Application of texture simulation to understanding mechanical behavior of Mg and solid solution alloys containing Li or Y

The viscoplastic self-consistent model was used to interpret differences in the mechanical behavior of hexagonal close packed magnesium alloys. There are only subtle differences in the compression textures of magnesium and its solid solution alloys containing lithium or yttrium. However, the plane strain compression textures of the alloys showed an increasing tendency for the basal poles to rotate away from the “normal direction” towards the “rolling direction”. Texture simulations enabled these distinctions to be attributed to the increased activity of the non-basal c+a slip mode. The alloys had improved compressive ductilities compared to pure magnesium, and the increased c+a slip mode activity provides a satisfying explanation for this improvement, since it can accommodate c-axis compression within individual grains. Accounting for individual deformation mode hardening enabled the flow curves to be simulated and the anisotropic plastic response of textured wrought alloys to be mechanistically understood and predicted.     

Dependence of Microstructure Evolution and Mechanical Properties on Loading Direction for AZ31 Magnesium Alloy Sheet with Non-basal Texture During In-Plane Uniaxial Tension

In-plane uniaxial tension of AZ31 magnesium alloy sheet with non-basal texture has been conducted in order to demonstrate the effects of loading direction on the microstructure evolution and mechanical properties at ambient temperature. Loading axes are chosen to be along five directions distributed between rolling direction (RD) and transverse direction (TD), allowing various activities in involved slip and twinning modes to take place. As for twinning modes, electron backscattered diffraction observations confirm that the contribution of ${{\{ 10\overline{1}1\} }}$ compression twinning is minimal to the plastic deformation of all deformed samples. By comparison, ${{\{ 10\overline{1}2\} }}$ extension twinning (ET) not only serves as an important carrier on sustaining and accommodating plastic strain but also contributes to the emergence of TD-component texture with the progression of plastic strain. In terms of slip modes, analysis on Schmid factor demonstrates that the increasing tilted angle between loading direction and RD of sheet is unfavorable to the activation of basal <a> slip, whereas it contributes to the activation of prismatic <a> slip. These observations consequently explain the increasing tendency of 0.2% proof yield stress. Moreover, the activations of basal <a> slip and ${{\{ 10\overline{1}2\} }}$ ET collectively contribute to the concentration of two tilted basal poles toward normal direction. With increasing angle between loading direction and RD, the activations of basal <a> slip and ${{\{ 10\overline{1}2\} }}$ ET are gradually weakened. This leads to a weakening tendency about concentration of two tilted basal poles, a generally increasing tendency about Lankford value (r-value) and a generally decreasing tendency about strain-hardening exponent (n-value).     

The effects of the combined substitution of Y and Ga on the crystallographic structure of Nd2−xYxFe17−yGay intermetallic compounds

The effects of the combined substitution of Y and Ga on the crystallographic structure of Nd_2−xY_xFe_17−yGa_y compounds with x = 0, 0.5, 1.0, 1.5 and y = 0, 1, 2, 3 have been investigated using X-ray and neutron powder diffractions. Rietveld refinements of the diffraction data indicate that all the samples crystallize in the rhombohedral Th₂Zn₁₇-type structure with only small amounts of alpha iron. It is found that the addition of Ga atoms lessens the decreasing rates of the a-axis and unit cell volume V on the Y content but almost does not affect the decreasing rates of the c-axis. However, the substitution of Y has a positive effect on the increasing rates of the a-axis and unit cell volume V on the Ga content but has a very slight effect on the increasing rate of the c-axis. The c/a ratio of Nd_2−xY_xFe_17−yGa_y as a function of Ga content exhibits a different increase for different Y content owe to the combined effects of Y and Ga on the crystallographic structure. The substitution of Y is found to have little effect on the site occupancy of Ga in Nd_2−xY_xFe_17−yGa_y. The combined effects of Y and Ga on the bond lengths and ASBL of Nd_2−xY_xFe_17−yGa_y indicate that more bonds detrimental to ferromagnetic exchange can be modulated into the desirable ferromagnetic exchange distance range through suitable combined substitution, which provides a valuable way to improve the magnetic properties of rare earth-transition intermetallic compounds.     

