热处理对铸态Mg-4.2Zn-1.5RE-0.7Zr镁合金显微组织和力学性能的影响

摘要

研究热处理工艺对铸态Mg-4.2Zn-1.5RE-0.7Zr镁合金显微组织和力学性能的影响。结果表明：铸态Mg-4.2Zn-1.5RE-0.7Zr镁合金的显微组织主要由α-Mg、T相和Mg51Zn20相组成；单级等温时效(325°C，10 h)以及双级时效(325°C，4 h)+(175°C，14 h)处理均未能使T相和Mg51Zn20相溶入基体，且晶粒也未明显长大。在325°C下时效10 h，晶内析出大量短杆状β′1相，延长时效时间将导致β′1相粗化及数量减少。Mg-4.2Zn-1.5RE-0.7Zr镁合金在325°C下时效10 h后具有最高的屈服强度(153.9 MPa)和抗拉强度(247.0 MPa)，相比铸态合金分别增加48 MPa和23 MPa，伸长率降低至15.6%。Mg-4.2Zn-1.5RE-0.7Zr合金经双级时效(325°C，4 h)+(175°C，14 h)处理后的屈服强度和抗拉强度与单级等温时效处理(325°C，10 h)的相当，但伸长率有所下降。此外，不同状态下Mg-Zn-RE-Zr镁合金的断裂主要表现为准解理断裂，但局部特征有差别。%The microstructures and mechanical properties of Mg-4.2Zn-1.5RE-0.7Zr alloy were investigated under different heat treatment conditions. The as-cast alloy consisted ofα-Mg phase, T-phase and Mg51Zn20 phase. After aging treatment (single-step (325 °C, 10 h) and two-step (325 °C, 4 h)+(175 °C, 14 h)), neither T-phase nor Mg51Zn20 phase dissolved into the matrix and the coarsening ofα-Mg phase was not significant. When peak-aged at 325 °C for 10 h, dense short rod-likeβ′1 phase precipitated in the matrix. Further ageing at 325 °C led to coarsening ofβ′1 phase and a decrease in number density. Alloy aged at 325 °C for 10 h achieved the highest yield strength (YS) and ultimate tensile strength (UTS) of 153.9 MPa and 247.0 MPa, which were increased by 48 MPa and 23 MPa from as-cast condition, respectively. While the elongation slightly decreased to 15.6%. Comparatively, the YS and UTS of alloy two-step aged by (325 °C, 4 h)+(175 °C, 14 h) showed little difference from those of single-step aged alloy, but with a lower elongation of 13.4%. In addition, the fracture surfaces of Mg-4.2Zn-1.5RE-0.7Zr alloy under different thermal conditions were mainly characterized by quasi-cleavage feature, but with differences in the details.