The effect of η phase precipitates on the creep behavior of alloy 263 and variants

Phase stability is an important design parameter for Ni-based superalloys to be used in future advanced ultra-supercritical (AUSC) power plants as exposure times in this type of environment are considerable. In this investigation, microstructures based on candidate alloy 263 were obtained with varying amounts of η precipitates using isothermal exposure at 800 °C for times ranging from 1000 h to 10,000 h. The effect of η phase stability on the creep properties was determined using creep specimens isothermally aged at 800 °C for 8 h, 3000 h, 5000 h and 10,000 h prior to creep screening. The creep life was found to exponentially decrease with increasing density of η phase while the elongation to failure was found to increase. Furthermore, the minimum creep rate was related to the density of η phase; a relationship that did not depend on the alloy formulation. Modification to the Ti and Al concentrations slowed down the γ′ to η transformation while doubling the γ′ fraction after standard heat treatment. By modifying the Ti and Al content, and thereby improving γ′ stability over η, the creep lives of specimens isothermally aged for up to 5000 h were greater than that of the nominal alloy in its standard aged condition.