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 degrees C) 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 10.5 mu m to 7.4 mu m 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.