type of film is required to produce the highly mobile grain boundaries that facilitate single-crystal conversion. The measured grain boundary mobilities correspond reasonably well with the mobilities calculated from data for a grain boundary containing a film with properties of the bulk glass, although some deviation from bulk behavior is indicated by the difference in activation energy. The grain boundaries are the most highly mobile alumina grain boundaries measured to date. This suggests that extrinsic effects produce the highest grain boundary mobility, rather than intrinsic behavior, which has conventionally been assumed to be the fastest.
Distribution of TiO2 dopants and SiO2 impurities in the bimodal microstructure of Al2O3 with anisotropic and equiaxed grains is systematically analyzed using analytical electron microscopy (AEM). The TiO2-doped ceramic materials were hot-pressed at 1500°C in a reducing environment. Different amounts of Ti solutes in the anisotropic or equiaxed grains were observed after removal of the contamination signal stemming from Ti on the surface. SiO2 and TiO2 exhibit a selective segregation behavior. The boundary between the equiaxed grains is segregated mainly by TiO2 but the boundary at the (0001) basal plane of anisotropic grains is covered with a thin amorphous film made up of mostly SiO2. Precipitation of Al2TiO5 occurs at high TiO2 doping levels.
A bimodal microstructure develops in three stages, characterized successively by segregation, solution, and precipitation. The preferential adsorption of SiO2 to the (0001) basal plane initiates the anisotropic grain growth, starting at low TiO2 doping level. At higher TiO2 doping level, bi-level Ti solution occurs, either as a result of equilibration between segregants and solutes, or incorporated as transient Ti solutes in the anisotropic grains due to fast-moving fronts. Further doping starts Al2TiO5 precipitation, which may result in de-wetting of the basal boundary, possibly due to a change of interface energy. The correlation and competition between segregation, solution, and precipitation characterize and dictate the evolution of microstructure, as monitored by the aspect ratio of anisotropic grains.
The microstructure of sol-gel-derived {alpha}-alumina (Al{sub 2}O{sub 3}) doped with 0.6 wt% titania, sintered at 1450 C for 1 h, consisted of thin platelets with (0001) faces in a matrix of equiaxed grains. Short facets at the edges of the platelets developed primarily parallel to the {l_brace}10{bar 1}2{r_brace} planes, while some were parallel to the {l_brace}11{bar 2}3{r_brace} planes; other edges showed irregular, curved boundaries. The basal surfaces of the platelets were coated with thin layers (0.5--6 nm) of an amorphous titanium-containing aluminosilicate phase, which also was present at triple points. No amorphous phase was found on the short faceted boundaries, on curved boundaries at platelet edges, or at grain boundaries of equiaxed, matrix grains. However, titanium enrichment was observed at all examined boundaries, suggesting that titanium segregation alone did not account for the development of anisotropic microstructure. Curved incursions on basal facets were associated with occasional particles of aluminum titanate (Al{sub 2}TiO{sub 5}).
It is shown that in any two-phase mixture of fluids near their critical point, contact angles against any third phase become zero in that one of the critical phases completely wets the third phase and excludes contact with the other critical phase. A surface layer of the wetting phase continues to exist under a range of conditions when this phase is no longer stable as a bulk. At some temperature below the critical, this perfect wetting terminates in what is described as a first-order transition of the surface. This surface first-order transition may exhibit its own critical point. The theory is qualitatively in agreement with observations.
Several low-index symmetrical tilt grain boundaries in α-Al2O3 were investigated by combining high-resolution transmission electron microscopy and spatially resolved energy dispersive X-ray analysis. Bicrystals with a well-defined orientation relation and interface plane were produced by diffusion bonding under ultrahigh vacuum. Grain boundaries like the prismatic and rhombohedral twin develop an atomically sharp and clean interface, whereas higher index grain boundary (GB) planes show a more complex structure and are able to accommodate both Si and Ca in the interface. Additionally, it was found that asymmetric GB planes which occur as facets at rhombohedral and prismatic twins also accommodate impurity atoms.
There is a growing body of the literature that suggests that there are grain boundary structures that exist in ceramic systems that are not predicted by bulk thermodynamics. Transitions between the various grain boundary structures are not well understood either experimentally or theoretically. This study identifies six different types of grain boundary structures present in the alumina system. These grain boundary structures were directly correlated with significantly different grain boundary mobilities. There is a general trend towards increasing grain boundary disorder and increasing grain boundary mobility with increasing temperature. This is the first time such a broad range of behavior has been observed in a single system. The results have many implications on understanding grain boundary transport phenomena in ceramic systems.
