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 (α_p), lamellar α (α_l) and β transformed matrix (β_t), were explored in this work. The results show that, at different strain amplitude (ε_ta) levels, the cyclic hardening/softening is determined by the competition (ε_ta < 0.9%) or superposition (ε_ta ≥ 0.9%) effect of the variations of back stress and friction stress. As well, the fractography shows remarkably different features at different ε_ta levels. When ε_ta < 0.9%, there exists only one fatigue crack initiation site activated by the dislocation pile-ups at α_p/β_t and α_l/β_t interfaces. Besides, narrow fatigue striation space in fatigue crack propagation region implies a relatively slower crack propagation. When ε_ta ≥ 0.9%, the additionally high-stress-induced crossed slip bands and coarse slip bands in α_p 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 α_p 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 α_l content would produce α_l 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.