Article

Strength enhancement induced by grain boundary solute segregations in ultrafine-grained alloys

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Abstract

A model is proposed explaining enhanced strength of ultrafine-grained alloys that contain grain boundary (GB) solute segregations. In the framework of the proposed model these segregations are treated as homogeneous ellipsoidal inclusions and act as the sources of elastic stresses affecting the emission of lattice dislocations from GBs. These segregations pin the ends of lattice dislocation segments at the initial stage of dislocation propagation along GBs, and the unpinning requires a load increase, leading to the enhanced yield strength. We calculate the contribution of GB segregations to the yield strength for the ultrafine-grained 1570 Al alloy. We demonstrate that the maximum yield strength of this alloy is achieved in the case of clustered, nearly spherical Mg segregations with a high Mg concentration and a diameter to thickness ratio of 1.0–1.4, depending on the Mg concentration inside segregations. We also briefly discuss the possible role of GB dislocations in the formation of such concentrated solute segregations as well as the influence of GB segregations on the strengthening of alloys containing nanoscale twins. The results of the calculations agree well with experimental data.

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... The alloying elements may form grain boundary (GB) solute segregations and, therefore, influence GB relaxation, GBS intensity, and superplastic properties [13][14][15][16][17][18][19][20][21]. Atomic segregations of Mg and Zn are confirmed by atom probe tomography analysis [22][23][24]. ...
... The GB segregations of Mg and Zn atoms and their difference in the atomic clusters formation [35,36] explain different GBS contribution to the alloys studied. The Mg-Al clusters [35] and GB segregations of Mg atoms, having a larger atomic size than Al atoms [17,19,20,36,37], are the reasons for a decrease in Al atoms diffusivity and weak GBS in Zn-free Al-Mg-based alloys. In the presence of Zn, which segregates on GB [17,18,38], the threshold stress level for GBS decreases, and GBS dominates. ...
... The Mg-Al clusters [35] and GB segregations of Mg atoms, having a larger atomic size than Al atoms [17,19,20,36,37], are the reasons for a decrease in Al atoms diffusivity and weak GBS in Zn-free Al-Mg-based alloys. In the presence of Zn, which segregates on GB [17,18,38], the threshold stress level for GBS decreases, and GBS dominates. This theory corresponds with the data of the first principle calculations in [14]. ...
Article
Al-Mg alloys (AA5083-type) are widely used for superplastic forming. Limited grain boundary sliding as compared to other superplastic alloys limits the formability of Al-Mg based alloys. Grain boundary sliding intensity depends on the grain size, grain boundaries structure, and chemical composition of alloys. To improve superplastic properties of Al-Mg based alloys, we investigated the influence of a minor addition of Zn on the grain boundary relaxation effect and grain boundary sliding ability. The temperature dependence internal friction, superplastic deformation behavior, and microstructural changes during superplastic deformation for the AA5083 alloy and Zn modified alloy were compared. A minor Zn addition of 0.7 wt.% does not influence the mean grain size but decreases the relaxation strength and the activation energy of grain boundary relaxation. The mechanisms of superplastic deformation and their contributions to the total strain were analyzed using FIB-milled grids evolution on the samples’ surface. Grain boundary sliding, grain rotations, and intragranular strain included both dislocation slip/creep and diffusional creep were involved in the deformation process. The contribution of grain boundary sliding increased from 10-25% for the Zn-free AA5083 alloy to 30-50% for the Zn-modified alloy. In result, a minor Zn addition proved to stimulate grain boundary sliding that decreases stress and increases strain rate sensitivity and elongation-to-failure of the studied alloy.
... Bobylev et al. [36] recently developed theoretical model explaining experimentally observed enhanced strength of UFG alloys containing GB segregations. Unlike previous models it takes into account not only concentration of the solute but also a shape and size of segregations. ...
... To overcome these forces external stress must by increased by some value  seg . Using theory of dislocation and inclusions Bobylev et al. [36] calculated dependences (Fig. 7) of the value  seg on aspect ratio a 1 /a 3 (it was assumed a 1 = a 2 ) of the inclusion in the exemplary case of Al alloy 1570 with Mg GB segregations using following values of parameters (corresponding to experimentally measured [15,16] values): c m = 0.064, gb c = 0.073, a 3 = 5 nm, c s = 0.1, 0.15, and 0.2, where gb c -average solute concentration inside GB (assumed to be constant). From Fig. 7 it is seen that in the case of the constant concentration gb c stress  seg reaches maximum at low aspect ratio values a 1 /a 3 (approximately 1.0-1.4 ...
... Also Fig. 7 shows that increase in concentration c s leads to higher values of  seg . The model [36] allows making a conclusion that maximum strengthening due to GB solute segregations is achieved when solute atoms are accumulated in small concentrated clusters as opposed to spreading uniformly over GBs. As it follows from Fig. 7 the value of  seg at c s  0.15 is close to experimentally measured strengthening of alloy 1570 equal to ~200 MPa [16]. ...
... Such unusually high strength for the alloy with less than 1.5 wt.%Mn content has been explained by the notably decreased grain size and increased dislocation density as well as by the formation of Mg nanoclusters and/or segregations at grain boundaries. Such segregations have been observed earlier in Al-Mg alloys subjected to HPT [40], and they can entail considerable extra-strengthening of the UFG alloys by inducing additional resistance to dislocation motion [41]. However, such an outstanding combination of physical and mechanical properties has been achieved mostly on small specimens using the HPT technology, which presently is not suitable for mass production, while attempts are being made to upscale it to industrial scale [42]. ...
... The estimations performed on the example of Alloy 2 showed a good agreement between the calculated and experimentally obtained values of strength and electrical conductivity (Table 4). A certain underestimation of the experimental data can be related to the neglected contribution of Al 3 Zr precipitates situated in the grain boundaries, which can be an additional strengthening factor in UFG materials [40,41]. As is seen, the main contribution to a high level of Al-Mg-Zr wire strength is provided by grain boundary hardening, the contribution of which is about 40%. ...
Article
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We propose pathways to produce high-strength thermal resistant Mg-doped Al–Zr-based conductors to be manufactured in the form of ultrafine-grained wires via deliberate thermomechanical treatment, including aging, continuous equal-channel angular pressing and cold drawing. Al–0.97Mg–0.35Zr and Al–1.17Mg–0.34Zr (wt.%) alloys were chosen as objects for investigation to reveal the effect of Mg on the alloys’ properties in the processed states. Prior to deformation, the initial rods of the studied alloys were aged at 400 °C for 72 h to attain precipitation of nano-sized particles of the metastable phase Al3Zr (L12) in the aluminum matrix. The following refinement of microstructure in the aged alloys by severe straining followed by cold drawing allows achieving a promising combination of ultimate tensile strength over 320 MPa, plasticity with elongation to failure over 2% and electrical conductivity about 50% IACS. We show that the produced wires exhibit thermal resistance similar to that of the commercial high-strength heat-resistant aluminum alloy (KTAL) type AT2 wire, but with a markedly superior strength. The Mg concentrations and parameters of thermal and mechanical processing that maintain a reasonable trade-off between high strength and acceptable electrical conductivity are discussed on the basis of the presented and earlier reported data to produce high-performance heat-resistant UFG Al–Mg–Zr wires. Graphical Abstract
... One is that interfacial enrichment at GBs significantly reduces the GB energy and suppresses GB activities [4,14]. Another is that the elastic stress in the segregation zone interacts with the expanding dislocation near the GB to improve the yield strength [15,16]. For nanocrystalline Al alloys, the normal width of normal GB segregation zone is 2-3.4 nm [17]. ...
... Generally, there are both mirror symmetrical local strain regions and long-range strain regions formed near the GBs. Among them, mirror symmetrical local strain is generated by geometrically necessary dislocation while long range strain (eigenstrain) is generated by external dislocation [15]. Since the screw dislocations near GBs do not create dilatational stresses and hence, will not interact with Mg atoms. ...
... Очевидно, в этом случае дополнительный вклад вносит механизм упрочнения, связанный с состоянием границ зерен 1 . Недавние расчеты, представленные в [38], показывают, что образование сегрегаций легирующих элементов на границах зерен может значи-__________________ 1 С учетом аналогичных исследований, авторами планируется провести дополнительную оценку природы упрочнения УМЗ титана, обусловленного границами зерен. тельно тормозить зарождение дислокаций на них, внося вклад в дополнительное упрочнение УМЗ материалов. ...
Article
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This paper discusses the formation of an ultrafine-grained (UFG) structure and nanosized second phase precipitates in commercially pure Grade 4 titanium subjected to severe plastic deformation by torsion at room temperature and subsequent heat treatment. It was found that the combined treatment of Grade 4 titanium produces very high tensile strength (σB ≈ 1500 MPa), which significantly exceeds the previously obtained values for this material. Analysis of the strengthening mechanisms showed that the superstrength of commercially pure titanium is due to several factors: formation of a UFG structure, dispersion strengthening from nanosized second phase particles, high dislocation density, and grain boundary segregation. The contribution of these strengthening mechanisms is evaluated and compared with experimental data.
