Abstract
Ultrafine grained (UFG) metals and alloys typically exhibit mechanical and thermal instabilities, partially due to the high density of lattice defects, which limits their engineering applications. Annealing represents a simple and effective way to regain strain hardening, ductility and thermal stability, and stabilize the UFG structures. In this study, we systematically investigated the mechanical behavior, microstructural evolution, fracture and deformation mechanisms of UFG Al during recovery via low-temperature annealing. More specifically we report that low-temperature annealing at 250 °C for 20 min increased the ultimate tensile strength by 10% from 190 to 208 MPa and tensile ductility by 50% from 4.5 to 6.8% without any changes in yield strength (180 MPa). Microstructural analyses indicate that the annealing increased the average grain size from 740 to 840 nm, dislocation density decreased from 5 × 1014 m-2 to 1 × 1014 m-2, while the nature of the grain boundaries and associated precipitated phases remained unchanged. Moreover, annealing led to modification of statistically stored dislocations into low-energy dislocation walls (sub-grain boundaries). Results from the fracture surface morphology indicated that the enhanced ductility of the annealed sample was related to the activation of numerous homogeneous micro shear bands, which were controlled by cooperative grain boundary sliding. These observations suggest that the dislocation walls formed during recovery promoted the formation of micro shear bands/cooperative grain boundary sliding and thereby enhanced the ductility.