Formation of γ′/β interface morphologies in Ni–Al–X ternary diffusion couples

The effect of alloying elements on the morphological stability of the interface between γ′ (L12) and β (B2) phases in the Ni–Al–X system was investigated using the Ni–Al–X ternary diffusion couples annealed at temperatures ranging from 1173 to 1573 K. Planar interfaces were observed in the Ni–Al–X couples with X=Co, Cr, Cu, Fe, Mn, Si and V, while nonplanar interfaces with the Widmanstätten-like structure were formed in the couples with X=Mo, Nb, Ta, Ti and W. The roughness of the unstable interface was found to depend not only on the kind of alloying element, but also on the annealing time and temperature. Such a dependency of interface morphological stability on the alloying element X is related to the thermodynamic properties and the diffusivities of the element Al and the particular alloying element X.