Low-cycle fatigue behavior and property of TA15 titanium alloy with tri-modal microstructure

Low-cycle fatigue (LCF) behavior and property characteristics of titanium alloy with a tri-modal microstructure, consisted of equiaxed α (αp), lamellar α (αl) and β transformed matrix (βt), were explored in this work. The results show that, at different strain amplitude (εta) levels, the cyclic hardening/softening is determined by the competition (εta < 0.9%) or superposition (εta ≥ 0.9%) effect of the variations of back stress and friction stress. As well, the fractography shows remarkably different features at different εta levels. When εta < 0.9%, there exists only one fatigue crack initiation site activated by the dislocation pile-ups at αp/βt and αl/βt interfaces. Besides, narrow fatigue striation space in fatigue crack propagation region implies a relatively slower crack propagation. When εta ≥ 0.9%, the additionally high-stress-induced crossed slip bands and coarse slip bands in αp cause the multiple fatigue crack initiations. Moreover, wider fatigue striation space in propagation region indicates a faster crack propagation. The above divisional LCF behavior and fracture features determine a two-part linear Coffin-Manson relationship. On the other hand, increasing αp content could delay the fatigue crack nucleation and propagation due to its positive effect for improving deformation compatibility. This will effectively increase the LCF life. However, increasing αl content would produce αl colonies, which promote the dislocation slip and reduce the slip reversibility. So, the crack nucleation and propagation will be facilitated and then the LCF life is decreased.