Article

# In situ transmission electron microscopy investigation of quasicrystal-crystal transformations in Mg–Zn–Y alloys

Authors:
• Ji Hua Laboratory
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... Gd. It has been widely reported that the stoichiometry composition of I phase in Mg-Zn-RE alloys is about Mg 3 Zn 6 RE 1 with a Zn/RE atomic ratio of 6 [27,29,33]. The nanoscale I phase in this study has a slightly higher Mg content but basically the same Zn/RE ratio as that in the previous studies. ...
... Since the W phase is not a crystalline approximant (such as Mg 7 Zn 3 [39],which can transform to I phase through slight adjustment of their common atomic clusters) of I phase, the transition from W phase to I phase takes place by firstly accumulating Zn at the interfacial region between W phase and Mg matrix and then the rearrangement of atoms, leading to the transformation from cuboctahedron to icosahedron [30]. In the meantime, it has also been confirmed that the phase transition from I phase to W phase could also take place at 400°C in Mg-Zn-Y alloy [33], which is only 80°C higher than the temperature required for the transformation of W to I phase. This indicates that the phase transition between I phase and W phase is reversible, depending on specific conditions. ...
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Nanoscale precipitations in deformed dilute alloying Mg-Zn-RE alloys usually play critical positive roles in mechanical properties, while characterizing them still poses a significant challenge due to their small size and low volume fraction. Here, we conduct a systematic structural analysis of the nanoscale secondary phase particles, including W phase, a small amount of I phase and a handful of Mg3Gd phase, in hot deformed dilute alloying Mg-Zn-Gd alloy by combining atomic-resolution transmission electron microscopy with first-principles calculations. The investigation of atomic structure of nanoscale W phase reveals that the stoichiometric composition of W phase is determined by the quantity of Mg atoms which are replaced by Zn at certain positions. Furthermore, nanoscale W phase, I phase and Mg3Gd phase particles could exhibit certain orientation relationships and coherent or semi-coherent interface with Mg matrix, which contributes to atomic bonding at their interfaces. We also identify a phase transition from Mg3Gd phase to W phase, which is further supported by first-principles calculations showing that Mg3Zn3Gd2 phase is energetically more favorable than Mg3Gd phase. The phase transition between W phase and I phase could also take place during hot deformation and is reversible by absorbing or releasing Zn atoms at the interfacial region.
... Besides comprehensive studies on the formation of quasicrystals, transformation from quasicrystals to crystals have also been investigated 13,[26][27][28][29] . Thermally induced transformations from quasicrystals to their approximants have been found in various systems 26 , such as IQCs of Al-Cu-Fe, Ti-Zr-Ni and Mg-Zn-Al, decagonal quasicrystals of Al-Co-Ni and Al-Pd, as well as octagonal quasicrystals of Cr-Ni-Si and Mn-Si-Al. ...
... Therefore, studies on solid-state crystal-to-quasicrystal transformations are of engineering importance 29,46,47 . Our in situ observations showed that the starting temperature for the formation of IQC particles at Zn 3 MgY/Mg interfaces was about 573 K which is almost 100 K below the phase transition temperature of Zn 6 Mg 3 Y IQC in Mg-Zn-Y alloys 28 . It is thus expected that no transformation of IQC to other deleterious crystalline phases will happen upon annealing such IQC-strengthened Mg alloys at the temperature range of 573-623 K. Our results may provide useful information to evoke new processing methods to optimize mechanical properties of this kind of Mg alloys through controlling transformation of deleterious crystals to IQCs. ...
Article
Phase transformation of quasicrystals is of interest in various fields of science and technology. Interestingly, we directly observed unexpected solid-state epitaxial nucleation and growth of Zn6Mg3Y icosahedral quasicrystals in a Mg alloy at about 573 K which is about 300 K below the melting point of Zn6Mg3Y, in contrast to formation of quasicrystals through solidification that was usually found in many alloys. Maximizing local packing density of atoms associated with segregation of Y and Zn in Mg adjacent to Mg/Zn3MgY interfaces triggered atomic rearrangement in Mg to form icosahedra coupled epitaxially with surface distorted icosahedra of Zn 3 MgY, which plays a critical role in the nucleation of icosahedral clusters. A local Zn:Mg:Y ratio close to 6:3:1, corresponding to a valence electron concentration of about 2.15, should have been reached to trigger the formation of quasicrystals at Mg/Zn3MgY interfaces. The solid-state icosahedral ordering in crystals opens a new window for growing quasicrystals and understanding their atomic origin mechanisms. Epitaxial growth of quasicrystals onto crystals can modify the surface/interface structures and properties of crystalline materials.
