High-resolution digital image correlation study of the strain localization during loading of a shot-peened RR1000 nickel-based superalloy

Abstract

The improved crack nucleation resistance of shot-peened materials has been usually studied from the point of view of the residual stresses. However, experimental limitations have made difficult to quantify strain localization during further deformation. In this study, we have used high-resolution digital image correlation (HRDIC) and electron backscatter diffraction (EBSD) in a large area covering shot-peened material and bulk to study quantitatively the strain distribution at the submicron scale. We found that, at the earlier deformation steps (e < 2.5%), the slip intensity and distribution at the shot-peened region differ from that observed at the bulk. In the former, well-spaced, sharp and homogeneously distributed slip bands are observed. On the other hand, in the shot-peened region, slip is less evident, and a more homogeneous diffuse deformation is observed along the majority of the grains. However, this region also contains a small number of slip bands that accumulate high strains, mainly along Σ3 annealing twin boundaries. Additionally, the border between both regions, at a depth with no residual stress, present the minimum strain values. During further deformation (e > 2.5%), the strain distributions become similar along the material simultaneously to the residual stresses relaxation. A further analysis of the HRDIC maps allowed for the first time to quantify strain heterogeneity. This study reveals the importance of the local stress state and the strain history on the strain localization during loading of peened materials.