AZ31镁合金再结晶退火的准原位EBSD研究

对AZ31镁合金热挤压板进行室温轧制(形变量为8%)后,利用背散射衍射技术原位(in-situ EBSD)观测了轧制试样中不同类型的孪晶组织在再结晶退火过程中的取向演变。结果表明:退火过程中拉伸孪晶区域形成尺寸相对粗大的再结晶新晶粒,再结晶晶粒取向与拉伸孪晶的取向较为接近;压缩孪晶/双孪晶区域形成了细小的再结晶晶粒,再结晶晶粒偏离基面取向。孪晶再结晶显著影响镁合金在退火过程中的织构演变,轧制样品中,拉伸孪晶再结晶使得基面织构强度增强,压缩孪晶再结晶则可以在一定程度上弱化镁合金的基面织构。     

孪晶类型对AZ31镁合金静态再结晶的影响

对轧制下压方向平行和垂直晶粒c轴的两类板材进行150℃轧制(5%下压量)后,利用背散射电子衍射分析(EBSD)研究了轧制试样中不同类型的孪晶组织对静态再结晶的晶粒形核、微观组织及织构的演变的影响。结果表明:含有大量{1011-}-{1012-}双孪晶的样品中,二次孪生有效促进再结晶形核,显著细化晶粒。再结晶晶粒取向规律性不强,有效削弱基面织构。而含有大量{1012-}拉伸孪晶的样品,拉伸孪晶不能有效促进再结晶形核。退火过程中基体不断长大,当再结晶驱动力足够大时,基体会吞并周围拉伸孪晶,同时诱发织构改变,基体取向的织构逐渐增强,拉伸孪晶取向的织构逐步减弱。     

金属铍静态再结晶行为

通过热压缩和真空退火实验系统研究了金属铍低温形变(应变温度350 ℃,应变速率10_^-3 s_^-1,应变量30%)后在680 ℃至880 ℃温度区间退火组织演变规律。结果表明：金属铍具有独特的静态再结晶行为,再结晶晶粒首先在10 2}<10 >拉伸孪晶界处形核,机理为应变诱导的孪晶界弓出形核。晶界“弓出”形核落后于孪晶界“弓出”形核的原因是BeO杂质对原始晶粒晶界钉扎,阻碍了其界面的迁移。孪晶界和原始晶粒晶界“弓出”形核是金属铍主要的形核方式,晶内直接形核和杂质处形核是其次要的形核方式。低温形变铍在680 ℃至880 ℃内退火均能够获得晶粒细化的完全再结晶组织,且没有再结晶织构形成。金属铍的再结晶晶粒不易长大,原因也是由于BeO杂质的对晶界迁移的钉扎作用。在680 ℃,730 ℃,780 ℃,830 ℃和880℃退火,完成再结晶时间分别大约为2160 min,180 min,20 min,5 min,4 min。金属铍350℃下压缩发生{0001}基面滑移和{10 2}类孪晶变形,形变机理与室温相同,没有随温度升高而发生改变,仍保持金属铍特有的反常变形行为。     

孪晶界对AZ31镁合金静态再结晶的影响

对铸态AZ31镁合金进行不同变形量的锻造或压缩室温变形,然后经过不同温度和时间的退火保温,研究孪晶界对合金静态再结晶过程的影响。结果表明,锻造产生的孪晶较短且取向错乱,而压缩变形产生的孪晶则较狭长,同取向的孪晶大量聚集、平行排列,这与锻造和压缩两种变形方式的形变速率不同有关。退火保温实验结果表明,在相同条件下,锻造变形镁合金孪晶处比压缩变形镁合金孪晶处更易发生再结晶。研究不同孪晶处的形核方式,提出低温"孪晶界凸出形核"机理,发现某些孪晶变体即二次孪晶处并不是优先形核的位置,这与经典的孪生形核理论的观点不同。     

AZ31镁合金室温多道次压缩过程微观组织和织构演变的研究

对AZ31镁合金在室温进行多道次压缩变形,利用EBSD技术研究其微观组织和织构演变,分析孪晶在细化晶粒和调控织构方面发挥的作用。结果表明:多道次压缩过程中的组织和织构演变主要受{10孪生影响,道次应变量越大,织构变化越明显,每道次压缩后,利于拉伸孪生的晶粒取向发生孪生转到压缩轴附近,从而弱化初始基面织构,而退孪晶的发生则不利于细化晶粒和弱化织构。在多道次压缩过程中,孪生Schmid准则支配着变形中的{10与后续变形中产生的孪晶片层相互交叉,分割细化晶粒;道次变形量会影响多向变形过程每道次孪晶的激活量和孪晶片层的形貌,从而影响晶粒的细化程度。     

