Electron microscopical investigation of as cast AZ91D alloy

A detailed examination of a magnesium alloy, AZ91D, in as cast condition was carried out by using electron microscopy techniques. Particles of Al₁₇Mg₁₂, known as β and Al₈Mn₅ precipitates have been identified by electron diffraction. It was found that Al₈Mn₅ particles adopted coarse angular shapes, while β phase existed in several distinct morphologies, namely grain boundary blocky particles, lamella of the colonies of discontinuous precipitation, irregular ones in the α constituent of the cellular nodules, and two types of intragranular continuous precipitates, i.e. plates and plaques. The crystallographic relationships between the α and β constituents of the discontinuous precipitation, and between the intragranular β and the α matrix have also been determined. It was observed that the morphologies and the crystallographic relationship observed are identical to the one that was reported in the literature for an aged structure. Based on the dark field electron micrographs a model has been proposed to explain the lengthening of plates of β. Basic morphological differences between the structures of the ingot and aged AZ91 are discussed.