Phase transition of nanocrystalline-amorphous Sm10.9Co66.5Fe22.6 alloy: Nucleation, growth kinetics and atomic process

The nucleation, growth kinetics and atomic process of the preexisting α-(Fe,Co) and newly-formed SmCo7 nanocrystals in the annealed nanocrystalline-amorphous Sm10.9Co66.5Fe22.6 alloy have been investigated. The preexisting α-(Fe,Co) phase possesses a growth activation energy of Eg = 0.6 eV, smaller than the nucleation activation energy En = 0.8 eV and growth activation energy Eg = 1.4 eV for the newly-formed SmCo7 phase. This demonstrates an easy growth of the α-(Fe,Co) nanocrystals and a relatively difficult formation of the SmCo7 phase, which leads to a large grain size for the α-(Fe,Co) soft phase in the α-(Fe,Co)/SmCo7 nanocomposite magnets. The activation volumes ΔVg∗ = 0.67 Ω and ΔVg∗ = 0.99Ω (Ω = average atomic volume) for the growth of α-(Fe,Co) and SmCo7 nanocrystals were obtained, indicating that the atom diffusion during the growth of both the α-(Fe,Co) and the SmCo7 are assisted by thermal vacancies. These findings provide a fundamental understanding to the crystallization of the SmCo based nanocomposite magnets.