Nickel-doped tungsten specimens were prepared with high purity chemicals and sintered. Although activated sintering starts more than 400°C below the bulk eutectic temperature, the nickel-rich crystalline secondary phase does not wet the tungsten grain boundaries in the solid state. These results contrast with the classical activated sintering model whereby the secondary crystalline phase was presumed to wet grain boundaries completely. High resolution transmission electron microscopy and Auger electron spectroscopy revealed the presence of nanometer-thick, nickel-enriched, disordered films at grain boundaries well below the bulk eutectic temperature. These interfacial films can be regarded as metallic counterparts to widely observed equilibrium-thickness intergranular films in ceramics. Assuming they form at a true thermodynamic equilibrium, these films can alternatively be understood as a class of combined grain boundary disordering and adsorption structures resulting from coupled premelting and prewetting transitions. It is concluded that enhanced diffusion in these thin intergranular films is responsible for solid-state activated sintering.
Microstructural changes that occurred during the sintering of alumina doped with TiO2 and SiO2 have been investigated. The kinetics of normal grain growth at the initial stage is retarded by the dopant segregation at the grain boundaries. However, due to the accumulation of dopants at the grain boundaries during grain growth, its concentration at the boundaries ultimately exceeds the solubility limit and an intergranular liquid film emerges. The appearance of the liquid and the resulting increase in boundary mobility are confirmed to be the main cause of abnormal grain growth. For the abnormal grains, a liquid phase is observed at the basal surface in most cases, while the edges of these grains were partially wetted. Anisotropic or directional growth of the abnormal grains is explained in terms of the enhanced growth kinetics due to the re-entrant edges formed by grain boundaries at the non-basal planes.
Grain boundary segregation of Y in α-Al2O3 and evolution of the structural environment around the Y atoms have been investigated using high resolution STEM and EXAFS. The stages of incorporation of Y atoms by α-Al2O3 grain boundaries, on average, are characterized by three composition regimes: (I) dilute to saturated; (II) supersaturated [where the degree of supersaturation is determined by the nucleation barrier for Y3Al5O12 (YAG)]; and (III) equilibrium with YAG precipitates. The average Y grain boundary concentration in equilibrium with YAG precipitates has been determined to be ∼1/4 equivalent monolayer, and the maximum supersaturation concentration has been determined to be ∼1/2 equivalent monolayer. EXAFS revealed that accompanying the supersaturation of grain boundaries with Y is an increasing Y–O nearest neighbor coordination number and, simultaneously, a significantly increased degree of ordering of Y with respect to Al ions beyond nearest neighbor O. This Y–Al distance is the same as that for Y absorbed on the free surface of α-Al2O3, and the same as that expected for the Y–Al distance when Y substitutes for Al with the Y–O distance relaxed to that in Y2O3. This compositional and structural information has led to a clearer picture of how the grain boundary segregated Y concentration influences grain boundary structure. For dilute Y concentrations, Y ions preferentially fill sites in the grain boundary core which have well defined order only within the nearest neighbor shell of oxygens. As the Y concentration increases, Y begins to occupy near-boundary sites, forming two near-boundary layers, each adjacent to a grain surface. The near-boundary layer has nearest neighbor ordering extending at least to nearest neighbor cations. Nucleation of the YAG phase leads to the depletion of Y from these partially ordered layers.
The Potts lattice-gas model is examined via Monte Carlo simulations and a mean-field approximation to obtain phase diagrams for multilayer adsorbed films. The advantage of the model is that it has three bulk phases, interpreted here as solid, liquid, and gas, allowing one to study the relationships between the phase transitions of the film and the bulk triple point. For our choices of coupling parameters, we find a solid film that wets the substrate and that melts, forming a liquid film, below the bulk triple-point temperature and either above or below the bulk roughening temperature. The liquid film wets the substrate above the bulk triple point but not below.
This paper presents a systematic classification of multilayer-adsorption phenomena on attractive substrates, with emphasis on the buildup of thick films. The approach is based on statistical mechanics and includes adsorption-desorption effects and the interrelation of bulk and surface behavior. The surface phase diagram depends qualitatively on the relative strengths and ranges of adatom-adatom and adatom-substrate attractions. When the adatom-substrate attraction dominates (strong substrate), the film builds up uniformly, as the bulk adatom density increases, and the excess surface density diverges at coexistence (complete wetting). The buildup proceeds via an infinite sequence of discrete layer transitions (layering) at low temperatures (below the roughening temperature TR and smoothly at higher temperatures, as originally noted by de Oliveira and Griffiths. Substrates of intermediate strength are characterized by a wetting temperature TW above which wetting at coexistence is approached. The relative values of TW and TR define three subregions: When TW<TR layering occurs, with an infinite sequence of transitions between TW and TR when TR≲TW, layer transitions have coalesced into a single thick-film—thin-film transition (prewetting); when TR≪TW, prewetting may disappear, leaving only a critical-wetting transition on the coexistence axis. For still weaker substrates, wetting is incomplete at all temperatures; however, a variety of drying phenomena may occur on the high-density side of bulk coexistence. Specific calculations are given for a lattice-gas model at T=0 and in the mean-field approximation. Conclusions are informed, in addition, by certain exact results and symmetries. The last section includes a critical discussion of the relation of the lattice-gas model to the real world and a brief review of relevant experimental data.
