Pt diffusion in B2-type ordered NiAl intermetallic compound and its diffusion mechanisms

Diffusion of platinum element which increase protectivity of NiAl layer for hot corrosion and oxidation of Ni-base superalloys was investigated in NiAl compound with compositions of 43.7-51.9 at% Al at 1373-1673 K by the diffusion couple method. The diffusion coefficients, D_Pt, exhibit a large concentration dependence of valley type-shape; the D_Pt has the minimum value quite near the stoichiometric composition and it increases with deviation from the stoichiometry. On the other hand, the activation energy, Q, shows the maximum value near the stoichiometry. Q slightly decreases with Ni composition in Ni-rich side, while it decrease dramatically with Al composition in Al-rich side. Diffusion of platinum element in NiAl occurs mainly by the Triple defect mechanism at stoichiometry, by the anti-site atom bridge mechanism in considerably off-stoichiometry in Ni-rich side and by the next nearest neighbor-jump mechanism with constitutional Ni vacancies in quite off-stoichiometry in Al-rich side.