The Role of Competing Mechanisms in the Fatigue-Life Variability of a Titanium and Gamma-TiAl Alloy

摘要

The variability in fatigue lives of an alpha+beta titanium alloy (Ti-6Al-2Sn-4Zr-6Mo) and a gamma-TiAl-based alloy in stress vs. life space resulted from superposition of variability associated with two separate mechanisms. The mean lives of the two mechanisms diverged with decreasing stress level, giving rise to the variability. A life-prediction methodology based on the variability in the worst-case mechanism is suggested. The potential for reducing uncertainty and increasing the utilization of the useful life as compared to more traditional approaches is discussed. Titanium alloys are used in many fatigue-critical structural applications in the aerospace industry, '~3 and variability in fatigue life is often the life-limiting factor for such components. In addition, gamma titanium aluminide (y-TiAl)-based alloys are important candidates for structural applications (especially high-temperature applications) in the aircraft industry. However, widespread transition of TiAl alloys has been hampered due to, among other factors, significant uncertainty in fatigue behavior. Current approaches to life management of fracture-critical components are largely empirical in nature, often resulting in non-utilization of a significant part of the useful life. A more accurate, physically based assessment of fatigue life and its variability is crucial for reliable utilization of the capability of titanium-alloy components currently in service, as well as for insertion of materials such as y-TiAl-based alloys in fracture-critical applications.