Plastic strain-induced grain refinement in the nanometer scale in a Mg alloy

By means of surface mechanical attrition treatment, nanometer-sized grains (with an average size of 30 ± 5 nm) were generated in the surface layer of a single-phase AZ91D alloy. Transmission electron microscopy investigations showed that the strain-induced grain refinement process in AZ91D alloy includes three steps. At the initial stage twinning dominates the plastic deformation and divides the coarse grains into finer twin platelets. With increasing strain, double twins and stacking faults form and a number of dislocation slip systems are activated, including basal plane systems, prismatic plane systems and pyramidal plane systems. As a result of the dislocation slip along these systems and of the cross slips, high-density dislocation arrays are formed which further subdivide the twin platelets into subgrains. Obvious evidence of dynamic recrystallization were identified within the high-strain-energy subgrains with a further increase of strain, leading to the formation of nano-sized grains in the surface layer.     

The relation between ductility and stacking fault energies in Mg and Mg–Y alloys

The underlying mechanisms that are responsible for the improved room-temperature ductility in Mg–Y alloys compared to pure Mg are investigated by transmission electron microscopy and density functional theory. Both methods show a significant decrease in the intrinsic stacking fault I₁ energy (I₁ SFE) with the addition of Y. The influence of the SFE on the relative activation of different competing deformation mechanisms (basal, prismatic, pyramidal slip) is discussed. From this analysis we suggest a key mechanism which explains the transition from primary basal slip in hexagonal close-packed Mg to basal plus pyramidal slip in solid solution Mg–Y alloys. This mechanism is characterized by enhanced nucleation of 〈c + a〉 dislocations where the intrinsic stacking fault I₁ (ISF₁) acts as heterogeneous source for 〈c + a〉 dislocations. Possible electronic and geometric reasons for the modification of the SFE by substitutional Y atoms are identified and discussed.     

Microscopic description of plasticity in computer generated metallic nanophase samples: a comparison between Cu and Ni

Simulations are reported on the plastic behavior of two model f.c.c. metals, Ni and Cu, with different stacking fault energies, and average grain sizes in the range of 3–12 nm. A change in deformation mechanism is observed: at the smallest grain sizes all deformation is accommodated in the grain boundaries. At higher grain sizes intragrain deformation is observed. Analysis of the atomic configurations shows that intrinsic stacking faults are produced by motion of Shockley partial dislocations generated and absorbed in opposite grain boundaries. In Cu the stacking faults are observed at smaller grain sizes than in Ni (8 nm in Cu, 12 nm in Ni) which is attributed to the lower stacking fault energy. Shockley partial dislocations appear on slip systems that are not necessarily those favored by the Schmid factor. Atomic displacement analysis shows that deformation starts at triple points, with grain boundary sliding followed by the creation of intragrain partial dislocations.     

Comparative study of the deformation behavior of hexagonal magnesium–lithium alloys and a conventional magnesium AZ31 alloy

Specimens of a conventional magnesium AZ31 alloy and a binary α-solid solution Mg4Li alloy with similar starting textures and microstructure were subjected to plane strain deformation under various deformation temperatures ranging from 298 K to 673 K. Lithium addition to magnesium exhibited remarkable room temperature ductility improvement owing to enhanced activity of non-basal slip, particularly, 〈c + a〉-slip mode. Furthermore, the addition of lithium to magnesium seemed to reduce the plastic anisotropy, typical for commercial magnesium alloys. This was evident in the flow curves and texture development obtained at 200 °C and 400 °C. At 400 °C prismatic slip gains strong influence in accommodating the imposed deformation. In terms of thermal stability against microstructure coarsening at elevated temperatures, the lithium containing alloy undergoes significant grain growth following recrystallization.     

