The high temperature fatigue characteristics of U72OLi have been investigated over the temperature range 65O'C to 725'C under imposed dwell times (at maximum load) of 1 and 20 seconds in vacuum and air conditions. The effect of varying grain size and coherent precipitate size under these conditions has been assessed.Testing in air resulted in oxidation dominated intergranular crack growth at all temperatures and dwell times with the slope (m-values) of the crack growth rate curves remaining constant. Increased crack growth rates are seen at the higher temperatures and at longer dwells, although no effect of dwell was observed at 650°C in the as-received fine grained variant. In vacuum crack growth rates were much lower than in air and a purely cyclic dependent regime was evident at 65O'C. As temperature and dwell time at maximum load was increased, mvalues increased and were accompanied by a change in crack growth mechanism iiom transgranular to intergranular cracking. This indicated that true, time-dependent, creepfatigue processes were occurring.
Superalloys 2000 Edited
Fatigue crack propagation at elevated temperatures (650 and 725°C) has been studied in a powder processed high strength nickel based superalloy, U720Li, in air and vacuum environments, using trapezoidal loading waveforms. Constant load (increasing D K) tests were used to generate da/dN ± D K curves, while interrupted constant D K tests were used to study crack propagation paths and mechanisms. At 650°C in air there is little effect of any dwell at maximum load while at 725°C such a dwell causes a signi® cant increase in the crack growth rate coupled with a transition from mixed mode to fully intergranular crack growth. In vacuum the growth rates were signi® cantly lower, and the dwell caused little or no change in crack growth rate at 650°C, but an increase at 725°C. The crack path in vacuum changed from fully transgranular at 650°C to incorporate increasingly mixed mode growth at 725°C with dwell. The intergranular failure in vacuum was through creep cavitation of grain boundaries, while in air, static failure of oxygen embrittled boundaries dominated.MST/4835
The effects of temperature, dwell and environment on the fatigue crack growth behaviour of two nickel base turbine disc superalloys RR1000 and Udimet 720Li are presented in the present paper. Fatigue tests were carried out at room temperature in air using a 20 Hz sinusoidal loading waveform and at elevated temperatures of 650 and 725°C in both air and vacuum environments using a trapezoidal loading waveform with dwell times at a maximum load of 1 or 20 s. The fatigue crack propagation resistance of both materials is rationalised in terms of the influence of microstructure as well as the effects of alloy chemistry.
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