Microstructure-based tensile indicator for assessing high-cycle and very-high-cycle fatigue of titanium alloys with additive and conventional manufacturing

Empirically, there is a quick estimate for the traditional fatigue limit of metallic materials at 10 7 cycles, which is a proportional value of the ultimate tensile strength (UTS), i.e. the so-called “fatigue ratio”. However, fatigue failure can still occur after 10 7 cycles, known as very-high-cycle fatigue (VHCF). In this case, the “true” fatigue limit is no longer a fixed proportion of the UTS, e.g. for the inclusion induced VHCF, the fatigue limit remains almost constant at 10 9 , 10 10 or 10 11 cycles and does not vary with the UTS. This phenomenon is more pronounced in additively manufactured alloys, where fatigue resistance is not solely related to the UTS in VHCF and even in high-cycle fatigue (HCF). Here, we summarize the basic metallurgical defects and typical microstructures of α+β titanium alloys produced by conventional and additive manufacturing, then extract their quantitative characteristics, compare their fatigue behaviors and correlate them with their tensile properties. Finally, we propose the possible form of an indicator that can rapidly assess the HCF and VHCF of the materials by combining UTS and elongation based on the detailed microstructure and the resulting specific failure type.