Precipitation behavior of Mg₁₇Al₁₂ in monolithic and Al₂O₃ short fiber reinforced Mg-Al-Zn alloys

The precipitation behavior of Mg₁₇Al₁₂ in monolithic and Al₂O₃ short fiber reinforced Mg-Al-Zn alloys was investigated by optical and transmission electron microscopies and hardness measurements. The maximum hardness was obtained when the long and short axes of the platelet type continuous Mg₁₇Al₁₂ precipitates were about 0.3 μm and 0.04 μm, respectively. The area fraction of the discontinuous Mg₁₇Al₁₂ precipitate nodule reached about 0.23. The coarsening behavior of the discontinuous Mg₁₇Al₁₂ precipitate nodule was found to obey the relationship suggested by the Johnson-Mehl-Avrami model. The slope of the Johnson-Mehl-Avrami plot for the Al₂O₃ short fiber reinforced Mg-Al-Zn alloy was four times larger than that for the monolithic alloy due to the increased number of nucleation sites, i.e. nucleation at the interface between the reinforcing material and the α-Mg matrix as well as at the α-Mg grain boundaries.