Effects of microstructure on the short fatigue crack initiation and propagation characteristics of biomedical α/β titanium alloys

This article presents the results of a study of the effects of microstructure on the fatigue strength and the short fatigue crack initiation and propagation characteristics of a biomedical alpha/beta titanium alloy, Ti-6Al-7Nb. The results are compared to those obtained from a Ti-6Al-4V extra-low interstitial (ELI) alloy. Fatigue crack initiation occurs mainly at primary alpha grain boundaries in an equiaxed alpha structure, whereas, in a Widmanstatten alpha structure, initiation occurs within the alpha colonies and prior beta grains, where alpha plates are inclined at around 45 deg to the stress-axis direction. In an equiaxed alpha structure, the short fatigue crack initiation and propagation life, where the length of the crack (a) is in a microstructurally short fatigue-crack regime (2a < 50 mu m), occupies around 50 pct of the total fatigue life. On the other hand, the fatigue crack in a Widmanstatten alpha structure initiates at very early stages of fatigue, and, therefore, the fatigue crack-initiation life occupies a few percentages of the total fatigue life in an alpha structure. Then, the short fatigue crack propagates rapidly and is arrested at the grain boundaries of alpha colonies or prior beta grains for a relatively long period, until the short crack passes through the boundaries to specimen failure. Therefore, the short fatigue crack-arrest life occupies more than 90 pet of the total fatigue life in a Widmanstatten alpha structure. These trends are similar between the Ti-6Al-7Nb and Ti-6Al-4V ELI alloys and biomedical alpha/beta titanium alloys. The total fatigue life for the Ti-6Al-7Nb alloy with an equiaxed alpha structure is changed by the volume fraction of primary alpha phase and the cooling rate after solution treatment. By increasing the volume fraction of the primary alpha phase from 0 to 70 pct, the fatigue limit of the Ti-6Al-7Nb alloy is raised. Changing the cooling rate after solution treatment by switching from air cooling to water quenching improves the fatigue limit of the Ti-6Al-7Nb alloy significantly.