Splitting of γ′ Precipitates in the Context of Phase Equilibrium

Abstract

The splitting of γ′ (Ni3Al and Ni3Si) precipitates in five binary Ni-Al alloys and one Ni-Si alloy is reviewed in the context of phase equilibrium. Two mechanisms are considered: Purely Elastic (PE) splitting, driven solely by competition between elastic and interfacial free energies; Thermodynamically Driven (TD) splitting, involving precipitation of the Ni-Al or Ni-Si solid solution γ phases within supersaturated γ′ particles. The main assertion is that TD splitting is responsible for all the observations, with the possible exception of dendritic growth of Ni3Si precipitates; dendritic morphologies can mimic split configurations. In three of the six investigations splitting was reported for alloy compositions lying within the single-phase γ regions of the binary Ni-Al and Ni-Si phase diagrams wherein the γ′ phases are unstable. For the three others the aging temperatures were at or barely below the solvus temperatures, suggesting that five of the alloys were compositionally heterogeneous, “solution treatment” having failed to dissolve pre-existing γ′ particles. TD splitting was thus a byproduct of slow cooling to the aging temperatures, as in the formation of hierarchical microstructures. The nature of secondary γ′ precipitation in some of the alloys indicates that their compositions exceeded the authors’ quoted values, the enrichment enabling precipitation of γ′ during solution treatment followed by TD splitting on slow cooling. PE splitting is the only possible mechanism in solution-treated specimens that are quenched and subsequently isothermally aged. Splitting under such conditions has never been reported, lending further support to the viability of the TD mechanism.