... Consequently, driven by the imperative of minimizing system free energy, solutes or impurities inevitably exhibit a propensity to segregate towards these locales, engendering interface structures (also known as interfacial complexions) characterized by solute/impurity segregation He et al., 2021;Nie et al., 2013;Peter et al., 2018;Xie et al., 2021a,b). The segregation of solutes/impurities at interfaces assumes a pivotal role in dictating the material properties (Bobylev et al., 2019;Chen et al., 2022;Du et al., 2022;Hu et al., 2018;Nie et al., 2013;Zhao et al., 2019). A paradigmatic illustration of this phenomenon is hydrogen embrittlement, whereby the segregation of hydrogen at alloy interfaces can precipitate catastrophic brittle fracture (Liang et al., 2021;Wan et al., 2019;Zhou et al., 2021). ...
Article
Understanding the interfacial structures and the induced solute segregation behaviors at the atomic scale can facilitate the design of high-performance materials. However, the diversity and concealed nature of interfaces often make it challenging to reveal the interfacial microstructures and the accompanying solute segregation phenomena. Here, we report the discovery of 8 types of asymmetric tilt grain boundaries (GBs) with solute segregation in a deformed and annealed Mg-rare-earth binary model alloy, by means of Z-contrast high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) observations. Molecular dynamics (MD) simulations provide a visual depiction of the atomic-scale characteristics of the periodic extension and compression sites in these tilt GBs. The first-principles density functional theory (DFT) calculations unveil the energetics of the well-ordered segregation behavior of solutes at these interfaces. It is confirmed that the selective segregation of solutes at substitutional sites in the periodic misfit-dislocations separated tilt GBs leads to the formation of 8 types of unique two-dimensional interfacial superstructures. Additionally, the MD simulations have revealed that solute segregation plays a pivotal role in governing the shear strength of tilt GBs by influencing the nucleation of dislocations at interfaces. Our atomic-scale insights provide valuable guidance for understanding the formation of tilt GBs within the wrought alloys and their pivotal role in facilitating solute segregation, enabling the creation of specific two-dimensional superstructures at interfaces. Furthermore, these insights hold promise for advancing the design of high-performance alloys through the application of GB engineering.
... The phenomenon can be explained by the presence of segregations/clusters of alloying elements at grain boundaries in the NS state HPT. These segregations/clusters hinder the emission of dislocations from the grain boundaries, which increases the stress required for plastic flow in this material [35,36]. ...
Article
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Homogeneous nanostructured and ultrafine grained (NS, UFG) states with mean grain sizes of 95 and 200 nm, respectively, have been formed in a 1565ch Al–Mg alloy (Al–5.66Mg–0.81Mn–0.67Zn–0.09Zr–0.07Cr–0.04Ti–0.001Be–0.3(Fe + Si) wt %). Microstructure of both states is represented by grain boundaries with predominantly high-angle misorientations. The alloy, produced both by high pressure torsion at room temperature and equal channel angular pressing at 200°C using the Conform approach, exhibits superplasticity at low temperatures in the range 250–300°C and strain rates in the range of 5 × 10 –4 –10 –2 s –1 . Elongation values range 170–560%, while the rate sensitivity coefficient ( m ) varies from 0.3 to 0.73 at low flow stress for both NS and UFG structures. The temperature range for the stability of strength properties of the 1565ch alloy in NS and UFG states after thermal and thermal mechanical treatments has been determined. The material in both structural states maintains a high level of strength after undergoing deformation under SP conditions. The deformation relief formed on the gage surface of the NS and UFG specimens of the 1565ch alloy during the established SP yield stage has been analyzed.
... There is only one significant discrepancy between the numerical and experimental data for the processing route HPT + 700°C annealing + HPT + 350°C annealing, where the highest strength of the material is observed. Obviously, in this case, an additional contribution results from the strengthening mechanism associated with the state of grain boundaries 1 ions [38] showed that the segregation of alloying elements at grain boundaries can significantly slow down the generation of dislocations on them, contributing to additional strengthening of UFG materials. It was also found that the SPD processing and lowtemperature annealing of Grade 4 titanium can lead to active segregation of oxygen and nitrogen at grain boundaries [39]. ...
Article
Full-text available
This paper discusses the formation of ultrafine-grained (UFG) structure and nanosized second-phase precipitates in commercially pure Grade 4 titanium subjected to severe plastic deformation by high pressure torsion at room temperature with subsequent heat treatment. It was found that the combined processing of Grade 4 titanium provides very high tensile strength (σu ≈ 1500 MPa), which significantly exceeds the previous results for this material. Analysis of the strengthening mechanisms showed that the superstrength of commercially pure titanium is due to several factors: UFG structure formation, dispersion strengthening from second-phase nanoparticles, high dislocation density, and grain boundary segregation. The contribution of these strengthening mechanisms is evaluated and compared with experimental data.
... It restrains the dislocation relaxation at the grain boundary, improving the strength of the test steels [44,45]. The increase in the content of the carbon atoms at grain boundaries also improves the plastic deformation stress; thus, the strength of the test steels is enhanced [46], and the yield strength of the test steels after aging treatment is significantly increased. For the NbV-ALDS, the addition of Nb-V will reduce vacancies in the specimens and thus reduce the segregation of carbon atoms [43]; on the other hand, the precipitated (Nb,V)C particles will intensify the segregation of carbon atoms. ...
Article
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In present study, a novel Nb-V microalloyed Fe–Mn–Al–C steel and a non-microalloyed Fe–Mn–Al–C austenitic steel were treated with different thermo-mechanical processes. The microstructure of the test steels was observed by scanning electron microscope, transmission electron microscope, electron probe micro-analyzer, and X-ray diffractometer. A tensile test was conducted to estimate the mechanical properties of the test steels. Results show that the equiaxed austenitic grains are obtained in both test steels, while hot rolling reduces the grain size significantly. The grain size of the Fe–Mn–Al–C steel decreases by 40%–55% after the addition of Nb-V due to the precipitation of nanoscale (Nb,V)C particles within the austenite matrix. Compared with the solid solution treated specimens, the strength of the hot-rolled or aged specimens is improved. Meanwhile, the strength of the Fe–Mn–Al–C steel with Nb-V microalloying is also increased by 55 MPa due to the precipitation strengthening and fine grain strengthening, while the elongation is decreased. The Nb-V microalloyed Fe–Mn–Al–C steel, after hot rolling + aging treatment, obtains the maximum strength, with the yield and tensile strength of 669 MPa and 1001 MPa, respectively. The strengthening mechanisms that contribute significantly to the yield strength are solid solution strengthening and dislocation strengthening. They are 185 MPa and 211 MPa, respectively, for the Nb-V microalloyed Fe–Mn–Al–C steel at hot rolling + aging conditions. Meanwhile, the segregation of carbon atoms after aging treatment also improves the yield strength significantly. Further, it is revealed that the deformation mechanisms are microband-induced plasticity and dynamic slip band refinement.
... 22) Important to note that several studies reported about grain boundary segregation in SPD-processed AlMgSi alloys. 2325) These atomic-scale features are essential for the SPD-processed alloys and may considerably contribute to additional impact in strength 26,27) as well as to depletion of solid solution. 25) Most of the research above was carried out with small specimens mostly processed by HPT. ...
Article
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This paper presents an overview of fundamentals and potential applications of ultrafine-grained Al-based conductors developed with the help of severe plastic deformation (SPD) techniques. Based on deliberate formation of nanoscale features (such as nanoprecipitates, segregation of solutes along crystallographic defects and so on) within ultrafine grains, it is possible to optimise their mechanical and functional performance enhancing the combination of strength and electrical conductivity to produce advanced lightweight conductors required by modern industries. Guidelines related to SPD-driven development of Al alloys with properties superior to those exhibited by traditionally processed conductors are discussed.
... Recent model calculations performed in [24] have shown that the formation of segregations of impurities or alloying elements at grain boundaries can substantially slow the generation of dislocations at grain boundaries and contribute to additional hardening of UFG materials. At the same time, the computer simulation data [25,26] and experimental study results [13,14,16] provide convincing evidence of the formation of grain boundary segregations during the formation of UFG structures in metallic materials by the SPD methods, while their nature and morphology are closely related to the treatment regimes. ...
Article
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Ultrafine grained (UFG) metallic materials obtained by severe plastic deformation (SPD) typically exhibit very high strength properties, whose values are much higher than those predicted by the well-known Hall–Petch relation. Our studies show that the basis for this to occur is that SPD not only forms the UFG structure, but also leads to the formation of other nanostructural features, such as dislocation substructures, nanotwins, and nanosized precipitates of second phases, which additionally contribute to strengthening of materials. At the same time, this analysis of hardening mechanisms indicates that the structure and condition of grain boundaries, namely, their nonequilibrium state and the presence of grain boundary segregations, also substantially contribute to hardening. Taking this into consideration, approaches are discussed to achieve very high strengths in metallic materials by SPD.
