Fatigue-induced evolution of nanograins and residual stress in the nanostructured surface layer of Ti–6Al–4V

Using ultrasonic deep rolling process, we have formed a surface layer of nanograins in Ti–6Al–4V alloy. The effect of fatigue deformation under uniaxial loading on the evolution of average grain size and Tresca stress in the surface layer of nanograins is studied. Increasing the strain amplitude of the uniaxial fatigue test increases the relaxation rate of the Tresca stress and the growth rate of the nanograins. A simple relation between the grain coarsening of the nanograins and the cycle number is used to describe the fatigue-induced grain growth of the nanograins in the surface layer. There exists strain-assisted grain growth of the nanograins, which is driven by strain energy and associated with the relaxation of residual stress. The rate of the grain growth can be approximately expressed as an exponential function of the strain amplitude.