Interfacial strain driven nucleation and growth of grain boundary phases

Secondary phases precipitation in grain boundaries (GBs) are ubiquitous presence in multicomponent polycrystalline materials, and play a crucial role in determining the materials performances. However, the atomistic formation mechanism remains a matter of conjecture since the GB phase is difficult to well characterize. Here, using a typical Mg-rare-earth binary alloy as the model system, we systematically investigated the precipitation behaviors of nano-sized secondary phases in a partially coherent tilt GB. Atomic-scale observations combined with theoretical calculations show that GB elastic strain minimization drives the segregation of solutes to the interfacial strain zones, and the selective substitution of solutes resulting in the nucleation of face-centered-cubic (FCC) phase from the bi-crystalline hexagonal closed-packed (HCP) lattices. Interphase boundary strain driven solute diffusion plus atomic shuffling can realize the growth of parent phase { 1 1 ¯ 00 } _HCP lattices to the {110} _FCC atomic-planes. These findings not only confirm a novel HCP to FCC phase transition pathway, but also provide a mechanistic insight into the nucleation and growth of GB phases.