The relationship between (Mg,Zn)3RE phase and 14H-LPSO phase in Mg–Gd–Y–Zn–Zr alloys solidified at different cooling rates

Abstract

Mg–10Gd–3Y–1.8Zn–0.5Zr (wt.%) (GWZ1032K) alloys are prepared by permanent mold casting at cooling rate of 5K/s, or further prepared by melt spinning at cooling rate of 104K/s, or by slow solidification at different cooling rates (0.5K/s, 0.1K/s, 0.01K/s and 0.005K/s). (Mg,Zn)3RE phase and 14H-LPSO structure in alloys under different conditions are measured by XRD and observed under electron microscope. It shows there is no LPSO structure in the alloy prepared by melt spinning at cooling rate of 104K/s. In the alloy prepared by permanent mold casting at cooling rate of 5K/s, fine lamellar 14H-LPSO structure appears in the matrix nearby grain boundaries. With the cooling rates slowing down from 0.5K/s to 0.005K/s, (Mg,Zn)3RE phase is gradually replaced by 14H-LPSO phase at grain boundaries, and lamellar 14H-LPSO structure also propagates in α-Mg matrix. Both (Mg,Zn)3RE phase and 14H-LPSO phase are present at grain boundaries in the alloys solidified at cooling rates of 0.5K/s and 0.1K/s. When the cooling rate is very slow (0.005K/s), lamellar 14H-LPSO structure penetrates throughout the matrix grain. It suggests the cooling rate is an important factor for the formation of 14H-LPSO structure in as-cast GWZ1032K alloys. The orientation relationship between (Mg,Zn)3RE phase and 14H-LPSO phase is determined by the composite SAED patterns, which is expressed as (110)(Mg,Zn)3RE//(0014)14H-LPSO phase, [3¯ 3 2](Mg,Zn)3RE//[1 1 0]14H-LPSO phase and [1¯ 1 2](Mg,Zn)3RE//[2 1 0]14H-LPSO phase.