Gamma prime (γ′) precipitating and ageing behaviours in two newly developed nickel-base superalloys

Abstract

Two cast nickel-base superalloys with superior creep rupture lives at high temperatures and high stresses were developed. In the present study, the microstructural characteristics of γ′-precipitates and the hardening behaviour were studied by electron microscopy and micro-hardness testing. The alloys were solution-treated at 1553 K for 2 h and air-cooled followed by step ageing heat treatments. Ageing up to 300 h at 1143, 1223 and 1293 K was carried out to examine the effects of ageing temperature and ageing time on the growth of γ′-precipitates. The growth kinetics of γ′-precipitates was analysed. The experimental evidence reveals that the morphology of γ′ does not change in shape during heat treatments. The γ′ is usually cubic in both Alloy B and Alloy C. Growth of γ′ precipitates proceeds by Ostwald ripening controlled by volume diffusion of solute atoms despite the high volume fraction of γ′ and complex compositions in these two alloys. The activation energies for the growth are evaluated as 272 and 277 kJ mol^-1 for Alloy B and Alloy C, respectively, which correlate well to coarsening of γ′ in other commercial and developed superalloys. The hardness increases to a peak value at about 20 to 60 h for Alloy B and at about 10 to 20 h for Alloy C during ageing. The hardness of the alloys hardly decreases and still maintains a high value at high temperatures after passing the hardness peak.