Grain boundary and intragranular deformations during high temperature creep of a PM nickel-based superalloy

Abstract

Grain boundary and intragranular deformations have been analysed during high temperature creep of a PM nickel-based superalloy. Local deformation has been quantified using an original microextensometry technique. This technique consists in calculating deformation of ceramic square grids deposited on flat specimens thanks to an electron lithography technique. Grain boundary sliding (GBS) was quantified by measuring microgrid offsets at grain boundaries. Strain maps and GBS maps have been computed. Superimposition with electron back scattered diffraction (EBSD) maps allowed studying the influence of grain boundary character on their deformation behaviour. The effectiveness of this technique was demonstrated through creep test performed at 700, 750 and 800°C under an applied stress of 700 MPa. The results showed that grain boundary and intragranular mechanisms of deformation accommodate one's each other to preserve the continuity of deformation in the material. Modification of deformation mechanisms within grains was related to coarsening of tertiary γ′ precipitates with increase of temperature. Simultaneously, decrease of GBS contribution was observed when temperature increases.