镁合金中形变孪晶间的应变协调效应及其变体选择机制

发布时间：2018-05-27 23:24

本文选题：镁合金 + 孪生　；参考：《重庆大学》2016年博士论文

【摘要】：本文以研究镁合金中形变孪晶间的应变协调效应及其变体选择机制为主要目的。以商业AZ31热轧板为初始材料,通过压缩、拉伸、轧制等多种变形方式对其进行一定量的塑性变形。变形组织中观察到晶内多孪晶、孪晶对、孪晶链、长孪晶带等多种孪晶图案以及不同类型的二次孪晶。重点利用(原位)电子背散射衍射(EBSD)技术表征了孪晶图案的形貌和取向关系,结合(广义)Schmid定律和局部应变协调效应(m￠判据)研究了孪晶发生与变体选择机制。迹线分析是取向旋转法鉴定孪晶变体的重要补充,但对观察面依赖。研究发现,AZ31板压缩中Schmid factor(SF)最大相对变体的迹线角在平行c轴和压缩方向的观察面上分布在高角度(79-86°),利于从中区分实际变体,因此镁合金压缩中迹线分析变体的最佳观察面总平行c轴和压缩方向。轧制中孪晶活度对晶粒取向高度依赖,通过分析轧制中的侧向应力,发现文献中应用于孪晶的修正Schmid判据对应相同的应力状态,具有一致性。考虑侧向应力的广义Schmid定律不仅能解释孪晶的变体选择,而且能预测孪晶活度随织构的变化规律。AZ31板沿横向(TD)压缩中出现了多种{101_2}拉伸孪晶图案,如晶内多孪晶、晶间孪晶对和孪晶链。沿接近晶粒101_0压缩时,晶内六个孪晶变体中一对相对变体的SF非常大,因此这两个变体都容易激活。当接近112_0压缩时,晶内四个变体的SF比较大且接近,此时相邻变体因为应变相容性最好(m￠最高)而经常激活。晶界处的孪晶传递形成孪晶对或孪晶链,孪晶对通常有较高的m￠和SF,其中被诱导孪晶也可能有较低的SF。统计结果显示,孪晶间的应变协调效应在孪晶对中普遍存在,可以激活一定量的non-Schmid孪晶;高m￠不是Schmid定律主导变体选择的结果。细晶强织构轧制中孪晶传递能力进一步增强,形成了长孪晶带。长孪晶带中的孪晶都有较高的SF且相连孪晶间有高m￠(通常0.8),说明应变协调效应与Schmid定律协同决定孪晶变体选择且促进长孪晶带形成。原位EBSD获得的图像质量图显示,孪晶应变在拉伸过程中穿过晶界传递到相邻晶粒,证实了晶间孪晶传递现象和孪晶间的应变协调效应。另外,原位观察到孪晶独立在孪晶界和晶内成核等现象。AZ31板沿法向(ND)拉伸中超大晶粒的初级{101_2}孪晶交割处出现了大量{101_2}-{101_2}二次孪晶。统计发现,(交割孪晶-二次孪晶)晶界取向关系择优分布在49°9_094,这与初级和二次孪晶变体选择有关。相互交割的初级孪晶通常来自晶内应变相容性最好的相邻变体;二次孪晶经常在与其母体孪生面相邻的两个孪生系统上激活,其中与交割孪晶应变不相容性较低的孪生系统激活频率更高。{101_1}-{101_2}二次孪晶变体与初始母体有四种不同的取向关系,对应四个二次变体,它们的择优出现依赖具体的变形方式。单独利用Schmid定律或局部应变协调效应(m￠判据)都无法解释这种依赖性。本文基于初级和二次孪晶变体间的应变相容性和Schmid定律提出一种复合Schmid因子mc,能够统一解释三种变形方式下二次孪晶变体的激活规律及择优差异。
[Abstract]:In this paper, the main purpose of this study is to study the strain coordination effect and the selection mechanism of the deformation twins in the magnesium alloy. The commercial AZ31 hot rolled plate is used as the initial material, and a certain amount of plastic deformation is carried out by compression, tensile, rolling and other deformation modes. The intragranular twins, twin pairs, twin chains and long twin bands are observed in the deformation structure. With a variety of twin patterns and different types of two twins, the relationship between the morphology and orientation of the twin patterns is characterized by (in situ) electron backscatter diffraction (EBSD), and the mechanism of twin generation and variation selection is studied with the (generalized) Schmid law and the local strain coordination effect (M criterion). The trace analysis is the identification of the orientation rotation method. It is an important supplement for the twins, but it is dependent on the observation surface. It is found that the trace angle of the largest relative variation of the Schmid factor (SF) in the AZ31 plate compression is distributed at the high angle (79-86 degrees) in the parallel C axis and the compression direction, which is beneficial to distinguish the actual variant from it, so the best observation surface of the optimal observation surface of the trace analysis variant in the compression of the magnesium alloy is the total parallel C. The twinning activity in rolling is highly dependent on the grain orientation in rolling. By analyzing the lateral stress in the rolling, it is found that the revised Schmid criterion applied to twins in the literature is consistent with the same stress state. The generalized Schmid law considering the lateral stress can not only explain the selection of the twins but also predict the twins. A variety of {101_2} tensile twin patterns, such as intragranular twins, intercrystalline twins, and twin chains, are found in.AZ31 plates along the transverse (TD) compression. The SF of a pair of relative variants in the six twin variants in the crystal is very large, as these two variants are easily activated when they are close to 112_0 compression. The SF of the four variations in the crystal is large and close, and the adjacent variants are often activated because of the best strain compatibility (the highest m). The twins at the grain boundary form twin pairs or twin chains, and the twins are usually higher in M and SF, and the induced twins may also have lower SF. statistics showing the strain coordination effect between twins. It is common in twin pairs that a certain amount of non-Schmid twins can be activated; high m is not the result of the selection of the leading variant of Schmid's law. The twin transfer ability of the fine crystal strong textured rolling is further enhanced and the long twin crystal band is formed. The twins in the long twinning bands have high SF and high m (usually) between the twins (usually 0.8), indicating the strain covariance. The modulation effect and Schmid's law coordinate the selection of twins and promote the formation of the long twinning bands. The image quality map obtained by in situ EBSD shows that the twin strain passes through the grain boundary to the adjacent grain during the stretching process, which confirms the phenomenon of intergranular twins transfer and the strain coordination between twins. In addition, the twins are observed in situ that twins are independent in twins. A large number of {101_2}-{101_2} two twins appeared at the primary {101_2} twinning at the primary {101_2} twin in the normal direction (ND) stretching of the boundary and the intragranular nucleation. It was found that the grain boundary orientation of the two twin crystals (the intertwinning twins) was preferentially distributed at 49 degree 9_094, which was related to the selection of the primary and two twinning variants. The two twinning is often activated in two twin systems adjacent to the twin surface of the mother, and the twin system with the lower incompatibility of the twinning strain is higher in the activation frequency of the twin system, and there are four different orientation relationships between the.{101_1}-{101_2} two twin and the initial parent. Four two variations, their selection depends on the specific deformation mode. Using the Schmid's law or the local strain coordination effect (M criterion) can not explain this dependence. Based on the strain compatibility between the primary and two twin variants and the Schmid law, a compound Schmid factor MC is proposed, which can explain the three variations in a unified way. The activation pattern and the preferred difference of the two twinning variants under the shape.
【学位授予单位】：重庆大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TG146.22

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