On the effect of Ti addition on microstructural evolution, precipitate coarsening kinetics and mechanical properties in a Co–30Ni–10Al–5Mo–2Nb alloy

Abstract

In this work, the effect of varying Ti content on the γ/γ' microstructure, precipitate coarsening kinetics, and mechanical properties in a series of Co-30Ni-10Al-5Mo-2Nb-xTi alloys (where x = 0, 1, 2, 3, and 4 at. %) is presented. An addition of 2 at. % Ti and beyond changes the γ′ morphology from rod shape (alloys containing 0 and 1 at. % Ti) to cuboidal shape (alloys containing 2 to 4 at% Ti) when aged beyond 500 h at 900 °C. The composition profiles obtained by STEM-EDS analysis reveal a strong partitioning of Ni, Ti, Al, and Nb in the γ′ precipitate, whereas partitioning of Mo changes from (KMo>1) γ′ precipitates for Co-30Ni-10Al-5Mo-2Nb alloy to equal proportion in both the phases (KMo∼1) for alloys containing 1 at.% Ti or more. Among the alloys studied, Co-30Ni-10Al-5Mo-2Nb-4Ti alloy exhibits an excellent combination of low mass density (8.19 g/cc) and high γ′ solvus temperature (1117 °C). The addition of Ti to the base 2Nb0Ti alloy introduces a 0.2% proof stress anomaly with temperature with a peak in strength was observed at 770 °C. The experimentally determined temporal evolution of average γ′ precipitate size suggests an LSW model-based evaporation-condensation (EV) coarsening mechanism in alloys containing 1 and 2 at% Ti. In contrast, a combined EV and coalescence/coagulation coarsening mechanisms operates in alloys with 3 and 4 at% Ti at the aging temperature of 900 °C. Despite the increase in γ/γ′ stability, the coarsening rate constant, calculated using modified LSW based coarsening model, increases by six times with an increase in Ti content from 1 to 4 at.%.