... Some researches have reported the transformation of I-phase to W-phase during heat treatment process [25,26]. Liu et al. [26] took a systematical investigation on the transform of I-phase and found that I-phase began to transform to W-phase at 400 C in TEM samples and 447 C in bulk samples and the growth of W-phase was controlled by diffusion. ...
... Some researches have reported the transformation of I-phase to W-phase during heat treatment process [25,26]. Liu et al. [26] took a systematical investigation on the transform of I-phase and found that I-phase began to transform to W-phase at 400 C in TEM samples and 447 C in bulk samples and the growth of W-phase was controlled by diffusion. During the FSP of cast rare-earth Mg, the peak temperature of SZ can reach 520 C [24] corresponding to the eutectic temperature of W-phase, which guarantees the temperature requirement of phase transformation during FSP. ...
Article
Here, a Mg-6Zn-1Y0.5Zr casting was subjected to friction stir processing (FSP). The effect of thermal and mechanical effects on the microstructural evolution and mechanical properties of the casting was studied. FSP resulted in remarkable grain refinement, dissolution and dispersion of intergranular eutectic I-phase (Mg3Zn6Y) networks and strong basal texture. Based on mechanically activated effect of FSP, I-phase transformed to W-phase (Mg3Zn3Y2) and dispersed particles with a core-shell structure formed. The increase of travel speed caused greater grain refinement and higher fraction of dispersed particles, which greatly improved yield strength, ultimate tensile strength, and elongation, 93.1%, 53.0%, and 151.4% higher than that of the cast materials, respectively.
... For the formation of heat-resistant RE-containing phases (RE, rare earth), the addition of RE elements such as Nd, Ce, and La is a promising way to improve the high-temperature mechanical properties of Mg-Zn alloys in particular [7][8][9][10]. As one of the more cost-effective RE elements, yttrium (Y) has been widely used in Mg-Zn alloys [11][12][13]. Its maximum solid solubility is 12.4 wt.%, which falls to nearly zero at room temperature. ...
... Its maximum solid solubility is 12.4 wt.%, which falls to nearly zero at room temperature. The addition of Y to an alloy improves the mechanical properties; this is primarily attributed to the Y-containing phases and fine microstructures [13,14]. These Y-containing phases have been shown to exhibit good heat resistance, with the capacity to improve high-temperature mechanical properties [7,15]. ...
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The microstructures and high-temperature tensile properties of as-aged Mg-6Zn-1Mn-4Sn-(0.1, 0.5 and 1.0) Y (wt.%, ZMT614-Y) alloys were investigated by optical microscopy (OM), X-ray diffractometer (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-temperature tensile tests. The tensile temperatures were 150 °C, 200 °C, 250 °C and 300 °C, respectively. The results showed that the phase compositions of as-aged alloys were α-Mg, α-Mn, MgZn2, Mg2Sn, and MgSnY phases. The Mg2Sn and MgSnY high-temperature phases inhibited grain growth in the heat treatment and tensile processes. The as-aged ZMT614-0.5Y alloy has the best high-temperature mechanical properties, with yield strength (YS), ultimate tensile strength (UTS), and elongation values of 277 MPa, 305 MPa, and 16.7%, respectively, at 150 °C. As the tensile temperature increased to 300 °C, the YS and UTS decreased to 136 MPa and 150 MPa, and elongation increased to 25.5%. The fracture mechanism changed as the tensile temperatures ranged from 150 °C to 300 °C, from the transgranular fracture type at temperatures of 150 °C and 200 °C, to the transgranular and intergranular mixed-mode fracture type when tensile temperatures increased to 250 °C, to an intergranular fracture mechanism at 300 °C.
... TEM has been intensively involved in the study of QCs since Shechtman et al. [1] discovered the icosahedral Al-Mn phase through its electron diffraction pattern analysis [28]. This is due to the Crystals 2016, 6, 105 2 of 16 significant advantages of TEM in the characterization of QCs: the bright and dark field images can provide the morphological and microstructural information of QCs [29,30]; the forbidden symmetries (5, 8, 10, 12-fold) of the electron diffraction (selected area electron diffraction (SAED) and convergent beam electron diffraction (CBED)) patterns are the proof of the presence of QCs [31,32]; high-resolution TEM (HRTEM) allows for phase-contrast imaging of the atoms in QCs [33,34]; scanning transmission electron microscopy (STEM) combined with a high angle annular dark field (HAADF) detector provides the intuitive atomic positions in QCs [35,36]. For the critical question in the study of quasicrystalline structure-how atoms are arranged-electron diffraction patterns, HRTEM and STEM HAADF can provide distinct answers. ...
