Significance of surface layer integrity for sustaining the ductility of gradient-structured nickel

Gradient-structured nickel (Ni) was processed by rotationally accelerated shot peening (RASP). The microstructures and mechanical properties of the RASP-processed samples were studied. Nanostructured layers with a thickness of ~50 μm were formed on the sample surfaces. Beneath the surface layer there is a transition region in the depth range of ~50–150 μm. The transition region contains high densities of dense dislocation walls (DDWs), microbands (MBs), sub-grain boundaries and dislocations, and is susceptible to plastic instability. High-speed RASP processing created some small cracks which deteriorate the integrity of the nanostructured surface layer. During tensile deformation, local stress concentrations are created beneath the surface cracks to trigger plastic instability in the nearby transition region. As a result, interior cracks nucleated and grew in the transition region, and eventually agglomerated with surface cracks to cause premature failure of the bulk material.