Deformation and fracture mechanisms of gradient nanograined pure Ti produced by a surface rolling treatment

Gradient nanograined Ti (GNG Ti) samples were produced by a surface rolling treatment. The mechanical properties were evaluated by microhardness and tensile tests. The results indicated that GNG Ti achieved an optimized combination of strength and ductility relative to the free-standing nano/ultrafine-grained Ti. The deformation and fracture mechanisms were studied using transmission electron microscope (TEM) and scanning electron microscope (SEM). As the grain size reduced from the coarse-grained (CG) matrix to the nano/ultrafine-grained (NG/UFG) layer, the transition from conventional plastic deformation mechanisms, such as dislocation activity and twinning deformation, to grain boundary-mediated deformation mechanisms, such as stress-inducing grain growth, was identified. At the early stage of tensile deformation, the microcracks were first initiated in the CG and deformed grain (DG) regions rather than in the NG/UFG region and the CG/DG interface had a deflection effect on crack propagation. After tensile failure, the fracture morphology showed a mixture of ductile dimples and shear band-like regions.