Abstract
Growth behaviour of small fatigue cracks has been investigated on three materials with different microstructures of a beta Ti-15Mo-5Zr-3Al alloy which were solution treated at 735°C, 850°C and 1000°C, followed by aged at 500°C (STA735, STA850 and STA1000) at a stress ratio of-1 under axial loading in laboratory air. The da/dN-Kmax relationships for STA735 and STA850 were almost the same as the da/dN-ΔK relationships for large cracks, while STA1000 showed initially lower growth rates than large cracks, and then the growth rates rapidly increased and became faster at an intermediate Kmax regime than those of large cracks. After allowing crack closure, STA850 and STA1000 still exhibited enhanced growth rates, which was found to be attributed to the crystallographic nature of small crack growth based on a close examination of the fracture surfaces. The trace of the crack front remained on the fracture surfaces of STA850 and STA1000 represented the transition from the microstructurally small crack to the mechanically small crack, which has occurred at a/d=2.63.9 for STA850 and a/d= 1.62.1 for STA1000, where a and d were the crack depth and the beta grain size, respectively. Of three materials, STA1000 showed the lowest growth resistance of small cracks at intermediate Kmax and ΔKeff regimes. Furthermore, it was found that all the materials of the beta alloy exhibited lower growth rates at low Kmax regime than an annealed Ti-6Al-4V alloy, but the difference disappeared in terms of ΔKeff, indicating that crack closure was responsible for the difference in growth behaviour between both alloys.
