Shear transformation of coupled β1/β′ precipitates in Mg–RE alloys: A quantitative study by aberration corrected STEM

The coupled distribution of β1 and β′ particles is frequently observed in aged microstructures of Mg–RE (RE represents rare-earth elements) alloys. The crystallography of these coupled β1/β′ particles is characterized using atomic-resolution high-angle annular dark-field scanning transmission electron microscopy, and the shear and dilatation strains associated with the migration of the β1/β′ interface are analysed. It is found that the orientation relationship between the β1 and β′ phases, and the irrationally oriented and yet fully coherent interface associated with this orientation relationship, can both be well described by the crystallography of phase transformations. It is also found that there exists a large shear strain for the β1/β′ transformation or the migration of the fully coherent β1/β′ interface. The magnitude and sign of this shear strain are compatible, but opposite to those of the shear strain involved in the α-Mg/β1 transformation. Hence the coupled distribution of β1/β′ particles offers a unique opportunity to almost fully self-accommodate the shear strains involved in the α-Mg/β1 and β1/β′ structural transformations. This is perhaps why the β1 plates with β′ particles attached to their ends are the most commonly observed distribution in aged microstructures of Mg–RE alloys.