Evidence of grain boundary dislocation step motion associated to shear-coupled grain boundary migration

The present work reports dynamical observations of the grain boundary (GB)-mediated plasticity during in situ transmission electron microscopy straining experiments at moderate temperature (400C) both in a 76.4 bicrystalline and a polycrystalline Al sample. We show that the GB migration occurs by the lateral motion of elementary GB dislocation steps. The accumulation of GB dislocation steps eventually form macro-steps. This observation agrees with the idea that GB dislocation steps generally operate in high angle GBs similarly as in twinning or martensitic transformations. The coupling factor, i.e. the strain produced by the motion of the steps was measured using fiducial markers and image correlation. The migration process involves different types of GB dislocation steps, producing different amounts of strain both parallel (coupling factor) and perpendicular to the GB plane.