Molecular Dynamics Simulations of the Orientation Effect on the Initial Plastic Deformation of Magnesium Single Crystals

Molecular dynamics simulation is employed to study the tension and compression deformation behaviors of magnesium single crystals with different orientations.The angle between the loading axis and the basal a direction ranges from 0° to 90°.The simulation results show that the initial defects usually nucleate at free surfaces,but the initial plastic deformation and the subsequent microstructural evolutions are various due to different loading directions.The tension simulations exhibit the deformation mechanisms of twinning,slip,crystallographic reorientation and basal/prismatic transformation.The twinning,crystallographic reorientation and basal/prismatic transformation can only appear in the crystal model loaded along or near the a-axis or c-axis.For the compression simulations,the basal,prismatic and pyramidal slips are responsible for the initial plasticity,and no twinning is observed.Moreover,the plastic deformation models affect the yield strengths for the samples with different orientations.The maximum yield stresses for the samples loaded along the c-axis or a-axis are much higher than those loaded in other directions.     

Twinning and texture development in two Mg alloys subjected to loading along three different strain paths

The evolution of twinning and texture in two Mg-based (+Al, Mn, Zn) alloys was investigated using uniaxial tension, uniaxial compression and ring hoop tension testing at temperatures from ambient to 250 °C and a strain rate of 0.1 s⁻¹. The results indicate that the initial extrusion texture plays an important role in the formation of different types of twins and that the twinning behavior also depends on the strain path. Contraction and double twinning are the dominant twinning mechanisms in uniaxial tension, while extension twinning prevails in uniaxial compression and ring hoop tension testing. Schmid factor analysis indicates that only components that are favorably oriented (i.e., with the highest SF values) can undergo rapid and complete twinning. The different twinning behaviors are shown to be responsible for the sharply contrasting strain hardening characteristics of the experimental flow curves and dramatic texture changes.     

镁合金塑性变形力学行为与微观组织研究进展

介绍了镁合金在单轴压缩、单轴拉伸、轧制和挤压条件下塑性变形的力学行为及微观组织结构演变规律。简述了镁合金中二次拉伸孪生现象以及各种变形条件下孪生与孪生变体类型的选择规律。基于对镁合金位错滑移、机械孪生及动态回复与再结晶行为的认识,对镁合金力学行为的各向异性、轧制与挤压成型能力的影响规律进行了探讨,强调了初始织构对变形机制、动态再结晶及成型能力的重要影响。最后讨论了析出强化镁合金塑性变形与强韧化机理。     

Temperature Effects on the Microstructures of Mg-Gd-Y Alloy Processed by Multi-direction Impact Forging

A high strain rate multi-directional impact forging(MDIF) was applied to a solutionized Mg-Gd-Y-Zr alloy in the temperature range of 350-500℃.Results demonstrate that the dominant deformation mode is twinning at a temperature below 400℃,whereas at a medium temperature of 450℃ considerable continuous dynamic recrystallization was promoted by{10-12} extension twins.At a higher temperature of 500℃,twinning activation was suppressed.New DRX grains were observed but their sizes were much bigger than those resulting from the MDIFed 50 passes at 450℃,which are ascribed to the larger grain boundary mobility and atomic diffusion at 500℃.Moreover,a non-basal weak texture was gained afterward MDIF at each temperature,which is credited to the MDIF process and the minor strain applied in each pass.     

Influence of Sc on high temperature strengthening behavior of Ti-6A1-4V alloy

Vacuum arc melting technique was used to prepare Ti-6A14V alloy containing Sc （0.3% and 0.5%, mass fraction）. The ingots were melted twice by vacuum self-consumable electrode arc furnace. Forging of ingots was started in β-phase region and finished in high （a＋β-phase region. Annealing after forging was performed in low （a＋β）-phase region for 30 min. Isothermal high temperature compression tests were conducted using thermal simulation machine under Ar atmosphere at 850℃ and 1 000 ℃, and the strain rate were 0.001, 0.01, 0.1 and 1.0 s^-1. Optical microscope（OM）, scanning electron microscopy（SEM）, energy dispersive spectrum（EDS） and transmission electron microscope（TEM） were used to study the microstructure evolution during high temperature deformation. The results show that, the peak stress value of alloys increases with increasing Sc content after deformation at 850℃, however, there is no obvious strengthening of Sc when the alloys are deformed at 1 000 ~C. Sc exists as Sc203 forms by internal oxidation during forging procedure, only minor Sc solutes in matrix. At 850 ~C, the interaction between dislocation and participated particles and twinning mechanism controls the deformation procedure accompanied recrystallization. At 1 000 ℃, the deformation of alloys containing Sc is mainly controlled by twinning, while the deformation of alloy without Sc is not only controlled by twinning, but also the interaction between dislocation and precipitated particles inside the twinning lamellar.     

