Phase gradient in a laser peened TC6 titanium alloy analyzed using synchrotron radiation

The titanium alloy TC6 was laser peened without coating (LPwC) with a pulsed Nd:YAG laser (1064 nm) at power densities of 3, 6 and 9 GW cm(-2). Microhardness data revealed that hardness does not linearly scale with power density. Residual stress analysis indicated that the profile has two distinct regions, one near surface ( < 100 mu m) showing thermal effects due to laser beam-material interaction and a far-surface region ( > 100 mu m) with decreasing compressive residual stresses with increasing power density. Transmission electron microscopy (TEM) showed high dislocation density with dislocation tangles and dislocation walls without dislocation cell or nanocrystals in the near surface region. Synchrotron radiation showed the increase in volume fraction of beta phase with increase in power density in the far-surface region. Fatigue performance of the samples (number of cycles to failure) subjected to LPwC at a power density of 6 GW cm(-2) was found to have increased by a factor of 2-4, in the load range of 12.5-15 kN. Further, it was observed that in addition to the normally observed sub-surface crack nucleation of LPwC samples, additional formation of beta phase in the far-surface region improves fatigue performance.