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

Abstract 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 (ΔK cr ), 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 ΔK cr values. The results indicate that the increase in propagation rates is actually governed by the static K max 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.