Microstructure,Texture Evolution,and Mechanical Properties of ECAP-Processed ZAT522 Magnesium Alloy

In this work,the high-strength Mg-5Zn-2Al-2Sn(ZAT522,in wt%) Mg alloys was obtained at 220℃ and 130℃ by a two-step equal channel angular pressing(ECAP).For each stage,two passes were used.The results showed a remarkable grain refinement after the first stage of ECAP(A2 samples),leading to a fine-grained structure with average size of 1.40 μm.The additional stage(A4 samples) caused further grain refinement to 1.18 μm,and an ultra-fine grain structure(700 nm)appeared in the precipitate-rich region.The grain refinement mechanism for both samples was discussed in detail.To this end,the original extrusion fiber texture evolved into a new strong texture characterized by the base planes tilted toward the ECAP shear plane,with a higher Schmid factor value of 0.34.Compared with the as-extruded alloy,the yield strength of the A2 samples increased from 180 to 245 MPa,which was mainly attributed to the combined effects of grain boundary strengthening and precipitation strengthening.In the case of A4 samples,the dislocation strengthening resulted in a net increase in yield strength to 335 MPa,while the ductility was significantly reduced.     

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).     

变形镁合金中的织构及其优化设计

针对变形镁合金存在的典型织构以及织构优化设计方面的研究工作和进展进行综合评述。镁合金由于基面滑移和{1 012}孪生是最容易开动的变形模式,在变形镁合金中容易形成挤压丝织构及轧制板织构。通过引入剪切变形,改变成型过程中外加应力的取向,能够有效地改变变形镁合金的织构,同时通过添加微量稀土元素Nd、Ce和Y等,能够明显弱化或随机化变形镁合金织构。织构随机化后的镁稀土合金具有较好的强韧性,合金的变形各向异性得以改善。添加稀土元素后会改变稀土元素与Mg原子间的键能,改变稀土元素周围Mg-Mg原子之间的结合能等,增加非基面滑移的可能性,减弱基面滑移及{1 012}孪生所占的比率,有效地弱化镁合金的织构。     

Thermal Stability of Nanocrystalline AZ31 Magnesium Alloy Fabricated by Surface Mechanical Attrition Treatment

A nanocrystalline layer(NL) was fabricated on the surface of AZ31 magnesium(Mg) alloy sheet by surface mechanical attrition treatment(SMAT). The microstructure of the Mg alloy was characterized by optical microscopy,X-ray diffraction and microhardness test. The results showed that both the microstructure and microhardness of AZ31 Mg alloy sheet after SMAT revealed a gradient distribution along depth from surface to center. The thermal stability of the NL was investigated through characterizing the microstructure evolution during the post-isothermal annealing treatment within the temperature range from 150 to 250 ℃. The NL exhibits a certain degree of thermal stability below 150 ℃, while it disappears quickly when annealing at the temperature range of 200–250 ℃. The grain growth kinetics of the nanocrystalline of AZ31 Mg alloy induced by SMAT was investigated. The activation energy of nanocrystalline AZ31 Mg alloy was obtained with a value of 92.8 k J/mol.     

Effect of Nanoscale Cu-Riched Clusters on Strength and Impact Toughness in a Tempered Cu-Bearing HSLA Steel

The effect of Cu-riched clusters on strength and impact toughness in a tempered Cu-bearing high-strength low-alloy (HSLA) steel is investigated. With increasing the tempering temperature, it is found that the yield strength increases firstly, achieving the maximum value (~ 1053 MPa) at the tempering temperature of 450 °C, and then decreases significantly with the rise of tempering temperature. The tempering temperature-dependent yield strength is closely related to the precipitation of Cu-riched clusters. When tempering at 450 °C, the peak strength will be reached as the nanoscale Cu-riched clusters with small size and high number density will cause a strong precipitation strengthening (~ 492 MPa) due to the dislocation shearing mechanism. However, the Cu-riched clusters will coarsen with further increasing tempering temperature, resulting in obvious decrement of yield strength owing to the dislocation bypassing mechanism. Compared with the yield strength, the variation in impact energy displays an inverse tendency and the impact energy is only 7 J for the sample tempered at 450 °C. The fracture mode can be well explained by the competition between the cleavage fracture strength (σ_F) and “yield strength” (σ_Y). Although transgranular cleavage fracture can be found in samples tempered at 450 and 550 °C, the crack propagation along the lath boundaries is prevented in the sample tempered at 550 °C. The reason is that the number density of Cu-riched clusters at lath boundaries decreases and the segregation of Mo element at the lath boundaries is induced, which will increase the bonding energy.     