Methodology for estimating the critical resolved shear stress ratios of α-phase Ti using EBSD-based trace analysis

A novel method for calculating the critical resolved shear stress (CRSS) ratios of different deformation system types in polycrystalline non-cubic metals has been developed. The mean CRSS ratios between different deformation systems were calculated for both commercially pure (CP) Ti and Ti–5Al–2.5Sn (wt.%) tensile deformed at ambient temperature and 455 °C using an in situ scanning electron microscope-based testing technique combined with electron backscattered diffraction. It was found that the relative activity of the different deformation systems changes as a function of alloying composition and deformation temperature. Prismatic slip was the most active deformation mode for CP Ti. CP Ti exhibited a lower resistance to prismatic slip at both ambient and elevated temperatures compared with Ti–5Al–2.5Sn. For Ti–5Al–2.5Sn, prismatic slip was the most active deformation system at ambient temperature although the basal slip activity significantly increased compared to CP Ti, mostly likely due to an increased c/a ratio resulting in a closer packed basal plane. At 455 °C, basal slip exhibited a lower CRSS than prismatic slip for Ti–5Al–2.5Sn. The relative activity of other deformation systems was also affected by alloying and temperature. The statistical resampling technique of bootstrapping was used to generate multiple equivalent data sets from which mean CRSS ratios between different deformation systems, and associated confidence intervals, could be deduced. It was found that the mean CRSS ratios at low and high strains varied slightly for the same testing conditions. Moreover, lesser activated slip systems resulted in relatively larger confidence intervals for the CRSS means. This variability may be attributed to a number of potential factors, including measurement errors, rotations of grains during deformation, local stress state variations, and work hardening. The analysis further suggests that awareness of the intrinsic statistical variability in CRSS ratios should be considered when formulating crystal plasticity constitutive models.     

Separate contributions of texture and grain size on the creep mechanisms in a fine-grained magnesium alloy

The influence of texture and grain size on the creep behavior of a fine-grained magnesium alloy, over the temperature range 423–723 K was investigated. Equal channel angular pressing and rolling were used to produce samples with different textures. Two deformation regimes could be distinguished by their stress exponents. A stress exponent close to 2 and activation energy of 91 kJ mol⁻¹, close to that for grain boundary diffusion, were found at the lower strain rates. In this range, there is no detectable effect of texture. In the high stress exponent regime, within the range 3 < n < 12, a noticeable effect of texture and grain size has been found. The texture effect is related to the orientation of the basal planes. The influence of grain size distribution on flow stress is satisfactorily explained by modeling the deformation as a combination of grain boundary sliding and slip creep.     

Effects of plastic deformation on lamellar structure formation in Ti–39 at.% Al single crystals

The effects of plastic deformation on lamellar structure formation in solution-treated Ti–39 at.% Al single crystals were investigated, focusing on the role of dislocations of different slip systems. The dislocations were introduced by indentation on the surfaces of solution-treated single crystals with different crystallographic orientations. Traces of basal and prism slips were observed, depending on the position relative to the indentation. During annealing at α₂ + γ dual-phase temperatures, lamellar structures were formed faster where basal slip had occurred than where prism slip had occurred. After long annealing, the length scale of lamellar structures formed depends on the slip system operated during prior deformation: in the region where only one of either basal or prism slip had occurred the lamellar structure was coarser than in undeformed crystal, while in the region where both basal and prism slips occurred the lamellar structure was finer than those formed in undeformed crystal. The reasons for the differences in lamellar structures are discussed on the basis of the frequencies of stacking fault formation on (0 0 0 1) planes as precursors to γ-precipitates. The results suggest that the cross-slip of dislocations between basal and prism planes, which gives rise to the formation of multiple stacking faults on many parallel (0 0 0 1) planes, is responsible for the refinement of lamellar structures.     

α-Al2O3 sapphire and rubies deformed by dual basal slip at intermediate temperatures (900–1300 °C): II. Dissociation and stacking faults

Sapphire and rubies (undoped and Cr-doped α-Al₂O₃ single crystals) have been deformed in compression at temperatures lower than those previously used in studies of dislocations in the basal slip plane. Above 1400 °C, several features associating stacking faults out of the basal planes and partial dislocations (dissociation and faulted dipoles) have been observed in previous transmission electron microscopy investigations. The formation of these features involves climb controlled by atomic diffusion. Properties of climb-dissociated dislocations are discussed in relation to dislocation dynamics. Transmission electron microscopy examination of dislocation structures at lower deformation temperatures (1000–1100 °C) shows that similar features are formed but that they often imply cross-slip. A new mechanism for the formation of faulted dipole by glide is presented and an explanation for the 30° Peierls valley orientation is proposed. The presence of chromium has a small influence on stacking fault energies on planes perpendicular to the basal plane.     