The structural characterization of the whisker-matrix interface in a β-Si3N4 whisker-Al alloy 6061 composite was investigated by high-resolution transmission electron microscopy (HRTEM). It was shown that there existed a nearly amorphous layer (2–3 nm thickness) between the whiskers and the Al matrix. Mg segregation at the whisker-Al interface was revealed by energy-dispersive X-ray analysis. HRTEM observations indicated that MgO and MgAl2O4 nanocrystalline particles were easily formed at the whisker-Al interface in the composite owing to Mg segregation at the interface during the manufacturing of the composite. The Mg2Si particles (strengthening phase in the matrix) precipitated preferentially at the whisker-Al interface when the composite had been given a T6 heat treatment. Specific orientation relationships between the MgAl2O4 or Mg2Si particles and the β-Si2N4 whiskers were found. The problems associated with characterizing the microstructure of a whisker-matrix interface in an Al alloy composite reinforced by whiskers are also discussed.
Two different electron energy loss spectroscopy (EELS) quantitative analytical methods for obtaining complete compositions from interface regions are applied to ultrathin oxide-based amorphous grain boundary (GB) films of similar to 1 nm thickness in high-purity HIPed Si3N4 ceramics. The first method, 1, is a quantification of the segregation excess at interfaces for all the elements, including the bulk constituents such as silicon and nitrogen; this yields a GB film composition of SiN0.49+/-1.4O1.02+/-0.42 when combined with the average film thickness from high resolution electron microscopy (HREM). The second method, II, is based on an EELS near-edge structure (ELNES) analysis of the Si-L-2.3 edge of thin GB films which permits a subtraction procedure that yields a complete EELS spectrum, e.g., that also includes the O-K and N-K edges, explicitly for the GB film. From analysis of these spectra, the film composition is directly obtained as SiN0.63+/-0.19O1.44+/-0.33, close to the one obtained by the first method but with much better statistical quality. The improved quality results from the fewer assumptions made in method II; while in method I uniform thickness and illumination condition have to be assumed, and correction of such effects yields an extra systematic error. Method II is convenient as it does not depend on the film thickness detected by HREM, nor suffer from material lost by preferential thinning at the GB. in addition, a chemical width for these films can be deduced as 1.33 +/- 0.25 nm, that depends on an estimation of film density based on its composition. Such a chemical width is in good agreement with the structural thickness determined by HREM, with a small difference that is probably due to the different way in which these techniques probe the GB film. The GB film compositions are both nonstoichiometric, but in an opposite sense, this discrepancy is probably due to different ways of treating the surface oxidation layers in both methods.
The effect of abnormal grain growth on the formation of amorphous films at grains boundaries was studied in a model system BaTiO3. 0.4 mol% TiO2-excess BaTiO3 powder compacts were sintered at 1380°C for various times up to 16 h. During the sintering, abnormal grains formed. With the growth of the abnormal grains, amorphous films formed and eventually thickened up to 19.2 nm at grain boundaries. The film formation is attributed to the accumulation of Ti solutes at grain boundaries with the grain growth, while the film thickening is mostly caused by the redistribution of liquid at triple junctions. Extended annealing of the 16-h-sintered sample at 1350°C for 15 days resulted in a thinning of the film to nearly 1.7 nm without a change in the grain size, showing an equilibrium thickness. This result demonstrates that the film thickness observed during the growth of the grain may not be the equilibrium thickness. The result further suggests that the shape of the abnormal grains, even when equiaxed, can differ from the equilibrium shape.
In a given batch more than 30%–40% of polycrystalline, MgO-doped Al2O3 tubes were converted into single crystals of sapphire by abnormal grain growth (AGG) in the solid state at 1880°C. Most crystals grew 4–10-cm in length in tubes with wall thicknesses of 1/2 and 3/4 mm and outer diameters of 5 and 7 mm, respectively, and had their c-axes oriented ∼ 90° and 45° to the tube axis. Initiation of AGG was associated with low values of bulk MgO concentration near 50 ppm. The unconverted tubes did not develop centimeter-size crystals but instead exhibited millimeter-size grains. The different grain structures in converted and unconverted tubes may be related to nonuniform concentration of MgO in the extruded tubes. The growth front of the migrating crystal boundary was typically nonuniformly shaped, and the interface between the single crystal and the polycrystalline matrix was composed of many “curved” boundary segments indicative of classical AGG in a single-phase material. The average velocities of many migrating crystal boundaries were quite high and reached ∼1.5 cm/h. The average grain boundary mobility at 1880°C was calculated as 2 × 10−10 m3/(N·s), representing the highest value reported so far in Al2O3 and within a factor of 2.5 of the calculated intrinsic mobility. Under similar experimental conditions sapphire crystals did not grow when a codopant of CaO, La2O3, or ZrO2 was added in concentrations of several hundred ppm.