Article
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Quasicrystals (QCs) possess rotational symmetries forbidden in the conventional crystallography and lack translational symmetries. Their atoms are arranged in an ordered but non-periodic way. Transmission electron microscopy (TEM) was the right tool to discover such exotic materials and has always been a main technique in their studies since then. It provides the morphological and crystallographic information and images of real atomic arrangements of QCs. In this review, we summarized the achievements of the study of QCs using TEM, providing intriguing structural details of QCs unveiled by TEM analyses. The main findings on the symmetry, local atomic arrangement and chemical order of QCs are illustrated.
... Such transformation was also reported by previous reports using heat treatment (Yan et al., 2014) and FSW (Xie et al., 2007). Liu et al. (2015) adopted in situ TEM technique to investigate the evolution of I-phase in elevating temperature, and found that the transformation from I-phase to W-phase began at 447°C in bulk samples. The transformation was dominated by element diffusion. ...
Article
Here, the influence of the Y form (eutectic phase and solute) on microstructural evolution of as-cast and solid solution Mg-Zn-Y-Zr alloys during friction stir processing (FSP) is studied. The solid solution of Y decreases the activation of twinning as well as mobilities of dislocations and grain boundaries, resulting in less dynamic recrystallization (DRX). During FSP of the as-cast sample, greater DRX and more dispersed particles contribute to the formation of finer and more uniform grains, achieving excellent mechanical properties with yield strength of 170 MPa, ultimate tensile strength of 300 MPa and elongation of 27%, improved by 90%, 50% and 150%, respectively.
... Transient behaviors through temperature can be investigated in a number of different material systems during in situ heating TEM experiments, where samples are heated (and cooled), so that the dynamics of phase transformations and reactions at high temperatures can be directly observed in situ [231]. In situ heating experiments have been utilized to investigate thermal stability in fine-grained metals [203,232], phase transitions [233,234], and for many other applications [231,[235][236][237][238]]. ...
Thesis
The unique behaviors of fine-grained metals have attracted significant interest in recent years due to their increased hardness and enhanced radiation tolerance manifested through annihilation of defects and He gas retention. However, their far from equilibrium state arising from their high interfacial densities renders fine-grained metals unstable at high temperatures, potentially vitiating the potential benefits of nanostructuring. Targeted doping has emerged as a strategy for alleviating thermal instabilities through reduction of the intrinsic driving force for grain growth by targeting the excess interfacial free energy or limiting the kinetic behavior. However, the addition of dopants has implications for the mechanical behavior and radiation tolerance of fine-grained metals. In this dissertation, numerous topics related to thermal stability, radiation defect production, and gas behavior are investigated in fine-grained BCC refractory metals and alloys. First, in situ TEM is utilized to study the transitions in the dominant thermodynamic stabilization mechanism in a fine-grained Mo alloy. In situ heating of a Mo-Au nanometallic multilayer microstructure demonstrated a transition from thermodynamically dominated stabilization at low temperatures to a kinetically stabilized microstructure at higher temperatures, coincident with predicted Au segregation and diffusion at those temperatures. Then, the influence of dopants and dispersoids on microstructural stability and defect production in fine-grained W alloys under ion irradiation are investigated through in situ TEM, with results indicating that the addition of dopants stabilizes the microstructure against irradiation induced grain growth and modifies the stress state around grain boundaries, augmenting their effective sink strength. Finally, the influence of helium gas implantation at grain boundaries on mechanical behavior in fine-grained tungsten is investigated through TEM and nanoindentation. The formation of helium filled cavities at the grain boundaries is shown to correlate to a significant reduction in material strength along with a reduction in dislocation activity and the initiation of abnormal dislocation behavior.
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The main goals of this work were to manufacture novel Al-Zn extruded alloys by varying the Zn content (0, 10, 20, 30 wt%), investigate the microstructural evolutions, hot deformation, and work hardening behaviour by hot compression test at different temperatures (25 °C, 75 °C, 150 °C, 225 °C, 300 °C). Al-20Zn alloy microstructure revealed α-Al and uniform distribution of (α + η) phases, coherent (α + η) crystals in GBs with casting defect-free surfaces, and effective interactions of pinning dislocations which led to improve mechanical performance of Al-20Zn alloy, as compared to the other alloys. The observed engineering stress-strain curve results revealed the decrease of stress with increasing of temperature due to flow softening, dynamic recovery and dynamic recrystallization. These results displayed also an increase of stress value with increasing of Zn content due to the precipitation of high density (α + η) phase in the matrix and GBs, increasing of forest and mobile dislocations density with strain fields, and the formation of fine dendrites. Work hardening rate (WHR) of extruded samples displayed three stages: stage I, WHR decreased slightly with increasing of temperature up to 75 °C and decreased drastically from 75 °C to 300 °C due to softening; stage II, WHR maintained constant due to balance between dislocation generations and dislocation annihilation; stage III, WHR slightly increased due to strain hardening of (α + η) phase. WHR was observed to increase considerably with increasing of Zn content due to the formation and dispersion of high density of (α + η) phase in the Al matrix and GBs. Deformation micro-localization in terms of different characteristics was examined and reported on the deformed samples after hot-compression test through SEM micrographs.
