Recrystallization-induced transition from brittle to ductile fracture in severe plastic deformed copper

Abstract

Severe plastic deformation (SPD) has been widely used to improve the strength of metallic materials. The improvement in strength comes with sacrifice of ductility. The poor ductility of SPD-materials is generally accompanied by brittle fracture. Here, we demonstrate that, once heat treatment (HT) is conducted over the recrystallization temperature, SPD-copper under quasi-static uniaxial tension can fracture via ductile dimples, showing a transition from brittle to ductile fracture. This transition is reflected by the diagram of ultimate strength versus uniform elongation, as well as the changes in the macroscopic fracture angle from incline to perpendicular and in the microscopic fracture morphology from shear dimples to tensile dimples. Microstructural observations show that the inhomogeneous microstructure of recrystallization surrounded by elongated substructure affects the fracture behavior. An exponential expression describing the relationship between fracture parameter and the volume fraction of recrystallization is obtained by performing numerical simulations. Combining the exponential expression and the ellipse criterion, we proposed a theoretical model to analyze the brittle-to-ductile transition in materials processed by SPD and HT. Comparative studies show that the proposed failure criterion can accurately predict the fracture angles. It is revealed that normal stress sensitivity of failure in SPD-materials depends on HT temperature.