Investigating Microstructural and Environmental Effects on the Very High Cycle Fatigue Behavior of TI-6242S.

Abstract

Extending the service lifetimes of existing components, and designing new mechanical systems capable of withstanding longer lifetimes, is critically important for applications in the aerospace, energy generation, and transportation sectors. These applications require an understanding of the coupled interactions between local microstructure and environmental effects. Towards this end, the effects of microstructure and environment on fatigue damage accumulation behavior in the VHCF regime were investigated in the near alpha titanium alloy Ti-6Al-2Sn-4Zr-2Mo-0.1Si (Ti-6242S). The influence of microstructure and environment on fatigue crack initiation from micro-notches, and on early crack growth behavior, was characterized using ultrasonic fatigue. Fatigue crack growth was studied in laboratory air, high vacuum, and in various pressures of water vapor, high purity oxygen, and hydrogen gas. A new experimental methodology was developed that combines ultrasonic fatigue at 20 kHz and environmental scanning electron microscopy to examine small crack growth behavior as a function of the local microstructure, in-situ. A number of new findings have resulted from this study. Natural fatigue crack initiation occurred at or very near grain boundaries between two similarly oriented primary alpha grains that were not favorably oriented for slip. Small fatigue cracks took longer to initiate from micro-notches in vacuum than in laboratory air or low pressure water vapor environments (65 Pa – 665 Pa). Water vapor was also found to be significantly more deleterious to fatigue life than either pure hydrogen or pure oxygen at equivalent pressures. Similar fatigue crack growth rates were obtained for cracks grown in ambient air and 1330 Pa water vapor environments (corresponding to a relative humidity of 40%-60%), suggesting a dominance of water vapor effects in ambient air as well. Fatigue crack growth rates were found to significantly increase with increasing water vapor pressure. Increased fatigue crack growth rates in water vapor and oxygen were determined to be due to adsorption-assisted crack propagation, which stems from atoms or molecules being adsorbed onto fresh surfaces at the crack-tip causing increased irreversibility.