Hydrogen-induced diffusion phase transformations in Nd2Fe14B and Sm2Fe17 magnetically hard alloys

Abstract

The Nd 2 Fe 14 B and Sm 2 Fe 17 magnetically hard alloys become thermodynamically unstable under the action of hydrogen and suffer hydrogen-induced direct and inverse phase transformations at elevated temperatures. The kinetics of these transformations is investigated. It is shown that they are diffusion-controlled and develop according to the mechanism of nucleation and growth. The hydrogen treatment of materials is a new branch of materials science [1] based on the specific features of hydrogen and hydrogen-induced phase transformations. In Nd 2 Fe 14 B- and Sm 2 Fe 17 -type magnetic materials, the hydrogen-induced diffusion phase transformations occur due to the different affinities of the atoms of different components of materials to hydrogen. Since the alloys consist of strong hydride- and nonhydride-forming elements, they lose their thermodynamic stability as a result of saturation with hydrogen. At low temperatures, these materials exist in the nonequilibrium (metastable) state. At elevated temperatures (T > (0.20 – 0.45) T m , where T m is the melting point), the diffusion of large atoms of the material becomes possible, and we observe the realization of hydrogen-induced diffusion phase transformations. The alloys decompose into rare-earth hydrides ( Nd H x or Sm H x ) and the α -phase of iron. In the Nd 2 Fe 14 B alloy, we also observe the formation of iron boride (Fe 2 B). The evacuation of hydrogen from the alloy after the direct transformation requires the thermodynamically inverse phase transformation resulting in the formation of the Nd 2 Fe 14 B or Sm 2 Fe 17 initial phase. As a result of the direct and inverse phase transformations, the microstructure is refined, which improves the magnetic properties of permanent magnets prepared from the alloys treated as indicated above [2]. In what follows, we generalize the results of investigations of the kinetics of hydrogen-induced direct and inverse diffusion phase transformations in the Nd 2 Fe 14 B or Sm 2 Fe 17 alloys and discuss the nature of these transformations within the framework of solid-state physics.