Effect of superplastic forming exposure on tensile and S-N fatigue behavior of Ti64 alloy

The effect of superplastic forming (SPF) on tensile and S (stress)-N (number of cycles to failure) fatigue properties of Ti64 alloy was examined at 298 and 473 K. For simulating the superplastic forming exposure, millannealed Ti64 alloy sheet was heated in a vacuum chamber with a pre-determined temperature profile. For some as-exposed specimens, the alpha-case formed on the surface during expousre was mechanically removed to understand the effect of alpha-case on the mechanical properties of Ti64 alloy. It was found that the presence of alpha-case significantly affected the tensile and the fatigue properties of Ti64 alloy at 298 and 473 K by providing an easy initiation site for both tensile and fatigue fracture. The microstructural change during the SPF exposure was marginal in affecting the S-N fatigue properties of Ti64 alloy. Different testing temperature of 298 and 473 K affected the S-N fatigue behavior of as-received and as-exposed (alpha-case removed) Ti64 specimens, but not that of as-exposed specimen.