Characterisation of strain localisation under cyclic loading at 450 °C by SEM-DIC in a PM Ni-based superalloy

Abstract

In this study, the strain localisation under cyclic loading in an advanced powder metallurgy Ni-based superalloy at 450 °C was characterised by SEM-DIC, and the related deformation substructures were characterised by electron channelling contrast imaging under controlled diffraction conditions (cECCI). The results indicate strain mainly accumulates in slip/strain bands due to dislocation slip on the {111} plane, and the strain saturates at 1000 cycles with no evident increase in strain value within strain bands or strain band density from 1000 cycles to 10,000 cycles. Analysis of the strain components in the local coordinates associated with each strain band shows the strain band mainly consists of shear strain ϵxy along the strain band and transverse strain ϵyy normal to the strain band. The strain bands can transmit through, be deflected, or be blocked at grain boundaries depending on the grain boundary characteristics and the neighbouring grain orientation. Comparing the strain bands obtained by SEM-DIC and deformation substructures obtained by cECCI shows that the strain bands originate from underlying continuous slip bands which show the sheared γ′ within the slip bands and dislocation pairs looping around the γ′ precipitates, but the deformation localised in the discontinuous slip bands is not well characterised by SEM-DIC at this lateral resolution of strain.