On the effect of deep-rolling and laser-peening on the stress-controlled low- and high-cycle fatigue behavior of Ti–6Al–4V at elevated temperatures up to 550°C

The effect of surface treatment on the stress/life fatigue behavior of a titanium Ti–6Al–4V turbine fan blade alloy is investigated in the regime of 102–106 cycles to failure under fully reversed stress-controlled isothermal push–pull loading between 25 and 550°C at a frequency of 5Hz. Specifically, the fatigue behavior was examined in specimens in the deep-rolled and laser-shock peened surface conditions, and compared to results on samples in the untreated (machined and stress annealed) condition. Although the fatigue resistance of the Ti–6Al–4V alloy declined with increasing test temperature regardless of surface condition, deep-rolling and laser-shock peening surface treatments were found to extend the fatigue lives by factors of more than 30 and 5–10, respectively, in the high-cycle and low-cycle fatigue regimes at temperatures as high as 550°C. At these temperatures, compressive residual stresses are essentially relaxed; however, it is the presence of near-surface work hardened layers, with a nanocystalline structure in the case of deep-rolling and dense dislocation tangles in the case of laser-shock peening, which remain fairly stable even after cycling at 450–550°C, that provide the basis for the beneficial role of mechanical surface treatments on the fatigue strength of Ti–6Al–4V at elevated temperatures.