Ti-2Al-2.5Zr单道次皮尔格轧制过程中显微组织和织构的演化

研究了单道次皮尔格轧制过程中Ti-2Al-2.5Zr材料的变形行为和织构的演变规律。结果表明,在轧制过程中,｛102｝孪晶和柱面滑移是最容易被激活的2种变形模式,｛102｝孪晶的产生使得晶粒在轴向上的位向从<100>转向<110>。并且,在不同瞬时Q值和等效应变量下,滑移和孪生导致｛0001｝极图中最大极密度点在切向上发生变化。     

应变路径变化对AZ31镁合金力学行为的影响

以挤压态AZ31镁合金棒材为原材料,在室温下沿着∥ED和⊥ED的方向进行预变形实验,模拟二辊皮尔格冷轧过程中减壁段横截面瞬时变形应力状态,接着对预变形试样取样进行二次压缩,利用电子背散射衍射（EBSD）对2次变形之后的微观结构进行表征。研究了应变路径变化情况下组织和织构对力学行为的影响。结果表明,预变形使AZ31镁合金的屈服强度提高,其主要原因是预变形产生的拉伸孪晶导致晶粒细化和位错密度增加。并且孪晶的出现会改变晶粒的取向,基面织构弱化（或孪生织构增强）在改善AZ31镁合金力学性能方面可能起到更重要的作用。∥ED-3%和⊥ED-3%试样的屈服强度分别提高了66.7%和6.6%。     

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。     

{1012}拉伸孪晶对AZ31B镁合金动态力学行为及应变速率敏感性的影响

采用分离式Hopkinson压杆和反射式拉杆装置在室温对挤压态AZ31B镁合金进行了动态压缩和拉伸试验,分析了AZ31B镁合金沿挤压方向压缩和拉伸时的不对称性和应变速率敏感性.结果表明:沿挤压方向压缩时,拉伸孪晶{1012}<1120>首先启动,屈服强度对应变速率不敏感;沿挤压方向拉伸时,拉伸孪晶不能启动,位错滑移参与变形,应变速率敏感性有所提高;由于拉伸孪晶只能单向启动,AZ31B镁合金在挤压方向的动态拉压不对称性显著.     

工业纯钛板材的轧制织构及微观组织对其孪晶和再结晶行为的依赖性

将工业纯钛（CP-Ti）板轧制至不同程度,随后进行退火以及进行20%的再轧制。通过电子背散射衍射（EBSD）对合金微观组织的变化进行表征。重新轧制后,｛112｝<23>压缩孪晶和｛102｝<101>拉伸孪晶产生。可以观察到孪晶的层状结构,这是由变形孪晶的缠结以及二次和三次孪晶的产生引起的。平均晶粒尺寸和孪晶量之间没有简单的关联。0.5 h退火样品中的晶粒尺寸随其预变形程度的增加而显著减小。重新轧制倾向于使晶格的重新取向更接近法线方向。虽然织构变化和孪晶体积分数很小,但平行于RD方向的｛100｝纤维织构仍然保留。     

挤压态AM30镁合金的动态力学行为及变形机制

采用分离式Hopkinson压杆和反射式拉杆装置在室温对挤压态AM30镁合金进行动态压缩和拉伸试验,分析AM30镁合金在沿挤压方向(ED)和横向(TD)压缩和拉伸时的变形机制,计算AM30镁合金在ED和TD方向压缩和拉伸时的应变速率敏感系数,并通过SEM观察断口形貌。结果表明:沿ED方向压缩时,拉伸孪晶{1012}1120是主要变形机制,屈服强度对应变速率不敏感;沿ED方向拉伸以及TD方向压缩和拉伸时,拉伸孪晶不能启动,位错滑移参与变形,应变速率敏感系数提高;AM30镁合金在ED方向表现出很强的拉压不对称性,压缩/拉伸屈强比约为0.38,在TD方向则无明显的拉压不对称性;AM30镁合金在动态压缩和拉伸时断口形貌呈韧脆混合的断裂特征。     

挤压态AZ31B镁合金动态拉压不对称性分析

为了研究挤压态AZ31B镁合金在高应变速率下的拉压不对称性,对挤压态AZ31B镁合金进行了织构分析.采用分离式Hopkinson压杆和反射式拉杆装置分别沿挤压方向和垂直挤压方向进行了动态压缩和拉伸试验,应变速率范围在500～2650 s-1之间.结果表明,由于在挤压过程中形成了基面织构,沿挤压方向压缩时,拉伸孪晶{1012}<1120>容易启动,屈服强度对应变速率不敏感,且屈服强度较低;沿挤压方向拉伸时,拉伸孪晶不能启动,压缩孪晶{1011}<1120>和非基面滑移是其主要的塑性变形机制,合金屈服强度较高;合金在压缩和拉伸时表现出很强的拉压不对称性,压缩屈服强度与屈服强度的比值约为0.30.垂直于挤压方向拉伸和压缩时,没有表现出拉压不对称性.     

