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

The beta(1) 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 beta(1) 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) over bar0}(hcp) zig-zag monolayers along a direction parallel to < 001 >(fcc) with a shear angle of similar to 5.26 degrees, followed by atoms shuffling on the non-close-packed {10 (1) over bar0}(hcp) planes, can transform the hexagonal close-packed Mg3Sm structure (beta or beta(H)' intermediate phase) to the face-centered-cubic structure Mg3Sm ( beta(1) phase). Besides, the formation of the beta(1) phase can also be realized from the other beta(S)' or beta(L)' intermediate phase (c-bco, Mg7Sm) via solute diffusion coupled with shuffle transformation manner. A new habit plane of {10 (1) over bar0}(hcp) // {1 (1) over bar0}(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.

Automated summary

This study revealed the diffusional-displacive dominated formation mechanism of beta(1) phase in Mg-Sm alloys using aberration-corrected scanning transmission electron microscopy observation and first-principles calculations. Shear and atomic shuffling can transform hexagonal close-packed structure to face-centered-cubic structure, and beta(1) phase formation can also be realized from other intermediate phases through solute diffusion and shuffle transformation. A new habit plane on the non-close-packed plane has been identified, enriching the diffusional-displacive transformations.