Abstract
In order to better understand the fundamental fatigue behaviour of a beta Ti 15Mo 5Zr 3Al alloy, rotating bending fatigue tests have been carried out using materials with three different microstructures, i.e. beta grain sizes, which were solution treated at 735°C (STA735), 850°C (STA850), and 1000°C (STA1000) followed by aging at 500°C. It was found that STA735 showed the highest fatigue strength while the materials solution treated above the beta transus, STA850 and STA1000, had fatigue lives which decreased with increasing solution treatment temperature, i.e. beta grain size. There was no significant difference in fatigue limit between the materials. Fatigue cracks were initiated as a result of cyclic slip deformation in STA735, but the crack initiation site was related to beta grain boundaries in both STA850 and STA1000. In particular, STA850 exhibited an unique initiation behaviour in which cracks were initiated on the specimen surface at higher stress levels, but subsurface crack initiation was observed at lower stress levels. Since STA735 indicated considerably slower crack initiation than STA850 and STA1000 and the growth rates of small cracks were almost the same for all the materials, the excellent fatigue strength of STA735 was primarily attributed to higher crack initiation resistance of its microstructure.
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