Crack initiation mechanism and fatigue life of titanium alloy Ti–6Al–2Sn–2Zr–3Mo-X: Effects of stress ratio and loading frequency

Abstract

In this paper, the fatigue behavior of Ti–6Al–2Sn–2Zr–3Mo-X alloy with basketweave microstructure is investigated at stress ratios R = −1, −0.5, 0.1 and 0.3 and loading frequencies f = 0.4 Hz, 4 Hz and 35 Hz. It is shown that the Ti–6Al–2Sn–2Zr–3Mo-X alloy presents two different crack initiation mechanisms. One is due to the cleavage of α grains and the other is due to the microstructure inhomogeneity and deformation incompatibility. For the former case, the fracture surface exhibits facet feature in the crack initiation region. For the latter case, the crack nucleates and initiates at α grains or interfaces and the fracture surface presents no facet feature in the crack initiation region. The higher stress ratio and the lower frequency tend to induce the higher probability for the occurrence of multiple crack initiation mode. Moreover, the paper indicates that the effect of stress ratio on fatigue life could be expressed as Nf = A [(1–R)/2]lσma,R, where A, l and m are material parameters. The scatter of fatigue life is related to both the stress amplitude and the loading frequency. For relative higher stress amplitude, the fatigue life and the frequency at R = −1 could be correlated by a linear relation in log-log scale.