Diffusional-displacive transformation mechanism for the β1 precipitate in a model Mg-rare-earth alloy

Abstract The β1 precipitate is a key strengthening-phase in Mg-rare-earth alloys, however, the formation mechanism concerning it has not yet been addressed. Herein, we have uncovered a diffusional-displacive dominated formation mechanism for the β1 phase, using aberration-corrected scanning transmission electron microscopy observation combined with first-principles calculations, in an aged Mg Sm model alloy system. Shear of the nucleated { 10 1 ¯ 0 }hcp zig-zag monolayers along a direction parallel to fcc with a shear angle of ~5.26°, followed by atoms shuffling on the non-close-packed { 10 1 ¯ 0 }hcp planes, can transform the hexagonal close-packed Mg3Sm structure (β” or βH’ intermediate phase) to the face-centered-cubic structure Mg3Sm (β1 phase). Besides, the formation of the β1 phase can also be realized from the other βS’ or βL’ intermediate phase (c-bco, Mg7Sm) via solute diffusion coupled with shuffle transformation manner. A new habit plane of { 10 1 ¯ 0 }hcp // { 1 1 ¯ 0 }fcc and [0001]hcp // [110]fcc on the non-closed packed plane has been identified, which is completely different from the traditional displacive transformation mechanism usually happened on the closed-packed plane. This finding enriches the diffusional-displacive transformations.