Precipitated structures and mechanical properties of AZ91D magnesium alloy

The magnesium alloys are high in specific strength. Mg-Al-Zn alloys, in particular, have the excellent balance between strength and ductility. A magnesium alloy containing over 5% aluminum yields the cellular structures through the grainboundary reaction. AZ91D is modified for the heat treatment with killing the gaseous pore, since the strength is improved with the aging. The cellular reaction takes place heterogeneously along the grain boundary at the early stage of aging. The spacing of the lamellar, actually does not alter during the aging. The hardness has a linear relationship with the reciprocal spacing. The effect of the microstructures was observed on the mechanical properties; tensile and fatigue ones. Both the yield and tensile strengths increase in the aged structures, while the ductility decreases. The S-N curve appears to have little to do with the aging processes, though the fatigue crack propagation rate is fairly reduced for the aged materials. The homogeneous precipitation, however, would not bring large degradation in the fatigue properties. Although both homogeneous and heterogeneous structures can contribute to the strength. of AZ91D, we observed the difference in the effect on the fatigue properties. The interface of the cellular precipitations could not be an effective barrier against the propagating fatigue crack.