THE ROLE OF MICROSTRUCTURE ON VARIOUS STAGES OF THE VERY HIGH CYCLE FATIGUE BEHAVIOR OF AN a + ß TITANIUM ALLOY, TI-6Al-2Sn- 4Zr-6Mo (PREPRINT)

The very high cycle fatigue behavior (10-10 cycles) of Ti-6246 has been investigated using ultrasonic fatigue techniques. In this regime of fatigue (0.4-0.6σYS), 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 αp grains favorably oriented for basal slip in microtextured regions of the microstructure. However, many microtextured regions are observed throughout the microstructure and it is not clear 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, FIB notches were machined in fatigue specimens 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.