Effect of shot peening on the fatigue and fracture behaviour of two titanium alloys

The fatigue and fracture behaviour of two titanium alloys, the near-alpha IMI-685 and alpha-beta IMI-318, were studied in the machined and polished (MP) as well as the machined, polished and shot (glass-bead) peened (MPS) conditions. Glass-bead peening reduced the room-temperature as well as the high-temperature (450 degrees C) fatigue life of alloy lMI-685 at high stress amplitudes, sigma(a), approaching the proof stress, sigma(ps), of the material (LCF region). When the applied stress amplitude (0-770 MPa, HCF region) was comparable to the peen-induced peak longitudinal residual stress, sigma(LP), i.e. (sigma(LP)/sigma(a))=0.92, an improvement in the room-temperature fatigue life of lMl-685 was observed. When the (sigma(LP)/sigma(a)) ratio was less than this value, decreases in the fatigue life were seen. The room-temperature fatigue behaviour of IMI-318 at high stress amplitudes was similar to that of IMI-685. The decrease in the fatigue life of this alloy, at a stress amplitude (770 MPa) where improvement was observed for IMI-685, could be attributed to the higher relaxation of peen-induced residual stresses in IMI-318 compared with IMI-685. Glass-bead peening improved the high-temperature (450 degrees C) fatigue life of IMI-685 at a low stress amplitude (465 MPa; (sigma(a)/sigma(PS))=0.87). The crack-initiation sites in the MP and the MPS conditions were at the surface for both the alloys. However, fracture in the surface layers of the alloys appeared more brittle in the peened (MPS) rather than in the unpeened (MP) condition.