Microstructure Evolution of Multicomponent γ′-Strengthened Co-Based Superalloy at 750 °C and 1000 °C with Different Al and Ti Contents

Abstract

Three novel Co-30Ni-(7 − 1.5x)Al-2W-1.5Mo-1Ta-(3.5 + 0.7x)Ti-12Cr-0.1Hf-0.08C-0.08B-0.4Si-0.01Zr (x = 0, 1, 2 at. pct) γ′-strengthened wrought Co-based superalloys with varying concentrations of Al and Ti were investigated to clarify the effects of the Al and Ti concentration on the microstructural stability at 750 °C and 1000 °C. The results show that the 7Al-3.5Ti and 5.5Al-4.2Ti alloys maintain a γ/γ′ two-phase microstructure after aging at 1000 °C for 600 hours, but the 4Al-4.9Ti alloy forms a η(D0₂₄)-(Co,Ni)₃Ti phase after aging at 1000 °C for 312 hours. There exist stacking faults in the γ′ phase adjacent to the D0₂₄ needles, which form during the aging process, and the growth of the η(D0₂₄) phase is controlled by the diffusion of W. The γ′ phase and η(D0₂₄)-(Co,Ni)₃Ti phase exhibit the following OR: $\left. {\left\langle { 1 1 {\bar{\text{2}}}0} \right\rangle \left\{ {000 1} \right\}\eta } \right\|\left\langle { 1 10} \right\rangle \left\{ { 1 1 1} \right\}\gamma \prime$. All the alloys possess good intermediate-temperature microstructural stability, with γ/γ′ two phases after aging at 750 °C for 1500 hours. With increasing Ti content and decreasing Al content, the coarsening rate of γ′ at 1000 °C first decreases and then increases; however, it gradually decreases at 750 °C. This difference in the coarsening behavior is attributed to the interaction of the interfacial energy, diffusion of the solute atoms and coherent stress.