Mg-Zn-Gd合金中纳米准晶增强相的析出调控及其变形机理

In previous studies about Mg-Zn-Gd-based alloys reinforced with icosahedral quasicrystals (I-phase), to prevent over-heated and over-melted, the homogenized heat treatment temperature before extrusion is about 400℃. Dendritic I-phase remain in the Mg matrix after homogenization and then broken into microscale particles during thermal-mechanical processing. The volume fraction of nanoscale I-phase precipitation is limited because these microscale I-phase particles occurrence reduces the contents of alloying elements in Mg matrix solid-solution. In addition, many researchers have studied deformation behavior of Mg alloys strengthened by I-phase, and most of these studies are focused on constitutive equation establishment and secondary phase precipitation. Although these studies are useful and important for understanding deformation behaviors of Mg alloys reinforced with I-phase, few studies have been done on the deformation and fracture behavior of I-phase themselves. Therefore, increasing the heat treatment temperature was proposed in the project by using the way of multi-stage solid solution, and then control the precipitation of nanoscale I-phase by using the method of statical aging or thermal-mechanical processing. Finally, Mg-Zn-Gd alloy with large volume fraction of nanoscale I-phase and high properties was obtained. Baseing on the developing of high performance Mg-2.4Zn-0.4Gd（at.%）alloy reinforced with large volume fraction of nanoscale I-phase, we focus on the deformation and crack behavior of nanoscale I-phase in the alloy during deformation at room temperature. The results will help to provide theoretical and technical supports for revealing the strengthening and toughening mechanisms of magnesium alloys reinforced with I-phase, which is meaningful for accelerating the research and application of deformation magnesium alloys with high strength and toughness.

现有研究中Mg-Zn-Gd基准晶增强镁合金热塑性变形前均匀化热处理的温度较低，热处理后合金中粗大的准晶相依旧存在，挤压后破碎成微米尺度的准晶颗粒，微米级准晶的存在使得镁固溶体中合金元素含量较低而导致析出的纳米准晶分数较低。此外，在揭示合金的变形行为时多研究本构方程、准晶析出等问题，然而对这些纳米准晶增强相自身的变形及开裂行为及其对Mg-Zn-Gd基准晶增强镁合金的性能的影响未做系统深入研究。针对上述问题，本项目提出采用多级固溶热处理新工艺逐步递进式实现铸态合金中的粗大第二相完全固溶，再采用时效和轧制变形两种方式来调控纳米准晶的析出，据此得到大体积分数纳米准晶增强Mg-2.4Zn-0.4Gd变形镁合金。在此基础上，再拟集中研究合金中纳米准晶相在合金室温变形过程中的变形和开裂行为，为揭示纳米准晶增强变形镁合金的强韧化机制提供理论支持，对推动我国新一代高强韧变形镁合金的研究和应用具有重要意义。

现有研究中Mg-Zn-Gd基准晶增强镁合金热塑性变形前均匀化热处理的温度较低，热处理后合金中粗大的准晶相依旧存在，挤压后破碎成微米尺度的准晶颗粒，微米级准晶的存在使得镁固溶体中合金元素含量较低而导致析出的纳米准晶分数较低。此外，在揭示合金的变形行为时多研究本构方程、准晶析出等问题，然而对这些纳米准晶增强相自身的变形及开裂行为及其对Mg-Zn-Gd基准晶增强镁合金的性能的影响未做系统深入研究。针对上述问题，本项目提出采用多级固溶热处理新工艺逐步递进式实现铸态合金中的粗大第二相完全固溶，再采用时效和轧制变形两种方式来调控纳米准晶的析出，据此得到大体积分数纳米准晶增强Mg-2.4Zn-0.4Gd变形镁合金。在此基础上，再拟集中研究合金中纳米准晶相在合金室温变形过程中的变形和开裂行为。在本项目的资助下经过三年的研究，得到了如下重要结果：（1）获得了固溶热处理对合金组织演变的影响，确定了最佳的二级固溶处理工艺460℃×8h+490℃×4h；获得了高性能准晶增强变形镁合金样品，在100℃挤压温度下进行了Mg-1.92Zn-0.08Gd （at%）和Mg-2.70Zn-0.47Gd（at%）合金的挤压，挤压后合金的屈服强度分别为410MPa和417MPa，延伸率分别为14.6%和13.1%，完全满足所要达到的考核指标要求的屈服强度≥400MPa，延伸率≥10%。（2）研究中观察到铸态合金中的大尺寸准晶析出相与镁基体之间的一种位向关系，即[11-20]Mg∥[2-fold]I-phase，(0001)Mg∥(5-fold)I-phase。通过TEM观察变形后合金的准晶相发现大尺寸的准晶相主要沿着2-fold原子面脆性开裂的方式及开裂行为，也发现了纳米准晶析出相中位错的形成，以及纳米准晶相与基体的取向关系、界面原子匹配关系等。研究结果还表明纳米准晶相析出后，长大到一定尺寸时可以激发PSN效应，弱化合金变形后的织构；综合上述各方面的研究结果初步揭示了纳米准晶的强韧化机制这一科学问题。