On the role of tertiary γ′ precipitates in the creep behaviour at 700°C of A PM disk superalloy

Abstract

The creep behaviour of the PM NR3 nickel-based superalloy has been studied at 700°C in a wide stress range as a function of its microstructure. It was first confirmed that at this critical temperature a coarse-grained microstructure was preferable to a fine-grained one to obtain the highest creep resistance. The role of the tertiary γ′ phase precipitates in the creep behaviour of coarse-grained NR3 was investigated at two stress levels typical of both low stress and high stress creep regimes. Careful identification of dislocation mechanisms by transmission electron microscopy was performed in creep strained specimens in order to correlate the macroscopic behaviour with the creep controlling mechanisms. Prior elimination of the tertiary γ′ precipitates using an adequate overageing heat treatment promotes easy glide of a/2〈110〉 perfect matrix dislocations between the secondary γ′ precipitates and therefore confers to the alloy a poor creep resistance. On the contrary the presence of fine tertiary γ′ particles impedes propagation of the perfect matrix dislocations which are forced to cut the γ′ precipitates through two different modes of dissociation depending on the local precipitate distribution. Reduction of the γ phase channel width promotes the decorrelation in the matrix of the two a/6〈112〉 partial dislocations constituting an a/2〈110〉 perfect dislocation. This mechanism which leads to stacking fault configurations extending through both γ and γ′ phases evidences the strong resistance of the microstructure against creep deformation. Coupled analysis of the creep behaviour and of the dislocation structures in the low stress creep regime indicates a growing involvement of the grain boundary areas in the macroscopic creep deformation.