Effect of stress ratio on very high cycle fatigue properties of Ti-10V-2Fe-3Al alloy with duplex microstructure

In fatigue critical applications, Ti-10V-2Fe-3Al alloy components are expected to endure cyclic loading with cycles above 10⁹. To assess their operating safety, S–N relations of Ti-10V-2Fe-3Al alloy in very high cycle fatigue (VHCF) regime are of concern and have been investigated in this work. Fatigue behavior including S–N curves and crack initiation mechanisms is reported. Two transitions of fatigue crack initiation mechanism, from internal crack initiation to surface crack initiation and from α_p cleavage to α_s/β decohesion, occur when the stress ratio (R) and stress level are reduced. Fatigue limits exist at N_f = 6 × 10⁷ cycles for all stress ratios except for 0.5. In the VHCF regime two kinds of internal crack initiation mechanisms exist, i.e., coalescence of cluster of α_p facets and α_s/β decohesion. Their mutual competition depends on the stress ratio and can be interpreted in terms of different stress character required for promotion on different internal crack initiation mechanism. Small crack propagation is discussed to be life controlling process under the stress ratio range from ?0.5 to 0.1 during VHCF regime while under the stress ratio 0.5 VHCF, life almost refers to the life required for crack initiation.