A theoretical investigation of the slip systems of Ta2C

Tantalum carbides show a wide range of interesting properties including plasticity at elevated temperatures. In a companion paper, the plastic deformation of high-temperature polycrystalline Ta₂C was examined using transmission electron microscopy. Here, we investigate the different potential slip systems in α-Ta₂C using density functional theory to compute generalized stacking fault (GSF) energy curves. The GSF curves show that the preferred slip system is the basal plane. The results further show that slip occurs preferentially between the Ta–Ta layers, or between metal–metal bonds. GSF energy curves for the basal, prismatic and pyramidal planes show a hierarchy of preferred slip systems and pyramidal slip is likely enabled by the high temperature and the need for additional slip systems caused by the general deformation. These results are not only useful in understanding deformation in α-Ta₂C but can be used to understand plasticity in general transition metal hemicarbides.     

Theoretical Calculation of Schmid Factor and Its Application Under High Strain Rate Deformation in Magnesium AlloysEI北大核心CSCD

As an important parameter, the Schmid factor has been widely applied to analyze the deformation modes in metals. In order to analyze the deformation mechanisms of magnesium alloys under high strain rate, the Schmid factors of four slip modes (basal, prismatic, pyramidaland pyramidal slips) and two twinning systems ({10 (1) over bar2} tension and {10 (1) over bar1} contraction twinnings) were systematically calculated in this work. The experimental values of Schmid factor of as-received AZ31 rolling magnesium alloy sheets were obtained by electron backscatter diffraction (EBSD) technique, and then the theoretical calculated values were compared with those values. The high strain rate compression test of AZ31 rolling magnesium sheets was conducted by using split Hopkinson pressure bar at the strain rate of 1600 s(-1), and the microstructures after compression were observed by optical microscopy. The Schmid factors and microstructures are combined to discuss the predominant deformation mechanisms for different orientation samples under different loading directions. The results showed that the theoretical calculated values of Schmid factors are in good agreement with their experimental values. Therefore, the Schmid factor, owing to its simplicity and convenience, could be used to analyze the predominant deformation mechanism and interpret the unique characteristics of "true stress-true strain" curves in magnesium alloys. Furthermore, since the Schmid factor and its variation trend associated with deformation behavior in magnesium alloys are related, the calculation result of Schmid factor can provide a theoretical analytic approach to understand anisotropic phenomena caused by strong texture in magnesium alloys.     

New experimental insight into the mechanisms of nanoplasticity

The evolution of microstructure and texture of a nanocrystalline Pd–10 at.% Au alloy (initial grain size 16 nm) subjected to severe plastic deformation by high-pressure torsion (HPT) at room temperature is investigated by X-ray line profile analysis and X-ray microdiffraction, respectively. In addition, changes in the microhardness are measured and the texture is modeled. During HPT the microstructure changes: the crystallite size goes over the maximum, the dislocation density goes through a minimum and the density of stacking faults decreases at/up to a shear strain of ～1, corresponding to a grain size of 20 nm. Starting with a random texture, typical brass-type shear components develop at a shear strain above ～1. The microhardness with decreasing crystallite size goes over a maximum at ～20 nm. The correlated changes in microstructure, texture and strength strongly suggest the transition from a dislocation slip to a grain boundary sliding (GBS)-dominated deformation mechanism. The unexpected brass-type texture and its deviation from the ideal position can be simulated with the Taylor model assuming dominant partial dislocation slip and a certain contribution of GBS, respectively. Taken together, the results of many techniques applied to the same material, in particular those of the texture investigations, provide a more comprehensive and consistent picture of nanoplasticity than reported before for face-centered cubic metals.     