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。     

Effect of Initial Grain Size on the Hot Deformation Behavior and Microstructural Evolution of Pure Copper

The influences of initial grain size (IGS) with 20 µm and 50 µm on the hot flow behavior and microstructural changes of pure copper were investigated using hot compression tests at a temperature range of 623-1073 K and strain rate range of 0.001-0.1 s^-1. The effects of critical stress and corresponding critical strain were studied based on the internal and external processing parameters. The critical stress and strain decreased with increasing temperature and decreasing strain rate. The investigation results of the microstructure and true strain-stress diagrams showed that dynamic recovery, dynamic recrystallization (DRX), and twinning mechanisms were caused during the hot deformation of pure copper. Microstructure evolution indicated some DRXed fine-grain took place around grain boundary of hot deformed samples with IGS of 20 µm whereas DRXed fine-grain took place in interior grains for samples with larger IGS. The results also showed that grain growth is also dependent on IGS as the grain growth rate for samples with the larger IGS is greater than the smaller IGS. The critical strain rate and the temperature were obtained at 0.01 s^-1 and 973 K, respectively, for the sudden change in the grain growth rate. Also, twinning highly depended on IGS which almost did not happen in fine grain size while the volume fraction of twinning increased with increasing grain size.     

Orientation factor analysis of deformation mechanisms under special processing techniques in AZ31 magnesium alloys

Experiments show that special processing techniques such as asymmetrical rolling(ASR),equal channel angular pressing(ECAP) and equal channel angular rolling(ECAR) can weaken the basal texture of the magnesium alloys and therefore improve their plasticity.However,the deformation mechanisms related are different.In this paper,we determine the deformation mechanisms activated during ASR,ECAP and ECAR by calculation of orientation factors.Analysis shows that during ASR the shear stress σ13 on the rolling plane ...     

Non-crystallographic shear banding in crystal plasticity FEM simulations: Example of texture evolution in α-brass

We present crystal plasticity finite element simulations of the texture evolution in α-brass polycrystals under plane strain compression. The novelty is a non-crystallographic shear band mechanism [Anand L, Su C. J Mech Phys Solids 2005;53:1362] that is incorporated into the constitutive model in addition to dislocation and twinning. Non-crystallographic deformation associated with shear banding leads to weaker copper and S texture components and to a stronger brass texture compared to simulations enabling slip and twinning only. The lattice rotation rates are reduced when shear banding occurs. This effect leads to a weaker copper component. Also, the initiation of shear banding promotes brass-type components. In summary the occurrence of non-crystallographic deformation through shear bands shifts face-centered-cubic deformation textures from the copper type to the brass type.     

Low cycle fatigue properties and cyclic deformation behavior of as-extruded AZ31 magnesium alloy

The low cycle fatigue(LCF)properties of as-extruded AZ31 Mg alloy were investigated under total strain amplitudes in the range of 0.4%-1.2%with strain rate of 1×10- 2s -1.Due to the twinning effect in compression during loading and the detwinning effect during unloading,the alloy showed an asymmetric hysteresis loop.The cyclic stress response exhibited cyclic hardening at high total strain amplitudes.The cyclic deformation behaviors were discussed using the Coffin-Manson plot,which divided the plastic strain amplitudes into the tension side and the compression side.Through the LCF tests that were started from either tension or compression under a total strain amplitude of 1.0%,the interaction between the twinning effect and dislocation was analyzed.The twinning effect during the LCF test and the variation of the dislocation density were investigated using optical microscopy and transmission electron microscopy,respectively.     

