Effect of predeformation on the globular grains in AZ91D alloy during strain induced melt activation (SIMA) process

The effect of predeformation on the alpha-Mg grains in the AZ91D alloys was investigated in different stages during the semisolid processing. The result showed that, with the increase of compressive ratio, the alpha-Mg dendrites in the as-cast microstructures gradually oriented themselves in the direction that was vertical to the compressive direction, and exhibited an obvious directional character. During the sequent heat treatment at 310 degrees C for 20 min. the recrystallization was induced in the deformed alloys by the increasingly accumulated systemic strain energy. The metallographical observation showed that the average size of recrystal alpha-Mg grains gradually reduced with the increase of compressive ratio. The reason might be that the nucleation to growth ratio of the renascent recrystal alpha-Mg grains gradually increased in the recrystallization process. Due to the recrystallization mechanism. the predeformation made the alpha-Mg grains particularly rounder and finer after the semisolid isothermal heat treatment at 560 degrees C for 20 min. The average diameter of the globular alpha-Mg grains in the semisolid microstructures also reduced with the increase of compressive ratio and this descending trend was almost similar to that in the recrystal microstructures, which further demonstrated that the recrystallization had an inevitable role on the evolution process of globular grains during SIMA process. While the compressive ratio surpassed 20%, the above descending trend was not so evident as that below 20%. In addition, the effect of predeformation on the tensile strength and elongation of the semisolid AZ91D alloys was also discussed in the present paper. (c) 2006 Elsevier B.V. All rights reserved.