Microstructure and mechanical behavior at elevated temperatures of a novel nickel base superalloy heavily alloyed with γ' forming and substitution elements

A novel nickel base superalloy heavily alloyed with γ' forming and substitution elements, particularly with rhenium and tungsten, has been proposed in the present work as a potential die material for die forging of the most hard-to-deform nickel base superalloys. The superalloy composition is Ni-13(Al, Ti, Nb, Ta)-27(Cr, Co, Mo, W, Re, Si, C, B) (wt.%). The superalloy ingot was manufactured by vacuum induction melting. Microstructure examination showed that the as-cast condition was characterized by a coarse grained dendritic structure consisted of coarse γ grains containing γ' precipitates with a size 0.3 –1 μm and primary γ' particles with a size 5 –100 μm. A small amount of carbides and topologically close packed (TCP) phases was also detected. The microstructure examination suggests that the general solidification pathway was L → γ → γ', as it is in nickel base superalloys. At the same time, the primary γ' particles were probably formed via the peritectic reaction Lnoneq. + γ → γ', where Lnoneq. is the nonequilibrium liquid, which was retained at the peritectic temperature. The superalloy was subjected to heat treatment including solid solution treatment and ageing. The heat treatment led to coagulation of the primary γ' phase and to a slight decrease in the volume fractions of carbides and TCP phases. The heat treated condition was used to prepare samples for compression tests, which were performed at 900 –1200°C up to the true strain e = 0.1. The yield strength of the novel superalloy was generally appreciably higher as compared with those of known nickel base superalloys and alloys based on the Ni3Al intermetallic phase.