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。     

挤压镁合金AZ31的压缩过程中的组织及织构演化

对常规挤压态镁合金AZ31压缩过程的组织及织构演化进行了扫描电镜-电子背散射衍射(SEM-EBSD)原位观察。结果表明材料的初始组织为等轴晶,晶粒的平均尺寸为76微米,晶粒内部未发现形变孪晶。材料的初始织构类型为典型的{11-20}丝织构,即大多数晶粒的<11-20>晶向平行于棒材的挤压方向(ED)。在压缩过程中,多数晶粒内部开始出现拉伸孪晶,随着压缩应变的增加,孪晶片层不断增厚,导致晶内的孪晶合并成大的孪晶并占据晶粒内部的大部分区域进而使孪晶的体积分数不断增加。随着压缩压缩应变的增加初始丝织构不断减弱并有新的基面织构形成。实验表明压缩过程中的{10-12}<10-11>孪生而非滑移是引起压缩过程中织构演化的主要原因。     

Al—Mn合金中粒子促进形核及初期再结晶织构 Ⅱ.粒子与其它形核位置的交互作用及初期再结晶织构

本文在分析了Al—Mn合金形变时不同形变取向基体中粒子周围亚晶转动规律及对应的粒子促进形核晶粒的取向特征的基础上,进一步测定了处在晶界,立方带上的粒子周围新晶粒的取向分布,最后分析了再结晶织构的形成．结果表明,处在这些位置上的粒子周围新晶粒一方面保留了晶内PSN晶粒的取向特征,另一方面也带有晶界及立方带形核的特点再结晶织构反映了不同类型形核位置形成的新晶粒的取向特征     

Mg-Gd-Y-Nd-Zr合金热压缩变形的动态再结晶机制（英文）

在变形温度为450°C和应变速率为2 s-1的条件下对均匀化退火后的Mg-7Gd-4Y-1Nd-0.5Zr合金进行热压缩试验。采用金相显微镜(OM)、扫描电镜(SEM)和透射电镜(TEM)综合分析合金变形过程中的动态再结晶机制。采用电子背散射衍射(EBSD)获得晶体微取向信息。结果表明:随应变逐渐增加到-1.88,合金流变应力先快速升高到某个峰值,随后下降到最低值,最后又开始逐渐上升。在低应变下,大量{1 012}拉伸孪晶诱发形核形成动态再结晶晶粒,导致晶粒明显细化。动态再结晶晶粒首先在孪晶边界进行形核,且与孪晶母体存在30°0001的取向差。在大应变下,合金组织中在原始大晶粒附近形成细小动态再结晶晶粒,且从原始大晶粒内部到其晶界处的累积微取向连续增加,从而确定合金发生了连续动态再结晶。合金中也发现了粒子激发形核的动态再结晶机制。     

利用道次间退火改善镁合金轧制成形性的研究

塑性较差的六方结构镁合金轧制时易出现裂纹,尤其是在1mm以下薄板带的终轧阶段。其原因是在较低温度下基面取向晶粒内形成的切变带不易扩展所致。研究了MB1,AZ31(MB2)镁合金在热模拟条件和实验室热轧过程中利用静态再结晶改善形变组织、细化晶粒、提高成形性的规律。实验表明,在选择的多道次轧制退火工艺下可顺利轧出0．3mm厚的薄板带,得到平均尺寸～7μm的等轴细晶。热模拟条件下得到的形变温度、形变量和形变组织的关系可帮助确定实际生产轧制过程中各道次轧制的温度。织构测定表明,各阶段退火前后都得到强的基面织构。终轧阶段无法利用{10^-12}拉伸孪晶的静态再结晶细化晶粒,而只能利用压缩孪晶／扩展的切变带的再结晶细化晶粒。本文对轧制时利用动、静态再结晶细化晶粒的潜力及工艺优化进行了讨论。     

基于原子尺度模拟研究HCP镁中的一次及二次孪生模式

通过分子动力学模拟（MD）,研究在HCP镁中的一个对称倾斜晶界与基面滑移的位错相互作用而激发的变形孪晶,也就是孪晶形核与长大的过程（或者是孪晶界迁移,TBM）。{1^-1^-21}孪晶在该过程中是最易被激发的孪生模式。一旦这样的孪晶形成了,它们就会不断长大。该种孪晶界迁移是由单纯的原子位置局域调整造成的。在模拟过程中同时也产生了二次孪晶{1^-1^-22}。该二次孪晶模型的孪晶形核与长大需要克服的能垒与{1^-1^-21}孪晶不同。同时,二次孪晶的孪晶界迁移过程是通过孪晶界上的锥形滑移而激发的。     

