Self-diffusion in Ni2MnGa

Lattice self-diffusion of Ni-63, Ga-67, and Mn-54 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 L2(1) structures, the diffusion coefficients of the transition metal components are close to each other within a factor of 2. Gallium diffuses in the L2(1) 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 L2(1)-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 L2(1) structures are explained qualitatively in the framework of a six-jump-cycle (6JC) mechanism. In the L2(1) 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 L2(1) 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. (C) 2007 Elsevier Ltd. All rights reserved.