Extraordinary fracture toughness in nickel induced by heterogeneous grain structure

The tensile properties and fracture toughness of both heterogeneous grain structures and homogeneous structures in pure nickel have been investigated and compared. The heterogeneous samples were found to show much larger uniform elongation and much higher fracture toughness at the similar level of yield strength, compared to the homogeneous structures. The enhanced ductility/toughness can be attributed to the stronger hetero-deformation-induced hardening for the heterogeneous structures. In sharp contrast to the cleavage-like and brittle fracture behavior in the homogeneous hot-extruded and cold-rolled samples, the fracture process in the heterogeneous structures shows ductile fracture by microvoid coalescence and dimples. The crack path in the heterogeneous structures was found to display a gourd shape, which can result in longer crack length and dissipate more energy for better fracture toughness. Hardening is obvious across a much larger area around the crack tip in the heterogeneous structures as compared to the homogeneous structures, indicating a much larger plastic zone for the heterogeneous structures. The strong strain hardening at the crack tip was found to be induced by the grain refinement of the coarse grains in the heterogeneous structures, resulting in extraordinary fracture toughness.