应力和温度耦合作用下Mg-Dy-Zn合金中LPSO相的析出行为和增强作用

Under the coupling of stress and temperature fields, the atomic diffusion rate, diffusion direction and volume fraction occur to change in solid solution/super solid solution, which leads the corresponding change in the volume fraction, scale, distribution and orientation relationship of the precipitate with Mg matrix. A new phenomena, law and property will be also found in the above two conditions but not do in the single one. In this study, the influnces of coupling of stress and temperature fields on the precipitation behavior and strengthening role of long period stacking ordered (LPSO) phase in the Mg-Dy-Zn alloy were investigated greatly. Composition, structure and orientation relationship of the LPSO phas with Mg matrix were characterized by using X-ray Diffraction, Scanning Electric Microscope and Transmission Electron Microscope to reveal the formation mechanism of the phase under coupling of the stress and temperature fields. Furthermore, Macro/micro hardness of the alloy and LPSO phase was measured by using Vickers hardness tester and Nanoindentation instrument to investigate the precipitation behavior of the LPSO phase. Tensile properties were tested by using INSTRON machine tester to study the deformation behavior of the alloy and clarify greatly the strengthening role of the LPSO phase at different strain rates and temperatures. Finally, the three-dimensional pattern of Stress-Temperature-Mechanical Property would be drawn.

应力和温度的耦合作用，可改变原子在固溶体/过饱和固溶体中的扩散速度、方向和数量，从而有效的控制析出相的数量、尺寸、分布及与基体的位相关系，获得单一作用下难以出现的新现象、新规律和新性能。本项目拟深入分析应力和温度的耦合作用对Mg-Dy-Zn合金中长周期有序相(LPSO)的析出行为和增强作用的影响。采用X射线、扫描和透射电镜表征LPSO相的组成、结构及与镁基体的位相关系，揭示LPSO相在应力和温度耦合作用下的形成机理，进而采用维氏硬度计及纳米压痕仪测定合金及LPSO相的宏观/微观硬度，研究LPSO相的析出行为，并利用INSTRON 拉伸机测试合金在不同温度和应变速率下的拉伸性能，研究合金的变形行为，深入分析LPSO相的增强作用，最终绘制出应力-温度-性能的三维谱图。

镁合金低的高温性能限制了其发展和应用。析出强化是提高镁合金高温性能的有效途径之一。析出相的性质（结构、尺寸、分布和形貌）决定了合金的性能。因此，本项目利用热处理优化合金中析出相的性质进而提高合金的耐热性能，并探讨析出相的性质对合金力学性能及变形行为的影响。本课题采用应力-温度-时间耦合时效优化合金中LPSO相的性质，研究了合金在铸态、热处理态和挤压态中LPSO相的性质及对合金性能的影响，并揭示了LPSO相及合金的变形机理。研究结果表明，Zn含量为0.5at%，粗大片层状的18R-LPSO相分布在Mg-2Dy合金的枝晶间界。高温热处理(525 oC，8 h)后，18R-LPSO相转变为晶内细小条形的14H-LPSO相。时效过程中压缩应力的加载有效的促进了LPSO相和堆垛成错的析出，提高了LPSO相的体积分数，在保证时效峰值硬度的同时，显著的缩短了时效峰值产生的时间，提高了合金的时效硬化效率。LPSO相的性质也影响了合金的变形行为。挤压后合金中LPSO相发生了细化和弯曲。变形过程中沿挤压方向分布18R-LPSO相形成Kink带，阻碍合金中位错的滑移，提高了合金的力学性能。同时沿挤压方向的LPSO相和大部分晶粒均处在硬取向，提高了激发晶内滑移系开启所需的临界剪切应力，因此合金在挤压方向显示了最高的压缩屈服强度和最大压缩强度；对于晶内分布的14H-LPSO相也发生了少量的弯曲，其主要抑制了晶内的位错滑移，其阻碍了合金在高温下的晶粒长大，促使合金在300 oC，应变速率为3×10-5s-1时显示了超塑性变形。在低温(RT~200 oC)和300 oC的高应变速率下，合金的变形主要是位错滑移为主；在300 oC的低应速率下，合金的变形为位错滑移协调的晶界滑移。