... Dislocations are known to be the main defects of crystal structures responsible for the plasticity of metals [1,2]. If a crystal's interior is initially devoid of dislocations, they have to be nucleated homogeneously inside perfect areas of crystal [3][4][5][6][7][8][9][10][11] or be emitted from other defects such as grain boundaries [12][13][14][15][16][17][18][19], phase inclusions [20][21][22][23], interfaces [24][25][26], and pores [27][28][29][30] in order to start the plastic flow. Even if a crystal is not absolutely devoid of dislocations, the pre-existing dislocation density can be not enough to accommodate either ultra high rate dynamic loading with the strain rate up to 10 9 s −1 [31][32][33][34][35] or small-scale localized loading at nanoindentation [36][37][38][39][40]. ...
Article
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Incipience of plastic flow in nanoporous metals under tension is an important point for the development of mechanical models of dynamic (spall) fracture. Here we study axisymmetric deformation with tension of nanoporous aluminum with different shapes and sizes of nanopores by means of molecular dynamics (MD) simulations. Random deformation paths explore a sector of tensile loading in the deformation space. The obtained MD data are used to train an artificial neural network (ANN), which approximates both an elastic stress–strain relationship in the form of tensor equation of state and a nucleation strain distance function. This ANN allows us to describe the elastic stage of deformation and the transition to the plastic flow, while the following plastic deformation and growth of pores are described by means of a kinetic model of plasticity and fracture. The parameters of this plasticity and fracture model are identified by the statistical Bayesian approach, using MD curves as the training data set. The present research uses a machine-learning-based approximation of MD data to propose a possible framework for construction of mechanical models of spall fracture in metals.
... In polycrystalline materials, a large number of grain boundaries (GBs) with various characters generally appear, where the lattice misorientation between adjacent grains leads to higher-energy state relative to the grain interior. To reduce the GB energy, certain solutes or impurity atoms can segregate at the GB region [1], which plays an important role in the material properties such as the strength, ductility, hardness, thermal stability and fracture toughness [2][3][4][5][6][7]. In particular, when the grain size is reduced to nanoscale, the increasing concentration of GBs provides more available lattice sites for solutes to segregate, resulting in more prominent influence on material properties, especially mechanical properties. ...
Article
The combined Monte Carlo and Molecular Dynamics simulations were employed to explore the influence of elemental segregation at grain boundaries (GBs) on the mechanical properties of FeNiCrCoCu high entropy alloys under uniaxial load. The chemical potential difference with respect to Ni atoms was first obtained, and used to reach the equilibrium elemental distributions in sample models with different Cu contents, grain sizes and twin thicknesses. By comparing the random and segregated configurations, the role of GB segregation was analyzed in detail. It is found that the GB segregation trend is Ni<Fe<Co<Cr<Cu. The Cu atoms have no preference to segregate at coherent twin boundaries, and their dominant segregation at GBs prevents the detwinning that usually occurs in softening stage, further increasing twin strength. The segregation with low concentration at GBs can improve the strength by suppressing dislocation emission from GBs and further enhance the GB-mediated plastic deformation; while excessive segregation at GBs results in harmful weakening effect with GB cracking. The transition from strengthening to weakening is closely related to the GB segregated Cu concentration. To estimate the critical segregation amount in case of maximum strength, a theoretical model was established, where three regions (strengthening, transition and weakening) are distinguished. It is revealed that the strengthening region occupies a small area, and the addition of Cu contents should be rigorously tailored. This work deepens the understanding of the strengthening mechanisms arising from GB segregation and provides insights into the design of ultrahigh-strength materials.
... Lightweight aluminum alloys, particularly 5xxx series Al-Mg alloys, are now widely used in marine, automotive and food packaging, owing to their high strength-to-mass ratio (i.e. high specific strength), attractive weldability, excellent resistance to corrosion, good formability and easy recyclability (Bobylev et al., 2019;Hirsch, 2011;Liu et al., 2020;Sanders Jr et al., 2004;Zha et al., 2021). Usually, the Mg content in 5xxx series Al alloys is below 6 wt.%, much less than the equilibrium solubility (~16.5 wt.%) of Mg in Al at 450 • C , (Baker and Handbook, 1992). ...
Article
Al-Mg based alloys are known to experience dynamic strain aging (DSA) at room temperature, which is often associated with the diffusion of mobile Mg atoms to dislocation segments temporarily arrested at obstacles (e.g., forest dislocations) upon deformation in the DSA regime. With the emergence of new promising high Mg-content (> 6 wt.%) Al-Mg alloys, the combined effects of Mg content and DSA on their tensile flow behavior need to be better understood. In the present work, the tensile deformation behavior of Al and Al-(5.3, 6.4, 7.6 and 8.7) wt.% Mg samples tested at room temperature and at strain rates ranging from 0.0001 to 0.1 s⁻¹ was investigated, based on the Kocks-Mecking-Estrin model combined with a DSA model, the Haasen plot as well as transmission electron microscopy. Particular focuses are placed on the influence of Mg solutes on DSA and associated strain hardening effects, on dislocation accumulation and annihilation as well as on apparent activation volume. We show that the synergistic effect of the Mg-dislocation interactions and DSA effectively reduces the rate of dynamic recovery via suppressing dislocation cross-slip, leading to an enhanced strain hardening capacity for the Al-Mg alloys with a higher Mg content. The influences of DSA on the yield strength, flow stress and apparent activation volume of thermally-assisted deformation of the Al-Mg alloys are also discussed to elucidate fundamental insights into the development of new high Mg-content Al-Mg alloys with superior mechanical performance.
... Tailored microstructural heterogeneities, including solute segregation along GBs (Bobylev et al., 2019;Liu et al., 2019) and heterogeneous structures, such as bimodal grain structure (Zhu and Lu, 2012), heterogeneous lamellar (Geng et al., 2020) and harmonic structure (Wang et al., 2020), have been demonstrated as effective strategies to improve mechanical properties of metallic materials. Thereinto, the heterogeneous microstructure offers the opportunity to reap benefits of the strong hardening capability from micron-sized coarse grains and high strength from nano/ultrafine grains, achieving satisfying strength-ductility synergy (Zhu and Lu, 2012). ...
Article
Achieving high superplasticity in single-phase Al alloys remains a challenge, since the fine-grained structure required for superplastic deformation coarsens rapidly in the absence of dispersed second-phase particles during tensile deformation at elevated temperatures. This paper concentrates on the superplastic response of a high solid solution Al–7Mg alloy processed by equal-channel angular pressing (ECAP) under uniaxial tension. The ECAP-processed Al–7Mg alloy features multi-scale microstructural heterogeneities including a bimodal grain structure and Mg solute segregation along grain boundaries (GBs) of nano/ultrafine grains. To identify effects of multi-scale microstructural heterogeneities on superplastic deformation behavior of the high solid solution Al–7Mg alloy, microstructural evolutions are studied systematically by combing electron backscatter diffraction (EBSD), ASTAR-transmission electron microscopy (TEM) orientation imaging and atom probe tomography (APT). During deformation at the optimal tensile condition of 573 K and 1 × 10⁻³ s⁻¹, the heterogeneous microstructure evolves to a stable uniform fine grain structure via continuous dynamic recrystallization (CDRX), and impressive superplasticity of ∼523% elongation is achieved. The high superplasticity is discussed in terms of the cooperated mechanism by dislocation slip accommodated by CDRX at the early tensile deformation stage and grain boundary sliding (GBS) at the late deformation stage. Our findings show that the evolution of microstructural heterogeneities in high solid solution Al–Mg alloys can be regulated, favoring for superplastic deformation, which offers an alternative strategy for developing low-cost Al alloys for enhanced mechanical properties.
... Note, that the segregation parameters (segregation heterogeneity, the size and shape of nanoclusters in GBs, etc.) can strongly affect the stress to emit dislocations from GBs [12]. At the same time, the extradensity of EGBDs can significantly enhance the emission of dislocations from the GBs [13]. ...
Article
We study the effect of annealing followed by additional deformation on the mechanical performance of an ultrafine-grained Al-Cu-Zr alloy with a mean grain size of 300 nm. We show that elongation to failure can be notably varied by modification of the alloy’s defect structure and discuss this phenomenon in terms of interfacial dislocation emission and interaction of dislocations with grain boundaries in different states.
... In order to unpin the dislocations and to start plastic flow, an extra-stress is required. This stress can be defined by the reduced length of the nucleating [14] or emitting dislocation segments [15] or by mismatch in a lattice parameter between the segregated inclusion and a matrix [16]. Since pinned dislocations are located in GBs, deformation of the annealed state leads to fast accumulation of critical stresses in the localized areas. ...
Preprint
We study three structurally different states of nanocrystalline 316 steel and show that the state, where boundaries containing excess concentration of alloying elements are combined with mobile dislocations in grain interiors, allows maintaining extraordinarily high strength and remarkably enhanced plasticity. Underlying mechanisms featuring interaction between the segregations and mobile dislocations are discussed.
... During the aging treatment, dislocations and the high strain region are beneficial to stimulate the formation of precipitates. A similar element segregation behavior at defects has been reported in Al and Mg alloys [26,27]. High-resolution TEM analysis was employed to clarify the interaction between the dislocations and the element segregation in the 945A-32h sample. ...