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The Mg-Zn-Y-Zr alloys with long-period stacking-ordered (LPSO) and W eutectic phases were investigated to develop new magnesium casting alloys. The temperatures for T6 heat treatment were selected based on the hardness and electrical conductivity measurements. The hot tearing susceptibility of the alloys with LPSO phase is lower than that of the alloys with W phase, which is associated with the freezing range of the alloys. However, the investigated alloys displayed the same fluidity. Under T6 conditions, increasing the Y content in the alloys resulted in increased yield strength, whereas other tensile properties were similar for the alloys. The corrosion resistance was higher for the alloys with LPSO phase compared to that of the alloys with W phase. Mg-2.5Zn-3.7Y-0.3Zr (mass fraction, %) alloy with LPSO phase possessed high castability and mechanical properties, with a corrosion rate of 2 mm/year.
Article
Ultrafine-grained structure in Mg-Zn-Y-Zr alloy was achieved towards extraordinary superplasticity induced by friction stir processing. Grain refinement and homogeneous dispersion of precipitates were promoted by the coupled thermo-mechanical effect. The ultra-fined grains size of 1.9 ± 0.4 μm and the superplastic elongation of 642% were obtained. Grain boundary sliding was considered as the predominant superplastic deformation mechanism in ultrafine-grained materials.
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Low-energy electron diffraction patterns, produced from quasicrystal surfaces by ion sputtering and annealing to temperatures below {approximately}700 K, can be assigned to various terminations of the cubic CsCl structure. The assignments are based upon ratios of spot spacings, estimates of surface lattice constants, bulk phase diagrams vs surface compositions, and comparisons with previous work. The CsCl overlayers are deeper than about five atomic layers, because they obscure the diffraction spots from the underlying quasicrystalline substrate. These patterns transform irreversibly to quasicrystalline(like) patterns upon annealing to higher temperatures, indicating that the cubic overlayers are metastable. Based upon the data for three chemically identical, but symmetrically inequivalent surfaces, a model is developed for the relation between the cubic overlayers and the quasicrystalline substrate. The model is based upon the related symmetries of cubic close-packed and icosahedral-packed materials. The model explains not only the symmetries of the cubic surface terminations, but also the number and orientation of domains. {copyright} {ital 1998} {ital The American Physical Society}
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A metallic solid (Al-14-at. pct.-Mn) with long-range orientational order, but with icosahedral point group symmetry, which is inconsistent with lattice translations, has been observed. Its diffraction spots are as sharp as those of crystals but cannot be indexed to any Bravais lattice. The solid is metastable and forms from the melt by a first-order transition.
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We have observed the icosahedral quasicrystals in the Zn- and rare earths-containing Mg alloys. The quasicrystals in these alloys contains Mg, Zn and rare earths elements. Three crystal phases, i.e. W phase, W′ phase and MgZn2-type Laves phase, coexist with the quasicrystals in this kind of alloys. XRD analysis suggest that the quasicrystals in experimental alloys may have the same structure but different compositions to those in the Mg-Al-Zn [5] and Ga-Mg-Zn [6] alloys.
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Phase transitions from and to the quasicrystalline state show a typical signature due to some structural peculiarities. Both quasicrystals and most of their translationally periodic transformation products, the approximants, consist of the same basic structural units ('clusters'). This is the cause of low-energy interfaces between the newly forming phase and the parent phase. It is also the origin of the rather high stability of the frequently resulting orientationally twinned nanodomain structures. Owing to topological incompatibilities between quasiperiodic and periodic structures, purely displacive phase transitions are impossible. Diffusion of a significant fraction of atoms, at least on the scale of the cluster diameters, always has to take place. Locally similar icosahedral structural ordering between parent phase and nucleating phase is also responsible for the frequently occurring formation of icosahedral quasicrystals from undercooled liquid alloys or during devitrification of metallic glasses. The different types of experimentally observed phase transitions are discussed, from amorphous to quasicrystalline, quasicrystalline to ordered/disordered quasicrystalline and quasicrystalline to crystalline as a function of temperature, pressure, irradiation and high-energy ball milling, respectively.
Article
Through investigating tensile properties of the duplex Mg-Li alloys with and without I-phase (Mg3Zn6Y, icosahedral quasicrystal structure) tested both at ambient and elevated temperatures, it demonstrates that I-phase can not only improve the tensile strength at room temperature, but also be beneficial for the mechanical improvement at elevated temperatures. Based on the microstructural observations of the sample surfaces before and after tests, the I-phase particles can effectively suppress the plastic flow of beta phase at 220 degrees C, resulting in the I-phase reinforced duplex Mg-Li alloy having higher mechanical properties at elevated temperatures. (c) 2013 Elsevier B.V. All rights reserved.
