Understanding the microstructural stability in a γ′-strengthened Ni-Fe-Cr-Al-Ti alloy

Ni-Fe-Cr-Al-Ti alloys, with Ni levels of about 45 at% have the potential to develop a microstructure consisting of a face-centered cubic (γ) matrix with homogeneously precipitated, nanoscale ordered γ′ precipitates similar to that found in traditional Ni-based superalloys with a significantly greater Ni content. Scanning electron microscopy, transmission electron microscopy, atom probe tomography, and CALPHAD-based thermodynamic modeling were employed to determine the phase stabilities and microstructural evolution in an age-hardenable 44.56Ni-26.6Fe-19.2Cr-1.0Co-3.4Al-4.4Ti-0.7Mo-0.14 C (at%) alloy. The primary heat treatment of solution annealing at 1121 °C for 4 h followed by age-hardening at 760 °C for 16 h resulted in a microstructure consisting of fine γ′ precipitates in an austenitic matrix along with grain boundary precipitates of carbides and other minor phases. Long-term aging at 900 °C for 250 h resulted in the coarsening of γ′ precipitates along with a change in the morphology from an initial spherical to a more cuboidal shape. In addition, the formation of plate-like η phase precipitates was observed, concomitant with the partial dissolution of the γ′ phase. The ability of computational thermodynamic models to predict microstructural characteristics is discussed.