Formation mechanism of [formula omitted] phase in age-hardenable magnesium-rare earth alloys: Insight from in-situ and ex-situ observations with HAADF-STEM.

• β F ′ precipitate acts as the nucleate site for β 1 precipitation. • β 1 is transformed from β F ′ by short-range solute diffusion, atomic shuffle and shear mechanism. • The transformation from β ′ to β 1 was not observed in the present work. The formation mechanism of β 1 phase has been a controversy in age-hardenable magnesium-rare earth alloys for over two decades. Its nucleation and growth were not clearly illustrated so far due to the lack of direct experimental evidence. In the present work, we unveil the formation mechanism of β 1 phase based on the in-situ and ex-situ observations using high-angle annular dark-field scanning transmission electron microscopy. The direct evidences reveal that β 1 phase is transformed from β F ′ phase by short-range solute diffusion, atomic shuffle coupled with shear mechanism. The β F ′ precipitate firstly converts to intermediate precipitate via short-range solute diffusion in its zig-zag array on (10 1 ¯ 0) α plane, and then transforms into β 1 precipitate via atomic shuffle in the modified β F ′ lattice and shear of (10 1 ¯ 0) α zig-zag array by ∼5° with respect to [ 11 2 ¯ 0 ] α orientation. These findings shed light on the formation mechanism of β 1 phase, and hopefully address the long-standing controversy. [Display omitted] [ABSTRACT FROM AUTHOR]