Alumina specimens with various amounts of CaO and SiO2 (1:2 ratio) were prepared, and their abnormal grain growth (AGG) kinetics were investigated. A plot of the area fraction covered by abnormal grains versus log (sintering time) had a sigmoidal shape with an apparent incubation period before the onset of AGG. The overall kinetics of AGG was similar to that of a phase transformation controlled by nucleation and growth. The incubation time and the end point of AGG were strongly dependent on the amount of liquid-forming additives. Correspondingly, the final microstructure was affected by the liquid content: a large grain size and a high aspect ratio at low liquid content and a small grain size and a low aspect ratio at high liquid content.
The microstructural features of platelike grains in liquid-phase-sintered Al2O3 were investigated. The flat boundaries of platelike grains wetted with a liquid phase were basal planes. After impingement of platelike grains, flat boundaries progressively changed to curved boundaries which consisted of both basal and rhombohedral planes as microscopic facets. Second-phase particles were observed for most doped samples. Small-angle subgrain boundaries inside platelike grains were also observed.
Analytical and high-resolution transmission electron microscopies were used to study the structure and chemistry of two-grain boundaries and three-grain junctions in polycrystalline alumina sintered with additions of between 0 and 10 wt% calcium silicate. Addition of calcium silicate greatly aided full densification and resulted in the presence of a continuous, amorphous grain-boundary film at the majority of the two-grain boundaries, the thickness of which was independent of the bulk level of additive. The chemistry of the glass at the grain boundaries and the three-grain junctions was notably different. The grain boundaries showed strong segregation of calcium, whereas both silicon and calcium appeared to favor triple pockets and larger-volume facets at the grain boundaries. Grain-triple-pocket interfaces also showed segregation of calcium. The overall extent of segregation appeared to be independent of the additive level. The amorphous grain-boundary film was of nominal composition CaO6Al2O3 and contained predominantly octahedrally coordinated aluminum within the glass. The triple pockets were generally of a composition within the primary-phase field of anorthite and contained tetrahedrally coordinated aluminum and silicon.
Abnormal grain growth (AGG) is not one of the intrinsic properties of alumina but rather is an extrinsic property that is controlled by certain impurities that are introduced during powder synthesis, processing, or sintering. When small amounts of glass-forming impurities are introduced, some portion beyond their solubility limits will accumulate at grain boundaries at the final stage of densification, form thin intergranular glass films of thermodynamically stable thickness, and induce the sudden appearance of abnormal grains by increasing the rate of grain-boundary migration abruptly. The proposition has been tested experimentally with small, but varying, amounts of silica in ultrapure alumina (99.999%) that has been sintered in a contamination-free condition. Average grain sizes for the appearance of AGG are inversely related to the doping concentration of silica. The thickness of intergranular silicate glass films at the onset of AGG in alumina is constant and estimated to be }3.7 nm.
The tensile creep behavior of two rare-earth dopant systems, lanthanum- and yttrium-doped alumina, are compared and contrasted in order to better understand the role of oversized, isovalent cation dopants in determining creep behavior. It was found that, despite some microstructural differences, these systems displayed qualitatively a similar improvement in creep resistance, supporting the hypothesis that creep is strongly influenced by segregation. Differences in primary creep behavior and activation energy for steady-state creep were, however, observed for these systems. Given these results, it is expected that creep behavior can be further optimized by adjusting the dopant level and by controlling the microstructure.
The microstructure of sol-gel-derived alpha-alumina (Al2O3) doped with 0.6 wt% titania, sintered at 1450°C for 1 h, consisted of thin platelets with (0001) faces in a matrix of equiaxed grains. Short facets at the edges of the platelets developed primarily parallel to the {102} planes, while some were parallel to the {113} planes; other edges showed irregular, curved boundaries. The basal surfaces of the platelets were coated with thin layers (0.5-6 nm) of an amorphous titanium-containing aluminosilicate phase, which also was present at triple points. No amorphous phase was found on the short faceted boundaries, on curved boundaries at platelet edges, or at grain boundaries of equiaxed, matrix grains. However, titanium enrichment was observed at all examined boundaries, suggesting that titanium segregation alone did not account for the development of anisotropic microstructure. Curved incursions on basal facets were associated with occasional particles of aluminum titanate (Al2TiO5).
