Crystallographic Orientation Relationship between Discontinuous Precipitates and Matrix in Commercial AZ91 Mg Alloy

Abstract

Discontinuous precipitates (DPs) have a random distribution at grain boundaries in commercial AZ91 Mg alloy, whereas continuous precipitates exist in the matrix. Previous studies have mainly focused on continuous precipitates and their orientation relationship with the matrix. In this study, thin cross sections of DPs in commercial AZ91 magnesium alloy aged at 489 K were prepared by focused ion beam milling. Their morphologies and the orientation relationship between the DPs and the matrix were investigated by transmission electron microscopy. The results reveal that the DPs have the shape of platelets. Selected-area electron diffraction patterns of the DPs allowed them to be identified as β-Mg₁₇Al₁₂. The DPs were found to have the same orientation relationship (i.e., Burgers, Potter or Crawley) as the continuous precipitates in the matrix. However, their longitudinal directions were not fixed, but were distributed about their (11\\bar1) zone axis.