热处理对真空压铸NZ30K镁合金微观组织及力学性能的影响

使用光学显微镜(OM)、扫描电镜(SEM)、能谱分析(EDS)、硬度测试和拉伸性能测试等方法，研究了热处理对真空压铸NZ30K镁合金微观组织及力学性能的影响。结果表明：铸态合金的宏观组织分为表层区和心部区，表层区组织由细小α-Mg等轴晶和分布在晶界的Mg₁₂Nd组成，心部区组织则由细小α-Mg等轴晶、粗大预结晶组织(ESCs)和分布在晶界的离异共晶Mg₁₂Nd组成。在固溶处理过程中心部区晶粒的长大比表层区更为显著，晶界迁移速率与晶粒尺寸不均匀呈正相关性，满足晶粒长大模型v=M₀ exp (-Q/RT) A (1/D₁-1/D₂)。合金的优化热处理工艺为540℃×6 h+200℃×8 h。与铸态合金(UTS=186.0±1.5 MPa，YS=131±2.5 MPa，EL=6.6±0.4%)相比，峰值时效态合金的抗拉强度和屈服强度分别提高到了223.6±4.1 MPa和172.8±2.9 MPa，但延伸率降低到了4.2±0.3%。其强度的提高主要得益于时效析出的片状纳米β"相能够有效地阻碍位错在基面上的滑移。铸态和热处理态合金的表层区断裂模式均为韧性断裂，而心部区的断裂模式在铸态下为准解理断裂、在固溶态下为解理断裂、在峰值时效态下为准解理断裂。

Effect of Heat Treatment on the Microstructure and Mechanical Property of Vacuum Die-casting NZ30K Mg-alloy

Effect of heat treatment on the microstructure and mechanical property of vacuum die-casting (VDC) NZ30K Mg-alloy were systematically investigated by means of optical microscope (OM), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), hardness test and tensile test. The results show that the as-cast alloy is composed of a surface zone and a central region. Fine α-Mg matrix and Mg₁₂Nd eutectic compounds were observed in the surface zone and the central region, besides, coarser externally solidified crystals (ESCs) existed in the central region. During solution treatment the grain growth of the central region was more significant than that of the surface zone, which can be explained by the grain growth model of unhomogenized structure, i.e.v=M₀ exp (-Q/RT) A (1/D₁-1/D₂). The optimized heat treatment of the alloy was 540^oC×6 h+200^oC×8 h. Compared with the as-cast alloy, the ultimate tensile strength and yield strength of the peak-aged alloy enhanced from 186.0±1.5 MPa to 223.6±4.1 MPa and from 131±2.5 MPa to 172.8±2.9 MPa respectively, with a decreased elongation (from 6.6±0.4 % to 4.2±0.3%). The strength enhancement may be mainly attributed to the plate-shaped β" precipitates, which could block the dislocation motion effectively. The fractography of surface zone exhibited ductile fracture pattern at different states. However, the fractography of central region showed quasi-cleavage, cleavage and quasi-cleavage fracture patterns for the as-cast, as-solutioned and peak-aged alloys, respectively.