The role of microstructure on sequential stages of the very high cycle fatigue behavior of an a + p titanium alloy, Ti-6AI-2Sn-4Zr-6Mo

摘要

The very high cycle fatigue behavior (10~5-10~9 cycles) of Ti-6246 has been investigated using ultrasonic fatigue techniques. In this regime of fatigue (0.4-0.6σ_(ys), only certain microstructural regions are susceptible to fatigue damage accumulation, and lifetimes are related to the distribution of fatigue critical microstructural neighborhoods. Three distinct categories of crystallographic crack initiation sites have been identified. Fatigue crack initiation and early growth is typified by a crystallographic facet formation process within α_p grains favorably oriented for basal slip in microtextured regions of the microstructure. However, many microtextured regions are observed throughout the microstructure and it is unclear why some of these regions are susceptible to fatigue damage accumulation while others are not. To identify the influence of microtexture on the small fatigue crack growth process, multiple micronotches were machined in fatigue specimens with a focused ion beam (FIB) microscope to serve as fatigue crack initiation sites in various microstructural neighborhoods. Regions with a texture favorably oriented for basal slip were observed to promote early fatigue crack initiation from FIB notches. However, small fatigue crack propagation rates did not correlate with the locally measured textures in individual microtextured regions.