Reverse Transformation in [110]-Oriented Face-Centered-Cubic Single Crystals Studied by Atomic Simulations

Bidirectional transformations, which are achieved by triggering both dynamic forward transformation from the face-centered-cubic (fcc) austenite to the hexagonal-close-packed (hcp) martensite and the reverse transformation from martensite to austenite during cold deformation, have been previously reported in FeMnCoCr-based high-entropy alloys (HEAs). This leads to the permanent refinement of microstructure and hence enhances the work-hardening capacity of alloys. In order to reveal the microscopic mechanism of the reverse transformation in HEAs under deformation, the effect of the sample aspect ratio, i.e., Z/X, on the evolution of deformation systems in the equi-atomic FeMnCoCrNi alloy with [110] orientation during uniaxial tensile loading along the Z direction is investigated by atomic simulations in this study. When the aspect ratio is 0.5, the reverse transformation is more significant compared with other models, while a good plasticity can still be maintained. We then compare the micromechanical behavior of three fcc single crystals, i.e., FeMnCoCrNi, FeCuCoCrNi, and pure Cu. The results show that the stacking fault energy plays a major role in the activation of different deformation mechanisms; however, the lattice distortion in the HEA does not significantly affect the activation of deformation systems. Furthermore, for all materials dislocation slip leads to the softening, while strain hardening is attributed to the initiation of multiple deformation mechanisms. The Shockley partials slip leads to bidirectional phase transition, twinning and detwinning in the three materials. Thus, the reverse transformation can occur in all metallic materials where the fcc to hcp phase transformation is the dominant deformation mechanism. These findings contribute to an in-depth understanding of the deformation mechanism in fcc-structured materials under severe plastic deformation and provide theoretical guidance for the design of alloys with superior strength-plasticity combinations.     

增材制造高强韧高熵合金中基于切变型相变的微裂纹抑制机理

研究选区激光熔化增材制造FeMnCoCrNi体系高熵合金的微裂纹行为,并采用XRD技术对激光打印后样品表面的残余应力进行分析。结果表明,经激光打印后等原子比FeMnCoCrNi高熵合金显示为稳定的单相面心立方(FCC)结构,出现残余拉应力,并产生微裂纹。相比之下,具有低层错能的非等原子比亚稳FeMnCoCr高熵合金在各种激光能量密度下均出现残余压应力,且无微裂纹形成。在激光熔化后的冷却过程中,亚稳高熵合金中发生的从FCC基体相到密排六方(HCP)相的切变型相变消耗了激光打印过程中的热应力,从而抑制微裂纹的产生。此外,相比于单相稳定高熵合金,亚稳高熵合金在拉伸变形过程中马氏体相变也有助于提高其抗拉强度和延展性。这些结果为增材制造领域设计开发高强、高韧、无裂纹的合金提供有益参考。     

纳米金属镍的冷轧变形与微结构

在室温条件下对纳米金属镍进行了不同程度的冷轧变形,利用X射线衍射分析和高分辨透射电镜的观察对纳米金属镍的微观结构演变以及塑性变形机制进行了研究。结果表明:形变量ε20%时,晶粒旋转为主要的变形方式;当20%ε30%时,位错活动与晶粒旋转共同协调变形;ε30%时,晶界发射的不全位错,形成变形孪晶与层错,主导变形。     

Effects of Mn Content on Mechanical Properties of FeCoCrNiMn_x (0≤x≤0.3) High-Entropy Alloys: A First-Principles Study

Effects of Mn content on mechanical properties of FeCoCrNiMn_x(0 ≤x≤0.3) high-entropy alloys(HEAs) are investigated via first-prmciples calculations combining EMTO-CPA method.Related physical parameters,including lattice constant.elastic constants,elastic modulus,Pugh's ratio,anisotropy factors,Poisson's ratio,Cauchy pressure,Vickers hardness,yield strength,and energy factor,are calculated as a function of Mn content.The results show that the resistances to bulk,elastic,and shear deformation decrease with increasing Mn content.Pugh's ratio B/G indicates that the ductility of FeCoCrNiMn_x HEAs has a remarkable reduction between 22 and 24% of Mn content.Meanwhile,Cauchy pressure suggests that the atomic bonding transforms from metallic to directional characteristic from 22 to 24% of Mn content.Vickers hardness and yield strength of FeCoCrNiMn HEA are intrinsically larger than those of FeCoCrNi HEA.Dislocation nucleation easily occurs in FeCoCrNiMn HE A compared to FeCoCrNi HEA,and large dislocation width in FeCoCrNiMnO_2 HE A results in low stacking-fault energy,which easily induces twinning deformation.This work provides a valuable msieht for further theoretical and experimental study on the mechanical properties of FeCoCrNiMn_x(0≤x≤0.3) HEAs.     

