Measurement of local plastic strain during uniaxial reversed loading of nickel alloy 625

The reverse loading behaviour of polycrystalline alloys is challenging to predict, as it depends on complex multi-scale interactions that are not well understood and difficult to study. We have used high resolution digital image correlation (HRDIC) to measure the local strain development during reversed loading of a nickel based superalloy with sub-micron spatial resolution. A specially designed rig ensured that deformation remained uni-axial during reversal, and measurements were made in-situ, with the sample under load. The deformation data was correlated with the underlying crystallographic microstructure using orientation maps from electron back-scatter diffraction (EBSD), which was transformed using an affine transformation, to account for measurement related distortion. The strain was found to be localised into crystallographic slip bands separated by regions with much lower, mostly elastic, strain. On unloading, no localised deformation reversal could be measured in slip bands or elsewhere, implying that most of the strain reversal is elastic and that any plasticity during unloading is small, evenly distributed, and could not be detected using HRDIC. These results imply that HRDIC studies on unloaded samples can provide representative measurements of the deformed state of the material during reversal. On reversal, deformation is accommodated primarily by slip on slip bands formed during forward loading. In some grains, however, the slip band patterns appear to change location and sharpen with further deformation. This suggests that slip extent of slip reversal at the scale of slip bands is limited and can be quantified, opening up a new way to study reverse loading in advanced alloys.