Article
The wear and friction properties of the MgZnYalloys with a dispersion of the quasi-crystalline phase were investigated using the pin-ondisk configuration. The wear loss rates and the friction coefficient were obtained to be 1.95 × 10-5mm3/mm and 0.26 in the casted alloy, 1.75 × 10-5mm3/mm and 0.24 in the heat-treated alloy, 2.40 × 10-5mm3/mm and 0.24 in the extruded alloy, respectively, under the dry wear condition. An effective method to enhance the wear properties in the magnesium alloys was determined to be a homogenous distribution of the quasi-crystalline phase in the coarse-grained matrix. Meanwhile, the grain refinement was not suitable due to grain boundary sliding, i.e., materials softening.
Article
Mg-Zn-Y icosahedral quasicrystals have been prepared from the Mg63.5Zn34Y2.5 (at.%) alloy by copper and steel molds casting respectively. The phase transformations under different heat treatments have been investigated successively. Although the cooling rates of the two samples are quite different, the icosahedral quasicrystalline (I-phase) can precipitate from liquid directly almost at the same temperature. The eutectic structure (alpha-Mg + I-phase) forms at the later stage, while the Mg7Zn3 phase forms under a larger cooling rate (copper mold). Moreover, with a lower cooling speed, micro shrinkage cavities and cracks are easily caused by solute partitioning at the edge of petals-like I-phase. The volume fraction of I-phase is effectively increased after heat treatment at 350 degrees C for 50 h because I-phase can grow further in a facet manner during the heat treatment process. The I-phase in the Mg63.5Zn34Y2.5 alloy is thermodynamically unstable at 420 degrees C, and therefore transformations of the I-phase to the W or H-phase occur due to slow transformation kinetics and low thermodynamics driving force. In addition, few decagonal quasicrystalline phases (D-phases) can form on the I-phase/Mg7Zn3 interface. Atomic diffusion and concentration also play a crucial role in these transformations.
Article
Recently, magnesium alloys with a dispersion of quasicrystalline icosahedral i-phase have been shown to exhibit very high strengths combined with ductility. However, effect of the i-phase and its amount has not been investigated comprehensively. To make a systematic investigation, two alloys of composition Mg–6x Zn–x Y, where x=0.5 and 1 at%, were chill cast and extruded at three different temperatures to produce various grain sizes with a dispersion of i-phase. The extruded alloys were tested in tension and compression. Very fine grains of micron and submicron size have been obtained, resulting in very high yield strengths of up to about 400 MPa accompanied by total elongations of over 12%. Microstructural features such as precipitation and texture have been studied, and mechanical properties such as strength and ductility in tension and compression have been determined. As the grain size is refined, depending on the alloy composition and extrusion temperature, the texture is also modified, such that a higher strength resulting from finer grain size is accompanied by a reasonable elongation, of over 12%. In aged condition the Hall–Petch plots for tensile and compressive yield strengths are nearly parallel, with slopes in the range of 237–307 MPa μm−1/2. The Hall–Petch slopes of critically resolved shear stress of slip and twinning are nearly the same at about 63 MPa μm−1/2.
Article
This work mainly investigated the influence of W-phase on the mechanical properties and damping capacities of as-cast Mg–Zn–Y–Nd–Zr alloys with Zn/RE (rare element) ratio about 1.0. Obtained results indicate that the alloys with Zn and RE addition are composed of α-Mg matrix and W-phase. With the contents of Zn and RE increasing, the diffraction peaks of W-phase are gradually intensified and the morphology of W-phase transforms from fine-network microstructure to coarse-network microstructure. The tensile strength and fracture mechanism are strongly dependent on the quality of W-phase and the alloy with W-phase content of 8.0% has the highest strength as a result of strong atomic bonding between the W-phase and the Mg matrix. The damping capacities of Mg–Zn–Y–Nd–Zr alloys decrease gradually with the increasing amount of W-phase and then maintain at high level at high strain amplitude. The decline of damping capacity can be explained by the forming of W-phase, which makes more phases and interfaces form in the alloys. And the mobile dislocation densities in the alloys increase as residual stress at the interface of W-phase/Mg matrix and long dislocations generate in the α-Mg matrix. Therefore, the damping of alloy with high W-phase content improves accordingly with the increasing amount of moving dislocations.
