热处理对GWZK94合金微观组织与力学性能的影响

Influence of Heat Treatment on Microstructure Evolution and Mechanical Properties of GWZK94 Alloys

为了改善铸态GWZK94合金微观组织的不均匀性,使用电阻加热炉在温度505~520℃范围内保温8~20 h进行热处理实验。采用光学显微镜(OM),差示扫描量热仪(DSC),X射线衍射仪(XRD),扫描电子显微镜(SEM),能谱仪(EDS),电子背散射衍射技术(EBSD),万能试验机和维氏硬度计进行微观组织演变及力学性能分析。结果表明,铸态合金组织主要包括树枝状α-Mg基体,含亚稳态层错(SFs)的片层结构,共晶相Mg24(Gd,Y,Zn)5,块状长周期有序堆垛结构(LPSO,Mg12(Gd,Y)Zn)和少量的富稀土相。在均匀化处理过程中,片层结构和共晶相Mg24(Gd,Y,Zn)5逐渐溶于基体中,同时块状LPSO相体积分数逐渐减小并伴随有片层状LPSO相向晶粒内部生长,颗粒状相在晶界附近析出。加热温度为520℃时出现复熔三角晶界,说明此时发生了合金的过烧现象。经过均匀化处理后,合金的极限抗拉强度(UTS)和拉伸屈服强度(TYS)表现出了与组织演变规律相同的变化趋势,同时得到了较为均匀的硬度分布。最佳的均匀化制度为515℃/16 h。

In order to eliminate the inhomogeneous structure of as-cast GWZK94 alloys, the homogenization was conducted by resistance heating furnace at 505~520℃ for 8~20 h. Optical microscope(OM), differential scanning calorimeter(DSC), X-ray diffraction(XRD), scanning electron microscope(SEM), energy dispersive spectroscopy(EDS), electron backscatter diffraction(EBSD), universal mechanical tester, and Vickers hardness tester were used to investigate the microstructure evolution and mechanical properties. The results show that the as-cast alloy mainly consists of dendritic α-Mg matrix, lamellae with metastable stacking faults(SFs), eutectic phase Mg24(Gd, Y, Zn)5, block-shaped long-period stacking ordered(LPSO) phases Mg12(Gd, Y)Zn, and a few RE-rich phases. During the homogenization, the lamellae and Mg24(Gd, Y, Zn)5 eutectic phases gradually dissolve into the matrix, and the volume fraction of block-shaped LPSO phases decreases while the lamellar-shaped LPSO phases grow into grains steadily, and some precipitated particles form near the grain boundaries. At 520 ℃, triangle-shaped remelted eutectic phases appear, which indicate the over burning of magnesium alloy. Ultimate tensile strength(UTS), tensile yield strength(YS) and fracture elongation show a tendency corresponding to microstructure evolution, and more uniform hardness is obtained. The homogenization condition is optimized to 515℃/16 h.