Microstructural study and mechanical properties of a thixoformed AZ91

The microstructure of a thixoformed AZ91 (Mg-Al-Zn system) alloys was investigated by different techniques and mechanical tests were performed after different heat treatments conditions. A correlation was established between microstructure and mechanical properties. The material was furnished in the thixocast state, and then solution treated and aged in order to study its ageing response. Optical microscopy, scanning and electron microscopies and X-rays were carried out to follow the structure evolution. Observations showed that the α-Mg developed into globules, while the β-phase (Mg₁₇Al₁₂) particles are present only in the eutectic area. During solution treatment at 415°C the original microstructure was produced by thixoforming transformed in a conventional structure of equiaxed grains with a redistribution of solute atoms. Different precipitates were observed during heat treatments. A quasicrystal phase of composition close to (Al,Mg)₈₆Mn₁₄ was also observed after solution treatment and quenching. A good compromise of mechanical properties is reached after 4-6h heat treatment at 415°C.