Thermomechanical cyclic response of an ultrafine-grained NiTi shape memory alloy

We focus on the effects of ultrafine grains on the thermomechanical cyclic stability of martensitic phase transformation in Ni_49.7Ti_50.3 shape memory alloy fabricated using equal-channel angular extrusion (ECAE). The samples were ECAE-processed between 400 and 450 °C resulting in average grain sizes of 100–300 nm. Tensile failure experiments demonstrated that the strength differential between the onset of transformation and the macroscopic plastic yielding increases after ECAE. Such increase led to a notable improvement in the thermal cyclic stability under relatively high stresses. The experimental observations are attributed to the increase in critical stress level for dislocation slip due to grain refinement, change in transformation twinning mode in submicron grains, the presence of R-phase, and multi-martensite variants or a small fraction of untransforming grains due to grain boundary constraints. The effects of these microstructural factors on the transformation behavior are discussed in the light of transformation thermodynamics.     

Quasicontinuum study of incipient plasticity under nanoscale contact in nanocrystalline aluminum

Atomistic simulations using the quasicontinuum method are performed to examine the mechanical behavior and underlying mechanisms of surface plasticity in nanocrystalline aluminum with a grain diameter of 7 nm deformed under wedge-like cylindrical contact. Two embedded-atom method potentials for Al, which mostly differ in their prediction of the generalized stacking and planar fault energies, and grain boundary (GB) energies, are used and characterized. The simulations are conducted on a randomly oriented microstructure with 〈1 1 0〉-tilt GBs. The contact pressure–displacement curves are found to display significant flow serration. We show that this effect is associated with highly localized shear deformation resulting from one of three possible mechanisms: (1) the emission of partial dislocations and twins emanating from the contact interface and GBs, along with their propagation and intersection through intragranular slip, (2) GB sliding and grain rotation and (3) stress-driven GB migration coupled to shear deformation. Marked differences in mechanical behavior are observed, however, as a function of the interatomic potential. We find that the propensity to localize the plastic deformation at GBs via interface sliding and coupled GB migration is greater in the Al material presenting the lowest predicted stacking fault energy and GB energy. This finding is qualitatively interpreted on the basis of impurity effects on plastic flow and GB-mediated deformation processes in Al.     

Mechanical behaviour and in situ observation of shear bands in ultrafine grained Pd and Pd–Ag alloys

An in situ tensile test with grey scale correlation has been performed to study the deformation process in ultrafine grained (UFG) Pd and Pd–x at.% Ag (x = 5 or 20) alloys produced by high-pressure torsion. Shear band nucleation and propagation was found to be an important deformation mechanism after strain localization during the tensile test. The underlying microscopic mechanism is related to cooperative grain boundary sliding. Moreover, the additional influence of stacking fault energy was found to change the nature of the deformation mechanism from localized strain in Pd to more homogeneous deformation in Pd–20% Ag. In situ analysis and the findings are new and give innovative insight into the basics of deformation in UFG face-centred cubic metals.     

Structure and stacking faults in layered Mg–Zn–Y alloys: A first-principles study

We use first-principles density functional theory total energy calculations based on pseudo-potentials and plane-wave basis to assess stability of the periodic structures with different stacking sequences in Mg–Zn–Y alloys. For pure Mg, we find that the 6-layer (6l) structure with the ABACAB stacking is most stable after the lowest energy hcp (2l) structure with ABAB stacking. Addition of 2 at.% Y leads to stabilization of the structure to 6l sequence whereas the addition of 2 at.% Zn makes the 6l energetically comparable to that of the hcp. Stacking fault (SF) on the basal plane of 6l structure is higher in energy than that of the hcp 2l Mg, which further increases upon Y doping and decreases significantly with Zn doping. SF energy surface for the prismatic slip indicates activation of non-basal slip in alloys with a 6l structure. Charge density analysis shows that the 2l and 6l structures are electronically similar which might be a cause for better stability of 6l structure over a 4l sequence or other periodic structures. Thus, in an Mg–Zn–Y alloy, Y stabilizes the long periodicity, while its mechanical properties are further improved due to Zn doping.     