挤压态AZ31镁合金的拉压不对称性及微观组织

在考虑滑移和孪生两大塑性变形机制的基础上,通过修正的粘塑性自洽（VPSC）模型,模拟挤压态AZ31镁合金轴向拉-压过程中的力学行为及微观组织。结合EBSD实验与模拟,分析了不同变形机制对初始挤压态丝织构镁合金产生拉压不对称的机理以及塑性变形过程中的微观组织。结果表明,轴向拉伸变形初期以基面滑移系为主,由于基面滑移的施密特因子较低,导致屈服应力较高;随着应变的增加,棱柱面滑移成为主导变形机制,应变硬化率降低,应力-应变曲线较平稳;轴向压缩变形初期,临界剪切应力较低的拉伸孪晶大量开启导致屈服应力较低;随着拉伸孪晶相对活性的快速降低,应变硬化率迅速提高;轴向压缩后期,随着应力的持续升高,压缩孪晶开始启动,塑性变形积累的应力得到释放,导致应变硬化率降低。另外,从典型晶粒的颜色和孪晶迹线方面解释了沿ED方向压缩时孪晶体积分数较小的原因。     

Tension-Compression Yield Asymmetry Influenced by the Variable Deformation Modes in Gradient Structure Mg Alloys

Surface mechanical attrition treatment(SMAT) was carried out on hot-rolled AZ31 Mg samples along two orthogonal directions;as a result,two types of gradient structures with different grain sizes and texture components in different layers were produced.The tension-compression yield asymmetry(YA) was studied using samples with different thicknesses,in order to elucidate the effect of combinations of variable deformation modes operating in different layers of the two oriented SMAT samples.The 0° oriented SMAT sample containing layers with strong basal texture displayed significant YA,because of either dislocation slip or extension twinning domination during tension or compression.By contrast,the 90° oriented SMAT sample containing layers with coexisting orthogonal texture components had an obviously weakened YA,which was attributed to the multi-deformation modes cooperating during tension or compression,i.e.,extension twinning or detwinning in conjunction with dislocation slips,leading to close yield stresses compared between tension and compression.     

Improving Ductility and Anisotropy by Dynamic Recrystallization in Ti/Mg Laminated Metal Composite

Ti/Mg laminated metal composites (LMCs) were processed by hot roll bonding and subsequent annealing at 200 °C and 300 °C for 1 h, and the effect of dynamic recrystallization on the microstructure and anisotropy behavior was investigated in detail. The results revealed that, in both as-rolled and annealed Ti/Mg LMCs, the inhomogeneous distribution in the microstructure of Mg layers near the interface and near the center was related to the effect of friction between the roller and sheet surface and uncoordinated deformation between constituent layers. With increasing annealing temperature, Ti/Mg LMCs exhibited excellent elongation without sacrificing strength properties, which was mainly due to the improvement in bonding strength and the increasing strain gradient at the interface between soft and hard layers after annealing treatment. Besides, the increasing Schmid factors (SFs) of prismatic <  a > slip and pyramidal <  c +  a > slip in Mg layers contributed to the improved plastic deformation ability of Ti/Mg LMCs. The experimental analysis indicated that the presence of Mg alloys resulted in the microstructure and mechanical properties of LMCs were different along various loading directions, and annealing treatment can effectively inhibit the anisotropic behavior of Ti/Mg LMCs resulting from the weakening of basal texture in the Mg layer.     

Plastic deformation of single crystals of TiSi2

Compression experiments have been performed at high temperatures for single crystals of TiSi₂ with the C54 (oF24) structure. Compression axes chosen are -, b- and c-axes, and intermediate directions between a- and c-axes, and b- and c-axes. Based on the slip line observation and the geometrical consideration, it has been concluded that one of the three slip systems, (001)[110], ( 10)[130] and (0 1)[011], is activated depending on the compression axis; in the former two systems dislocations are assumed to be dissociated into superpartials, i.e. 1/2[1101→1/3[100]1+1/6[130] and 1/2[130]→3 × 1/6[130]. The (001)[110] slip is active at room temperature and the other two slip systems are active only above 1000 K. The thermal-activation analysis of the plastic deformation has shown that the deformation is controlled by the Peierls mechanism for the three slip systems; the total activation enthalpy is 1.5 eV for the (001)[110] slip and 4–5 eV for the ( 10)[130] and (0 1)[011] slips. An asymmetry of the Peierls potential is suggested for the (3 0)[130] slip.     

高应变速率下AZ31镁合金的各向异性及拉压不对称性

采用分离式霍普金森拉杆及压杆装置,研究挤压态AZ31镁合金高速变形下的各向异性及拉压不对称性,并从微观变形机制的角度探讨具有强烈初始基面织构的挤压态镁合金各向异性及拉压不对称性产生的原因。结果表明:在高速变形条件下,依据加载方向及应力状态挤压态AZ31镁合金的拉伸行为表现出很强的各向异性,但压缩行为的各向异性不明显;在挤压方向表现出很强的拉压不对称性,而在垂直于挤压方向的拉压不对称性很低。挤压态AZ31镁合金宏观上的各向异性及拉压不对称性是由于不同的微观变形机制所引起的。沿挤压方向拉伸的主要变形机制为柱面滑移,沿垂直于挤压方向拉伸及压缩的主要变形机制为锥面滑移;沿挤压方向压缩时初始变形机制为拉伸孪晶,当变形量为0.08(8%)左右时由于孪晶消耗殆尽,变形变而以滑移的方式进行。     

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.     