Article
In the present work, the microstructure, texture and mechanical properties of as-extruded Mg–xZn–xEr (x = 2, 4 and 6) alloys were investigated. The results showed that the W-phase in the as-cast alloys was destroyed during hot extrusion process. The distribution of the W-phase is more uniform when the extrusion temperature is 400 °C, and the texture has also been a certain degree of weakening. With increase in the volume fraction of the primary W-phase, the particle-stimulated nucleation (PSN) of recrystallization was activated. It was suggested that the recrystallization via PSN should lead to weak texture. Good matching between strength and elongation is achieved in the Mg–6Zn–6Er alloy extruded at 400 °C, of which the ultimate tensile strength (σb) and the yield tensile strength (σ0.2) were 328 (±2.5) MPa and 283 (±2.2) MPa, respectively, companying with an elongation of 19.7% (±1.2).
Article
An aluminum film is deposited on the fivefold-symmetric surface of icosahedral Al-Pd-Mn quasicrystal. Its orientation with respect to the bulk structure is studied using low-energy electron diffraction. It is found that Al nanocrystals grow in the face-centered cubic structure in five different sets of domains composing the film, and that their [111] axes are aligned parallel to one of five threefold-symmetry axes of the substrate quasicrystal at $37.37\ifmmode^\circ\else\textdegree\fi{}$ away from the surface normal.
Article
The long-period stacking ordered structures 18R and 14H formed in Mg–Y–X (X = Zn, Cu, Ni) systems have received considerable interest over the past decade, but their thermal stability and relationships with other intermetallic phases in the Mg–Y–X systems remain to be unambiguously established. In this study, the occurrence and transformations of long-period stacking ordered structures 18R and 14H are clarified in as-cast and heat-treated Mg–Y–Zn alloys. The 18R structure is a stable equilibrium phase that forms directly from the melt whereas the 14H cannot form directly from the melt but forms in a solid-state transformation. That explains the absence of 14H in the as-cast microstructures of the alloys. These findings are embedded in the complete description of Mg–Y–Zn phase equilibria, generated by Calphad-type thermodynamic calculations and verified for a range of Mg-rich alloys by electron microscopy and thermal analysis. It is found that the 18R is a stable equilibrium phase that exists in the high temperature range from 753 to 483 °C, and that the 14H is an equilibrium phase below 537 °C. In the small temperature range between 537 and 483 °C, the 18R and 14H can co-exist in equilibrium in some special alloy compositions.
Article
Crystal defects in a plastically deformed Mg–Zn–Y alloy have been studied on the atomic scale using aberration-corrected scanning transmission electron microscopy, providing important structural data for understanding the material’s deformation behavior and strengthening mechanisms. Atomic scale structures of deformation stacking faults resulting from dissociation of different types of dislocations have been characterized experimentally, and modeled. Suzuki segregation of Zn and Y along stacking faults formed through dislocation dissociation during plastic deformation at 300 °C is confirmed experimentally on the atomic level. The stacking fault energy of the Mg–Zn–Y alloy is evaluated to be in the range of 4.0–10.3 mJ m−2. The newly formed nanometer-wide stacking faults with their Zn/Y segregation in Mg grains play an important role in the superior strength of this alloy at elevated temperatures.
Article
Experimental studies of recrystallization in deformed single crystals of pure iron are described. The results are used to analyze various parameters associated with the time evolution of the volume fraction of the growing phase during phase transformations in this system associated with the phenomena of nucleation and growth. In addition, using the experimental results, the phenomenon has been modeled by a new approach which may provide a different, and possibly more precise, description of the kinetics of the process. The proposed analytical approach uses easily measured metallographic parameters, obtained from a systematic two-dimensional surface examination, to provide a detailed description on the time dependence of nucleation, nucleation rate, growth rate and interfacial migration and compared with the classical approach based on Kolomogrov formalism.
Article
A process to obtain high strength in a Mg–Zn–Y alloy containing quasicrystalline phase is described. The process involves solutionizing at a high temperature, precipitation of the quasicrystal phase during extrusion, followed by ageing. Tensile yield strengths of over 350MPa are obtained with grain sizes of 14–20μm.
Article
Order, Order The structure of glassy materials, which are known to have short-range order but no long-range pattern, continues to be a puzzle. One current theory is that some glassy materials possess icosahedral ordering, a motif that cannot show translational periodicity. Hirata et al. (p. 376 , published online 11 July) obtained diffraction patterns from subnanometer volumes in a metallic glass, which show some, but not all, of the expected features of an icosahedron. Simulations suggest that the patterns arise from icosahedrons distorted to include features of the face-centered cubic structure. This observation is different from the predictions of molecular dynamics simulations and provides pivotal information in understanding the competition between the formation of the globally inexpensive long-range order and the locally inexpensive short-range order.
