Comparative studies on evolution behaviors of 14H LPSO precipitates in as-cast and as-extruded Mg-Y-Zn alloys during annealing at 773 K

The microstructural evolutions of as-cast and as-extruded Mg97Y2Zn1 (at.%) alloys during annealing at 773 K were systematically investigated in this work. The results show that the evolutions of 14H long period stacking ordered (LPSO) phases are obviously different for the two alloys. In cast alloy, 14H LPSO precipitates firstly and then dissolves into alpha-Mg, while 18R shows no obvious change. Only a few 14H phases remain near 18R/alpha-Mg interfaces of cast alloy after annealed for 240 h. As for extruded alloy, 14H precipitates and grows continuously with increasing annealing time, while 18R gradually dissolves. The extruded alloy exhibits an alpha-Mg/14H dual microstructure after annealed for 240 h. The formation of 14H in cast alloy follows the reaction of 18R + alpha-Mg -> 14H, while it is generated in extruded alloy through a precipitation process. Moreover, the 14H preferentially forms near alpha-Mg/18R interfaces where SFs and solute atoms are enriched in cast alloy, but they are uniformly generated within alpha-Mg grains of extruded alloy. The differences between microstructural evolutions of cast and extruded alloys during annealing could be mainly attributed to the segregations of solute atoms and SFs within alpha-Mg grains. (C) 2015 Elsevier Ltd. All rights reserved.