The Effect of Elevated Temperature Pre-Exposure on the Creep-Fatigue Crack Growth Behaviour of a Polycrystalline Nickel-base Superalloy

摘要

The demand for increased gas turbine operating temperatures has led to the design and use of nickel-base superalloys of increasing chemical complexity. The most recent generations of turbine disc alloys are often thermodynamically unstable at their operating temperatures. In this paper, the effect of microstructural degradation caused by thermal exposure on the creep-fatigue crack growth behaviour of a new polycrystalline nickel superalloy is investigated. Test specimens have been evaluated in two conditions: One with a standard solution and ageing heat treatment, one with an exposure at 800°C. Prolonged exposures at this temperature lead to precipitation of the topologically close packed phase σ as well as some M_(23)C_6 along grain boundaries. Both these phases are rich in solid solution strengtheners and are characterised by their low ductility. The effect of this grain boundary modification on the relative contribution of creep, fatigue and environmental damage mechanisms was evaluated by testing both in air and in vacuum at 725°C. An increase in crack growth rate up to an order of magnitude was observed in specimens with the most severe exposure when fatigue tested in air. The results are discussed in relation to the mechanisms of crack growth observed from fractographic studies of both air and vacuum specimens.