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。     

初始织构对差厚板非均匀塑性变形的影响

CR340轧制差厚板（TRB）在轧制过程中,其不同的厚度区形成了不同的织构,分别是薄区的{111}<01>和{141}<22>织构,过渡区的{225}<10>和{211}<01>织构,厚区的{876}<5>和{411}<01>织构。根据EBSD测试结果,建立了各厚度区的多晶体塑性有限元模型,研究了单向拉伸时各厚度区的晶粒织构对滑移系开动情况和应力应变分布的影响规律。结果表明,薄区的{111}<01>织构和厚区的{876}<5>织构有利于滑移系的开动,开动的数量分别为9和8组,这使得等厚区在变形中的应力集中弱化,具有良好的塑性变形行为。而过渡区的{225}<10>、{211}<01>织构的晶粒滑移系开动较少,开动的数量分别为6和7组,导致应力集中,其塑性变形行为较差。差厚板各厚度区织构的差异导致其塑性变形呈现明显的不均匀性,其断裂位置发生在单轴拉伸时塑性变形较差的过渡区。     

AZ31B镁合金薄板室温拉压不对称性能研究

通过室温下单向拉伸和压缩试验研究了AZ31B镁合金挤压薄板的拉压不对称性.研究表明,在压缩过程中,孪生较拉伸易开动,单向压缩变形过程中产生的孪晶体积分数比拉伸大,使镁合金压缩屈服强度低于拉伸,导致镁合金薄板的拉压不对称性;对于挤压镁合金薄板,沿挤压方向和横向的压缩屈服强度均低于拉伸.     

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.     

Simulation of deformation twins and deformation texture in an AZ31 Mg alloy under uniaxial compression

To investigate deformation twins and the evolution of deformation texture during plastic deformation, uniaxial compression tests on a hot-rolled AZ31 Mg alloy were carried out at 200 °C. Cylindrical specimens were then compressed in both the rolling and the normal directions. The findings revealed that texture evolution, work hardening and macroscopic anisotropy are strongly dependent on the loading direction. Electron backscattered diffraction analysis was used to examine the orientation of parent grains and twin bands in the AZ31 Mg alloy under uniaxial compression. A viscoplastic self-consistent model (VPSC) was theoretically employed to calculate the relative activities of slip and twin systems in polycrystalline hexagonal aggregates under uniaxial compression. Each deformed grain exhibited an independent number and type of twin variants under uniaxial compression. Neutron diffraction was used to measure the macroscopic texture of the AZ31 Mg alloy. The VPSC model was used to simulate texture evolution, work hardening and macroscopic anisotropy during the uniaxial compression. A modified predominant twin reorientation (PTR) scheme was suggested to explain the gradual increase in twin volume in deformed grains.     

Tailoring Texture to Highly Strengthen AZ31 Alloy Plate in the Thickness Direction via Pre-tension and Rolling-Annealing

Conventional wrought Mg alloys, such as AZ31 and ZK60 rolled plates, usually exhibit significantly low tensile yield strength in the thickness direction. This can be attributed to the high activity of {10-12} tension twinning due to the strong basal texture (< 0001 > //ND, normal direction). In this work, the tensile yield strength in the ND of the as-rolled (AR) AZ31 plate increased from 50 to 150 MPa (increased by 200%) via simple processing, i.e., pre-tension and rolling-annealing (PTRA) treatment. The strong basal texture (< 0001 > //ND) of the AR plate was changed into a weakened fiber texture (< 0001 > ⊥ND). The evolution of microstructures during PTRA treatment and the activated deformation modes during uniaxial tension were studied quantitatively and statistically by the means of intergranular misorientation (IM) and in-grain misorientation axes (IGMA) analysis. The results indicate that various twin variants, as well as {10-12}-{10-12} secondary twins, were activated during pre-tension and rolling, and most residual matrix was consumed by twins after annealing. The dominated deformation modes in tension changed from {10-12} tension twinning (the AR sample) to prismatic slip (the PTRA sample) in the early tensile deformation. The underlying formation mechanism of the fiber texture and corresponding strengthening mechanism were discussed.     

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.     

Zr-4合金板材冷轧过程材料变形晶体塑性分析与织构演变

选取厚度为3.6mm具有典型双峰织构的Zr-4合金板材,利用电子背散射衍射（EBSD）技术对板材冷轧后的织构进行表征,利用粘塑性自洽（VPSC）模型对板材冷轧后的变形机理进行分析。VPSC模型预测了轧制道次数量、每道次压下量以及总变形量对冷轧织构以及变形机理的影响规律,结果表明Zr-4合金板材在冷轧后,织构保持典型的基面双峰织构;轧制道次数、单道次压下量对冷轧后的织构以及变形机理无明显影响;总变形量对冷轧后的织构有明显影响,随着轧制总变形量减小,大部分晶粒的c轴由法向（ND）向宽向（TD）转动;当变形量低于临近变形量39%时,法向科恩系数（Fn）随着变形量的增大而快速增大,柱面滑移开启快速降低,当变形量超过39%时,法向科恩系数（Fn）的增长趋于平缓,柱面滑移的开启趋于稳定。     

Influence of texture and test velocity on the dynamic,high-strain,tensile behavior of zirconium

Experiments were conducted to characterize the influence of texture and impact velocity on the dynamic, high-strain, tensile extrusion of zirconium. Bullet-shaped samples were machined from a clock-rolled, highly textured Zr plate. Specimens in two orthogonal directions were tested: the extrusion direction aligned with either the in-plane (IP) rolling or the through-thickness (TT) direction of the plate. The post-extrusion microstructure and texture evolution were examined using electron backscatter diffraction microscopy and modeled using the viscoplastic self-consistent model. It was found that extrusion deformation was accomplished through a combination of twinning and slip with their relative activity greatly depending on the initial texture. In this regard, higher elongations in the IP samples as compared to the TT samples were observed at similar test velocities. This difference in ductility is discussed in terms of the material’s ability to accommodate plastic deformation. Due to the availability of a larger number of slip systems with relatively high Schmid factors in the IP samples under this configuration, plastic deformation by prismatic slip can be easily achieved, resulting in larger elongations. On the contrary, for TT samples, twinning preceded deformation by slip. This sequential deformation process, driven by the need to reorient the microstructure favorably to slip, led to diminished elongations to failure.     

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.