Abstract:

Two novel types of irrational orientation relationships (ORs) between β-Mg₂Sn precipitates and the α-Mg matrix were observed in a Mg-4Sn alloy aged at 160℃. In contrast to that for the dominant β-Mg₂Sn laths lying on the (0001)_α basal plane of the matrix, the precipitates with the irrational ORs are inclined or normal to (0001)_α. Specifically, the precipitates that adopt the OR of [011]_β//${{\left[ 2\bar{1}\bar{1}0 \right]}_{\text{ }\!\!\alpha\!\!\text{ }}}$, with ${{\left( \bar{1}1\bar{1} \right)}_{\text{ }\!\!\beta\!\!\text{ }}}$ deviating approximately 4.7° from (0002)_α, have a long axis parallel to [0001]_α. The precipitates with the OR of [011]_β//${{\left[ 2\bar{1}\bar{1}0 \right]}_{\text{ }\!\!\alpha\!\!\text{ }}}$, with ${{\left( 11\bar{1} \right)}_{\text{ }\!\!\beta\!\!\text{ }}}$ deviating approximately 13.5° from (0002)_α, exhibit a long axis of ${{\left[ 02\bar{2}1 \right]}_{\text{ }\!\!\alpha\!\!\text{ }}}$. Both types of precipitates have a habit plane (HP) of ${{\left( 2\bar{1}\bar{1}0 \right)}_{\text{ }\!\!\alpha\!\!\text{ }}}$. The interface orientations of the precipitates associated with the irrational ORs are interpreted by a good matching sites (GMS) method. A thorough examination of the crystallographic features of the β-Mg₂Sn precipitates in the literature shows that the precipitates with most rational ORs have a HP parallel to (0001)_α, and those with most irrational ORs have a HP (or long axis) inclined or normal to (0001)_α. The preferences of the precipitates with different irrational ORs have also been analyzed by examining the lattice matching condition across the interfaces between the precipitates and the matrix.

Key words: Mg alloy, Non-basal precipitates, Crystallography, Good matching sites, Orientation relationship, β-Mg₂Sn