Low temperature superplasticity in a friction-stir-processed ultrafine grained Al–Zn–Mg–Sc alloy

Friction stir processing (FSP) was used to create a microstructure with ultrafine grains (0.68 μm grain size) in an as-cast Al–8.9Zn–2.6Mg–0.09Sc (wt.%) alloy. The ultrafine grained alloy exhibited superplasticity at relatively low temperatures and higher strain rates. Optimum ductility of 1165% at a strain rate of 3 × 10⁻² s⁻¹ and 310 °C was obtained. Enhanced superplasticity was also achieved at a temperature as low as 220 °C. Experimentally observed parametric dependencies and microstructural examinations indicated that the operating deformation mechanism might be the Rachinger grain boundary sliding accommodated by intragranular slip. The FSP microstructure became highly unstable at 390 °C onwards, thus, affecting ductility adversely. In situ transmission electron microscopy heating was used to understand the instability phenomenon, which has been attributed to the drop in particle pinning forces due to the dissolution of metastable precipitates and microstructural heterogeneity.     

Dominant Deformation Mechanisms in Mg-Zn-Ca Alloy

The coaddition of Zn and Ca has great potential to improve the ductility of Mg alloys. Herein, the mechanical properties of an extruded Mg-Zn-Ca solid-solution alloy were studied by quasi-in situ electron backscatter diffraction (EBSD)-assisted slip trace analysis. The dominant deformation mechanisms of the Mg-Zn-Ca alloy were studied, and the origins of enhanced ductility were systematically revealed. The results indicate that most grains deformed by basal slip. In addition, multiple non-basal slip traces were detected (particularly prismatic, pyramidal I <  a > , and pyramidal I <  c +  a > slip traces), and their activation frequency was promoted with increasing tensile strain. The enhanced participation of non-basal slip systems is believed to play a critical role in achieving homogeneous plastic deformation, thus effectively promoting the ductility of the Mg-Zn-Ca alloy. Furthermore, first-principle calculations revealed that the coaddition of Zn and Ca significantly reduces the unstable stacking fault energy for non-basal slip, which contributes to the activation of non-basal slip systems during plastic deformation.     

Deformation mechanisms in nanocrystalline palladium at large strains

The mechanical behaviour and microstructure evolution of nanocrystalline palladium was investigated. Material with an initial grain size ～10 nm was prepared by inert gas condensation. Instrumented high-pressure torsion straining was used to characterize the flow stress during plastic deformation to shear strains up to 300. A change in primary deformation mechanism was induced by stress-induced grain growth. For grain sizes <40 nm, grain boundary mediated processes (shear banding, grain boundary sliding and grain rotation) controlled the deformation, with dislocation slip, twinning, and grain boundary diffusion providing the accommodation. For larger grain sizes, the operative deformation mechanism was dislocation slip.     

Slip in directionally solidified Mo-alloy micropillars – Part I: Nominally dislocation-free pillars

In situ Laue analysis during microcompression reveals plasticity in [0 0 1]-oriented, directionally solidified Mo alloy pillars to start with slip on the {1 1 2}〈1 1 1〉 system having the highest Schmid factor followed by slip on the (1 1 0) plane containing the same Burgers vector. The results are interpreted in terms of the microstructure analyzed by scanning transmission electron microscopy and 3-D atom probe.     

Hardening of pure metals by high-pressure torsion: A physically based model employing volume-averaged defect evolutions

A physically based model to predict the increment of hardness and grain refinement of pure metals due to severe plastic deformation by high-pressure torsion (HPT) is proposed. The model incorporates volume-averaged thermally activated dislocation annihilation and grain boundary formation. Strengthening is caused by dislocations in the grain and by grain boundaries. The model is tested against a database containing all available reliable data on HPT-processed pure metals. It is shown that the model accurately predicts hardening and grain size of the pure metals, irrespective of crystal structure (face-centred cubic, body-centred cubic and hexagonal close packed). Measured dislocation densities also show good correlation with predictions. The influence of stacking fault energy on hardening is very weak (of the order of ?0.03 GPa per 100 J mol^?1).