Effects of microstructures on age-hardening of Mg-Al binary alloys

Magnesium alloys of AM30, AM60 and AM90 containing 3, 6 and 9 % Al, respectively, were prepared. The ingots of the alloys were warm-rolled and then solution heat treated. Microstructures of the alloys at as cast condition and after solution heat treatment were observed. Only α solid solution was observed in the solution heat treated specimens. Discontinuous precipitation was observed and age hardening occurred during aging at 489 K in the alloys except the AM30 alloy. The area fraction of discontinuous precipitates increased with increasing aging time, which resulted in increasing hardness. Hardness on areas containing discontinuous precipitates and those containing continuous precipitates were separately measured. The former showed higher value at the early stage of aging and then decreased, while the latter increased with aging. Calculated hardness through the rule of mixtures using values of hardness for pure magnesium and β-Mg₁₇Al₁₂ phase is consistent with the experimental results. Spacing between lamellars in the discontinuous precipitation increased with increasing aging time, although the hardness in the area composed of discontinuous precipitates decreased.