Valence electron concentration and ferromagnetism govern precipitation in NiFeGa magnetic shape memory alloys

Abstract

Precipitation is one of a few effective ways enabling to reduce the brittleness of NiFeGa magnetic shape memory alloys. However, the physical origin behind precipitation and the key factors deciding precipitation remain unknown. There is still a lack of available methods to control the precipitation of NiFeGa. To this end, the precipitation of Ni2FeGa is systemically studied by combining first-principles calculations and experimental examinations from the following four aspects: the stability of the matrix, the crystal structure and stability of precipitates, the key factors dominating precipitation and the strategy of tailoring precipitation. Results show that the L21 matrix phase possesses weak elastic and dynamic stabilities, which may be linked to the occurrence of precipitation. It is identified theoretically and experimentally that the precipitate has L12 rather than the previously reported FCC structure. The underlying mechanism was clarified from electron density of state and chemical bond. Valence electron concentration e/a and magnetism are found to be the dominant factors affecting the relative stability of precipitate against matrix with sensitivity coefficients of ‒175.1 and 67.4 meV, respectively. The increase of e/a prefers stabilizing the precipitate, but the enhanced magnetism favors the matrix. Conversely, the influence of lattice volume on precipitation is weak from the aspects of both sensitivity and adjustable range. Lastly, an effective strategy of tuning precipitation, i.e., by tailoring e/a and magnetism, is proposed and verified. This work is expected to lay a theoretical foundation for tailoring precipitation and further optimizing the mechanical and functional performances of ferromagnetic Heusler alloys.