Zn含量对反挤压Mg-8Sn-Zn合金组织演变和力学性能的影响

利用OM,SEM,TEM,EBSD,XRD和电子材料试验机研究了Zn含量（1%-4%,质量分数）对反挤压Mg-8Sn-Zn合金组织、织构演化和力学性能的影响.结果表明,所有合金均可在相对较低的挤压温度（250℃）和较高的挤压速度（2 m/min）下成形.在反挤压过程中,所有在均质化处理后残留的粗大第二相在挤压过程中破碎并沿着挤压方向被拉伸成条带状;所有的粗大晶粒均转变为细小的等轴晶,其平均晶粒尺寸分别为7.4,8.3和10.5μm.随着Zn含量的增加,在挤压态合金晶内和晶间分布的细小弥散第二相的体积分数增加,这些第二相主要由亚微米级的Mg₂Sn相和纳米级的富Zn相组成.弥散分布在晶界上的第二相有效地钉扎了晶界,从而细化了晶粒尺寸.另外随着Zn含量的增加,合金的织构强度降低,这和变形晶粒的体积分数减小有关.组织细化、织构弱化和第二相弥散化是Mg-Sn-Zn合金强度提高和拉伸/压缩屈服点各向异性减弱的主要因素.

EFFECTS OF Zn CONTENT ON MICROSTRUCTURE EVOLUTION AND MECHANICAL PROPERTIES OF INDIRECT-EXTRUDED Mg-8Sn-Zn ALLOYS

The influence of Zn content on the microstructure evolution,texture and mechanical properties of indirect-extruded Mg-8Sn-Zn alloys have been investigated by OM,SEM,TEM, EBSD,XRD and a standard universal testing machine.The studied alloys were demonstrated to be extrudable at a relatively low temperature（250℃） and a high extrusion speed（2 m/min）.During the extrusion process,most of the remained second phase particles present in the homogenized alloy are found to be aligned along the extrusion direction（ED） in the form of stringers after being broken into fragments.While most of the coarse grains were changed into fine equiaxed grains with average sizes ranging from 10.5μm to 7.4μm.The volume fractions of the second phase particles increase with increasing Zn content while the grain size and texture strength decrease with increasing Zn content.These second phase particles are mainly composed of Mg₂Sn,having a diameter of submicron and some nano-meter Zn-rich phases.Furthermore,the decrease in grain size can be explained by the Zener drag of fine particles.While,the textural weakening with Zn addition is associated with the decreased fraction of elongated grains retain strong fiber texture.The improvement in strength and reduction in yield asymmetry of the studied alloys were associated with finer grain size,higher fraction of second phase and weaker texture.