摘要
为揭示触变加工Mg-Bi-Sn系合金微观组织和力学性能特点,选取细晶Mg-3Bi-5Sn-2Zn(BTZ352)合金进行半固态触变锻造加工.采用OM、SEM、XRD、EPMA和室温拉伸试验对触变锻造BTZ352合金微观组织和力学性能进行表征分析.结果表明:触变锻造BTZ352合金由近球状粗晶和细小二次凝固组织构成,近球状粗晶体积分数和平均尺寸分别为59.3％与(120.5±6)µm,而二次凝固组织由平均二次枝晶臂间距为3.2µm的细小等轴树枝晶与晶间化合物相(Mg3Bi2、Mg2Sn、Mg2Zn和BiSn)构成.触变锻造BTZ352合金中的近球状粗晶与二次凝固组织中细小的等轴树枝晶构成双峰结构.归因于双峰结构中细晶组织显著的晶界强化作用以及粗、细晶区之间的协同作用,触变锻造BTZ352合金表现出优异的力学性能,其屈服强度和伸长率为分别达到(196.5±5)MPa和(14.9±0.6)％,与铸态合金相比分别提升39.7％和12.8％;其断口组织整体上呈现典型穿晶韧性断裂特征.
Abstract
In order to reveal the microstructure and mechanical properties of thixoformed Mg-Bi-Sn alloys,fine-grained Mg-3Bi-5Sn-2Zn (BTZ352) alloys were selected for semi-solid thixoforging. The microstructure and mechanical properties of thixoforged BTZ352 alloy were studied by OM,SEM,XRD,EPMA and tensile test. The results indicate that a bimodal structure composed of near-spherical coarse grains and fine secondary solidification microstructure is formed in the thixoforged BTZ352 alloy,different from the uniform and fine dendrites in the as-cast BTZ352 alloy. The volume fraction of spherical α1-Mg grains is about 59.3％ with an average grain size of (120.5±6) μm. The secondary solidification region is composed of greatly refined α2-Mg with secondary dendrite arm spacing of about 3.2 μm and continuous network-like compound phases including Mg3Bi2,Mg2Sn,Mg2Zn,and BiSn. A bimodal structure is formed in the thixoforged BTZ352 alloy by the uniform distribution of the nearly spherical coarse α1-Mg grains and the fine equiaxed dendrites in the secondary solidification structure. Due to the significant grain-boundary strengthening effect from fine α2-Mg grains in the bimodal structure and the synergistic effect between coarse and fine grain regions,the thixoforged BTZ352 alloy exhibits superior mechanical properties. The tensile yield strength,and elongation are (196.5±5) MPa,and (14.9±0.6)％,respectively,which are 39.7％,and 12.8％ higher than those of the as-cast alloy,respectively. The fracture surface exhibits typical transgranular and ductile fracture characteristics.
维普
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