Article
Formation of icosahedral quasicrystalline phase on the W cubic phase in α-Mg matrix has been studied in a Mg97Zn2.5Y0.5 alloy. The orientation relationship of the W phase with the matrix is determined to be [001]W∥[1¯ 0 0]Mg, (110)W∥(001)Mg, equivalent to Burgers orientation relationship. The icosahedral phase forms curved interfaces with the W phase, but planar interfaces on five-fold and two-fold planes with the matrix. The icosahedral phase shows its unique orientation relationship to the W phase, which results in two orientation relationships WOR1 and WOR2 with the matrix. WOR1 is a slight deviation of the orientation relation, OR1, of icosahedral phase precipitates in the matrix. In WOR2, one of the five-fold axes occur along the hexagonal axis. The resulting morphology and the interface structures are described here. It is shown that WOR1 and WOR2 are related to each other by twinning of the icosahedral phase.
Article
The crystal structure of the hexagonal Zn3MgY phase has been determined by single-crystal X-ray diffraction. The structural model, refined to a final R value of 0.047, has the composition Zn60.68Mg18.28Y21.04,a=9.082(2)Å and c=9.415(5)Å and the space group P63/mmc. Among the 36 atomic sites 28 (or 77.8%) are icosahedrally coordinated (heavily distorted by the large Y atoms) and occupied by Zn and Mg (not all) atoms. As the interatomic distance between the centers of a pair of icosahedra increases from 2.6 to 3.1Å, 4.7 to 4.8Å, to 6.4Å, their connection changes from interpenetration, face-sharing, to vertex-sharing. The structure of Zn3MgY is characterized by a layer structure consisting of FP(FP)’ layers stacked along the c axis, where F and P denote flat and puckered layers, respectively, and (FP)’ is related to FP by a 63 screw. The Zn3 icosahedra, in the PFP’ layer block, are fused into pairs in the 〈100〉 directions. On the other hand the Zn1, Zn2, and Mg/Zn icosahedra form vertex-sharing, face-sharing, or interpenetrated chains in the [001] direction.
Article
A new group of stable icosahedral phases (i-phases) in the Zn-Mg-RE system were found to have an ideal composition close to Zn50 Mg42RE8 (RE ≡ Y, Gd, Tb, Dy, Ho or Er). The new i-phases exhibit a highly ordered and nearly perfect face-centred icosahedral lattice as revealed by electron and X-ray diffraction and high-resolution electron microscopy. Powder X-ray diffraction indicates that the i-phase has long-range structural order with a correlation length over 1000 Å, which is the largest found in the Frank-Kasper group. These new i-phases have the common valence concentration (about 2.08) and could be regarded as a new class of Hume-Rothery alloys.
Article
A coincidence of reciprocal lattice planes model was developed to calculate the interfacial energy in quasicrystal-crystal epitaxy. This model allows a quantitative description of the interface as opposed to previously employed qualitative models that consider symmetry relations and alignment of rotation axes. Computations were carried out on several types of quasicrystal-crystal systems, namely, the crystalline structures on various surfaces of single quasicrystals (Al-Cu-Fe, Al-Ni-Co, and Al-Cu-Co) caused by ion bombardment, the crystalline thin films grown on quasicrystal substrates, and the quasicrystalline thin films grown on crystalline substrates. This model can also be extended to include quasicrystal-quasicrystal epitaxy.
Article
Isothermal sections of the Zn-Mg-Y equilibrium phase diagram at 700 K, 773 K and 873 K have been determined in the region of 30-70 at.% Zn, 20- 60 at.% Mg and 0-20 at.% Y using scanning electron microscopy (SEM), wavelength dispersive X-ray, X-ray diffraction and transmission electron microscopy techniques. The icosahedral phase has equilibrium phase fields with three distinct phases (L,Z,H ) at 873 K, four distinct phases (L,W,Z,H) at 773 K and five distinct phases (L,W,Z,H,alpha) at 700 K. The equilibrium range of composition for the i-phase at these temperatures was determined.
Article
All stable quasicrystals known so far are composed of at least three metallic elements1, 2, 3, 4. Sixteen years after the discovery of the quasicrystal5, we describe a stable binary quasicrystalline alloy in a cadmium–ytterbium (Cd–Yb) system. The structure of this alloy represents a new class of packing of 66-atom icosahedral clusters whose internal structure breaks the icosahedral symmetry. The binary quasicrystal offers a new opportunity to investigate the relation between thermodynamic stability and quasiperiodic structure, as well as providing a basis for the construction of crystallographic models.
Article
A reaction is identified in formation of icosahedral phase in dilute Mg–Zn–Y alloys, involving a hexagonal phase H-Zn3MgY (a = 0.918, c = 0.95 nm). The crystallographic relationship and the interface between the H and the icosahedral phase are described. Formation of an icosahedral–H phase nano-composite in a Mg93Zn6Y alloy is shown.