The influence of the minor impurity atoms S, Se and Te on the rate of the normal grain growth in the primary recrystallization of 0.8 % Si-iron has been investigated. The rates of the grain growth are observed to be appreciably retarded in the order, S, Se, Te. Since theoretical treatments applicable to the rate of the normal grain growth controlled by the impurity-drag effect are lacking, a model based on the absolute reaction rate theory is proposed. Comparing the experimental results with the model, the values of the unknown physical parameters in the theory are determined to be as follows; the free energy of activation for the boundary diffusion of iron atom is 31.88 kcal mol-1, and the interaction energy of a solute atom in the iron lattice with a boundary is -0.946, -1.000 and -1.140 eV for S, Se and Te, respectively. The validity of the present model is shown by discussing the reasonableness of these determined values.
Aluminum nitride is not so extensively applied because of low productivity and high cost due to necessity of high sintering temperature over 1900 °C and long sintering time around 10 h. In the present study, high thermal conductivity over 200 W/(m K) was attained by sintering with 28 GHz millimeter-wave heating at 1700 °C for 2 h under nitrogen/hydrogen mixed gas atmosphere. Attainment of such a high thermal conductivity of aluminum nitride sintered by millimeter-wave at low temperature for a short time is attributed to a characteristic microstructure induced by millimeter-wave heating. From the results of the observation by high resolution TEM, the intergranular film layer between aluminum nitride grains in the sintered body by millimeter-wave heating was as thin as difficult to be observed, resulting in remarkable enhancement of heat transfer at the thinner intergranular phase. Therefore, high thermal conductivity was attained in the millimeter-wave-sintered aluminum nitride in spite of short sintering time and low sintering temperature, compared with the conventional sintering method.
Hot-pressed alumina samples, with 5 wt.% additions of CaO:SiO2 possessing molar ratios ranging between 1:5 and 10:1 were studied by electron microscopy. Marked differences in microstructure (e.g. grain size, secondary crystalline phases and grain boundary film thickness) were observed, which depended on the composition of sintering additives. The compositions of glassy phases in triple pockets and in grain boundaries varied markedly depending on sintering additives, but also within individual specimens. High residual compressive stresses were measured in alumina grains of samples which contained low thermal expansion crystalline phases such as anorthite and grossite, while gehlenite-containing samples were correspondingly less stressed.
Segregation of Bi at ZnO grain boundaries in the presence of a small amount of Bi-rich liquid has been studied. Below the eutectic temperature (740 °C), a partial monolayer of Bi segregation is observed in equilibrium with a non-wetting solid phase. Above the eutectic temperature, the behavior is more complex. Partial monolayer segregation of Bi is observed in equilibrium with a Bi-rich grain junction liquid which meets many grain boundaries with a zero contact angle. Although at larger volume fractions, the liquid fully wets and penetrates along boundaries, when the liquid fraction is small, a continuous liquid film is not detectable. The grain boundary coverage of Bi has been quantified using STEM, and the interplay between capillary pressure, temperature, and composition in determining boundary coverage and the distribution of liquid is discussed. An alternative interpretation of the segregation as the remnant of a high-temperature intergranular liquid film is considered.
The conditions for structural transitions at the core of a grain boundary separating two crystals was investigated with a diffuse interface model that incorporates disorder and crystal orientation [Kobayashi , Physica D 140, 141 (2000)]. The model predicts that limited structural disorder near the grain boundary core can be favorable below the melting point. This disordered material is a precursor to a liquid phase and therefore the model represents grain boundary premelting. This model is shown to be isomorphic to Cahn's critical point wetting theory [J.W. Cahn, J. Chem. Phys. 66, 3667 (1977)] and predicts first- and higher-order structural grain boundary transitions. A graphical construction predicts the equilibrium grain boundary core disorder, the grain boundary energy density, and the relative stability of multiple grain boundary "complexions." The graphical construction permits qualitative inference of the effect of model properties, such as empirical homogeneous free energy density and assumed gradient energy coefficients, on properties. A quantitative criterion is derived which determines whether a first-order grain boundary transition will occur. In those systems where first-order transition does occur, they are limited to intermediate grain-boundary misorientations and to a limited range of temperatures below the melting point. Larger misorientations lead to continuously increasing disorder up to the melting point at which the disorder matches a liquid state. Smaller misorientation continuously disorder but are not completely disordered at the melting point. Characteristic grain boundary widths and energies are calculated as is the width's divergence behavior at the melting point. Grain boundary phase diagrams are produced. The relations between the model's predictions and atomistic simulations and with experimental observations are examined.
The Orowan, Petch, and Knudsen equations were examined against 46 sets of strength-vs-grain-size data. For the 30 sets which are most discriminating, represented by 229 averaged-data points, the variances of the Orowan-Petch and Knudsen treatments are ∼4.5 and ∼17.1 kpsi2, respectively. Statistical considerations give preference to the Orowan-Petch treatment at a high confidence level in these 30 cases, showing that the Knudsen equation does not remove systematic variations of strength as a function of grain size. The Orowan-Petch treatment also appears to provide a sounder basis for extrapolation. The identification of the Orowan and Petch equations with postulated physical models permits interpretation of data in terms of probable underlying causes. Several examples are discussed and in most cases agree at least qualitatively with present understanding. A satisfactory physical model for Petch behavior in “nonyielding” ceramics is needed.
