Morphology of γ′ Precipitate in NKH71 Nickel Based Superalloy Depending on Heat Treatment

Abstract

NKH71 is a Re free nickel based single crystal superalloy. The long term creep strength of this alloy is superior to that of the practical used 2 nd generation superalloy, CMSX 4, containing Re but the short term creep strength is inferior to that of CMSX 4. This may be caused by the initial microstructure before creep. In order to arrange the initial morphology of the g′phase in NKH71, eight kinds of processes in heat treatment are carried out. After a series of heat treatments, each specimen is examined with scanning electron microscopy (SEM) and transmission electron microscopy (TEM). From SEM, it is found that the initial morphology of the g′ phase is different by the cooling rate after solution treatment and temperatures of 1 st step aging. It is also found by TEM that dislocation density at the interface between the g and g′ phase is different. In order to make clear the origin of the morphological difference in the each heat treatment, the phase field simulation is carried out on the basis of Gibbs free energy calculated using thermodynamic date of Ni Al binary system.