Effects of manganese on the precipitation process and mechanical properties of AZ91 magnesium alloy

Abstract

A Mg-9 % Al-1%Zn ternary alloy was prepared from high purity magnesium, aluminum, and zinc. The precipitation and age hardening characteristics of the alloy and the mechanical properties of as-cast, solution heat-treated, and aged specimens were compared with those of a commercial AZ91 alloy containing manganese. Active growth of discontinuous precipitates that occupied the majority of the grain areas was observed in the high purity alloy, while in the commercial alloy, growth of the discontinuous precipitates ceased after the early stage of the precipitation process, resulting in only about 20% of the grain areas being occupied by the discontinuous precipitates and the remainder occupied by continuous precipitates. These differences in the microstructure were reflected in the peak hardness of the alloys. It was also found that there was preferential precipitation of the β-phase in the areas of the grains that were richer in manganese. Thus, these results showed that manganese promotes the precipitation of the β-phase within the grains while hindering the growth of discontinuous precipitates. From measurements of the room temperature tensile properties of as-cast, solutiontreated, and aged specimens of the high purity and commercial alloys, it was found that the high purity alloy exhibited higher yield strength, tensile strength, and elongation in the aged condition and higher elongation after solution heat treatment compared to the corresponding properties of samples from the commercial alloy.