Article
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Ni-based superalloys have attracted much attention due to their good resistance to high-temperature and -pressure environments. Compared with the traditional 718 Ni-based superalloy, 945A Ni-based superalloy with a lower Ni content showed better performance in terms of precipitated hardening and corrosion resistance. In this study, the aging behavior and the evolution of mechanical properties of the wrought 945A Ni-based superalloy were investigated. Microstructures were analyzed by scanning electron microscopy (SEM), bright field transmission electron microscopy (TEM), high-resolution TEM and high-angle annular dark field scanning TEM. Mechanical properties were measured by tensile and compressive tests. The results illustrated that the compressive yield stress was significantly improved by increasing aging time from 229 to 809 MPa. The increase was greater than 220%. This improvement was mainly attributed to the precipitates of the γ′ phase and carbides during the aging treatment. The residual dislocations generated by the plastic processes stimulated the formation of these precipitates. The precipitation behavior and the strengthening mechanism are discussed in detail.
... First, Zr doped at GBs (at low Zr addition) or Zr segregation at GBs (at high Zr addition) will both strengthen the GBs. This will definitely increase the strength, as demonstrated in simulations [65] and found in previous experiments . This strengthening effect, however, is not covered in present calculations. ...
Article
Manipulating the twin morphology to achieve the reinforcement of strength is a great challenge in Al with high stacking fault energy. In this work, the influence of Zr addition on the twin morphology and strengthening response of nanostructured Al films was symmetrically studied. The results showed that, for low Zr addition (≤ 4.0 at.%), the Zr atoms were homogeneously distributed within the matrix, while Zr segregation at grain boundaries was evident at higher Zr addition (> 4.0 at.%). Twins were substantially observed in all the films, and the twin morphology was highly dependent on the Zr addition. In the pure Al film, only twins with a single coherent twin boundary were detected. In comparison, nanotwins with coplanar twin boundaries (C-nanotwins) and 9R phase were predominant in the Al-Zr films with Zr addition ≤ 4.0 at.%. Further raising the Zr content, multiple nanotwins (M-nanotwins) coexisted with the C-nanotwins and 9R phase. In particular, a zero-strain twinning mechanism was applied to account for the C-nanotwins and 9R phase formation, and a zig-zag grain boundary feature induced by Zr segregation was responsible for the M-nanotwin formation. The hardness also exhibited a strong Zr dependence that increased monotonically with the Zr addition. The Al-13.4 at.% Zr film displayed a hardness of ∼4.3 GPa, about 11 times greater than the pure Al film. Strengthening mechanisms were quantitatively evaluated, and the highly-promoted hardness was mainly ascribed to the 9R phase and solid solution strengthening.
... Bobylev et al. proposed a model to explain the enhanced strength of UFG Al alloys provided by Mg segregation. In this model, segregated Mg atoms were treated as homogeneous ellipsoidal inclusions acting as the sources of elastic stresses affecting the emission of lattice dislocations from GBs [91] . It is plausible that in the present study, the interstitial C atoms segregating on GBs in the C2N3 alloy provided extra strength comparing with that of the C0 alloy after HPT processing, however it is difficult to estimate this effect as the physical mechanisms behind it are still not clear. ...
Article
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In this study, a Cantor type high entropy alloys with the addition of C interstitials (, 0, 0.5 and 2 at. %) were processed via high pressure torsion (HPT) under 6.5 GPa by 0.5, 1 and 3 turns at room temperature. The microstructures and mechanical properties of samples before and following HPT were investigated. In all compositions studied, HPT deformation led to a dramatic grain size refinement down to a nanoscale range and also resulted in a considerable increase in dislocation density. Atom Probe Tomography studies reveal that in the alloy with 2 at. % C, C atoms segregated at the boundaries of the nano-grains. The hardness of specimens with 0, 0.5 and 2 % at. C approached a maximum value at a plateau at 490 HV, 550 HV and 640 HV, respectively. This plateau was reached at an HPT shear strain corresponding to more than ∼20. The yield and ultimate tensile strength values of the as-cast alloys increased with increasing C content. The uniform elongation of all three alloys studied exceeded 30%. The yield strength values of the HPT processed samples increased significantly and reached 1.7 GPa, 1.9 GPa and 2.4 GPa, respectively, with 0, 0.5 and 2 at. % C; however, a dramatic decrease of ductility was observed. Analysis of the factors contributing to the strengthening of HPT processed alloys revealed that the conventional approach based on dislocation motion resulted in significantly overestimated yield stress values in comparison with the experimentally obtained ones. It was proposed that the discrepancy between the theoretical estimate and experimental results is related to the emergence of grain boundary sliding. The present results provide new insights on the leverage of strength versus ductility by combining HPT processing with alloying with an interstitial-type element.
... This also explains why the Al-Li-Cu alloys always exhibit the highest hardness under different ε. Recently, many studies have confirmed the deformation induced solute segregation at grain boundaries [24][25][26][27][28], for instance, Xu found that Cu atoms segregate at grain boundaries during plastic deformation due to dynamic interaction between Cu atoms with gliding dislocations, segregation of Cu atoms at grain boundaries play a crucial role in stabilizing the nanostructures [24]. Segregation of Mg has also been investigated in Al\ \Mg alloys [27,28], it shows that the HPT processed Al\ \Mg alloys exhibit higher strength than the Hallpetch relationship predicts, the enhanced strength is attributed to segregation of Mg at the grain boundaries, it is concluded that the addition of Mg can amplify the Hall-Petch type GB strengthening. ...
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Structural features and aging behavior of AlLi, Al-Li-Cu and Al-Li-Mg alloys under different equivalent strains (ε ) were investigated. Following solid-solution treatment, high-pressure torsion (HPT), asymmetric rolling (ASR) and cold rolling (CR) were adopted to introduce high, middle and low amount of strains to Al-Li-(Cu, Mg) alloys. After deformation, for the HPT processed alloys under high equivalent strains, the highest as-deformed hardness was obtained. Transmission electron microscopy (TEM) revealed that the grain size was refined to 210 nm, 120 nm and 150 nm, respectively.  Under severe plastic deformation condition (ε > 30), the AlLi alloy lost age-hardenability, however, the aging of the asymmetric rolled AlLi alloys increased the hardness further and the highest hardness was obtained in this alloy. For the Al-Li-Cu and Al-Li-Mg alloys, a further increase in hardness was achieved by aging the as-deformed alloys, regardless of the equivalent strains. Meanwhile, the peak hardness increases with increasing the equivalent strains. During aging treatment, the behavior of the precipitates was discussed in the present work.
... The similar effect can be also suggested in Al-Mg-Mn alloys with weak GBS [35] with grain boundaries impeded by high number density of fine Mn-enriched precipitates and Mg segregations. The segregations/clusters of Mg and Zn atoms on the grain boundaries of the aluminum solid solution were confirmed in [63][64][65][66][67]. ...
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Titanium and its alloys are popular materials for medical application, particularly in implant devices, where high mechanical properties and osseointegration are critical factors for successful implantation. In this work, the progress in the studies of nanostructured commercially pure Grade 4 titanium (nanoTi) is demonstrated, in which an ultrafine‐grained structure with nanoscale grain size is formed using severe plastic deformation processing. Nanostructured Grade 4 Ti has a very high strength, and its physical nature and strengthening mechanisms are analyzed herein. NanoTi proved also to have very high osseointegration during in vivo experiments. At the same time, the highest biofunctionality is demonstrated by the etched nanoTi samples with pronounced surface roughness, the latter being revealed from precise roughness measurements. The present study provided convincing evidence of accelerated bone formation on nanoTi, which is very promising for manufacture of dental and maxillofacial implants.
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Ultrafine-grained and heterostructured materials are currently of high interest due to their superior mechanical and functional properties. Severe plastic deformation (SPD) is one of the most effective methods to produce such materials with unique microstructure-property relationships. In this review paper, after summarizing the recent progress in developing various SPD methods for processing bulk, surface and powder of materials, the main structural and microstructural features of SPD-processed materials are explained including lattice defects, grain boundaries and phase transformations. The properties and potential applications of SPD-processed materials are then reviewed in detail including tensile properties, creep, superplasticity, hydrogen embrittlement resistance, electrical conductivity, magnetic properties, optical properties, solar energy harvesting, photocatalysis, electrocatalysis, hydrolysis, hydrogen storage, hydrogen production, CO2 conversion, corrosion resistance and biocompatibility. It is shown that achieving such properties is not limited to pure metals and conventional metallic alloys, and a wide range of materials are currently processed by SPD, including high-entropy alloys, glasses, semiconductors, ceramics and polymers. It is particularly emphasized that SPD has moved from a simple metal processing tool to a powerful means for the discovery and synthesis of new superfunctional metallic and nonmetallic materials. The article ends by declaring that the borders of SPD have been extended from materials science and it has become an interdisciplinary tool to address scientific questions such as the mechanisms of geological and astronomical phenomena and the origin of life.
