Modeling effects of nonmetallic inclusions on LCF in DS nickel-base superalloys

Inclusions often play a dominant role in limiting fatigue life in both cast and wrought alloys under nominally elastic cyclic loading conditions. The hierarchical scales of fatigue crack formation and their correspondence to strain-life relations were considered for cast A356-T6 Al in recent work by McDowell and co-workers using a microstructure-property framework that related cyclic plasticity near debonded silicon particle and large casting pores to formation and early growth of microstructurally and physically small fatigue cracks. In this paper, we turn our attention to the analogous role of carbides, oxides or other hard ceramic particles in fatigue crack formation in nickel-base superalloys. Cyclic plasticity of a directionally solidified (DS) nickel-base superalloy is evaluated in the [0 0 1] orientation. The effects of loading parameters (strain amplitude, R-ratio) and microstructural parameters (nonmetallic inclusion shape, spacing, etc.) on local cyclic plasticity at microstructure-scale notches are evaluated. The effect of neighboring grain orientation on intensification of plastic strain around inclusions is analyzed, and functions are determined for relating applied strain to cyclic plastic shear strain near the inclusion/matrix debond (micronotch) that contributes to LCF crack formation. Use of such a framework for predicting microstructure-dependence of fatigue crack initiation life and its variability are discussed, as are additional mechanisms of microfracture associated with cumulative plastic deformation.