The Role of Local Microstructure on Small Fatigue Crack Propagation in an α + β Titanium Alloy, Ti-6Al-2Sn-4Zr-6Mo

Microstructural origins of the variability in fatigue lifetime observed in the high- and very-high-cycle fatigue regimes in titanium alloys were explored by examining the role of microstructural heterogeneity (neighborhoods of grains with similar crystallographic orientations or microtexture) on the initiation and early growth of fatigue cracks in Ti-6246. Ultrasonic fatigue of focused ion beam (FIB) micronotched samples was used to investigate long lifetime (10⁷ to 10⁹) behavior for two microstructural conditions: one with microtexture and one without microtexture. For specimens containing notches of nominally 20 μm in length, fatigue crack initiation in the microtextured material was most likely to occur from notches placed in neighborhoods with a microtexture favorably oriented for easy basal slip. Initiation lifetimes in the untextured material with similar sized notches were, on average, slightly greater than those for the microtextured condition. In both materials, the crack-initiation lifetime from micronotches of length 2c > 20 μm was a very small fraction (<1 pct) of the measured fatigue lifetime for unnotched specimens. Furthermore, in the microtextured condition, small fatigue crack propagation rates did not correlate with the microtextured regions and did not statistically differ from average small crack growth rates in the untextured material. As the micronotch size was reduced below 20 μm, fatigue crack initiation was controlled by microstructure rather than by FIB-machined defects. Finally, predictions of the fraction of life consumed in small and long fatigue crack growth from preexisting cracks nominally equivalent in size to the micronotches was compared with the measured fatigue life of unnotched specimens. The predicted range of lifetimes when factoring in the experimentally observed variability in small fatigue crack growth, only accounted for 0.1 pct of the observed fatigue lifetime variability. These findings indicate that in the high-and very-high-cycle fatigue regimes, fatigue life is dominated by crack initiation and that the variation in the initiation lifetime is responsible for the observed variation in total fatigue life.