镁合金变形机理研究中的Schmid因子计算及应用

基于EBSD技术所测得晶粒的欧拉角,建立了Schmid因子计算公式,用来计算HCP晶体中单个晶粒的Schmid因子。在此基础上,系统分析了镁合金板材拉伸变形时晶粒取向变化与不同微观变形模式的Schmid因子的关系。对镁合金棒材中的基面织构的Schmid因子分布特征进行了计算分析,结合镁合金棒材压缩变形的实测结果,分析了镁合金棒材压缩变形的力学性能及镁合金棒材径向压缩过程晶粒取向变化特征。结果表明：镁合金板材中的基面织构不利于基面滑移系和拉伸孪晶启动,而利于柱面和锥面滑移系启动,异步轧制对镁合金板材力学性能的提升有限。晶粒Schmid因子分布的差异导致镁合金棒材轴向和径向压缩时的微观变形机制存在差异,从而导致宏观力学性能差异明显。     

镁合金织构与各向异性

介绍了镁合金变形及退火织构的组分与特点,论述了在挤压、轧制、等径角挤压等塑性变形及退火过程中镁合金织构的演变规律及形成机理,分析了织构与镁合金力学性能的基本关系,探讨了合金元素、变形温度、应变速度、外加应力及晶粒度等基本因素对镁合金织构特征与各向异性的影响.结果表明:织构对镁合金力学性能的影响,其实质是通过改变各滑移系特别是{0001}[1120]基面滑移系的Schmid因子、产生织构强化或软化而实现的.     

镁合金棒材扭转变形的孪晶机制

室温下对镁合金挤压棒材进行了最大扭转角分别为90°、180°和320°的扭转变形,依据扭转变形过程数值模拟结果,基于切应力作用下的等效施密特因子计算模型,结合扭转变形后晶粒取向的EBSD测试结果,对扭转过程中的拉伸孪晶启动机制进行了分析。结果表明:在大扭转角条件下,多数拉伸孪晶的启动遵循Schmid定律,一些Schmid因子值低的拉伸孪晶也启动。原始棒材中的晶粒c轴垂直于棒材轴向,而拉伸孪晶的启动使多数晶粒的c轴向转向平行于棒材轴向。晶粒中虽然启动较多孪晶带,但孪晶启动并没有改变扭转变形过程中应力应变曲线的硬化特征,扭转变形后棒材中仍保持为晶粒的基面平行于棒材轴向的织构特征。     

织构对AZ31镁合金温热压缩变形机制影响

在AZ31轧制板材上截取4种具有不同晶粒取向特征的试样,在170℃温热条件下,分别对上述试样进行压缩变形,分析了变形过程中力学性能变化、晶粒取向演化和织构变化,并基于压缩过程施密特因子的分布特征,对上述变形过程中各种微观变形模式的启动趋势进行了分析,最后对上述变形过程的微观启动模式进行了定量计算。分析结果表明:织构明显影响镁合金压缩变形过程的微观变形机制,从而对宏观力学性能产生明显影响,平行于晶粒c轴压缩因大量的柱面滑移系启动而呈现最高应力值。随着晶粒c轴与压缩方向夹角的增大,拉伸孪晶启动量增加而导致织构明显改变。     

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%。     

应力状态对纯钛织构演化机制影响研究

采用拉伸、压缩的试验方法,结合Schmid因子计算和晶体塑性模拟计算研究了TA2纯钛在不同应力（拉应力、压应力）状态下织构的演化机制。结果表明：在拉伸变形过程中,较大的应变量也难以使织构发生显著变化,相对而言,压缩变形过程中织构变化较为显著。在不同应变路径下,变形初期启动的变形方式有一定的差异。在不同应变量下,随着变形程度的增加,发生基面滑移或锥面滑移或■拉伸孪生的晶粒数变多是导致形成不同织构的主要原因。     

