High-temperature low-cycle fatigue of a gamma titanium aluminide alloy Ti-46Al-2Nb-2Cr

The low-cycle fatigue (LCF) behavior of a gamma titanium aluminide alloy Ti-46Al-2Nb-2Cr in fully lamellar (FL) and nearly lamellar (NL) microstructural conditions is studied at 650 °C and 800 °C, with and without hold times. At 650 °C and 800 °C, the alloy in either condition exhibits cyclic stability at all strain levels studied, excepting the NL structure which shows slight cyclic hardening at higher strain levels at 650 °C. Fracture in the FL condition occurs by a mixed mode comprising delamination, translamellar fracture, and stepwise fracture. On the other hand, fracture occurs mostly by translamellar mode in the NL condition. At both test temperatures, the alloy in the FL condition obeys the well-known Manson-Coffin behavior. The fatigue resistance of the alloy at 650 °C in the FL condition is very much comparable to, while in the NL condition it is superior to, that of Ti-24Al-llNb alloy. At 650 °C, a 100-second peak tensile strain hold doubles the fatigue life of the alloy in the FL condition, while a 100-second hold at compressive peak strain or at both tensile and compressive peak strain degrades fatigue life. The observed hold time effects can primarily be attributed to mean stress. Irrespective of the nature of the test, the hysteretic energy (total as well as tensile) per cycle remains nearly constant during the majority of its life. The total and tensile hysteretic energy to fracture, at both test temperatures, increase with cycles to failure, and the variation follows a power-law relationship. Formerly NRC Senior Resident Associate, Wright Laboratory.