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To investigate the impact of Twinning Boundary Spacing (TBS) on the mechanical characteristics and deformation behavior of nanotwinned Cu–Ag alloys with solute gradient segregation structure during nanoindentation, Voronoi’s method is utilized to create a nanotwinned Cu–Ag alloy model with solute gradient segregation structure. Then, the indentation process of the indenter pressed into the model with varying TBS is simulated using the Molecular Dynamics (MD) simulation, yielding depth-stress curves of the corresponding indenter. According to the findings, yield stresses are lower in Cu–Ag alloys with smaller twin spacing. Grain boundary activities play a governing role in the plastic deformation of the matrix during the small deformation stage, although dislocations also play a part. Grain boundaries have less of an impact on material deformation during the big deformation stage, and dislocations are primarily responsible for the matrix’s plastic deformation. The stacking faults and deformation twin are more prone to occur inside grains with higher solute-atom concentrations compared to grain boundaries with lower solute-atom concentrations.
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Ultrafine-grained (UFG) metallic materials processed by severe plastic deformation (SPD) techniques often exhibit significantly higher strengths than those calculated by the well-known Hall-Petch equation. These higher strengths result from the fact that SPD processing not only forms the UFG structure but also leads to the formation of other nanostructural features, including dislocation substructures, nanotwins and nanosized second-phase precipitations, which further contribute to the hardening. Moreover, the analysis of strengthening mechanisms in recent studies demonstrates an important contribution to the hardening due to phenomena related to the structure of grain boundaries as a non-equilibrium state and the presence of grain boundary segregations. Herein, the principles of the nanostructural design of metallic materials for superior strength using SPD processing are discussed.
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We study three structurally different states of nanocrystalline 316 steel and show that the state, where boundaries containing excess concentration of alloying elements are combined with mobile dislocations in grain interiors, allows maintaining extraordinarily high strength and remarkably enhanced plasticity. Underlying mechanisms featuring interaction between the segregations and mobile dislocations are discussed.
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The mechanical strength of commercial Al alloys rarely exceeds 700 MP. Recent studies show that nanocolumnar metals exhibit superb mechanical behaviors but a discrepancy between tensile and hardness measurements often emerges due to the absence of investigation to the influences of loading conditions (tension versus compression and in-plane versus out-of-plane orientation) on deformation mechanisms. Here, we inspect the mechanical response of columnar nanotwinned Al–Fe alloys with emphasis on response of grain boundaries to in-situ tension and compression tests along both the in-plane and out-of-plane directions inside a scanning electron microscope. Our studies reveal ultra-high out-of-plane tensile and compressive stress, exceeding 1.8 GPa, and an in-plane tensile and compressive stress of 1.1 and 1.6 GPa, respectively. Post-mortem TEM analyses were performed to elucidate the orientation-dependent plastic anisotropy and tension-compression asymmetry in columnar nanotwinned Al–Fe. This study provides an important forward step towards the understanding of deformation mechanisms in high-strength nanotwinned Al alloys and nanocolumnar metals.
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The strengthening mechanism of the metallic material is related to the hindrance of the dislocation motion, and it is possible to achieve superior strength by maximizing these obstacles. In this study, the multiple strengthening mechanism-based nanostructured steel with high density of defects was fabricated using high-pressure torsion at room and elevated temperatures. By combining multiple strengthening mechanisms, we enhanced the strength of Fe-15 Mn-0.6C-1.5 Al steel to 2.6 GPa. We have found that solute segregation at grain boundaries achieves nanograined and nanotwinned structures with higher strength than the segregation-free counterparts. The importance of the use of multiple deformation mechanism suggests the development of a wide range of strong nanotwinned and nanostructured materials via severe plastic deformation process.
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Nanocrystalline materials, as a new generation of advance materials, have outstanding mechanical properties such as high strength and hardness, low elastic modulus, good ductility and excellent fatigue and wear resistance. In this paper, an overview of the synthesis and mechanical properties of nanocrystalline materials is provided. Furthermore, the grain size dependent plastic deformation mechanism is discussed to correlate with their mechanical behavior.
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Grain boundary complexions have been observed to affect the mechanical behavior of nanocrystalline metals, improving both strength and ductility. While an explanation for the improved ductility exists, the observed effect on strength remains unexplained. In this work, we use atomistic simulations to explore the influence of ordered and disordered complexions on two deformation mechanisms which are essential for nanocrystalline plasticity, namely dislocation emission and propagation. Both ordered and disordered grain boundary complexions in Cu-Zr are characterized by excess free volume and promote dislocation emission by reducing the critical emission stress. Alternatively, these complexions are characterized by strong dislocation pinning regions that increase the flow stress required for dislocation propagation. Such pinning regions are caused by ledges and solute atoms at the grain-complexion interfaces and may be dependent on the complexion state as well as the atomic size mismatch between the matrix and solute elements. The trends observed in our simulations of dislocation propagation align with the available experimental data, suggesting that dislocation propagation is the rate-limiting mechanism behind plasticity in nanocrystalline Cu-Zr alloys.
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Refining a metal’s grain size can result in dramatic increases in strength, and the magnitude of this strengthening increment can be estimated using the Hall–Petch equation. Since the Hall–Petch equation was proposed, there have been many experimental studies supporting its applicability to pure metals, intermetallics and multi-phase alloys. In this article, we gather the grain-size strengthening data from the Hall–Petch studies on pure metals and use this aggregated data to calculate best estimates of these metals’ Hall–Petch parameters. We also use this aggregated data to re-evaluate the various models developed to physically support the Hall–Petch scaling. © 2016 Institute of Materials, Minerals and Mining and ASM International
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The control of interfaces in engineered nanostructured materials has met limited success compared with that which has evolved in natural materials, where hierarchical structures with distinct interfacial states are often found. Such interface control could mitigate common limitations of engineering nanomaterials. For example, nanostructured metals exhibit extremely high strength, but this benefit comes at the expense of other important properties like ductility. Here, we report a technique for combining nanostructuring with recent advances capable of tuning interface structure, a complementary materials design strategy that allows for unprecedented property combinations. Copper-based alloys with both grain sizes in the nanometre range and distinct grain boundary structural features are created, using segregating dopants and a processing route that favours the formation of amorphous intergranular films. The mechanical behaviour of these alloys shows that the trade-off between strength and ductility typically observed for metallic materials is successfully avoided here.
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The experimental evidence for the Hall-Petch dependence of strength on the inverse square-root of grain size is reviewed critically. Both the classic data and more recent results are considered. While the data can be fitted to the inverse square-root dependence excellently (but using two free fitting parameters for each dataset), it is also consistent with a dependence on the simple inverse of grain size (with one free fitting parameter for each dataset). There have been difficulties, recognised for half-a-century, in explaining the inverse square-root expression. A Bayesian analysis shows that the data strongly supports the simple inverse expression proposed. Since this expression derives from underlying theory, it is also more readily explicable. It is concluded that the Hall-Petch effect is not to be explained by the variety of theories found in the literature, but is a manifestation of, or underlain by, the general size effect observed throughout micromechanics, due to the inverse relationship between the stress required and the space available for dislocation sources to operate.
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High quality bulk ultrafine-grained/nanocrystalline titanium samples were prepared through room temperature mechanical milling and conventional consolidation processes. The prepared bulk samples showed high purity, very low porosity and high ductility under compression. The dependency of yield stress and post-yielding behavior on grain size, strain rate and temperature were comprehensively studied. The texture evolution of the ufg/nc samples under compression was measured by synchrotron XRD. On the macroscopic scale, the viscoplastic phenomenological Khan–Liang–Farrokh (KLF) model was used to correlate the experimental results of the ufg/nc Ti.
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The present work deals with the atomic mechanism responsible for the emission of partial dislocations from grain boundaries (GB’s) in nanocrystalline metals. It is shown that in 12 and 20 nm grain size samples GB’s containing GB dislocations can emit a partial dislocation during deformation by local atomic shuffling and stress-assisted free volume migration. The free volume is often emitted or absorbed in a neighboring triple junction. It is further suggested that the degree of delocalization surrounding the grain boundary dislocation determines whether atomic shuffling can associate displacements into the Burgers vector necessary to emit a partial dislocation. Temporal analysis of atomic configurations during dislocation emission indicates that creation and propagation of the partial might be separate processes.
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A dislocation bow-out model has been developed to explain the strength of ultra-fine grained (UFG) materials with grain size roughly between 20 nm to 500 nm. In the model, perfect dislocations are assumed to be nucleated at grain-boundary sources and bow out between two pinning points on a boundary. Yielding is considered to occur when a dislocation takes a semi-circular shape under applied stress. Statistical consideration is introduced to evaluate the most probable pinning-point distance as a function of grain size. Comparison with experimental results is made for fee UFG metals. It is found that yield stress as well as thermal activation parameters can be explained reasonably by the present theoretical model.
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The microstructure of commercially available nanocrystalline (nc) electroplated Ni foils is studied by means of X-ray diffraction and transmission electron microscopy. It is shown that the microstructure is inhomogeneous and batch-dependent. Tensile properties at strain rates between 10−5 and 103 s−1 are studied and compared with the results of coarse-grained Ni. Data on strength, strain-rate sensitivity and work hardening are presented. At the highest strain rates, shear banding with local grain growth is observed in the nc structure. It is also suggested that the differences found in nc Ni for 3 and 20 mm tensile specimens are the size effects related to the inhomogeneous microstructure.