Article
Single phase regions of the icosahedral quasicrystal and the hexagonal (Zn, Mg)58Y13, (Zn, Mg)5Y, and Z (a ternary phase containing 6–8 at.% Y) in the Zn–Mg (0–40 at.%)–Y (0–25 at.%) region have been determined. A tentative isothermal section at 600 °C was proposed. The (Zn, Mg)5Y and the icosahedral phase have wide ranges of Mg content and are the dominant phases in these alloys. The hexagonal (Zn, Mg)5Y is resulted from the substitution of Mg for Zn in the binary Zn5Y and therefore they are isostructural, so are also the hexagonal Zn58Y13 and (Zn, Mg)58Y13 phases. Reactions among the intermetallic phases in this composition range during cooling were studied. Similar phase reactions have also been observed in the Zn–Mg–Ho/Er alloys.
Article
New Mg-rich Mg–Zn–Y alloys, reinforced by quasicrystalline particles, have been developed by thermomechanical processes. The deformation behavior of these alloys at room and elevated temperatures has been investigated. Yield strength of these alloys, which increases with an increase in the volume fraction of quasicrystalline particles, is relatively high due to their strengthening effect. The variation of the flow stress in the alloys is characterized by linking the microstructural evolution during deformation at high temperatures. The flow softening is related to dynamic recrystallization developed under the dislocation climb controlled creep; the flow hardening is related to grain growth that occurs under the grain boundary diffusion controlled creep. Quasicrystalline particles in the Mg–Zn–Y alloys resist coarsening due to their low interfacial energy, thereby forming of stable quasicrystalline particle/matrix interface and also prohibit against microstructural evolution of the α-Mg matrix during deformation at temperatures up to near the eutectic temperature. Stability of both quasicrystalline particles and matrix microstructure in the Mg–Zn–Y alloys provides large elongation to failure with no void formation at the quasicrystalline particle/matrix interface.
Article
This review outlined the quantum mechanical basis for regarding Z-contrast imaging and EELS in the STEM as directly interpretable, column-by-column imaging and analysis. These techniques form a powerful basis for structure determination that provides a first-order model without the need to solve any phase problem. In the examples discussed, theoretical modeling was used to refine the structures and make the link to properties through calculation of impurity or vacancy segregation energies and electronic structure. Future developments in the correction of aberrations offer the potential for greatly improved sensitivity and signal-to-noise ratios, with single-atom sensitivity in both imaging and analysis. This sensitivity will lead to a new level of insight into the atomic origin of materials properties. It will give rise to the ability to understand the limiting factors in optical and electronic devices, the active sites and mechanisms in catalysis, the origin of strength and ductility in structural materials, and the origin of the unique properties of nanostructured materials. There is a bright future for electrons.
Article
Fine-grained magnesium alloys reinforced by quasicrystalline particles were easily developed by thermomechanical processes for as-cast Mg-rich Mg–Zn–Y and Mg–Zn–Y–Zr alloys. The deformation behavior of the alloys at room and high temperatures was investigated and compared with that of commercial AZ31, AZ61 and AZ91 alloys. The yield strength of the Mg–Zn–Y alloys, increasing with an increase of the volume fraction of the quasicrystalline phase, is relatively high due to the strengthening effect of the quasicrystalline particles. At high temperatures, the level of flow stress of the Mg–Zn–Y alloys is lower than that of commercial magnesium alloys due to the softness of the eutectic region, but the alloys exhibit much higher elongation since the large number of quasicrystalline particles in the Mg–Zn–Y alloys can effectively prohibit microstructural evolution of the α-Mg matrix during deformation. Icosahedral particles in the alloy are also stable against coarsening during deformation near the melting temperature of the eutectic due to their low interfacial energy, thereby forming a stable quasicrystalline particle/matrix interface. The stability of both the quasicrystalline particles and the microstructure of the Mg–Zn–Y alloys provides a large elongation with no void opening at the interface between the quasicrystalline particle and the α-Mg matrix.
Article
We review the stability of various icosahedral quasicrystals (iQc) from a metallurgical viewpoint. The stability of stable iQcs is well interpreted in terms of Hume-Rothery rules, i.e. atomic size factor and valence electron concentration, e/a. For metastable iQcs, we discuss the role of phason disorder introduced by rapid solidification, in structural stability and its interplay with chemical order and composition.
Article
It is shown that the alpha-(AlMnSi) crystal structure is closely (and systematically) related to that of the icosahedral Al-Mn-Si alloys. Using a modification of the 'projection' method of generating icosahedral structures from six-dimensional lattices, a simple description of the alpha-(AlMnSi) structure is found. This structure, and (it is conjectured) the icosahedral one, can also be described as a packing of 54-atom icosahedral clusters.
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13. (a) Fraction of transformed H and W as a function of time. (b) Modified Avrami plot of volume fraction ln{ln
• Fig
Fig. 13. (a) Fraction of transformed H and W as a function of time. (b) Modified Avrami plot of volume fraction ln{ln[1/(1-f)]} as a function of lnt.