Thermal relaxation of compressive residual stresses in surface gradient nanostructure of TC11 titanium alloy

The thermal relaxation behaviour of compressive residual stress (CRS) in the gradient nanostructure of TC11 titanium alloy treated with ultrasonic surface rolling process (USRP) was investigated by X-ray residual stress test, transmission electron microscope (TEM) and high-resolution TEM (HRTEM). The Zener-Wert-Avrami (ZWA) model was successfully used to simulate the CRS relaxation behaviour in nanocrystalline and non-nanocrystalline regions at different temperatures. The results show that the relaxation of CRS is thorough and fast at higher temperature (650 oC). There are two distinct relaxation stages at lower temperatures, and the relaxation rate in the first stage is much faster, dominated by the annihilation of lattice defects through short-range diffusion. The relaxation in the second stage is slow and dominated by dislocation rearrangement. In contrast to the situation at low temperatures, the nanocrystals appear to be a negative factor in stabilizing the CRS at high temperatures, as the unstable sub-grain boundaries act as sources of stress relaxation and the multiple grain boundaries act as high-speed diffusion channels and accelerate the annihilation of dislocations.