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Hardening on annealing (HOA) has been frequently observed in nanostructured metals and alloys. For nanostructured materials obtained by severe plastic deformation (SPD), HOA has been attributed to the reduction of dislocation sources within grains and grain boundary relaxation during annealing. In the present work, it is shown that when a bimodal grain structured (a mixture of micron-sized and ultrafine grains) Al-5Cu alloy prepared by equal channel angular pressing (ECAP) was subjected to post-ECAP natural and artificial aging treatments, the alloy shows a completely different precipitation behavior with an accelerated precipitation kinetics. No coherent θ'' or semi-coherent θ' precipitates form in the bulk of grains, while a large fraction of stable incoherent θ precipitates form along high angle boundaries. After artificial aging at low temperatures for a short time, a significant improvement of both ultimate tensile strength and uniform elongation was achieved without sacrificing the yield strength. A systematic microstructure characterization by EBSD, TEM and APT has been carried out to investigate the evolution of grain size, dislocation density and solid solution level of Cu as well as the precipitation of Al-Cu precipitates during natural and artificial aging treatments. A quantitative evaluation of different supposed strengthening mechanisms revealed that the segregation of Cu elements at grain boundaries plays a more important role than grain boundary relaxation and the dislocation source-limited strengthening to compensate the yield strength reduction caused by the decrease in dislocation density and solute content of Cu in solid solution.
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Nanostructured metallic materials having nanocrystalline and ultrafine-grained structures show exceptional mechanical properties, e.g. superior strength, that are very attractive for various applications. However, superstrong metallic nanomaterials typically have low ductility at ambient temperatures, which significantly limits their applications. Nevertheless, several examples of nanostructured metals and alloys with concurrent high strength and good ductility have been reported. Such strong and ductile materials are ideal for a broad range of structural applications in transportation, medicine, energy, etc. Strong and ductile metallic nanomaterials are also important for functional applications where these properties are critical for the lifetime of nanomaterial-based devices. This article presents an overview of experimental data and theoretical concepts addressing the unique combination of superior strength and enhanced ductility of metallic nanomaterials. We consider the basic approaches and methods for simultaneously optimizing their strength and ductility, employing principal deformation mechanisms, crystallographic texture, chemical composition as well as second-phase nano-precipitates, carbon nanotubes and graphene. Examples of achieving such superior properties in industrial materials are reviewed and discussed.
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Severe plastic deformation (SPD) has established itself as a potent means of producing bulk ultrafine grained and nanostructured materials. It has given rise to burgeoning research that has become an integral part of the present day materials science. This research has received a broad coverage in literature, and several recent publications (including reviews in Progress in Materials Science) provide a very good introduction to the history, the current status, and the potential applications of SPD technologies. There is one aspect of SPD-related research, though, which despite its enormous importance has not been covered by any substantive review, viz. the modelling and simulation work. Due to the complexity of SPD processing and the specificity of material behaviour at the extremely large strains involved, analytical and computational studies have been indispensable for process design, parameter optimisation, and the prediction of the microstructures and properties of the ultrafine grained materials produced. The pertinent literature is vast and often difficult to navigate. The present article addresses this aspect of SPD and provides a commented exposé of a modelling and numerical simulation toolkit that has been, or can potentially be, applied in the context of severe plastic deformation.
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Interfacial nucleation is the dominant process of dislocation generation during the plastic deformation of nano-crystalline materials. Solute additions intended to stabilize nano-crystalline metals against grain growth, may segregate to the grain boundaries and triple junctions where they can affect the process of the dislocation emission. In this Letter we demonstrate that the effect of solute addition in a nano-crystalline material containing competing solute segregation sites and dislocation sources can be very complex due to different rates of segregation at different interfaces. Moreover, at large concentrations, when the solutes form clusters near the grain boundaries or triple junctions, the interfaces between these clusters and the matrix can introduce new dislocation emission sources, which can be activated under lower applied stress. Thus, the strength maximum can occur at a certain solute concentration: adding solutes beyond this optimal solute concentration can reduce the strength of the material.
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A theoretical model is suggested which describes grain boundary (GB) sliding and its accommodation through dislocation slip in ultrafine-grained and nanocrystalline metals. The initial stage of the accommodating dislocation slip represents emission of lattice dislocations from triple junctions into grain interiors. The lattice dislocations emitted from a triple junction slip across a grain and are absorbed by an opposite GB where they are dissociated into GB dislocations that climb along the GB. In the situation where these GB sliding and accommodating processes are dominant, stress-strain dependences are calculated in ultrafine-grained copper. With the calculated dependences, we found that pronounced strain hardening occurs which is related to the accommodation processes and associated formation of disclinations at triple junctions of GBs. It is theoretically revealed that the special (new) strain hardening mechanism under discussion can play a significant role in enhancing ductility of ultrafine-grained and nanocrystalline metals at comparatively low temperatures.
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A review is given in honor of Ilya A. Ovid'ko beginning from a mutual interest in disclinations. The main part of the review, however, is based on association of dislocation pileups and the Hall-Petch relation for an inverse square root of grain size dependence of cleavage fracturing. Important thermal influence is considered. Special attention is given to small pile-up characteristics associated with nanopolycrystalline material behaviors. Other topics include: (1) the ductile-brittle transition; (2) the ductile true fracture strain; (3) the fracture mechanics stress intensity; (4) hardness; (5) creep; and, (6) fatigue behaviors. Experimental measurements are presented for a wide range of materials.
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When grain sizes are reduced to the nanoscale, grain boundaries (GB) become the dominant sources of the dislocations that enable plastic deformation. We present the first molecular dynamics (MD) study of the effect of substitutional solutes on the dislocation nucleation process from GBs during uniaxial tensile deformation. A simple bi-crystal geometry is utilized in which the nucleation and propagation of dislocations away from a GB is the only active mechanism of plastic deformation. Solutes with atomic radii both larger and smaller than the solvent atomic radius were considered. Although the segregation sites are different for the two cases, both produce increases in the stress required to nucleate a dislocation. MD simulations at room temperature revealed that this increase in the nucleation stress is associated with changes of the GB structure at the emission site caused by dislocation emission, leading to increases in the heats of segregation of the solute atoms, which cannot diffuse to lower-energy sites on the timescale of the nucleation event. These results contribute directly to understanding the strength of nanocrystalline materials, and suggest suitable directions for nanocrystalline alloy design leading toward structural applications.
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Under intense heating and/or deformation, pure nanocrystalline (NC) metals exhibit significant grain coarsening, thus preventing the study of length scale effects on their physical response under such conditions. Hence, in this study, we use in-situ TEM heating experiments, atomistic modeling along with elevated temperature compression tests on a thermally stabilized nanostructured Cu–10 at.% Ta alloy to assess the microstructural manifestations caused by changes in temperature. Results reveal the thermal stability attained in NC Cu-10 at.% Ta diverges from those observed for conventional coarse-grained metals and other NC metals. Macroscopically, the microstructure, such as Cu grain and Ta based cluster size resists evolving with temperature. However, local structural changes at the interface between the Ta based clusters and the Cu matrix have a profound effect on thermo-mechanical properties. The lattice misfit between the Ta clusters and the matrix tends to decrease at high temperatures, promoting better coherency. In other words, the misfit strain was found to decrease monotonically from 12.9% to 4.0% with increase in temperature, leading to a significant change in flow stress, despite which (strength) remains greater than all known NC metals. Overall, the evolution of such fine structures is critical for developing NC alloys with exceptional thermo-mechanical properties.
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A new crystal plasticity model based on the deformation mechanism for ultrafine-grained/nanocrystalline face-centered cubic (FCC) metals was developed. The deformation mechanism was that dislocations glide from grain boundary to grain boundary (GB). Constitutive equations on the slip system level were developed based on dislocation glide and all stages of dislocation activities were considered especially their interactions with GB. An Arrhenius type rate equation was established based on the thermally activated depinning of dislocations from GB obstacles. The new constitutive equations were incorporated into a 3D crystal plasticity formulation, and this crystal plasticity model was implemented into a UMAT subroutine of the ABAQUS finite element program. The uniaxial deformation responses of the two ufg/nc materials were simulated. Crystal plasticity finite element method (CPFEM) simulations gave flow stress predictions that were very close to the experimental results. The dislocation mechanism-based crystal plasticity UMAT is ready to be used for more advanced simulation studies.
Article
When the grain size in polycrystalline materials is reduced to the nanometer length scale (nanocrystallinity), observations from experiments and atomistic simulations suggest that the yield strength decreases (softening) as the grain size is decreased. This is in contrast to the Hall-Petch relation observed in larger sized grains. We incorporated grain boundary (GB) sliding and dislocation emission from GB junctions into the classical DDD framework, and recovered the smaller is weaker relationship observed in nanocrystalline materials. This current model shows that the inverse Hall-Petch behavior can be obtained through a relief of stress buildup at GB junctions from GB sliding by emitting dislocations from the junctions. The yield stress is shown to vary with grain size, d, by a d1/2 relationship when grain sizes are very small. However, pure GB sliding alone without further plastic accomodation by dislocation emission is grain size independent.
