A new insight into ductile fracture of ultrafine-grained Al-Mg alloys

It is well known that when coarse-grained metals undergo severe plastic deformation to be transformed into

nano-grained metals, their ductility is reduced. However, there are no ductile fracture criteria developed

based on grain refinement. In this paper, we propose a new relationship between ductile fracture and grain

refinement during deformation, considering factors besides void nucleation and growth. Ultrafine-grained

Al-Mg alloy sheets were fabricated using different rolling techniques at room and cryogenic temperatures. It

is proposed for the first time that features of the microstructure near the fracture surface can be used to

explain the ductile fracture post necking directly. We found that as grains are refined to a nano size which

approaches the theoretical minimum achievable value, the material becomes brittle at the shear band zone.

This may explain the tendency for ductile fracture in metals under plastic deformation.