动态压缩条件下高纯钛微观组织特征研究

利用光学显微镜(OM)、背散射电子衍射(EBSD)技术及透射电子显微镜(TEM)对高纯钛低-中应变动态压缩变形的微观组织特征进行了研究。结果表明：随着应变量(ε)的增加,晶粒内部通过孪晶与孪晶,孪晶与位错以及位错与位错之间的交互作用逐步细化原始晶粒;变形初期,形变孪生以{11-22}孪晶为主,当ε达到0.2后,{10-12}孪晶转变为主要形变孪生类型,孪生改变了原始晶粒的取向,进一步促进晶粒内部的位错滑移。高纯钛动态压缩变形经历了由位错滑移到形变孪生,再到位错滑移主导的过程,但位错滑移和孪生始终共同作用协调动态压缩变形。     

超声滚压剧烈塑性变形诱导金属材料结构演变的研究进展

首先,对表面完整性的基本概念和内涵进行了概述,同时简要介绍了超声实现滚压技术的基本原理及其优点。随后,对比分析了不同剧烈塑性变形方法的特点和局限性,引出了实现表面完整性的相关剧烈塑性变形协调机制。在此基础上,随后结合其他剧烈塑性变形强化工艺,重点总结了超声滚压剧烈塑性变形对金属材料表面微观结构演变的影响。具体探讨了剧烈塑性变形诱导晶粒细化机制、晶粒生长机制以及合金元素偏聚机制等,主要分别论述了不同层错能的面心立方、体心立方以及密排六方等不同金属晶体结构的晶粒细化机制(以位错滑移、变形孪晶为主导)、晶粒长大机制(以晶界迁移、晶粒旋转为主要)与合金元素偏聚机制(晶界偏聚、位错核心偏聚)等。最后,对以上内容进行了综合总结,并针对超声滚压技术研究中存在的问题给出进一步研究和发展的建议,从而为实现超声滚压金属材料的表面完整性的主动精准控制及提高其服役寿命与可靠性提供一定的参考。     

纳米Ni薄膜在摩擦过程中塑性行为的分子动力学模拟

用分子动力学模拟研究了金刚石压头在Ni晶体薄膜上的摩擦过程和薄膜塑性变形行为的纳观机制.结果表明:在摩擦过程中,穿晶层错和棱形位错环是纳米薄膜结构传递塑性变形的两种载体,纳米薄膜晶界捕获位错阻滞了塑性变形向薄膜晶界下方材料中传播.摩擦过程中易在较薄的薄膜表面和薄膜晶界之间产生穿晶层错,穿晶层错的产生增加了薄膜蓄积塑性变形的能力,从而抑制材料表面摩擦力在黏滑过程中的振荡幅度;在比较厚的薄膜中不易生成穿晶层错,在摩擦过程中位错环依次向体材料发射,并与晶界反应,湮灭于晶界,黏滑动摩擦响应与单晶相似.由于不同厚度薄膜塑性变形产生的位错结构不同,使得在摩擦过程中亚表面微结构的演化亦不同.     

Achieving a High-Strength CoCrFeNiCu High-Entropy Alloy with an Ultrafine-Grained Structure via Friction Stir Processing

High-entropy alloys(HEAs) are a new class of materials with a potential engineering application,but how to obtain ultrafine or nano-sized crystal structures of HEAs has been a challenge.Here,we first presented an equiatomic CoCrFeNiCu HEA with excellent mechanical properties obtained via friction stir processing(FSP).After FSP,the Cu element segregation in the cast CoCrFeNiCu HEA was almost eliminated,and the cast coarse two-phase structure(several micrometers) was changed into an ultrafine-grained single-phase structure(150 nm) with a large fraction of high-angle grain boundaries and nanoscale deformation twins.This unique microstructure was mainly attributed to the severe plastic deformation during FSP,and the sluggish diffusion effect in dynamics and the lattice distortion effect in crystallography for HEAs.Furthermore,FSP largely improved the hardness and yield strength of the CoCrFeNiCu HEA with a value of 380 HV and more than 1150 MPa,respectively,which were 1.5 times higher than those of the base material.The great strengthening after FSP was mainly attributed to the significant grain refinement with large lattice distortion and nano-twins.This study provides a new method to largely refine the microstructure and improve the strength of cast CoCrFeNiCu HEAs.     

