Varying tensile fracture mechanisms of Cu and Cu-Zn alloys with reduced grain size: From necking to shearing instability

In ultrafine-grained (UFG) or nanocrystalline (NC) materials, achieving high strength often induces loss of ductility due to the formation of shear fracture without obvious necking feature. To investigate mechanical properties and fracture mechanism with reduced grain size, the macroscopic tensile fracture behaviors of UFG or NC Cu and Cu-Zn alloys were systematically investigated. It is found that the tensile strength and uniform elongation of UFG or NC Cu and Cu-Zn alloys display simultaneously increasing trend. The limitation of ductility can be attributed to the occurrence of shear bands in these materials; their main characteristic, the shear fracture angle, decreases with decreasing grain size as well as the degree of necking. The macroscopic fracture mechanisms are explained using an Ellipse criterion in terms of modification of the stress state in the fracture zone. It is suggested that the reduction of necking degree results from the geometrical hardening effect due to the change of the stress state. As the degree of necking decreases with decreasing grain size, the tensile shear fracture angle decreases too in the UFG/NC materials. Thus improving the geometrical hardening ability may effectively inhibit the formation of shear fracture, as the nucleation of shear bands becomes difficult.