Microstructural Evolution and Anisotropic Weakening Mechanism of ZK60 Magnesium Alloy Processed by Isothermal Repetitive Upsetting Extrusion

The isothermal repetitive upsetting extrusion (RUE) was implemented to process ZK60 magnesium alloy at 380 °C. Then, the relationship between the microstructural characters, including grain refinement and texture evolution, and the mechanical performance of the alloy was investigated. Results showed that after 3 passes of RUE, the average grain size was refined from 115.0 to 26.5 μm, which was mainly caused by the continuous dynamic recrystallization and discontinuous dynamic recrystallization. Meanwhile, the elongation of the alloy increased from 13.8 to 21.6%, and the superplasticity (142%) of the alloy has been achieved in the following high temperature tensile test, which is very beneficial for the further processing of the alloy into components. In particular, the alloy formed a distinctive texture distributed between < 2-1-11 > and < 2-1-14 > , which was greatly related to the Schmid factor of extrusion direction (ED) and transverse direction (TD). This texture changed the initiation ability of basal and prismatic slip in both directions and inhibited the initiation of partial tensile twinning in TD; thus, the anisotropy in both directions was weakened. As expected, the tensile yield strength difference decreased from 25.9 to 3.4 MPa, but it was used as the cost of tensile yield strength in ED.     

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。     

Anisotropic Stress Rupture Properties of a 3rd-Generation Nickel-Based Single-Crystal Superalloy at 1100℃/150 MPa

The influence of orientation on the stress rupture behaviors of a 3 rd-generation nickel-based single-crystal superalloy was investigated at 1100℃/150 MPa.It is found that the stress rupture anisotropy is shown at 1100℃,but not so obvious compared with that at intermediate temperatures.The [001] specimens display the longest rupture life,[111] specimens show the shortest rupture life,and [011] specimens exhibit the intermediate life.Detailed observations show that the final fracture is caused by crack initiation and propagation,and the anisotropy of three oriented specimens is related to the fracture modes,γ/γ' microstructures,interfacial dislocation networks and cutting mechanisms in y' phase.For [001] specimens,N-type rafted structures are formed which can well hinder the slip and climb of dislocations.Besides,the regular interfacial dislocation networks can prevent dislocations from cutting into y' phase,leading to the improvement of the creep resistance.For [011] specimens,±45°rafted structures and irregular networks result in less strain hardening.For [111] specimens,a large number of crack propagation paths and inhomogeneous deformations caused by irregular rafted structures deteriorate the property and result in the shortest life.Furthermore,a[100] superdislocations with low mobility are widely formed in[001] and [011] specimens which suggests the low creep strain rate during steady creep stage,whereas superdislocations in[111] specimens possess high mobility,which indicates the high strain rate and corresponding poor stress rupture property.     

Influence of Selective Laser Melting Process Parameters on Microstructure and Properties of a Typical Ni-Based Superalloy

The influence of selective laser melting (SLM) process parameters on the microstructure and mechanical properties of a typical Ni-based superalloy was researched. The optimum parameters of P = 170 W, V = 0.8 m/s were determined, under which the SLMed samples exhibited both the largest relative density of 99.57% and the best mechanical properties, including the microhardness (329.3 ± 3.8 HV), yield strength (726 ± 8.1 MPa), ultimate tensile strength (900 ± 5.9 MPa) and elongation ((31.9 ± 0.24)%). The average grain size ranges of SLMed samples are from 15.2 to 17.4 μm, with a typical mixed grain structure. Owing to the high cooling rate and remelting during SLM process, a large number of low-angle grain boundaries (LAGBs), dislocations and sub-grains were formed, and the fraction of LAGBs reached above 65%. At the same time, the content of low-Σ coincidence site lattice (CSL) boundaries was mostly less than 1%, while there was almost no γ′ phase precipitated in the matrix. The texture of SLMed samples was weak, and there was no obvious preferred growth direction. Combining with the microstructure characterization, both grain refinement strengthening and dislocation strengthening were considered as the main strengthening mechanisms. Moreover, the fracture mechanism of the optimum sample belonged to ductile fracture.     

In Situ Electron Backscatter Diff raction Analysis for Microstructure Evolution and Deformation Models of Mg–Ce Alloy During Uniaxial Loading

Previous studies showed that signifi cant increases in elongation in Mg–Ce alloys due to the Ce addition and the solute drag eff ect by Ce addition were ascribed to the non-basal dislocation slip activating and the texture altering. The microstructure evolution and deformation models of extruded Mg-0.5 wt%Ce alloy rods under uniaxial tension have been studied using in situ electron backscatter diff raction. The basal and non-basal slips were characterized by using slip line trace analysis. The results provide evidence for that pyramidal slip activated during deformation, besides basal slip and extension twinning, which contributes to the texture weakening and ductility increasing in Mg-0.5 wt%Ce alloy.