Relationship between microstructures, facet morphologies at the high-cycle fatigue (HCF) crack initiation site, and HCF strength in Ti-6242S

The effect of microstructures on the high-cycle fatigue (HCF) properties of Ti-6242S (Ti-6Al-2Sn-4Zr-2Mo-0.1Si) was investigated. Five different microstructural conditions were generated through combinations of hot-deformation and heat treatments, and each was fatigue-tested using smooth round bar specimens at an R ratio of 0.1. The HCF behavior was found to be highly dependent on microstructures. The bimodal microstructure showed the highest HCF strength in the whole cycle range with 750 MPa at 1.0E+07 cycles. The fatigue ratio S of the HCF strength at 1.0E+07 cycles to tensile strength showed its highest value of 0.65 for the acicular α-microstructure whereas it showed its lowest value of 0.49 for the full lamellar microstructure. The flat facets were observed at the fatigue crack initiation sites irrespective of the microstructures, and were always located in the subsurface regions. Concurrent observation of the fatigue initiation facet and the underlying microstructure was made using the precision sectioning method. Based on these experimental results, the main factor contributing to the improvement of HCF strength was analyzed, along with the correlation between facet morphologies and fatigue life-controlling microstructural units, the fatigue initiation mechanisms and the dependence of HCF strength on the facet size.