Characterization of intermetallics in Mg–Al–Sr AJ62 alloys

The intermetallics of two AJ62 Mg alloys with low and high Sr content were characterized. The high Sr version contained a ternary intermetallic, in addition to Al4Sr eutectic present in all AJ62 Mg alloys. The chemical composition of the ternary phase corresponded to a stoichiometry of Mg9Al3Sr. Heat treatments were performed in order to study the stability of the intermetallics. Chemical analyses after heat treatment revealed an increased Al/Sr ratio of Al4Sr, while its Mg content in solid solution decreased. After an initial change of the Al/Sr ratio, the ternary intermetallic decomposed during heat treatment into α-Mg and Al4Sr. A microstructural model was proposed in order to explain the changes occurring during the heat treatments. The crystal structure of the ternary intermetallic was investigated by TEM electron diffraction. According to these observations the structure is tetragonal with the following lattice parameters: a = 1012 pm and c = 1169 pm.