轧制温度对AZ31镁合金轧制板材中的{1011}-{1012}双孪生行为的影响

在150-350℃温区内不同温度下轧制AZ31镁合金板,观察了不同温度下轧制变形量为9%的AZ31镁合金板材的显微组织,研究分析了轧制温度对轧制板材中{1011}-{1012}双孪晶的含量、类型以及高温轧制过程中双孪晶中的动态再结晶行为的影响,讨论了板材中的孪晶对其力学性能的影响.研究结果表明:在150-300℃温区内轧制时,板材组织中均有含量不等的{1011}-{1012}双孪晶,随着轧制温度的升高,孪晶含量下降.250℃以上轧制的板材中单片一次孪晶中出现的双孪晶类型较为单一,仅出现共面型双孪晶.在250℃以上轧制板材中的双孪晶晶界处中可以观察到明显的动态再结晶现象,这些动态再结晶晶粒对孪晶界和孪晶起到消除和吞噬的作用.350℃下轧制的AZ31镁合金板材中未观察到{1011}-{1012}双孪晶.随着轧制温度的升高,镁合金轧制板材的强度减弱而塑性增强.     

Role of {10–12} twinning characteristics in the deformation behavior of a polycrystalline magnesium alloy

The active twin variants during {10–12} twinning of magnesium alloys were dependent on the strain path (i.e., compression perpendicular to the c-axis or tension parallel to the c-axis), and their section mechanism was governed by the Schmid law. The activation of specific twin variants depending on the strain path induced a significant difference in twinning characteristics, such as twin morphology, volume fraction of twins with strain, and twin texture, and consequently gave rise to a totally different effect on the deformation. The differences in the deformation characteristics (flow stress and strain hardening) between both strain paths are explained in relation to activation stresses for twinning and slips, activities of twinning and slips in the deformation, the Hall–Petch effect by twinning-induced grain size change, and twinning-induced change in activities of slips.     

低温轧制纯锆的形变孪晶及织构演变

通过在77K温度下进行不同变形量的低温轧制实验,研究了具有强烈单轴织构的工业纯锆板材在低温轧制变形条件下的孪生行为及变形机理。采用扫描电镜（SEM）和电子背散射衍衬（EBSD）分析和表征了变形材料的微观组织和织构。结果表明,在沿C轴加载的低应变条件下{1022}〈1123〉压缩孪生是主要的变形机制,同时在{1022}〈1123〉压缩孪晶中产生了二次孪晶（{10}-2}〈10T1〉拉伸孪晶）以协调变形。施密特因子计算及孪晶分布的EBSD模拟结果表明,在低温变形条件下的孪生模式的选择是由施密特因子的数值大小决定的。探讨并解释了轧制过程中随着应变量增加由孪生所导致的织构演变。     

First-Principles Studies on Twinnability of Magnesium Alloys: Effects of Yttrium and Lithium on $$\left( {10\bar{1}1} \right)\left[ {\bar{1}012} \right]$$ Compression Twinning Deformation Processes

We investigate the energetics involved in the \(\left( {10\bar{1}2} \right)\left[ {\bar{1}011} \right]\) tension and \(\left( {10\bar{1}1} \right)\left[ {\bar{1}012} \right]\) compression twinning deformation processes in magnesium via first-principles studies. Through identification of structural changes associated with each deformation process, we study the energetics of each deformation process and the local instability in the twin boundary region. We observe that the energy barrier in the \(\left( {10\bar{1}1} \right)\left[ {\bar{1}012} \right]\) compression twinning deformation pathway is higher than that in the \(\left( {10\bar{1}2} \right)\left[ {\bar{1}011} \right]\) tension twinning deformation pathway, even though the \(\left( {10\bar{1}1} \right)\) compression twin boundary is more stable than the \(\left( {10\bar{1}2} \right)\) tension twin boundary. We extend our study to examine the effects of Y and Li as alloying elements on each twinning deformation process. Our calculations predict that the addition of Y causes a reduction in the probability of fracture by an order of magnitude when the twinning deformation occurs and weakening of the resistivity to twinning deformation. However, the effect of Li addition on the twinning deformations is weaker than that of Y addition.     

