Anomalous, pre-yield grain-boundary sliding in copper revealed with in-situ high-resolution strain mapping

Grain boundary sliding is typically associated with high temperature deformation in engineering alloys. Here, we examine grain boundary sliding at room temperature in oxygen-free high-conductivity copper under quasi-static tensile testing. By using high-resolution digital image correlation (HRDIC) conducted in-situ within a scanning electron microscope to produce time-series strain maps, we unexpectedly observe that grain boundary sliding occurs extensively prior to macroscopic yield, and before the onset of significant crystallographic slip. Extreme values in strain and in-plane rotation are found to be associated with grain boundaries immediately prior to yield and during the initial stages of plastic deformation, which are higher than those associated with crystallographic slip. By combining laser scanning confocal microscopy height mapping with the strain maps and orientation maps from electron backscatter diffraction, grain boundary sliding character is determined, finding evidence of pure in-plane, pure out-of-plane and mixed-mode sliding.