Lattice self-diffusion of 63Ni, 67Ga and 54Mn tracers was studied in the ferromagnetic Ni2MnGa shape-memory alloy (tables 9 to 11). 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 were close to each other to within a factor of 2. The Ga diffused in the L21 structure at a rate which was 2 to 4 orders of magnitude slower. At about 1100K, a change of the slope was detected in the Arrhenius plots of the transition-metal components. This sharp change was due to the L21–B2′ transformation. The Ga diffusivity did not seem to be affected by the transformation. The diffusion activation energy of Mn and Ni was definitely higher in the B2′ phase. The diffusion trends which were observed in the B2′ and L21 structures were explained qualitatively within the framework of a 6-jump-cycle

mechanism. In the L21 structure, it was possible to define Ga-migrating and transition metal migrating cycles. These cycles resulted in rather different activation energies for the diffusion of transition-metal atoms and for Ga atoms in the L21 structure. However, in the B2′ structure, all of the components (Ni, Mn, Ga) were expected to take part in every type of 6-jump cycle; resulting in similar diffusion parameters for all 3 components.

Self-Diffusion in Ni2MnGa. G.Erdélyi, H.Mehrer, A.W.Imre, T.A.Lograsso, D.L.Schlagel: Intermetallics, 2007, 15[8], 1078-83

Table 9

Diffusivity of 67Ga in Ni2MnGa