Electron microscopical investigation of as cast AZ91D alloy

A detailed examination of a magnesium alloy, AZ91D, in as cast condition wa s carried out by using electron microscopy techniques, Particles of Al17Mg1 2, known as beta and Al8Mn5 precipitates have been identified by electron d iffraction, It was found that Al8Mn5 particles adopted coarse angular shape s, while beta phase existed in several distinct morphologies, namely grain boundary blocky particles, lamella of the colonies of discontinuous precipi tation, irregular ones in the alpha constituent of the cellular nodules, an d two types of intragranular continuous precipitates, i.e. plates and plaqu es. The crystallographic relationships between the alpha and beta constitue nts of the discontinuous precipitation, and between the intragranular beta and the alpha matrix have also been determined, It was observed that the mo rphologies 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 t he lengthening of plates of beta. Basic morphological differences between t he structures of the ingot and aged AZ91 are discussed. MST/4325.