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1、 沉淀相与孪晶强化镁合金塑性变形行为 及各向异性研究 重庆大学博士学位论文 学生姓名：宋 波 指导教师：辛仁龙 教 授 专 业：材料科学与工程 学科门类：工 学 重庆大学材料科学与工程学院 二 O 一三年五月 Study on the Plastic Deformation Behavior and Anisotropy of Precipitation and Twinning Strengthening Magnesium alloys A Thesis Submitted to Chongqing University in Partial Fulfillment of the Requ
2、irement for the Doctors Degree of Engineering By Song Bo Supervised by Prof. Xin Renlong Specialty: Materials Science and Engineering College of Material Science and Engineering of Chongqing University, Chongqing, China May 2013 中文摘要 I 摘 要 本工作以研究析出强化镁合金的塑性变形行为以及开发低成本高强度镁合金的加工方法为主要目的。以 Mg-Y-Nd 合金为高强镁
3、合金原材料，通过挤压和轧制工艺获得了变形态 Mg-Y-Nd 合金，研究了其塑性变形行为；以小应变的冷变形引入孪晶作为手段，研究预置孪晶对镁合金强韧化的影响以及孪晶化镁合金的塑性变形机理。通过电子背散射衍射（EBSD）技术、扫描电镜（SEM） 、X 射线衍射技术（XRD）和透射电子显微技术（TEM）等考察热处理及变形过程中镁合金的组织演变。通过 EBSD 技术分析变形过程中的微观组织演变及孪生行为。通过室温拉伸和压缩试验考察沉淀相及预置孪晶对镁合金塑性变形行为及各向异性的影响。 通过挤压和轧制工艺获得了具有初始织构的 Mg-Y-Nd 合金。考察了织构和析出相对 Mg-Y-Nd 合金力学性能的影响
4、。研究表明：1）具有弱（0001）丝织构的挤压态 Mg-Y-Nd 表现出明显的压缩各向异性。强化相 的析出提高了强度并减弱了屈服各向异性。时效后，CD/ED 和 CDED 样品（CD：压缩方向，ED：挤压方向）间屈服强度的差异从 34MPa 降低到 7MPa；2）挤压轧制态 Mg-Y-Nd 合金没有表现出拉伸压缩不对称性。其原因是由于弱的织构以及稀土元素的固溶限制了拉伸孪生对 Mg-Y-Nd 合金塑性变形的贡献。 沿特定方向的预应变可制备孪晶化的 AZ31 镁合金。 分析了孪晶化镁合金在压缩过程中的孪生变体选择规律及孪晶化镁合金的塑性变形行为。结果显示：1）拉伸孪晶的变体选择极大地依赖 Sch
5、mid 因子。沿 TD 预轧 1.5%后，基体中约 70%的晶粒中仅产生了一种孪晶变体。 沿 TD压缩与沿TD预轧具有相同的孪晶变体选择，故沿 TD 压缩主要表现为预置孪晶的长大、增殖和合并。而沿 RD 压缩往往会产生与预置孪晶不同的孪晶变体；2）预孪生会在组织中引入大量的孪晶界且引起织构的改变。因此，预置孪晶会影响 AZ31 镁合金的塑性变形行为和各向异性。孪晶化镁合金的变形行为主要有以下特征：沿 TD 轧制小的变形量后，大量的拉伸孪晶片层的出现细化了晶粒并提高了沿 RD 拉伸和压缩的屈服强度且降低了拉伸压缩不对称性；沿 TD 预轧后，沿 RD 和沿 TD 的压缩屈服强度都得到提高，且其硬化
6、机制分别为细晶强化（孪晶切割晶粒）和织构硬化（孪生使晶粒转到硬取向） ；沿 TD预轧后，孪晶织构的引入产生了较大的平面各向异性，然而可以极大地增强厚度变形能力。PRA2.6%板材的平均 r 值为 0.81。 结合沉淀强化和孪晶强化的共同作用来提高镁合金材料的性能。研究了时效和预变形对 ZK60 合金变形行为的影响。 结果显示： 1） 随着时效时间的增加， ZK60重庆大学博士学位论文 II 轧板的拉伸压缩不对称性表现出不单调的变化。时效初始阶段，大量基面盘状 2相的增加改善了屈服不对称性。然而，当时效时间增加到 30h 后，c 轴棒状 1相的出现略微地恶化了屈服不对称性。2）预变形产生的位错和
7、孪晶界分别可以提供位错强化和细晶强化，从而提高 ZK60 合金的强度。然而孪晶界强化具有更好的热稳定性且可以极大地降低拉伸压缩不对称性。因此，通过预变形引入孪晶界可以更好地改善 ZK60 合金的力学性能。 最终结合使用预孪生和时效可以更有效地提高ZK60 合金特定方向的强度。 关键字：关键字：镁合金，析出相，孪生，织构，强韧化 英文摘要 III ABSTRACT This work aims to investigate the plastic deformation behavior of precipitation hardenable magnesium alloys and devel
8、op the methods of improving their mechanical properties with low cost. In this thesis, Mg-Y-Nd alloy was chosen as the starting material of precipitation hardenable magnesium alloys. The wrought Mg-Y-Nd alloy was obtained by hot extrusion and hot rolling, and then their plastic deformation mechanism
9、 was studied. Moreover, the effect of pre-twinning on strengthening and toughening of magnesium alloys and the deformation mechanism of twined magnesium alloys were also studied. The microstructure and texture evolution during heat treatment and deformation have been characterized using electron bac
10、kscatter diffraction (EBSD), scanning electron microscope (SEM), x-ray diffraction (XRD) and transmission electron microscope (TEM). The EBSD data has also been applied to analyze in detail the microstructure evolution and twinning behavior. The effects of precipitates and pre-existing twins on plas
11、tic deformation and anisotropy behavior were investigated by tensile and compressive tests. The wrought Mg-Y-Nd alloys were developed by extrusion and rolling. The influence of initial texture and precipitates on plastic deformation behavior of wrought Mg-Y-Nd was investigated. Results showed that,
12、1) the extruded Mg-Y-Nd alloy with weak fiber texture exhibited significant compressive anisotropy. With peak-aging at 210for 59 h, precipitates were primarily formed, which effectively weakened the yield strength anisotropy. The difference of yield strength between the CD/ED and CDED specimens was
13、reduced from 34 MPa to 7 MPa. 2) extruded+hot rolled Mg-Y-Nd alloy exhibited little yield symmetry. Weak texture and the solid solution of rare earth elements limited the contribution of extension twinning to plastic deformation. Twined AZ31 magnesium alloy can be easily obtained by pre-straining al
14、ong appropriate directions. The extension twin variant selection during compression and plastic deformation behavior of twined magnesium alloy were systematically studied. Results showed that, 1) extension twin variant selection greatly depends on Schmid factor. After 1.5% pre-rolling along TD, appr
15、ox. 70% of the grains only contain one twining variant. The compression along TD favored the identical twin variants selection with pre-rolling along TD. Thus it causes the propagation, growth or mergence of pre-existed twins. However the compression along RD usually generates new extension 重庆大学博士学位
16、论文 IV twin variants which differ from pre-existing extension twins. 2) pre-twinning can introduce a large number of twin boundaries and induce the change of texture, thus influences the plastic deformation behavior and anisotropy. The deformation behavior of twined magnesium alloy are as follows: Pre-rolling along TD at a small thickness reduction significantly enhanced the tensile and compressive yield strengths and reduced the level of tension-compression asymmetr
