Self-diffusion in Ni2MnGa

Abstract

Abstract Lattice self-diffusion of 63Ni, 67Ga, and 54Mn tracers was studied in the ferromagnetic Ni2MnGa shape-memory alloy. The composition of the monocrystalline alloy was slightly Ni-rich (Ni53.7Mn21.1Ga25.2) and the direction of diffusion was perpendicular to the (100) plane. In the B2′ and L21 structures, the diffusion coefficients of the transition metal components are close to each other within a factor of 2. Gallium diffuses in the L21 structure 2–4 orders of magnitude slower. At about 1100 K, a change of the slope was detected on the Arrhenius plots of the transitional metal components. This sharp change is due to the L21–B2′ transformation. The Ga diffusivity seems not to be affected by the transformation. Interestingly, the diffusion activation energy of Mn and Ni is definitely higher in the B2′ phase. The diffusion trends observed in B2′ and L21 structures are explained qualitatively in the framework of a six-jump-cycle (6JC) mechanism. In the L21 structure one can define Ga-migrating and transition metal migrating cycles. These cycles result in rather different activation energies for the diffusion of transition metal atoms and for Ga atoms in the L21 structure. In the B2′ structure, however, all components (Ni, Mn, Ga) should take part in every type of six-jump cycle, resulting in similar diffusion parameters for all 3 components.