Article
Grain size has a profound effect on the mechanical response of metals. Molecular dynamics continues to expand its range from a handful of atoms to grain sizes up to 50 nm, albeit commonly at strain rates generally upwards of 10 6 s À 1. In this review we examine the most important theories of grain size dependent mechanical behavior pertaining to the nanocrystalline regime. For the sake of clarity, grain sizes d are commonly divided into three regimes: d4 1 μm, 1 μm od o100 nm; and d o100 nm. These different regimes are dominated by different mechanisms of plastic flow initiation. We focus here in the region d o 100 nm, aptly named the nanocrystalline region. An interesting and representative phenomenon at this reduced spatial scale is the inverse Hall–Petch effect observed experimentally and in MD simulations in FCC, BCC, and HCP metals. Significantly, we compare the results of molecular dynamics simulations with analytical models and mechanisms based on the contributions of Conrad and Narayan and Argon and Yip, who attribute the inverse Hall–Petch relationship to the increased contribution of grain-boundary shear as the grain size is reduced. The occurrence of twinning, more prevalent at the high strain rates enabled by shock compression, is evaluated.
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The metals and alloys subjected to severe plastic deformation can possess not only ultrafine-grained (UFG) structure but also specific nanostructural features, such as nonequilibrium grain boundaries, nanotwins, grain boundary segregations and nano particles. The authors consider in the present work the role of these features in exhibition of high strength of nanostructured metals and alloys. In particular, it is demonstrated that the presence of grain boundary segregations and non-equilibrium boundaries can result in yield stress values that considerably exceed those predicted from the Hall-Petch relation for the given materials.
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The influence of grain boundary segregation on the strength of a nanostructured austenitic stainless steel was investigated. The steel was nanostructured by severe plastic deformation at two different temperatures to form ultrafine-grained states different by microstructure parameters. It is shown that despite the difference in grain size both nanostructured steels demonstrated the same level of strength. For the first time it is directly observed that severe plastic deformation at elevated temperature leads to formation of MO-Cr-Si rich grain boundary segregations in the steel. Considering different contributions to the material strengthening, we demonstrate that grain boundary segregations can lead to significant enhancement of the yield stress.
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The yielding behavior of interstitial-free steels and low-carbon steels with varying amounts of C and N were investigated in connection with the Hall-Petch relation. The Hall-Petch coefficient is as small as 150 MPa.mu m(1/2) in interstitial-free steels but it increases to 600 MPa.mu m(1/2) by adding solute carbon up to 60 ppm. Nitrogen does not have a significant effect on the Hall-Petch coefficient. The results of three-dimensional (3D) atom probe analysis indicated that carbon has 3-4 times greater segregation potential in comparison with nitrogen. The small effect of nitrogen on the Hall-Petch coefficient in steel is probably due to the small segregation potential of nitrogen. It was also confirmed that discontinuous yielding occurs when the difference between the yield stress and friction stress is increased by grain-refinement strengthening and that yielding occurs by dislocation emission from grain boundaries where primary dislocations have piled up. Carbon atoms segregated at grain boundaries seem to play a role in stabilizing dislocation emission sites at the grain boundaries, which enhances the Hall-Petch coefficient of iron. These results support the dislocation pile-up model of explaining yielding in polycrystalline metals.
Article
In this article, we examine the conditions that favour the emission of Shockley partial dislocations (SPDs) that standoff from a grain boundary (GB) plane by a few lattice parameters as part of the atomic structure of some GBs. To do so, we consider GBs to be formed by the operation of arrays of intrinsic grain boundary dislocations (GBDs) that create the tilt and twist misorientation, and the lattice mismatch between the two crystal grains adjoining the GB. The conditions to be considered that favour SPDs are the following: (1) Frank’s rule, (2) the proper sequential arrangement of partial dislocations to bound an intrinsic stacking fault and (3) the equilibrium stand-off distance (ESD). We apply an isotropic elasticity analysis to compute the ESD, in the absence of an applied stress, for SPDs emerging from asymmetric tilt GBs in two FCC metals, Cu and Al. The ESD is shown to be dependent on the glide plane orientation relative to the GB plane and on the position of the glide planes, relative to the position of the GBDs. An applied stress increases the ESD up to a critical stress that removes the SPDs without limit from the GB. We examine the effect of the stacking fault energy on the ESD and critical stress. The critical stress is effectively linearly dependent on the stacking fault energy. Finally, we present results of atomistic simulations of asymmetric tilt Σ11[1 0 1]{4 1 4}||{2 5 2} GBs in Cu bicrystal models subject to shock loading that behave in a manner similar to the elasticity predictions. The atomistic simulations reveal additional behaviour associated with elastic incompatibility between the two grains in the bicrystal models.
Article
A Mg–Zn–Zr alloy was processed by high-pressure torsion for up to 2 turns at room temperature to produce significant grain refinement together with enhanced plasticity and strength. Measurements were performed to determine the strain-rate sensitivity, shear yield strength and activation volume as a function of the processing conditions. The results suggest there is a significant contribution from grain boundary sliding to the flow process and the onset of plasticity is associated with heterogeneous dislocation nucleation.
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The formation of Mg segregations at and near deformation-distorted grain boundaries (GBs) in ultrafine-grained Al–Mg alloys is theoretically described as a process enhanced by stress fields of extrinsic dislocations existing at such GBs. The equilibrium Mg concentration profiles near low-angle and high-angle GBs containing extrinsic dislocations are calculated. The results of the calculations explain the experimental observations (reported in the scientific literature) of spatially inhomogeneous Mg segregations characterized by high Mg concentrations at and near GBs in ultrafine-grained Al–Mg alloys processed by severe plastic deformation.
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The hardness, tensile ductility and fatigue lifetime of nanostructured (NC) Cu-Zr alloyed thin films have been systematically measured at different Zr additions (0, 0.5, 2.0, 4.0, 8.0 at.%). Experimental results showed that the Cu-0.5 at.% Zr film exhibited the highest hardness, largest ductility and longest fatigue lifetime, which are increased by 120%, 80%, and above an order of magnitude, respectively, in comparison with its pure Cu counterpart. The simultaneous improvements of mechanical properties are rationalized with respect to microstructural evolutions related to Zr segregation at grain boundaries (GBs). Besides refinement in NC grains and enhancement in nanotwins, grain orientation was apparently modified by the GB Zr segregation. (1 1 0) grains were promoted and layer-like microstructure was formed with coexistence of three orientation of grains at the upper layer, facilitating Cu grain growth at room temperature. GB doping strengthening worked as an additional strengthening mechanism and mechanically driven grain growth became the predominant deformation mechanism and fatigue mechanism, which accounted for the optimal combination of mechanical properties achieved in the Cu-0.5 at.% Zr film. When the Zr content was >2.0 at.%, however, amorphous phases were formed at the GBs due to locally increased Zr concentration. The GB doping strengthening effect was concomitantly weakened and strength was gradually decreased. In addition, stress/strain localization occurred readily at the amorphous phase regions, triggering intergranular fracture and reducing ductility. During cyclic testing in the low stress range, GB amorphous phases strengthened rather impaired the GBs, which gave the Cu-Zr films a fatigue lifetime greater than that of the pure NC Cu film The present results clearly indicate that, once solute content is suitably explored, the solute segregation at GBs in NC metals can not only retain NC grains in loading-free conditions, but also impart enhanced mechanical properties when exposed to applied stress.
Article
Due to their interaction with crystalline defects, solute atoms play a critical role in the microstructure evolution of aluminum alloys during deformation. In addition, deformed structures often exhibit a modified aging response. For a better understanding of these mechanisms, we provide here a thorough study of deformation-induced segregation and precipitation mechanisms in an aluminum alloy containing 5.8 wt.% Mg subjected to severe plastic deformation (SPD). The solutionized alloy was processed by high-pressure torsion at room temperature and at 200 °C. The investigation of the microstructure and of the distribution of Mg after deformation by scanning transmission electron microscopy and atom probe tomography revealed that clustering and segregations occurred during severe deformation. Mg atoms agglomerate on grain boundaries (GBs), forming mostly nanoscaled clusters at room temperature and more uniform segregation along GBs at 200 °C. In any case, however, the equilibrium Al3Mg2 phase does not nucleate. Using post-deformation annealing treatments, it was found that it can proceed only through a very specific orientation relationship with the face-centered-cubic Al matrix. Both the contribution of dislocations and deformation-induced vacancies were considered to account for the enhanced mobility of Mg atoms. From theoretical estimations it is, however, concluded that Mg atoms are dragged by the vacancy flux toward GBs while dislocations should not play a significant role. These data provide new insights about mechanisms controlling dynamic precipitation and segregation during SPD of aluminum alloys. The segregation and formation of clusters that is revealed can additionally contribute to the strengthening of these alloys, leading to a new understanding of dynamic ageing in non-age-hardenable alloys.
