Effect of Superplastic Forming Exposure on Fatigue Crack Propagation Behavior of Ti-6Al-4V Alloy

The effect of superplastic forming (SPF) exposure on the. (strain)-N (number of cycles to failure) fatigue and fatigue crack propagation (FCP) behaviors of Ti-6Al-4V (Ti64) alloy was examined at 298 and 473 K. To simulate the thermal exposure during superplastic forming process, the mill-annealed Ti64 alloy sheet was heated in the vacuum chamber with the pre-determined temperature profile. Notable microstructural change during the SPF exposure included the shape of transformed beta phase from fine and round particles in the as-received specimen to coarse angular particles in the as-exposed specimen. The effective grain size tended to increase with the exposure, enhancing the slip reversibility and the resistance to FCP. However, the crack hindering effect by fine, particle-like beta phase became weak with the exposure, offseting the beneficial effect associated with the increment of effective grain size. The effect of SPF exposure on epsilon-N fatigue and FCP behavior of mill-annealed Ti64 alloy was therefore marginal, excluding the effect of alpha-case (the oxygen-enriched phase) on the surface.