Influence of microstructure on local intra- and intergranular deformations during creep of a nickel-based superalloy at 700 °C

Several microstructural parameters of the NR6 nickel-based superalloy were varied by applying four heat treatment procedures. Tensile creep tests were performed in air at 700 °C and 700 MPa on specimens exhibiting different grain sizes, strengthening γ′ precipitate sizes and distributions and grain boundary morphologies. Flat creep specimens were instrumented thanks to a dedicated microextensometry technique to analyze the local intra- and intergranular deformations after tensile creep under vacuum at 700 °C. Tertiary γ′ precipitate dissolution, grain boundary serration and larger grain size reduce grain boundary sliding contribution to overall strain. These effects are attributed to the promotion of Orowan by-passing instead of shearing of the γ′ strengthening particles by matrix dislocations, increase of grain boundary resistance or decrease of grain boundary fraction. Relationships between local deformations and macroscopic creep behavior have been evidenced.