The crack initiation behavior and the fatigue limit of Ti-5Al-5Mo-5V-1Cr-1Fe titanium alloy with basket-weave microstructure

The crack initiation behavior and the fatigue limit of Ti-5Al-5Mo-5V-1Cr-1Fe titanium alloy with basket-weave microstructure were studied by means of OM, SEM and EDS analysis. Five double annealing processes were conducted to achieve different basket-weave features of Ti-5Al-5Mo-5V-1Cr-1Fe alloy. The results show that even for microstructures with similar yield strengths, their fatigue limits can show a big variance. The difference of their crack initiation behaviors, namely the subsurface-crack initiation ratio in high cycle fatigue regime, is responsible for above phenomenon. For Ti-5Al-5Mo-5V-1Cr-1Fe alloy, the subsurface-crack initiation ratio in high cycle fatigue regime can exert positive influence on its fatigue limit. Moreover, the subsurface-crack initiation ratio of Ti-5Al-5Mo-5V-1Cr-1Fe alloy is determined by its microstructure features. For microstructures with limited volume fraction of coarse α phase, the coarse α phase is the most preferential crack initiation place. In this situation, the subsurface-crack initiation ratios of these microstructures are directly influenced by the distribution of the coarse phase in the fatigue specimen. However, for microstructures with homogeneous microstructure features and no obviously coarse α phase, the fatigue specimen surface is the most preferential crack initiation place. In this situation, the subsurface-crack initiation ratio is almost equal to zero. Generally speaking, the microstructure with high matrix strength and limited volume fraction of coarse α phase can generally get high subsurface-crack initiation ratio, which leads to high fatigue limit.