Liquid structure of SmFe12-based alloys

Abstract

Annealing of SmFe₁₂-based glasses is a potentially effective approach for producing ThMn₁₂-type crystals, which are promising materials for main phases of next-generation strong magnets. However, preparing these glasses is difficult. Glasses are typically made from their melt through rapid cooling. In this paper, we study the liquid structure of SmFe₁₂-based alloys by using ab initio molecular dynamics simulations and the $p_3$ code, as well as by developing a hard-sphere model of liquid SmFe₁₂. Our simulation results, combined with previous experimental observations, elucidate the glass-forming ability of SmFe₁₂-based alloys as follows. Two factors make it difficult to vitrify liquid SmFe₁₂: one is its structural similarity to liquid Fe which is known as a poor glass former; The other is chemical inhomogeneity. Clusters free of Sm atoms form in liquid SmFe₁₂ due to excluded volume effects, facilitating the nucleation of Fe crystals and thereby preventing glass formation. Doping liquid SmFe₁₂ with titanium reduces the structural similarity but increases Sm-free clusters. However, because the reduced structural similarity has a dominant effect, the overall glass-forming ability of SmFe₁₂ increases with titanium doping. Cobalt doping would also improve the glass-forming ability due to the reduced structural similarity and the decreased Sm-free clusters. If a glass is successfully made from a cobalt-doped SmFe₁₂ liquid, it would contain fewer Sm-free clusters than a pure SmFe₁₂ glass. The lower content of Sm-free clusters would facilitate the formation of ThMn₁₂-type crystals through annealing of the glass.