Microstructure and mechanical properties of thixoforged Mg-3Bi-5Sn-2Zn alloy
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.
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