Atomistic simulation of tension deformation behavior in magnesium single crystal

The deformation behavior in magnesium single crystal under c-axis tension is investigated in a temperature range between 250 K and 570 K by molecular dynamics simulations. At a low temperature, twinning and shear bands are found to be the main deformation mechanisms. In particular, the {1012} tension twins with the reorientation angle of about 90° are observed in the simulations. The mechanisms of {1012} twinning are illustrated by the simulated motion of atoms. Moreover, grain nucleation and growth are found to be accompanied with the {1012} twinning. At temperatures above 450 K, the twin frequency decreases with increasing temperature. The {1012} extension twin almost disappears at the temperature of 570 K. The non-basal slip plays an important role on the tensile deformation in magnesium single crystal at high temperatures.     

铸造Mg-Zn-Cu-Zr合金的生长孪晶行为(英文)

在铸造Mg-Zn-Cu-Zr合金100°C时效过程中观察到生长孪晶,采用光学显微镜和透射电子显微镜表征生长孪晶的形貌和位向。结果表明:在铸态和固溶态的Mg-Zn-Cu-Zr合金中未发现生长孪晶,仅在时效态合金中出现{10ī2}型生长孪晶。Zn含量和热处理工艺显著影响其孪生行为。最后,从空位角度讨论此类生长孪晶可能的生长机制。     

Effects of tensile twinning on the stretch formability of Mg

This study examined the influences of twinning during the Erichsen test. {10-12} tensile twinning plays the critical role in Mg alloys. Alloy elements and grain size are important factors that determine the formation of {10-12} tensile twins at room temperature. Mg-6.0Zn(Z6) and Mg-6.0Zn-0.3Ca(ZX60) alloys were fabricated and their grain size was varied under different annealing conditions. Tensile twinning is promoted by the addition of Ca, as assessed from measurements of the microstructure and the viscoplastic self-consistent calculations. The coarse-grain Ca-containing alloy showed the largest amount of tensile twinning. However, the stretch formability increased with grain size up to a certain point and then decreased. It can be inferred that microstructures with large grains activated the tensile twin, which became origins of cracks. The results of the small Erichsen test showed that tensile twinning contributes to high stretch formability releasing the stress concentration in the grain boundaries but the interaction between twin and slip causes cracks as the grain size increases.     

Mechanical and Corrosion Behavior of a Biomedical Mg-6Zn-0.5Zr Alloy Containing a Large Number of Twins

The strong texture of Mg alloys can lead to strong tension–compression yield asymmetry and corrosion anisotropy, and this will consequently affect the effectiveness of hard tissue implants. A biomedical Mg–6Zn–0.5Zr alloy containing a large number of {10$\overline{1}$2} primary twins and {10$\overline{1}$2}–{10$\overline{1}$2} secondary twins is successfully prepared by cross compression. The dual twin structure not only removes the tension–compression yield asymmetry completely, but effectively reduces the corrosion anisotropy without compromise of corrosion resistance. The difference between the largest corrosion rate and smallest one is ~ 1.2 times compared to ~ 1.6 times of the original materials. It is found that the reduced corrosion anisotropy is related to re-distribution of crystallographic orientations by twins.     

Theoretical Calculation of Schmid Factor and Its Application Under High Strain Rate Deformation in Magnesium AlloysEI北大核心CSCD

As an important parameter, the Schmid factor has been widely applied to analyze the deformation modes in metals. In order to analyze the deformation mechanisms of magnesium alloys under high strain rate, the Schmid factors of four slip modes (basal, prismatic, pyramidaland pyramidal slips) and two twinning systems ({10 (1) over bar2} tension and {10 (1) over bar1} contraction twinnings) were systematically calculated in this work. The experimental values of Schmid factor of as-received AZ31 rolling magnesium alloy sheets were obtained by electron backscatter diffraction (EBSD) technique, and then the theoretical calculated values were compared with those values. The high strain rate compression test of AZ31 rolling magnesium sheets was conducted by using split Hopkinson pressure bar at the strain rate of 1600 s(-1), and the microstructures after compression were observed by optical microscopy. The Schmid factors and microstructures are combined to discuss the predominant deformation mechanisms for different orientation samples under different loading directions. The results showed that the theoretical calculated values of Schmid factors are in good agreement with their experimental values. Therefore, the Schmid factor, owing to its simplicity and convenience, could be used to analyze the predominant deformation mechanism and interpret the unique characteristics of "true stress-true strain" curves in magnesium alloys. Furthermore, since the Schmid factor and its variation trend associated with deformation behavior in magnesium alloys are related, the calculation result of Schmid factor can provide a theoretical analytic approach to understand anisotropic phenomena caused by strong texture in magnesium alloys.