Periodic formations of deformation twins and dislocation arrays via grain boundary sliding at serrated grain boundary in cold-shear-strained AZ31 alloy

Abstract

It is well known that grain boundary sliding (GBS) provides stress concentration and initiates grain boundary fracture at the serrated grain boundary or grain triple junction during high-temperature creep. Recent reports on GBS occurring during deformation at room temperature suggest that stress concentration may occur due to GBS, which has a stress component parallel to the grain boundary plane, even in cold working. To identify if GBS can generate stress concentration and related defects at the grain boundary at room temperature, we experimentally investigated periodic defect formation along the serrated grain boundary during cold shear deformation. To obtain defect-free serrated grain boundary structure, optimal conditions of hot-torsion and annealing were determined. Cylindrical torsion specimens were then machined into notched tensile specimens, which provide the advantage of directly observing the GBS and lattice curvature. Extensive observations and orientation analysis on periodic defect formation along the serrated grain boundary suggest that GBS at room temperature can periodically nucleate twinning as well as dislocation arrays. Crystal plasticity simulation proved that serrated GB generates non-rotating region and split stress distribution with intensified value. Two stress concentration mechanisms during the accommodation of GBS at the serrated grain boundary were proposed: direct transfer from rotational stress and indirectly formed tensile component, namely “cleavage stress.” While direct energy transfer was likely to follow the local Schmid's law for direction of the grain rotation, cleavage accommodation represented geometrically necessary characteristics.