Alumina specimens with small amounts of CaO and TiO2 were prepared and their microstructural evolution during sintering was investigated. Because of the appearance of a liquid phase during sintering, a duplex microstructure of a few abnormal grains and fine matrix grains was obtained when the CaO + TiO2 content was small (≤0.04 wt%). When the CaO + TiO2 content was relatively high (≥0.1 wt%), many grains grew and impinged upon each other. As a result, a rather uniform and homogeneous microstructure was observed.
Various properties of ceramics can be significantly influenced by the presence of grain boundaries. The influence on the properties is closely related to the grain-boundary atomic structures. As different grain boundaries have different atomic structure, different grain boundaries have different influence on the properties. It is difficult to examine the atomic structure and properties of individual grain boundaries in ceramics. In order to understand the atomic–structure–property relationships, well-defined single grain boundaries should be characterized. In the present paper, we review our recent results on the investigations of atomic structures and electrical properties of ZnO single grain boundaries. The relationships between the atomic structures and the electrical properties were investigated using ZnO bicrystals, whose grain-boundary orientation relationship and grain-boundary planes can be arbitrarily controlled. The discussion focuses on the microscopic origin of nonlinear current–voltage (I–V) characteristics across ZnO grain boundaries. High-resolution transmission electron microscopy (HRTEM) observations and lattice-statics calculations revealed the atomic structures of the undoped ZnO [0001] Σ7 and Σ49 grain boundaries, enabling a comparison between coincidence site lattice (CSL) boundaries with small and large periodicity. These grain boundaries contained the common structural units (SUs) featuring atoms with coordination numbers that are unusual in ZnO. The Σ49 boundary was found to have characteristic arrangement of the SUs, where two kinds of the SUs are alternatively formed. It is considered that the characteristic arrangement was formed to effectively relax the local strain in the vicinity of the boundary. Such a relaxation of local strain is considered to be one of dominant factors to determine the SU arrangements along grain boundaries. I–V measurements of the undoped ZnO bicrystals showed linear I–V characteristics. Although the coordination and bond lengths of atoms in the grain boundaries differ from those in the bulk crystal, this does apparently not generate deep unoccupied states in the band gap. This indicates that atomic structures of undoped ZnO grain boundaries are not responsible for the nonlinear I–V characteristics of ZnO ceramics. On the other hand, the nonlinear I–V characteristic appeared when doping the boundaries with Pr. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of Pr-doped boundaries revealed that Pr segregates to specific atomic columns, substituting Zn at the boundary. However, the Pr itself was not the direct origin of the nonlinear I–V characteristics, as the Pr existed in the three-plus state and would not produce acceptor states in the boundary. First-principles calculations revealed that Pr doping instead promotes the formations of acceptor-like native defects, such as Zn vacancies. We believe that such acceptor-like native defects are microscopic origin of the nonlinear I–V characteristics. Investigations of various types of grain boundaries in the Pr and Co-codoped ZnO bicrystals indicated that the amounts of Pr segregation and the nonlinear I–V characteristics significantly depend on the grain-boundary orientation relationship. Larger amount of Pr segregation and, as a result, higher nonlinearity in I–V characteristics was obtained for incoherent boundaries. This indicates that Pr doping to incoherent boundaries is one of the guidelines to design the single grain boundaries with highly nonlinear I–V characteristics. Finally, a Pr and Co-codoped bicrystal with an incoherent boundary was fabricated to demonstrate a highly nonlinear I–V characteristic. This result indicates that ZnO single-grain-boundary varistors can be designed by controlling grain-boundary atomic structures and fabrication processes.Summarizing, our work firstly enabled us to gain a deeper understanding for the atomic structure of ZnO grain boundaries. Secondly, we obtained important insight into the origin of nonlinear I–V characteristics across the ZnO grain boundaries. And, finally, based on these results, we demonstrated the potential of this knowledge for designing and fabricating ZnO single-grain-boundary varistors.
Grain-boundary films 0.6 nm in size have been observed on the grain boundaries of neodymia (Nd2O3)-doped alumina (α-Al2O3) sintered at 1800°C. Direct observation by high-angle annular dark-field imaging in the aberration-corrected scanning transmission electron microscope shows that this type of grain-boundary structure is the result of multilayer adsorption. Neodymium cations adsorb onto the faces of each of the two grains that comprise the grain boundary by substituting for aluminum cations. The positions of these cations are slightly distorted relative to the perfect lattice, and a third atomic layer in the core of the grain-boundary resides between these two layers. The measurements also confirm that the thickness deduced from high-resolution transmission electron microscopy lattice images are accurate.
