Effect of Co partitioning to the {\gamma} matrix on the microstructural stablity of a Ti-rich Ni-Base Superalloy

TL;DR

This study investigates how cobalt partitioning into the gamma matrix affects the microstructural stability and coarsening resistance of a Ti-rich Ni-based superalloy, revealing increased stability with higher Co content through experimental and theoretical analysis.

Contribution

It provides new insights into the role of Co partitioning on gamma prime coarsening kinetics and stability, combining atom probe tomography and CALPHAD simulations.

Findings

Higher Co content increases coarsening resistance.

Co partitioning influences solute redistribution and flux.

Gamma prime volume fraction impacts yield strength.

Abstract

The microstructural stability and mechanical properties of superalloys at high temperatures are significantly influenced by the composition and nature of the solutes they contain. Most of the alloys with high solvus temperature have higher gamma prime coarsening resistance, while the larger lattice misfit is attributed to higher gamma prime coarsening rate. In this work, we explore the influence of Co on the microstructure evolution, thermophysical/mechanical properties and gamma prime precipitate coarsening kinetics in a Ti-rich Ni-Co-Cr-Al-Ti based alloy. More specifically, we focus on the effect of partitioning of Co into the gamma matrix on the redistribution of other solutes across the interface. We observe a significant increase in the coarsening resistance and a twofold increase in the activation energy with the increase in the Co composition from 10at.%Co to 30at.%Co, even though the gamma prime solvus reduces by 75C. As otherwise, a higher solvus, usually, indicates better microstructural stability at high temperatures. We employed a combined experimental and theoretical approach by atom probe tomography (APT) and CALPHAD simulations to probe the critical role of Co partitioning to gamma matrix on the solute transport in the gamma matrix and flux across the gamma/gamma prime interfaces, which is found to control the overall gamma prime coarsening behavior in the alloy. The observed behavior was rationlised by the proposition of a simplistic unified coarsening rate expression that successfully decouples thermodynamic and kinetic contributions. Additionally, we also observe that the gamma prime volume fraction dominates over the gamma prime precipitate size on the 0.2% yield strength (YS) of the alloys.