Effective Stacking Fault Energy in Face-Centered Cubic Metals

As a typical configuration in plastic deformations, dislocation arrays possess a large variation of the separation of the partial dislocation pairs in face-centered cubic(fcc) metals. This can be manifested conveniently by an effective stacking fault energy(SFE). The effective SFE of dislocation arrays is described within the elastic theory of dislocations and verified by atomistic simulations. The atomistic modeling results reveal that the general formulae of the effective SFE can give a reasonably satisfactory prediction for all dislocation types, especially for edge dislocation arrays. Furthermore, the predicted variation of the effective SFE is consistent with several previous experiments, in which the measured SFE is not definite, changing with dislocation density. Our approach could provide better understandings of cross-slip and the competition between slip and twinning during plastic deformations in fcc metals.     

Deformation twins in nanocrystalline body-centered cubic Mo as predicted by molecular dynamics simulations

This work studies deformation twins in nanocrystalline body-centered cubic Mo, including the nucleation and growth mechanisms as well as their effects on ductility, through molecular dynamics simulations. The deformation processes of nanocrystalline Mo are simulated using a columnar grain model with three different orientations. The deformation mechanisms identified, including dislocation slip, grain-boundary-mediated plasticity, deformation twins and martensitic transformation, are in agreement with previous studies. In 〈1 1 0〉 columnar grains, the deformation is dominated by twinning, which nucleates primarily from the grain boundaries by successive emission of twinning partials and thickens by jog nucleation in the grain interiors. Upon arrest by a grain boundary, the twin may either produce continuous plastic strain across the grain boundary by activating compatible twinning/slip systems or result in intergranular failure in the absence of compatible twinning/slip systems in the neighboring grain. Multiple twinning systems can be activated in the same grain, and the competition between them favors those capable of producing continuous deformation across the grain boundary.     

Deformation twinning in nanocrystalline Al by molecular-dynamics simulation

We use a recently developed, massively parallel molecular-dynamics code for the simulation of polycrystal plasticity to elucidate the intricate interplay between dislocation and GB processes during room-temperature plastic deformation of model nanocrystalline-Al microstructures. Our simulations reveal that under relatively high stresses (of 2.5 GPa) and large plastic strains (of ˜12%), extensive deformation twinning takes place, in addition to deformation by the conventional dislocation-slip mechanism. Both heterogeneous and homogeneous nucleation of deformation twins is observed. The heterogeneous mechanism involves the successive emission of Shockley partials from the grain boundaries onto neighboring slip planes. By contrast, the homogeneous process takes place in the grain interiors, by a nucleation mechanism involving the dynamical overlap of the stacking faults of intrinsically and/or extrinsically dissociated dislocations. Our simulations also reveal the mechanism for the formation of a new grain, via an intricate interplay between deformation twinning and dislocation nucleation from the grain boundaries during the deformation. The propensity for deformation twinning observed in our simulations is surprising, given that the process has never been observed in coarse-grained Al and that the well-known pole mechanism cannot operated for such a small grain size. It therefore appears that the basic models for deformation twinning should be extended with particular emphasis on the role of grain-boundary sources in nanocrystalline materials.     