Article
Ultrafine-grained metals (UFGMs) produced by warm- or cold-rolling under severe plastic deformation have attracted interest as high-strength structural materials. UFGMs with a grain size less than 1 μm exhibits remarkable material and mechanical properties, and a computational model predicting these properties is desired in the field of materials science and engineering. In order to clarify the utility of UFGM numerically, it is important to investigate the size effects of metallic materials that depend on initial grain size. It is assumed that such unusual mechanical properties originate in grain size and the enormous volume fraction of the grain boundary. When grains are of the submicron order, dislocation loops are hardly generated from Frank–Read sources smaller than the grain size. Grain boundaries play an important role in dislocation dynamics. In this study, we develop a crystal plasticity model considering the effect of the grain boundary and dislocation source. In order to predict variation of critical resolved shear stress (CRSS) due to grain boundaries or dislocation sources, information on dislocation source and grain boundary is introduced into a hardening law of crystal plasticity. In addition, FE simulation for FCC polycrystal is used to analyze stress–strain responses such as increased yield stress and yield point drop, from the viewpoint of grain size and dislocation density. We thoroughly investigate the effect of dislocation behavior on the material properties of UFGMs.
Article
Emission of lattice dislocations from grain boundaries (GBs) specified by deformation-distorted structures with periodic fluctuations of misorientation in deformed ultrafine-grained (UFG) materials is theoretically described. It is theoretically revealed that (i) the dislocation emission from deformation-distorted GBs is significantly enhanced as compared to that from structurally equilibrated GBs; and (ii) the enhancement effect depends on the parameters specifying the deformation-distorted GBs. The influence of deformation-distorted GBs as dislocation sources on the tensile ductility of UFG materials is discussed.
Article
The effects of high-pressure torsion (HPT) processing on an Al–Mg–Si alloy (AA6060) have been investigated comprehensively. We show that the processing temperature has complex effects on the strength, grain refinement and solute nanostructures of the alloy. Ten-revolution HPT processing at room temperature produced the highest yield strength of 475 MPa, which is similar to a high-strength Al alloy. However, processing at 100 °C produced the finest grains due to the strong solute segregation to grain boundaries and the formation of high-density precipitates that pin grain boundaries. Processing at 180 °C led to significant decomposition of the alloy and the formation of coarse precipitates. This research demonstrates that solute nanostructures provide key information for unravelling the origins of HPT-induced strengthening and grain refinement, and reveals the important opportunities for “engineering” solute nanostructures to enhance grain refinement in HPT processing.
Article
Metals and alloys produced by severe plastic deformation (SPD) are characterized by not only an ultrafine grain size, but also other structural features, such as nonequilibrium grain boundaries, nanotwins, grain-boundary segregations, and nanoparticles. The present work deals with the study of the effect of these features on the strength of SPD metals and alloys. In particular, it has been shown that, with segregations on grain boundaries and nonequilibrium boundaries, the yield stress of the material can exceed considerably the values extrapolated to the range of ultrafine grains using the Hall-Petch relationship.
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Nanoindentation hardness measurements were performed on nanocrystalline (nc-) Cu alloys to test recent molecular dynamics predictions that (i) solute segregation to grain boundaries can lead to significant strengthening and (ii) solutes with large size mismatch with Cu are most effective. Results show that the hardness of nc-Cu90Nb10 is greater than 5 GPa, more than double that of pure nc-Cu, whereas similar additions of Fe solute have nearly no effect. These results are in good agreement with simulations.
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This article presents our take on the area of bulk ultrafine-grained materials produced by severe plastic deformation (SPD). Over the last decades, research activities in this area have grown enormously and have produced interesting results, which we summarise in this concise review. This paper is intended as an introduction to the field for the “uninitiated”, while at the same time highlighting some polemic issues that may be of interest to those specialising in bulk nanomaterials produced by SPD. A brief overview of the available SPD technologies is given, along with a summary of unusual mechanical, physical and other properties achievable by SPD processing. The challenges this research is facing—some of them generic and some specific to the nanoSPD area—are identified and discussed.
Article
As an attempt to understand the temperature and strain-rate dependence of strength in nanocrystalline and ultrafine-grained materials, thermally activated depinning process has been incorporated into a dislocation bow-out model from a grain boundary. Such quantities as activation energy, activation volume, strain-rate sensitivity and yield strength are discussed and compared with experimental results available in literature. It is found that the present model analysis can reasonably explain the observed characteristics of these quantities.
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Grain boundary segregations were investigated by Atom Probe Tomography in an Al-Mg alloy, a carbon steel and Armco\trademark Fe processed by severe plastic deformation (SPD). In the non-deformed state, the GBs of the aluminium alloy are Mg depleted, but after SPD some local enrichment up to 20 at.% was detected. In the Fe-based alloys, large carbon concentrations were also exhibited along GBs after SPD. These experimental observations are attributed to the specific structure of GBs often described as "non-equilibrum" in ultra fine grained materials processed by SPD. The grain boundary segregation mechanisms are discussed and compared in the case of substitutional (Mg in fcc Al) and interstitial (C in bcc Fe) solute atoms.
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Our Editor's Choice [1] presents results of analytical calculations of the displacement and strain field induced by a linear inclusion such as a strained semiconductor quantum wire in an isotropically elastic half-space solid. The cover picture shows the total dilatation distribution in the plane perpendicular to a crescent-shape wire which is parallel to the free surface, marked by the straight red line, and, as thick curved red lines, the sections of the two truncated cylinders used to generate the wire. Frank Glas is currently in charge of the research group ‘Elaboration and Physics of Epitaxial Structures’ at the Laboratoire de Photonique et de Nanostructures, CNRS Marcoussis. His activities include experimental and theoretical studies of the structural, elastic and thermodynamical properties of III–V semiconductor alloys and epitaxial structures, analytical elasticity calculations as well as electron microscopy and microanalysis.
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The effect of sulphur on the strengthening and the thermally activated deformation of cold-worked Zr–1 Nb alloy was investigated. In the present study, the sulphur strengthening was observed even at room temperature unlike the previous study of Ferrer et al. The flow stress increased by 65 MPa at room temperature with the addition of sulphur as little as 20 ppm. With further increase of sulphur content up to 300 ppm, negligible change of the flow stress was observed. The additive strengthening behavior in which the entire stress–strain curve shift upward by the friction stress due to the addition of sulphur was observed in the Zr–Nb alloy of the present study. The acti-vation volume decreased slightly (from 110b 3 to 80b 3) with the addition of 300 ppm sulphur at room temperature. The rate-controlling mechanism of the deformation can best be explained by the dislocation interaction mechanism in which the segregation of alloying elements such as oxygen and sulphur atoms affects the activation length of dislocations.
Article
Direct observations of grain boundary ledges have been made in annealed Ni, Al, Cu, Mo, Ta, Ir, 304 Stainless Steel and Inconel 600 by transmission electron microscopy. Grain boundary ledges have been observed to be sources of dislocations during and after plastic deformation, and to resemble the appearance of dislocation pileups in the transmission electron microscope. Ledge density (number per unit length of grain boundary) has been observed to increase with an increase in grain boundary misorientation in Ni and 304 Stainless Steel, and the distribution of misorientations was observed to be continuous over the range 0 deg <θ< 90 deg at an annealing temperature of 1060°C. The mean grain boundary misorientation in 304 Stainless Steel was also observed to decrease with a decrease in the recovery temperature following cold reduction and to vary from 10 to 45 deg in the temperature range of 660 to 1060°C. An essential point of this investigation is that Li’s theoretical treatment of the flow-stress, grain-size relation based on the existence of grain boundary ledges and their action as sources of dislocations under stress is shown to be correct.
Article
The yield strength of dilute nc-Cu alloys was investigated using molecular dynamics simulations. Alloying additions that lower grain boundary energy were found to dramatically increase the yield strength of the alloy, with dilute Cu-Nb alloys approaching the theoretical strength of Cu. These findings suggest a new scaling behavior for the onset of plasticity in nanocrystalline materials, one that depends on the product of the specific grain boundary energy and molar fraction of grain boundary atoms, and not simply on grain size alone.
Article
The uniaxial tensile straining of stainless steel 304 sheet material and transmission electron microscope observations of representative electron-transparent thin sections prepared from variously strained samples showed that dislocation profiles extending from the grain boundaries, and associated with ledges on the boundary plane, increase in frequency (the number of profiles per unit length of grain boundary plane) with increasing strain. Because of the nature of these profiles, gleaned from numerous observations, the majority are considered to be emission profiles, particularly at low plastic strains (ϵp ⩽ 2%). Consequently, grain boundaries in stainless steel are concluded to be the principal sources for initial dislocations. These conclusions were supported by the in situ straining of thin microtensile specimens of stainless steel 304 and direct observations of dislocation emission from grain boundaries in a high voltage electron microscope. In these observations, dislocation profiles resembling dislocation pile-ups were observed to form at grain boundary ledges, and ledges were observed to form by the glide motion of dislocations in the grain boundary plane. Grain boundary dislocations moving in the interface plane were observed in some cases to dissociate apparently into lattice dislocations which were emitted from the grain boundary to form profiles gliding on the {111} planes. Although in situ high voltage electron microscopy experiments proved to be extremely difficult, unpredictable and lacking in the ability to record elegant and sequential images attesting to the dynamic features of dislocation emission from grain boundaries in response to an applied strain, the results obtained confirm the feasibility of the experiments and provide direct evidence and corroboration for conclusions drawn from standard post-deformation experiments.