The properties of diffusion couples in a ternary system were considered theoretically for the case where one end-member was a single-phase material (α) and the other end-member was a 2-phase material (α+β). By using the Morral model, cases in which the 2-phase region either grew or dissolved were examined. It was proved that, when the 2-phase material grew, there could be no jump in concentration (or β-fraction) at the boundary between the 2 materials. A jump was necessary when it dissolved, and numerical examples demonstrated this. It was also shown how the diffusion path varied; depending upon the composition of the single-phase α end-member for a given α+β end-member composition. Under some conditions, 3 solutions were found for the same initial end-member compositions. When these multiple solutions existed, either 2 or 3 of them were virtual; in that the path involved supersaturation. When all 3 solutions were virtual, a single solution with 3 interfaces could be found. In this case, an intermediate layer of α-phase formed within the α+β material in order to relieve the supersaturation.

On the Properties of α/α+β Ternary Diffusion Couples. W.J.Boettinger, S.R.Coriell, C.E.Campbell, G.B.McFadden: Acta Materialia, 2000, 48[2], 481-92