Simulation of stress concentration in Mg alloys using the crystal plasticity finite element method

A crystal plasticity finite element method (CPFEM), considering both crystallographic slip and deformation twinning, was developed to simulate the spatial stress concentration in AZ31 Mg alloys during in-plane compression. A predominant twin reorientation (PTR) model was successfully implemented to capture grain reorientation due to deformation twinning in twin-dominated deformation. By using the direct mapping technique for electron backscatter diffraction (EBSD) data, CPFEM can capture the heterogeneity of stress concentration at the grain boundaries in AZ31 Mg alloys during in-plane compression. The model demonstrated that deformation twinning enhances the local stress concentration at the grain boundaries between untwinned and twinned grains.     

钛单晶纳米柱拉压不对称性的分子动力学模拟

本文基于分子动力学模拟,通过研究钛单晶纳米柱在拉伸和压缩下的力学响应特征及晶体结构演化行为,揭示其塑性变形机制。结果表明沿[0001]晶向拉伸条件下主要塑性变形机制为伴生的{101 ?2}孪晶和基面层错;而沿[0001]晶向压缩条件下,基面位错作为优先形核的缺陷参与到塑性变形过程,随后锥面位错出现并协调了轴向和横向变形,压缩条件下无孪晶产生。拉伸模拟过程中观察到一种有别于传统孪生的晶体再取向现象,其孪晶与基体间呈现基面/柱面对应关系。     

Modeling lattice strain evolution during uniaxial deformation of textured Zircaloy-2

An elastoplastic self-consistent model was used to interpret the experimental lattice strain evolution previously reported for testing in three directions of a thick polycrystalline Zircaloy-2 slab. The model was used to infer the underlying deformation mechanisms. The influences of prism 〈a〉 slip, basal 〈a〉 slip, pyramidal 〈c + a〉 slip and tensile twinning were considered. The critical resolved shear stresses and hardening parameters for each mode were obtained by simultaneously fitting the macroscopic flow curves, Lankford coefficients and internal elastic strain development for all diffraction peaks, for the combination of three measurement directions and three loading directions, for compression and tension. The effects of dislocation interactions during deformation and hardening between deformation modes were considered. Tensile twinning inferred from the intensity changes of the diffraction peaks and its activity was qualitatively reproduced by the simulations for compression in the plate rolling and transverse directions and tension in the plate normal direction.     

Microstructural Evolution and Biodegradation Response of Mg-2Zn-0.SNd Alloy During Tensile and Compressive Deformation

The effect of deformation behavior on the in vitro corrosion rate of Mg-2Zn-0.5Nd alloy was investigated experimentally after uniaxial tensile and compressive stress.The microstructure and texture were characterized using electron backscattered diffraction and X-ray diffraction,while potentiodynamic polarization and immersion tests were used to investigate the cor-rosion response after deformation.The result reveals that applied compressive stress has more dominant effect on the corro-sion rate of Mg-2Zn-0.5Nd alloy as compared to tensile stress.Both tensile and compressive strains introduce dislocation slip and deformation twins in the alloy,thereby accelerating the corrosion rate due to the increased stress corrosion related to dislocation slips and deformation twins.The { 10(1)2} tension twinning and prismatic slip were the major contributors to tensile deformation while basal slip,and { 10(1)2} tension twin were obtainable during compressive deformation.The twinning activity after deformation increases with the plastic strain and this correlates with the degradation rate.     

Role of yttrium content in twinning behavior of extruded Mg—Y sheets under tension/compression

The influence of Y content on the grain-scale twinning behavior in extruded Mg—xY (x=0.5, 1, 5, wt.%) sheets under uniaxial tension and compression along the extruded direction was statistically investigated. An automatic twin variant analysis was employed, based on large data sets obtained by electron backscatter diffraction (EBSD), including 2691 grains with 977 twins. The {10

2} tension twinning (TTW) dominance and prevailing anomalous twinning behavior (Schmid factor (m) <0) under both tension and compression were found. The anomalous twinning behavior was more pronounced as Y content increased under tensile loading, indicating a promoted stochasticity of twin variant selection for more concentrated Mg—Y alloys. However, the trend for the Y-content dependent anomalous twinning behavior was opposite in compression. The fractions of the anomalous TTWs were found to be well correlated with the maximum Schmid factor (m_max) values of basal slip and prismatic slip in the corresponding parent grains for compression and tension, respectively, indicating that twinning and dislocation slip might be closely related in the present Mg—Y alloys.     

