Cobalt-Controlled Interphase Partitioning Regulates Matrix Solute Transport and ${\gamma }^{\prime }$ Coarsening in Ti-Rich NiCoCr-Based Superalloys

Abstract

The microstructural stability and mechanical response of $\gamma /{\gamma }^{\prime }$ superalloys at elevated temperatures are governed by solute chemistry and elemental partitioning across the $\gamma /{\gamma }^{\prime }$ heterophase interface. Conventionally, a lower ${\gamma }^{\prime }$ solvus temperature and a larger $\gamma /{\gamma }^{\prime }$ lattice misfit are expected to increase the coarsening rate. Here, Co-for-Ni substitution is used in Ni--Co--Cr--Al--Ti superalloys to systematically modify the $\gamma$-matrix chemistry and generate medium-entropy-alloy-type matrix environments with Ni:Co:Cr ratios of approximately 2:1:2, 2:2:1, and 1:2:1. Despite a $\sim$65~${}^{\circ }$C reduction in ${\gamma }^{\prime }$ solvus temperature, the activation energy for Ti-rich Ni$3$(Al,Ti)-type $L12$ ${\gamma }^{\prime }$ coarsening increases from $\sim$156 to $\sim$302~kJ~mol${}^{-1}$ with increasing Co content. This enhanced coarsening resistance is attributed to an increase in the multicomponent solute-transport resistance, $S={\sum }_i(\Delta C_i{)}^2/(D_i^{\gamma }{C}_i^{\gamma })$, from $\sim 8.811\times 10^{15}$ to $\sim 1.267\times 10^{16}$~s~m${}^{-2}$, together with a reduction in the apparent interfacial-energy term from 25.0 to 7.55~mJ~m${}^{-2}$. The dominant transport resistance correspondingly shifts from Ni/Cr in B--10Co to Ni/Co/Cr in B--30Co.

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