Influence of Mg17Al12 intermetallic compounds on the hot extruded microstructures and mechanical properties of Mg-9Al-1Zn alloy

Mg-9Al-1Zn (AZ91) cast ingots annealed at 420 degrees C for 2 and 24h were hot extruded at 270 degrees C, respectively. Microstructural observation showed that dynamic recrystallization (DRX) occurred in both cases resulting in fine equiaxed grains. It was revealed that the bar fabricated by extruding short-time annealed ingot had a much smaller average grain size (about 4 mu m) than that extruded with a long-time annealed ingot (about 30 mu m). It was found that in the former situation, large amounts of small spherical beta (Mg17Al12) particles (about 0.3-0.8 mu m) were broken directly from the unresolved P phases during the hot extrusion process, most of them finally distributing along the recrystallized grain boundaries. Tensile tests showed that the bars extruded from ingot with short annealing time had superior mechanical properties because of the refined microstructure and the small P particles. (c) 2007 Elsevier B.V. All rights reserved.