形变热处理和扭转变形对镁合金组织性能的影响

发布时间：2018-04-12 20:38

本文选题：镁合金 + 形变热处理　；参考：《重庆大学》2016年博士论文

【摘要】：镁合金作为最轻的金属结构材料,具有比强度高、比刚度高且易于回收利用等优点,在汽车、航空和电子产品领域得到广泛应用。但是,镁合金具有密排六方晶体结构,具有的独立滑移系较少,室温变形能力较差。而且,镁合金在塑性变形过程中极易产生孪生变形,导致屈服强度低和拉压屈服不对称等。这些缺点都严重限制了镁合金的广泛使用。目前,镁合金的主要研究方向有:制备高强度镁合金、改善拉压屈服不对称性和提高室温塑性变形能力等方面。本课题选用常用的AZ31和ZK60镁合金作为研究对象。本文的一个主要研究内容就是研究形变热处理工艺对镁合金的微观组织和力学性能的影响;另一个主要研究内容就是研究扭转变形对镁合金的微观组织和力学性能的影响。通过金相显微分析(OM)、X射线衍射分析(XRD)、电子背散射衍射技术(EBSD)、扫描电镜(SEM)、透射电镜(TEM)等对试样微观组织进行表征,对相关机理进行深入分析,取得的主要结论如下:(1)低温退火处理(170℃)能够提高预孪生化AZ31镁合金试样的压缩屈服强度。由于溶质原子在孪晶界偏聚,阻碍孪晶界扩展,增加了孪生长大的激活应力,从而导致预孪生化试样的压缩屈服强度增高。(2)时效处理(180℃)可以有效地降低预孪生化ZK60镁合金试样的拉压屈服不对称性。在预孪生化ZK60镁合金的{1012}拉伸孪晶内观察到三种形貌的析出相:(0001)_(_(twin))板状、[11-20]_(twin)条状和[0001]_(twin)棒状,分别由Mg4Zn7相或Mg Zn2相组成。由于在预孪生化ZK60镁合金的基体和孪晶内具有不同的析出行为,而且溶质原子在孪晶界偏聚,阻碍孪晶界扩展,导致时效处理试样的后续拉伸(退孪生变形机制)和压缩(孪晶长大机制)屈服强度增量不同。(3)扭转变形可以有效地降低挤压态AZ31镁合金试样的拉压屈服不对称性。试样的压缩屈服强度随着扭转应变量的增大而升高;但拉伸屈服强度基本不受扭转应变量的影响。当试样经受的切应变量较低时(γ0.35),其塑性变形主要以位错滑移为主;当试样经受的切应变量较高时(γ0.52),其塑性变形除了位错滑移之外,还会产生大量的{1012}拉伸孪晶。(4)扭转变形速度对挤压态AZ31镁合金试样的流变曲线具有重要影响。扭转变形速度越快,则流变曲线就越高。扭转变形使试样横截面产生梯度组织,而且能够提高试样的硬度。扭转变形产生的梯度应变是试样横截面产生梯度组织的主要原因。位错密度、硬度和孪晶面积分数沿试样横截面的半径方向逐渐增加。硬度的提高主要归因于位错强化机制,而织构强化和细晶强化的作用微乎其微。(5)热扭转变形也会使挤压态AZ31镁合金试样在横截面上产生梯度组织。在横截面边缘处产生的动态再结晶晶粒多,而心部产生的动态再结晶晶粒少。随着变形温度升高和切应变量增大,动态再结晶晶粒的面积分数也逐渐增加。热扭转变形试样的压缩屈服强度增高主要是由细晶强化(动态再结晶)引起的。
[Abstract]:Magnesium alloy is the lightest metal structural material, has high strength, high stiffness and advantages, easy recycling in the automotive, aerospace and electronic products are widely used in the field. However, magnesium alloy has close packed crystal structure with six party, the independent slip systems at room temperature is less, poor deformation capacity. Moreover, magnesium alloy is easy to produce deformation twinning during plastic deformation, resulting in low yield strength and tension compression asymmetry. These disadvantages seriously limit the widespread use of magnesium alloy. At present, the main research direction of magnesium alloys: preparation of high strength magnesium alloy, improve the tension compression asymmetry and improve ductility at room temperature deformation capacity and so on. This paper selected the commonly used AZ31 and ZK60 magnesium alloy as the research object in this paper. One of the main research content is the study of heat treatment process on Microstructure and mechanical properties of magnesium alloys. Ring; another main research content is to study the influence of torsion deformation on the microstructure and mechanical properties of magnesium alloy. By metallographic micro analysis (OM), X ray diffraction (XRD), electron backscatter diffraction (EBSD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) were used to characterize the microstructure of the specimen, in-depth analysis of relevant mechanism, the main conclusions are as follows: (1) annealing temperature (170 DEG C) can improve the biochemical twin of AZ31 magnesium alloy with compression yield strength. Due to segregation of solute atoms in the twin boundary, hinder twin boundary expansion, increase the twin growth activation of stress thus, the pre twin biochemical samples compressive yield strength increased. (2) (180 C) aging treatment can effectively reduce the twin biochemical ZK60 magnesium alloy pre tension compression asymmetry. In the pre twin biochemical ZK60 magnesium alloy {1012} tension twin gold was observed within three The morphology of precipitated phase: (0001) _ (_ (twin)) plate, [11-20]_ (twin) and [0001]_ (twin) strip rod, respectively by Mg4Zn7 or Mg Zn2 phases. Due to different precipitation behavior in pre twin biochemical ZK60 magnesium alloy matrix and twin, and segregation of solute atoms in the twin boundary, hinder twin boundary expansion, leading to subsequent tensile aging treatment samples (back twinning mechanism) and compression (twin growth mechanism) yield strength increment is different. (3) can effectively reduce the torsional deformation of extruded AZ31 magnesium alloy specimen tension compression asymmetry. The compression yield strength. With the increase of torsional strain; but the tensile yield strength did not reverse the effects of strain. When the specimen is subjected to shear strain is low (gamma 0.35), the plastic deformation is dominated by dislocation slip; when the specimen is subjected to high shear strain (gamma 0.52), the plastic deformation In addition to form dislocation slip, will produce a large number of {1012} twinning. (4) the torsional deformation speed has an important influence on the rheological curves of extruded AZ31 magnesium alloy. The torsional deformation speed more quickly, while the rheological curve is higher. The torsional deformation of cross section of the specimen has the gradient structure, but also can improve the hardness of the sample. Torsional strain gradient deformation is the main reason for the cross section of the specimen to produce gradient tissue. The dislocation density along the radius direction of cross section of the specimen hardness and twinning area fraction increases gradually. The increase in hardness is mainly attributed to the dislocation strengthening mechanism, and strengthening the role of very little texture and fine grain strengthening. (5) hot torsion deformation the extruded AZ31 magnesium alloy samples have the gradient structure in cross section. In the cross section at the edge of the dynamic recrystallization grain, and the core of dynamic recrystallization grain with less. With the increase of deformation temperature and shear stress, the area fraction of dynamic recrystallized grain also increases. The increase of compressive yield strength is mainly caused by fine-grained strengthening (dynamic recrystallization).

【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;TG166.4

【参考文献】