Thick-film resistors arc electrical composites containing ultrafine particles of ruthenate conductor (Pb2Ru2O7 in the present materials) distributed in a highly modified silicate glass. We show that conductor particles remain flocced in the absence of any applied or capillary pressures, but are separated at equilibrium by a nanometer-thick film of glass. Microstructures show evidence for liquid-phase sintering, i.e., contact flattening of particles, under van der Waals attraction alone. Titania addition, which in dilute concentrations markedly increases the resistivity, decreases the temperature coefficient of resistance, and improves voltage stability and noise, is found to increase the equilibrium film thickness between particles by a few angstroms. STEM analyses show that the added titania preferentially concentrates in the silicate-rich grain boundary film, as well as at particle–glass interfaces. The roles of interparticle forces and adsorption on the glass film thickness with and without titania are discussed. The large increase in resistivity caused by titania additions is attributed to the increase in Film thickness as well as to local chemical changes of two possible types. Titania enrichment within the glass film itself is expected to decrease the local ruthenium ion solubility, and this along with the possible formation of a more insulating titania-substituted surface layer on ruthenate grains will decrease the tunneling conductivity between conductor grains.
Single crystals of Al2O3 were reproducibly grown from an MgO-doped polycrystalline precursor. The single crystals were grown through controlled abnormal grain growth at temperatures between 1670° and 1945°C. It was observed that CaO impurities segregated to the boundary between the single crystal and the polycrystalline region, and formed a wetting intergranular film. This type of film is required to produce the highly mobile grain boundaries that facilitate single-crystal conversion. The measured grain boundary mobilities correspond reasonably well with the mobilities calculated from data for a grain boundary containing a film with properties of the bulk glass, although some deviation from bulk behavior is indicated by the difference in activation energy. The grain boundaries are the most highly mobile alumina grain boundaries measured to date. This suggests that extrinsic effects produce the highest grain boundary mobility, rather than intrinsic behavior, which has conventionally been assumed to be the fastest.
The kinetics of densification and grain growth of ultrapure alumina (> 99.999%) were measured for clean sintering conditions in a pure-sapphire tube, and compared with kinetics measured during normal sintering conditions in an alumina crucible of 99.8% purity. For the clean condition, the microstructure of sintered alumina remained homogeneous and only normal grain growth was observed up to 1900C for 5 H. However, under the normal sintering condition, both normal and abnormal grain growth were observed depending on the sintering temperature and time. Thus, abnormal grain growth in alumina could be effectively suppressed without introducing sintering aids (such as MgO) by using an ultrapure powder and by preventing the introduction of any impurities throughout the sintering process. This result strongly suggests that abnormal grain in commercially pure alumina ( 99.99%) is not an intrinsic property of alumina but an extrinsic property controlled by minor constituents that can be present in the original powder or introduced during powder processing and subsequent sintering.
Olivine grain boundaries and phase boundaries in xenoliths from San Carlos have been investigated by high-resolution transmission
electron microscopy (HREM) and analytical electron microscopy (AEM). Thin amorphous intergranular layers with variable width
(1–2 nm) were detected along olivine grain boundaries. The Al2O3, TiO2 and CaO concentrations of the amorphous layers increase with increasing width of the layer. The composition of the amorphous
intergranular layers depends on the interface type – grain or phase boundary. Morphology, amorphous state and chemical composition
of the intergranular layer suggest the presence of a melt film at olivine grain boundaries. Since the composition of the amorphous
phase strongly depends on the type of interface, the melt must have been generated at the grain boundary. Also, the melt chemistry
is different from the composition of partial melts produced from possible hydrous phases, such as phlogopite or amphibole,
and from the host basanite. The mobility of very thin melt films is assumed to be very limited due to the strong interface
forces between the melt and the grain boundary. It is concluded that grain boundary melting occurred at the interfaces due
to decompression during uplift. The melt wetted olivine grain boundaries as well as olivine-opx phase boundaries. The thin
amorphous layers formed melt microsystems. Mixing of melts from different microsystems is suggested to occur in wider melt
films, melt veins or melt pockets thus creating a magmatic melt that could be extracted from its source.
