Effect of long term aging on the fatigue crack propagation in the β titanium alloy Ti 17

This work aims to evaluate the impact of a prolonged thermal aging on the fatigue crack growth properties of a commercial Ti 17 (Ti-5Al-2Sn-2Zr-4Mo-4Cr) titanium alloy. Aging was operated at 450 °C for extended times, of 1000 h and 10,000 h respectively. In terms of room-temperature tensile properties, aging is mainly responsible for a strain hardening of the material and a decrease in ductility, the total elongation dropping from 5.1% in the as-received state to 1.5% after 10,000 h. A significant increase in crack propagation rates at room temperature for R = 0.1 is observed, emphasizing an embrittling effect of aging. This last effect is however only noticed beyond a critical value of ΔK (ΔKcr), which decreases with increasing aging times. It is observed that the effect of long-term aging on the mechanical properties is more pronounced during the early stage of the aging process, and tends to saturate for longer times. Tests with different load ratio of R = 0.4 and 0.7 were performed to gain a better understanding of the occurrence of these ΔKcr values. The results indicate that the increase in propagation rates is actually governed by the static Kmax component of the fatigue loading. Fractographical analysis of the failed specimens reveal that aging affects the material's resistance at a very fine scale, namely the α lamellae/β matrix interfaces. The detrimental role played by both grain boundary α phase and precipitate-free zones on the fatigue crack growth resistance of the material were finally discussed.