Spatial confinement-induced switchover in microstructure evolution during severe plastic deformation at micrometer length scales

Severe plastic deformation in micrometer scale sample volumes is investigated by performing large strain machining inside a scanning electron microscope. Digital image correlation of in situ secondary electron images of the deformation zone is used to measure the mechanics of deformation. The effect of length scales on microstructure evolution is characterized using orientation imaging microscopy/electron backscattered diffraction analysis of the deformation zone. A geometrically necessary switchover in microstructure evolution mechanisms in small length scales is identified and attributed to a coupled effect of geometric confinement and high strain gradients. A phenomenological model capturing the switchover is proposed. It is shown that contrary to conventional perception, a high crystallographic curvature caused by strain gradients impedes grain refinement in small length scales, resulting in an anomalous suppression of grain refinement and fragmentation.