AZ31镁合金挤压棒材变路径压缩的微观机理

室温下,对AZ31镁合金挤压棒材分别进行两种变路径压缩变形,第一种为压缩方向为正交径向的两道次压缩,称为RD1-RD2压缩变形;第二种为压缩方向依次为挤压方向和的径向的两道次压缩变形,称为ED-RD1压缩变形,并对上述压缩过程中晶粒取向和力学性能的变化特征进行了分析。分析结果表明,压缩变形过程中大量{10-12}拉伸孪晶和{10-12}-{10-12}二次孪晶启动,不同的孪晶变体启动使晶粒中存在较多的交叉孪晶界,孪晶启动总体上遵循施密特定律。对于RD1-RD2压缩变形,两道应力应变曲线几乎相同,而对于ED-RD1压缩变形,第二道次压缩的应力应变曲线对应的应力值明显高于第一道次。     

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。     

AZ31镁合金热压缩过程中晶粒取向和织构的演变

利用电子背散射衍射(EBSD)取向成像技术分析AZ31铸态镁合金在不同温度和真应变下热压缩的晶粒取向和织构特点,从晶粒取向和织构角度分析不同温度下其动态再结晶(DRX)的类型。结果表明:在热压缩过程中,350℃时,AZ31铸态镁合金表现为连续动态再结晶(CDRX)特征,新晶粒取向与基体相似,具有较强的{0002}基面织构,以基面滑移为主;500℃时,为旋转动态再结晶(RDRX)特征,真应变为0.5时,新晶粒取向与基体偏转成一定角度,具有两种主要的基面织构,由于动态再结晶的定向形核、择优核心长大和旋转动态再结晶造成这两种基面织构弱于350℃时的{0002}基面织构;且随着真应变的增加,其中一种织构由于滑移系的改变而逐渐消失。     

轧制镁合金中的板织构及其控制工艺

对轧制镁合金中的板织构及其控制工艺的研究进展进行评述。镁合金在轧制过程中形成(0001)平面平行于轧制平面的基面织构,其主要原因是基面α滑移及{10-12}孪生变形。通过弱化织构,可显著提高变形镁合金的塑性和成形性。弱化镁合金织构的手段有2种:添加稀土元素和设计特殊成型工艺。添加少量稀土就可对织构弱化起到明显效果。通过异步轧制、等径角轧制等引入剪切变形,改变再结晶晶粒基面取向,从而弱化变形镁合金织构。镁合金板材多道次轧制过程中或者轧制后一般都需进行退火,退火工艺对变形镁合金的织构有一定弱化效果。     

拉伸取向控制对AZ31镁合金孪生和织构演化的影响

以室温单轴拉伸实验与晶体塑性有限元相结合的方法,通过拉伸取向控制,研究了AZ31镁合金拉伸变形过程中孪生行为、织构演化规律、塑性各向异性之间的关系。基于率相关晶体塑性本构理论,建立了滑移和孪生机制耦合的具有不同取向的晶体塑性本构模型,引入孪晶体积分数研究孪生对AZ31镁合金塑性变形过程中织构演变和力学性能的影响。结果表明,2种不同取向的样品在塑性变形过程中呈现出明显不同的织构演变规律,表现出明显的各向异性。轴向拉伸时孪生被抑制,孪晶激活体积分数低,径向拉伸时孪晶极易产生,孪晶激活体积分数高。轴向试样在整个塑性变形过程中{0001}极图偏移较小,径向试样因大量拉伸孪晶的开启,使得{0001}棱柱面织构的极密度逐渐向RD的正反方向发生明显偏移。     

Effect of Forging on Microstructure,Texture, and Uniaxial Properties of Cast AZ31B Alloy

The effect of open-die hot forging on cast AZ31B magnesium alloy was investigated in terms of the evolution of microstructure, texture, and mechanical properties. A refined microstructure with strong basal texture was developed in forged material. A significant increase in tensile yield and ultimate strengths by 143 and 23%, respectively, was determined as well. When tested in compression at room temperature, the forged alloy displayed significant in-plane asymmetry and unchanged yield strength compared to the cast alloy owing to the activation of \(\left\{ {10\bar{1}2} \right\}\left\langle {10\bar{1}1} \right\rangle\) extension twins in both the cast and forged conditions. However, the ultimate compressive strength for the forged material increased by 22 percent compared to the as-cast material. Microstructure and texture analysis of the fracture samples confirmed that the deformation of the forged samples was dominated by slip during tension and twin in compression. In comparison, both slip and twin were observed in the cast samples for similar testing conditions. The increase in strength of forging was attributed to the refinement of grains and the formation of strong basal texture, which activated the non-basal slip on the prismatic and pyramidal slip systems instead of extension twin.     

