It was noted that ternary 2-phase infinite diffusion couples exhibited 1, 2 or 3 planar interface solutions to Fick’s equations; depending upon the relative magnitudes of the 4 diagonal diffusion coefficients, the phase diagram and the terminal compositions. This conflicted with the assumption that such solutions were unique. Supersaturated regions on both sides of the couples were associated with the maximum degeneracy of all 3 interface velocities and profiles. This implied instability, and a combined problem of mode selection and wave-number selection. A doublet of unstable solutions marked a transition, with terminal composition change, to singlet solutions of stable or unstable character.

P.Maugis, W.D.Hopfe, J.E.Morral, J.S.Kirkaldy: Journal of Applied Physics, 1996, 79[10], 7592-5