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Observations of isothermal fatigue, isothermal fatigue with superimposed load hold times, and thermomechanical fatigue (TMF) crack growth rate behavior of Ti-24Al-11Nb are presented and compared with results from previous studies on titanium and nickel-base superalloys. Elevated-temperature crack growth mechanisms in this alloy, which involve fatigue, oxidation and creep, and the influence of frequency, temperature, and hold-time are discussed. These mechanisms are used to interpret the observations of TMF crack growth. The limitations of current crack growth rate models based on the linear-elastic fracture mechanics parameter, K, are addressed.

The fatigue crack growth behavior in a titanium aluminide (Ti, Al) alloy under thermomechanical loading as well as under elevated temperature conditions was investigated. The thermal mechanical fatigue crack growth behavior appears, in a general sense, to follow the same trends observed in similar data obtained in tests on nickel-base superalloys. However, crack growth in Ti,Al appeared to be influenced by blunting of the crack tip due to creep in addition to a cyclic-dependent contribution together with time dependent or environmental enhanced degradation. This complex phenomenon in Ti,Al is unlike that in nickel-base superalloys where crack growth was found to be due to a linear combination of cycle and time dependent contributions. Thus, the linear cumulative modelling technique is not applicable to the tested Ti,Al.

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