Effect of Environment on Fatigue and Creep Crack Growth in Inconel X-750 at Elevated Temperature
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INTRODUCTION
I T is well known that at high temperature (T/TM >- 0.4) fatigue cracks grow from strain concentration regions associated with thermal gradients or from preexisting flaws. The fatigue performance of high temperature components is a strong function of the cyclic frequency and their fatigue lives are dramatically reduced with decreasing cyclic frequencies. This effect is more remarkable when the cyclic loads or strains are applied at very low frequencies and combined with a static load in an aggressive environment. The increase in fatigue crack growth rate (FCGR) with decreasing frequency has been attributed to the interactions of fatigue damage with time dependent effects such as creep and oxidation taking place at the crack tip. At this time we are not able to separate the fractions of the crack growth per cycle which are due to (1) time independent plasticity and rupture, (2) creep-fatigue interactions, and (3) environmental effects. In air tests the environmental effects seem to be principally due to oxidation at the crack tip. The water vapor contained in the ambient air may also play an important role when it dissociates at the crack tip into oxygen and hydrogen. Oxidation damage is mainly transgranular at high frequencies and intergranular at low frequencies. The effect of oxidation on FCGR has been studied in several alloys by comparing test data in air to the test data in vacuum or in an inert environment. Usually the crack growth rate in the inert environment (pure argon) is taken as the reference mechanical component of the growth. Any difference between the vacuum data and the pure inert environment data can be assumed to be due to rewelding at the crack tip during the unloading part of each cycle. The FCGR of different nickel based superalloys ( y / y ' ) have been measured over a wide range of temperatures (0.3 < T/TM
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