Subsurface microstructure effects on surface resolved slip activity

We investigate the influence of subsurface microstructure on the micromechanical and slip activity fields at the free surface on a polycrystalline Ni-based superalloy under deformation. The approach combines full-field crystal plasticity finite element simulations, high resolution three-dimensional electron back-scattered diffraction TriBeam technology, and high-fidelity mirroring of the microstructure to bring to the analysis statistically significant numbers of grains and a broad field of view. The analysis reveals substantial disparities in the spatially resolved fields of stress, lattice rotation, and slip activity at the surface between a columnar grain representation versus the experimental full-3D subsurface representation, with deviations intensifying and changing spatially with applied strain, after slip locally initiates. We show that the location and intensity of incipient slip, as well as choice of primary active slip system, are highly sensitive to the underlying subsurface microstructure. Detailed 3D analysis of exceptionally affected regions identifies that influential subsurface structures are grain boundaries inclined to the surface or near-surface quadruple points. A statistical analysis is conducted to correlate the micromechanical quantities and slip activity to several key microstructure features both on and beneath the surface. The analysis finds that influential subsurface microstructure features are primarily linked to proximity to triple junctions and tendency of free-surface grains to deform via multiple slip systems.