Exploring Plastic Deformation Mechanism of Multilayered Cu/Ti Composites by Using Molecular Dynamics ModelingEI北大核心CSCD

Multilayered metallic composites have attracted great interest because of their excellent characteristics. In recent years, the mechanical behavior of Cu/Ti composites is described in terms of macroscopic or mesoscopic scales, but the micromechanism regarding dislocation slip, twinning and shear banding at heterogeneous interfaces remains unclear. In this work, the molecular dynamics method is used to study the uniaxial tensile and plane strain compression deformation of the Cu/Ti multilayered composites with characteristic initial crystal orientations. The simulation results show that under the tensile load, dislocations are preferentially nucleated at the heterogeneous interface between Cu and Ti, and then slip along {111} plane within the Cu layers. The corresponding mechanism is confined layer slip. With the multiplication of dislocations, dislocations interact with each other, and intrinsic stacking faults and deformation twins are formed in Cu layers. However, no dislocation slip or twinning is activated within the Ti layers at this stage of deformation. As the load increases, the stress concentration at the Cu/Ti interface leads to the fracture of the composites. For the composites under plane strain compression, the stress concentration at the Cu/Ti interface triggers the formation of shear bands in the Ti layer, and there are only very limited dislocations within the shear bands and their adjacent area. With the increase of applied strain, the common action of various deformation mechanisms causes the grains to rotate, and the disorder degree of complex atoms increases. In addition, the micro-plastic deformation mechanism and mechanical properties of Cu/Ti complex with different initial orientations and strain rates are significantly different. The results reveal the microscopic deformation mechanism of the laminated composites containing hcp metals.     

纯钛板材冷拉成形过程中的微观组织与织构演变(英文)

研究纯钛板材冷拉成形过程中微观组织及织构演化规律。半球形壳体件在深拉延过程中由于各部位变形模式及应变形式的不同会形成胀形区、拉深区及法兰区等3个区域。结果表明,在拉深件的3个区域中塑性应变均由位错滑移与变形孪晶共同作用。纯钛板材及其拉深件中的织构包含轧制织构与再结晶织构。由于变形孪晶与位错滑移对织构的影响规律不同,初始板材织构的强度及类型在深拉过程中不断变化。变形孪晶对初始织构具有弱化作用,特别是对于再结晶织构,这种弱化效应更为明显。由于拉深区产生的孪晶较多,再结晶织构消失。此外,大拉伸变形时位错滑移为主导机制,织构强化效应明显。     

Direct evidence for Suzuki segregation and Cottrell pinning in MP159 superalloy obtained by FEG(S)TEM/EDX

The effects of Suzuki segregation on the plastic flow behaviour of MP159 alloy deformed at high temperature and on the resulting dislocation structure have been examined. Elemental concentration profiles across both stacking faults and slip bands have been measured in a FEG TEM in nano-probe using the line scanning mode and EDX. It was found that Suzuki segregation resulted in continuously serrated plastic flow for deformation at temperatures from 450–670 °C and at a slow strain rate such as 1.0×10⁻⁴/s. TEM examination showed an increased dissociation width for dislocations and larger and more stacking faults after deformation at high temperatures as compared with those after deformation at room temperature. This can be interpreted as being due to the reduction of stacking fault energy by Suzuki segregation and/or Cottrell pinning. The elemental concentration profiles across stacking faults and slip bands showed that Mo and Al were more often found than other solutes to segregate to stacking faults and slip bands. Occasionally, the segregation of Ti and Nb could also be detected at stacking faults and slip bands.     

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.     

分子动力学模拟不同层错能单晶Ni及其合金拉伸变形行为

采用分子动力学模拟了不同尺寸模型的单晶Ni及Ni₅₇Cr₁₉Co₁₉Al₅合金[100]晶向拉伸变形过程,确定了具有稳定塑性流变应力的模型尺寸,进一步研究了在具有稳定塑性流变应力的相同模型下单晶Ni及其合金拉伸变形行为。结果表明,层错能较低的单晶Ni₅₇Cr₁₉Co₁₉Al₅合金在小尺寸模型拉伸变形时,容易形成多层孪晶结构或变形孪晶;模型的横截面边长大于30倍的晶格常数时,塑性流变阶段流变应力、相结构及位错密度随应变起伏趋于平稳。具有稳定流变应力的相同尺寸单晶Ni及其合金拉伸时,层错能越低,塑性变形时层错面的面积越大。Shockley不全位错在单晶Ni及其合金塑性变形过程中起主导作用,多层孪晶的形成伴随着位错耗尽,变形孪晶的形成与湮灭则主要由位错饥饿机制主导。     

Deformation characteristics of polysynthetically twinned (PST) crystals during creep at 1150 K