Preparation and characterization of Mg alloy rods with gradient microstructure by torsion deformation

Extruded AZ31 Mg alloy rods were subject to free-end torsion deformation at room temperature. The microstructure features of the torsion deformed samples were characterized using electron backscatter diffraction technique. Mg rods with gradient microstructure can be fabricated by torsion deformation. Inhomogeneous distribution of microstructure along the radial direction of the twisted rods is attributed to the linearly increasing strain accumulation and strain rate from core to surface. With increasing equivalent strain, both the amount of {10-12} twins and dislocation density increase and the c-axes of texture tend to rotate towards torsion axis. Although both dislocation slips and {10-12} twinning can be activated during torsion, dislocation slips are considered as the dominated deformation mechanism and responsible for the change of macro-texture for present torsion deformation. {10-12} twins and dislocations in the twisted samples can generate refinement hardening and dislocation hardening, respectively, to increase the hardness value.     

Shear bands in magnesium alloy AZ31

During deformation of magnesium at low temperatures, cracks always develop at shear bands. The origin of the shear bands is the { 1011 } twinning in basal-oriented grains and the mobility of this type of twin boundary is rather low. The most frequent deformation mechanisms in magnesium at low temperature are basal slip and { 1012 } twinning, all leading to the basal texture and therefore the formation of shear bands with subsequent fracture. The investigation on the influences of initial textures and grain sizes reveals that a strong prismatic initial texture of (0001) parallels to TD and fine grains of less than 5 8m can restrict the formation and expansion of shear bands effectively and therefore improve the mechanical properties and formability of magnesium.     

微观组织对镁合金FSW焊缝应变硬化行为的影响

采用传统搅拌摩擦焊和冷源辅助搅拌摩擦焊对3 mm厚的AZ31B镁合金进行焊接. 利用电子背散射衍射、透射电子显微镜和静拉伸试验研究焊缝区的微观组织对力学性能的影响. 结果表明,液态二氧化碳不仅降低焊接峰值温度,还提高焊后冷却速度. 焊缝峰值温度的降低为激活{10-12}孪生行为创造了有利条件. {10-12}孪晶可降低基面织构的强度,也可进一步分割晶粒,起到细化晶粒的作用. 焊后冷却速度的提高使焊接过程中产生的大量位错保留在晶粒内部. 因此冷源辅助搅拌摩擦焊缝表现为具有大量{10-12}孪晶和位错的细晶结构. 在拉伸过程中,细晶强化和位错强化为主要强化机制. 孪晶界面可有效吸收和分解变形时产生的位错,从而协调应变和减小应力集中,使焊缝具有合理的应变硬化行为和强塑性匹配.     

一种5083铝合金中晶界面热轧织构的研究

通过电子背散射衍射技术,测定了一种5083铝合金中的热轧织构。结果表明：在结构中的不同位置,织构组分呈现非均匀发展的特点,特别地,晶界面亦呈现取向织构。在样品的中心位置,晶界面趋向于{111}取向,且其数量高出随机晶界50%;同时,在样品的边缘位置,晶界面趋向于{110}与{112} 取向,且其数量高出随机晶界28%。更具体地,小角和大角晶界中均呈现这种晶界面的各向异性分布。     

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).     

Cyclic deformation behaviors of Ti-46Al-2Cr-2Nb-0.15B alloy during thermo-mechanical fatigue tests

Thermo-mechanical fatigue tests were carried out on the gamma-TiAl alloy in the temperature range of 500-800 °C under mechanical strain control in order to evaluate its cyclic deformation behaviors at elevated temperature.Cyclic deformation curves,stress-strain hysteresis loops under different temperature-strain cycles were analyzed and dislocation configurations were also observed by TEM.The mechanisms of cyclic hardening or softening during thermo-mechanical fatigue(TMF) tests were also discussed.Results showed that thermo-mechanical fatigue lives largely depended on the applied mechanical strain amplitudes,applied types of strain and temperature.On the hysteresis loops appeared two apparent asymmetries:one was zero asymmetry and the other was tensile and compressive asymmetry.Dislocations configuration and slip behaviors were contributed to cyclic hardening or cyclic softening.