Effect of Gibbs Free Energies of Terminal Alloys on the Diffusion Paths and Diffusion Depths in Couples Assembled with γ-Phase Fe-Ni-Cu Alloys at 1000 °C

A novel concept for diffusion couples based on iso-Gibbs free energy of mixing (iso-\(\Delta G^{M}\)) is proposed, in which the two terminal alloys of the couple have the same Gibbs free energy of mixing. A few iso-\(\Delta G^{M}\) couples were assembled with FCC Fe-Ni-Cu alloys at 1000 °C. To enable the selection of the terminal alloys for iso-\(\Delta G^{M}\) couples, iso-\(\Delta G^{M}\) lines were constructed from the data on the thermodynamic activities of the constituent elements in FCC Fe-Ni-Cu alloys at 1000 °C. A few couples were also studied whose composition vectors were at about 90°, less than 90° and greater than 90° to one of the iso-\(\Delta G^{M}\) couples. A strategy is also proposed and utilized to construct a near iso-Gibbs free energy (iso-\(G^{soln}\)) couple. All but one iso-\(\Delta G^{M}\) couples and also, the iso-\(G^{soln}\) couple were characterized by almost linear diffusion paths. All the iso-\(\Delta G^{M}\) couples presented very low diffusion depths characterized in terms of effective penetration depth (\(\overline{x}\)), with the iso-\(G^{soln}\) couple exhibiting the lowest values of \(\overline{x}\). This experimental work clearly reveals the definite role played by the difference in Gibbs free energies of terminal alloys in dictating the diffusion paths and the relative diffusion depths in the diffusion couples.