Microstructural Development and Ternary Interdiffusion in Ni-Mn-Ga Alloys

NiMnGa alloys functioning as either ferromagnetic shape memory alloys or magnetocaloric materials have both practical applications and fundamental research value. In this study, solid-to-solid diffusion couple experiments were carried out to investigate the phase equilibria, microstructural development, and interdiffusion behavior in Ni-Mn-Ga ternary alloys. Selected diffusion couples between pure Ni, Ni₂₅Mn₇₅ and four ternary off-stoichiometric NiMnGa alloys (i.e., Ni₅₂Mn₁₈Ga₃₀, Ni₄₆Mn₃₀Ga₂₄, Ni₅₂Mn₃₀Ga₁₈, Ni₅₈Mn₁₈Ga₂₄) were assembled and annealed at 1073 K, 1123 K, and 1173 K (800 °C, 850 °C, and 900 °C) for 480, 240, and 120 hours, respectively. At these high temperatures, the β NiMnGa phase has a B2 crystal structure. The microstructure of the interdiffusion zone was examined by scanning electron microscopy and transmission electron microscopy. Concentration profiles across the interdiffusion zone were determined by electron probe micro analysis. Solubility values obtained for various phases were mostly consistent with the existing isothermal phase diagrams, but the phase boundary of the γ(Mn) + β two-phase region was slightly modified. In addition, equilibrium compositions for the γ(Ni) and α′ phases at 1173 K (900 °C) were also determined for the respective two-phase region. Both austenitic and martensitic phases were found at room temperature in each diffusion couple with a clear boundary. The compositions at the interfaces corresponded close to valence electron concentration (e/a) of 7.6, but trended to lower values when Mn increased to more than 35 at. pct. Average effective interdiffusion coefficients for the β phase over different compositional ranges were determined and reported in the light of temperature-dependence. Ternary interdiffusion coefficients were also determined and examined to assess the ternary diffusional interactions among Ni, Mn, and Ga. Ni was observed to interdiffuse the fastest, followed by Mn then Ga. Interdiffusion flux of Ni also has strong influences on the interdiffusion of Mn and Ga with large and negative cross interdiffusion coefficients, \( \tilde{D}_{MnNi}^{Ga} \) and \( \tilde{D}_{GaNi}^{Mn} \). The \( \tilde{D}_{NiNi}^{Ga} \) and \( \tilde{D}_{MnMn}^{Ga} \) ternary interdiffusion coefficients exhibited minimum values near 52 at. pct Ni concentration.