The room-temperature crack-growth properties of an in situ toughened, monolithic silicon carbide are reported. Hot pressing was performed at 1900°C with 3 wt.% Al, 2 wt.% C and 0.6 wt.% B additions. Compared to a commercial SiC (Hexoloy SA), significant improvements in both the fracture toughness and cyclic fatigue-crack propagation resistance have been achieved through control of the β to α transformation. Using fatigue-precracked, disk-shaped compact-tension specimens, marked rising resistance-curve behavior was measured over the first ∼ 600 μm of crack extension, leading to a “plateau” fracture toughness of Kc ∼ 9.1 MPa√m; this represents more than a threefold increase over the toughness of Hexology, where a Kc value of 2.5 MPa√m was measured with no evidence of a resistance curve. Cyclic fatigue-crack growth rates in the toughened SiC were found to be faster than those under sustained loads (static fatigue) at the same stress-intensity level. The cyclic fatigue-crack growth resistance was found to be far superior to that of Hexology. Whereas cracking in the commercial SiC became unstable when the maximum stress intensity Kmax exceeded ∼ 2 MPa√m, thresholds for fatigue-crack growth in the in situ toughened material exceeded a Kmax of 7 MPa√m. Such dramatic improvements in the crack-growth resistance of SiC are attributed to a microstructure consisting of a network of interlocking, plate-like predominantly α-phase grains, which combine to both bridge and deflect the crack. Under cyclic loads, fatigue-crack growth is promoted by the cycle-dependent decay in such crack-tip shielding due to frictional-wear degradation of the zone of grain bridging ligaments in the crack wake. These results represent the first reported evidence of cyclic fatigue behavior in a monolithic silicon carbide and the first direct measurement of the resistance curve properties in this ceramic.
The microstructures of two dense polycrystalline aluminas hot pressed with a total of 5 wt.% of liquid phase-forming sintering additives: calcium oxide (CaO) and silicon oxide (SiO2) but with differing CaO:SiO2 molar ratios, have been studied using analytical scanning electron microscopy (SEM) and transmission electron microscopy (TEM) combined with energy dispersive X-ray (EDX) spectrometry and electron energy loss spectrometry (EELS). These techniques allowed the direct imaging of secondary crystalline phases and amorphous films at grain boundaries and at triple pockets as well as both a qualitative and semi-quantitative determination of their composition and chemistry. Two samples CS5 and CSp, containing CaO:SiO2 molar ratios of 1:5 and 1:1.33 respectively, were studied in detail. SEM revealed big differences in median grain size between the CS5 sample, 0.56 μm, and the CSp sample, which was more than 2 times larger. TEM revealed that the CS5 sample exhibited a significant proportion of a secondary crystalline anorthite phase at triple pockets which resulted in a silica-rich glassy intergranular phase deficient in calcium possessing a composition lying in the mullite phase field of the ternary phase diagram. Conversely the CSp specimen possessed a heterogeneous glassy film which differed in composition between grain boundaries (where significant Ca was observed) and triple pockets (which were more silica-rich). The microstructure of CSp also showed evidence for dislocation arrays, strain and microcracks possibly due to the mismatch in thermal expansion coefficients between alumina grains, glassy and secondary and crystallised phases which, together with bigger grain size, may explain the reduced wear resistance relative to CS5.
In this work the structure and chemistry of intergranular films at metal–ceramic interfaces was investigated via detailed microstructural characterization of model metal–Al2O3 nanocomposites. We report here experimental results indicating the formation and stability of equilibrium nanometer-thick films at metal–ceramic interfaces. Thin ∼1 nm interface films were observed for two different metal–alumina systems (Ni and Cu) doped with glass-forming additives. High spatial resolution energy dispersive spectroscopy showed a difference in the chemical composition of the films at Ni–alumina and Cu–alumina interfaces. These results may have immediate ramifications on structural and functional properties of metal–ceramic interfaces.
A two-dimensional frame-invariant phase field model of grain boundaries is developed. One-dimensional analytical solutions for a stable grain boundary in a bicrystal are obtained, and equilibrium energies are computed. With an appropriate choice of functional dependencies, the grain boundary energy takes the same analytic form as the microscopic (dislocation) model of Read and Shockley [W.T. Read, W. Shockley, Phys. Rev. 78 (1950) 275]. In addition, dynamic (one-dimensional) solutions are presented, showing rotation of a small grain between two pinned grains and the shrinkage and rotation of a circular grains embedded in a larger crystal.
The grain boundary segregation of Bi in dilute polycrystalline Cu–Bi alloys was systematically studied as a function of temperature and composition. The temperature dependencies of the Gibbsian excess of Bi at the grain boundaries exhibited discontinuous changes at the temperatures close to, but different from the bulk solidus temperatures. The observed segregational phase transition was interpreted in terms of prewetting model.
Several low-index symmetrical tilt grain boundaries in α-Al2O3 were investigated by combining high-resolution transmission electron microscopy and spatially resolved energy dispersive X-ray analysis. Bicrystals with a well-defined orientation relation and interface plane were produced by diffusion bonding under ultrahigh vacuum. Grain boundaries like the prismatic and rhombohedral twin develop an atomically sharp and clean interface, whereas higher index grain boundary (GB) planes show a more complex structure and are able to accommodate both Si and Ca in the interface. Additionally, it was found that asymmetric GB planes which occur as facets at rhombohedral and prismatic twins also accommodate impurity atoms.