On the properties of alpha/alpha+beta ternary diffusion couples

The properties of diffusion couples in a ternary system are examined theore tically when one end member is composed of single-phase material (alpha) an d the other end member is composed of two-phase material (alpha + beta). Us ing the model of Morral and co-workers, the cases in which the two-phase re gion either grows or dissolves are examined. It is proved that when the two -phase material grows, there can be no jump in concentration (or phase frac tion of beta) at the boundary between the two materials. When it dissolves a jump is necessary. Numerical examples are presented to demonstrate this r esult. It is also shown how the diffusion path varies depending on the comp osition of the single-phase alpha end member for a fixed alpha + beta end m ember composition. Under some conditions, three solutions are found for the same initial end member compositions. When these multiple solutions exist, either two or three are virtual (path involves supersaturation). Whenever all three solutions are virtual, a single solution with three interfaces ca n be found. In this case an intermediate layer of alpha phase forms within the alpha + beta material to relieve the supersaturation. Published by Else vier Science Ltd on behalf of Acta Metallurgica Inc.