A First-Principles Study on Basal/Prismatic Reorientation-Induced Twinning Path and Alloying Effect in Hexagonal MetalsEI北大核心CSCD

In hexagonal metals and alloys, deformation twinning plays an important role, because it is closely relevant to the mechanical behaviors. Recent studies have proposed a new twinning mode via direct lattice reorientation, which results in the basal/prismatic boundary, however, some important details remain unanswered, e.g., the twinning path and alloying effect. In this work, first principles calculations were employed to systematically study the reorientation process from basal to prismatic orientation in hexagonal metals and corresponding alloying effect. The result indicates that different activation energies are required to reorient in various hexagonal metals, and among them, the energy in Mg is the lowest and Os is the highest. Shear and shuffle components compose the reorientation process, where the shuffle component always contributes a significant part of the activation energy in Mg, whereas in Ti with sufficient shear strain, subsequent transition becomes energy-downhill. The pure shear was effected by alloying elements in Mg alloys, but pure shuffle in Ti alloys. Under certain shear or shuffle, subsequent activation energy has a complex dependence on alloying elements.     

Analysis on Dissociation of Pyramidal I Dislocation in Magnesium by Generalized-Stacking-Fault Energy

The generalized-stacking-fault energies are calculated to illustrate the dissociation of〈c+a〉dislocation on pyramidal I plane in magnesium.The c surfaces of f10 "11g plane and its adjacent planes f30" 34 g and f30 "32g are presented using Liu embedded-atom-method potential method,and one possible dissociation path of 1=3h11" 23 i dislocation on f10 "11g plane with minimum energy is predicted.Meanwhile,another two reasonable dissociation paths of 1=3h11" 23 i dislocation successively on f30 "34g and f30" 32 g planes are also proposed.Moreover,based on molecular dynamics simulations of magnesium single crystals under c-axis compression,the possible slip path is further examined and discussed.     

On the phase diagram of erbium

We have examined the magnetic phase diagram of single crystal erbium in applied magnetic fields up to 5.5 T along the c and b-axes using electrical resistance versus temperature and magnetoresistance measurements. The phase diagram for the field along the c-axis generally agrees well with the previous neutron scattering studies by Lin et al. (Phys. Rev. B., 45 (1992) 12873) and McMorrow et al. (J. Phys. Condens. Matter, 4 (1992) 8599). We have observed a splitting of the longitudinal Néel transition as a function of applied magnetic field, and a multicritical point at a field of 2.65 T and 71 K leading to previously unobserved magnetic phases. The phase diagram for the field along the b-axis of Erbium has been determined for the first time. Recently, Jehan et al. (Phys. Rev. B, 50 (1994) 3085) have studied the a-axis phase diagram using neutron scattering. We have used their data along the a-axis in identifying the magnetic structure of our phase diagram for Er along the b-axis.     

Tension-compression asymmetry of a rolled Mg-Y-Nd alloy

In this work, tension and compression deformation behaviors of rolled and aged Mg-Y-Nd alloys were investigated. The microstructure evolution and plastic deformation mechanism during tension and compression were analyzed by combined use of electron backscatter diffraction and a visco-plastic self-consistent crystal plasticity model. The results show that both rolled and aged Mg-Y-Nd sheets show an extremely low yield asymmetry. Elimination of yield asymmetry can be ascribed to the tilted basal texture and suppression of {10-12} twinning. The rolled sheet has almost no yield asymmetry, however exhibits a remarkable strain-hardening behavior asymmetry. Compressed sample shows lower initial strain hardening rate and keeps higher strain hardening rate at the later stage compared with tension. The strain-hardening asymmetry can be aggravated by aging at 280 C. It is considered the limited amount of twins in compression plays the critical role in the strain hardening asymmetry. Finally, the relevant mechanism was analyzed and discussed.     

Twinning Behaviors During Thermomechanical Fatigue Cycling of a Nickel-Base Single-Crystal TMS-82 Superalloy

This paper provides further insight into the formation of deformation twins at different stages during the whole thermomechanical fatigue cycling in a nickel-base single-crystal TMS-82 superalloy. In general, it is found that twinning behaviors can always be associated with the applied stress orientation. The preferred twinning direction at the primary stage is 〈001〉-compression since the tangled dislocations which appear after the first plastic deformation provide an opportunity for twinning nucleation in compression. At the intermediate stage, the applied stress required for formation of twins in tension is much larger than that in compression; hence, twinning behaviors show distinct tension/compression asymmetry. A thick twin plate and a great many dislocations can be found after fatigue failure, and one can rationalize the reason for this twinning being associated with the TMF procedure. Twins at the tip of the crack in tension occur owing to the existence of compressive strain field.