Influence of Multi-Pass Hot Extrusion on Microstructure and Mechanical Properties of the Mg–4Zn–1.2Y–0.8Nd Alloy

Abstract

In the present research, a Mg–4Zn–1.2Y–0.8Nd (wt.%) alloy was heat treated and hot extruded with different passes. XRD, SEM, TEM and tensile testing were employed to characterize the microstructure evolution and mechanical properties. The results exhibited that the semi-continuously distributed W-Mg3Zn3Y2 phases formed the skeleton structure which separated the α-Mg matrix into a dual-size grain structure. In addition, the Mg24Y5, Mg41Nd5 and Y2O3 phase was also observed in the heat-treated alloy. Moreover, it was found that the Mg24Y5 phase had an orientation relationship with the α-Mg matrix of α[111]Mg24Y5//[0001]α-Mg and α(101¯)Mg24Y5//(101¯0)α-Mg, and the Mg41Nd5 phase had an orientation relationship with the α-Mg matrix of α[001]Mg41Nd5//[0001]α-Mg. The one-pass hot extrusion segmented the secondary phases into small ones and refined the α-Mg matrix. Due to the partly recrystallization and crystal orientation difference, the coarse elongated grain surrounded by fine recrystallized grain and secondary phase was the main feature of the one-pass hot extruded alloy. Furthermore, the secondary phases exhibited the linear distribution along the direction of hot extrusion. The two-pass hot extrusion refined the secondary phase and matrix further, which produced the ultrafine α-Mg matrix with uniform grain size and a well redistributed secondary phase. Due to the microstructure optimization by the multi-pass hot extrusion, the ductility and strength of the Mg–Zn–Y–Nd alloy were well improved, especially the two-pass hot extruded alloy which was significant improved in ductility and strength simultaneously.