Precipitation and growth of γ′ phase in an Fe-38Ni-13Co-4.7Nb superalloy

Abstract

An Fe-38Ni-13Co-4.7Nb base superalloy (alloy 909) is the latest low thermal expansion chromium-free superalloy with a good resistance to SAGBO (Stress Accelerated Grain Boundary Oxidation) embrittlement at elevated temperatures. This investigation is carried out to elucidate the relation between the age-hardening and the nucleation and growth behavior of γ′ precipitates in alloy 909 by micro-Vickers hardness test and transmission electron microscopy. The hardness of alloy 909 measured at aging temperatures is ca. HV100 lower than that measured at room temperature. The hardness of specimen aged at 893-1 033 K for durations up to 720ks closely relates to the mean size of γ′ precipitates. The growth kinetics of the γ′ precipitates in ε. phase free region is explained by Lifshitz-Slyozov-Wagner's theory of volume diffusion controlled growth at 943-1 033 K. The activation energy for the growth of γ′ precipitates is estimated to be 254kJ/mol which is nearly equal to those of diffusion of Ti or Fe atoms in γ-iron or nickel.