Effects of yttrium on microstructure and mechanical properties of hot-extruded Mg-Zn-Y-Zr alloys

This article studied the effects of yttrium on microstructure and mechanical properties of extruded Mg-Zn-Y-Zr alloys. Three kinds of Mg-5.5Zn-xY-0.4Zr alloys with variable yttrium content were hot-extruded at different temperature to investigate the microstructure evolution and dynamic recrystallization (DRX) process of these alloys. The alloys with low yttrium content as 0.74 or 1.35 wt.% consist of alpha-Mg and large I-phase particles, while the alloy with high yttrium content as 1.72% consists of alpha-Mg, I-phase and W-phase. For the alloys with low yttrium content as 0.74 or 1.35 wt.%, fine I-phase precipitates formed during the hot extrusion process. These fine precipitates and particles, which are from destroyed I-phase pockets, provide obvious strengthening effects on the alloys. For the alloy with high yttrium content as 1.72%, fine I-phase precipitates did not form during the hot extrusion process. Further, coarse W-phase pockets were difficult to be destroyed in the alloy extruded at moderate temperature. As a result, the yield strength of this alloy is moderate compared to that of the alloy with yttrium content of 1.35%, especially when extruded at low temperature. Furthermore, it was observed that the growth of DRX grains was suppressed due to the pining effect of particles or precipitates during the hot extrusion process in all the alloys studied. (C) 2004 Elsevier B.V. All rights reserved.