The creep deformation characteristics of a lamellar polysynthetically twinned (PST) crystal of the composition Ti–48 mol% Al was investigated as a function of the lamellar orientation with respect to the compression axis and the applied stress. The creep resistance of hard PST orientation with the lamellar plates parallel or perpendicular to the compression axis was substantially higher than that of soft orientations with their lamellar plates oriented at intermediate angles to the compression axis. This fact could be associated with the predominant deformation of the hard orientations by deformation modes with the slip plane inclined to the lamellar interfaces in contrast to the predominant deformation of the soft orientations by deformation modes with the slip plane parallel to the lamellar plates. In the soft orientations, mainly straight ordinary dislocations with the Burgers vector b=1/2[1-10] aligned parallel to the lamellar interfaces were encountered in domains with a high resolved shear stress as well as in domains with no resolved shear stress. In the hard orientations, ordinary dislocations b=1/2[110] and superdislocations with a Burgers vector of the type b=1/2〈112] were observed. Despite a high resolved shear stress in certain oriented domains, superdislocations of the type b=〈101] were not found to play a considerable role during creep deformation under the investigated conditions. Cross twinning contributed to the deformation in favourably oriented variants, but twinning parallel to the lamellar interfaces was much more pronounced in the soft orientations leading to a substantial lamellar refinement. This microstructural hardening during creep results in the observed stress exponents of the soft orientations near unity at high stresses. A change in stress exponent from near unity at high stresses to about 9 at low stresses occurs at about 200 MPa. A critical stress of about 200 MPa for parallel twinning is proposed as a reason for the change in stress exponent.     

Effect of grain size on the mechanisms of plastic deformation in wrought Mg–Zn–Zr alloy revealed by acoustic emission measurements

The present study has clarified the roles of dislocation slip and twinning as the deformation mechanisms in magnesium alloys, as well as the effect of grain size on their relative contributions. The details of these mechanisms were studied by monitoring acoustic emission (AE) in conjunction with a novel signal categorization technique in Mg alloy ZK60. Through the analysis of AE time series the sequences of predominant deformation mechanisms in coarse grained (～70 μm) and fine grained (～2 μm) specimens of the alloy were identified with a high degree of confidence. It was found that dislocation slip and twinning occur during tensile loading simultaneously for both microstructural states of the material, while a change from one predominant mechanism to the other occurs in the course of loading. Specifically, in the fine grained material plastic deformation is initially carried by dislocation slip, but deformation twinning takes over as the lead mechanism early on. In the coarse grained variant this sequence is reversed. The implications of the changing roles of the mechanisms of plastic deformation for the overall mechanical performance of ZK60 in the two contrasting microstructural states are discussed.     

应变速率对含空洞的镁孪晶界面塑性变形机制的影响

利用嵌入原子势的分子动力学模拟,研究了应变速率对含空洞的镁孪晶界面塑性变形机制的影响。结果表明,塑性变形的主要形式包括不全位错、滑移带和堆垛层错;应变速率不会改变试样的杨氏模量,应变速率愈大屈服应力愈大;随着应变速率增大,位错和滑移带的数量增加,堆垛层错的数目先增加后减小,位错运动自由行程的平均长度减小;随着变形进行,位错源不断产生新位错,导致位错密度提高;高应变速率时,晶界处容易形成应力集中,并会有微裂纹产生。     

Mechanisms of plastic deformation in microcrystalline and nanocrystalline TiNi-based alloys

Mechanisms of plastic deformation of a high-temperature B2 phase that act upon tension, compression, and high-pressure torsion in TiNi-based single crystals have been studied depending on the crystal orientation. For the crystals with orientations located near the [$ \bar 1 $ \bar 1 11] and [$ \bar 1 $ \bar 1 12] poles in the standard stereographic triangle, multiple dislocation slip prevails upon both compression and tension. In “hard” crystals with the deformation axis close to the [001] direction, in which the Schmid factors for dislocation slip are close to zero, the main deformation mechanisms are the mechanical twinning in the B2 phase and the stress-assisted B2 → B19′ martensitic transformation. All the above listed mechanisms take part in the formation of the {111}〈hkl〉 texture. The mechanism of the change in the orientation of “hard” polycrystalline grains upon the formation of a nanocrystalline and amorphous-crystalline state has been demonstrated on the example of the evolution of the structure of [001] crystals upon severe plastic deformation in a Bridgman cell.