Effect of Microstructural Degradation on Crack Tip Stress Fields in Two-Phase Single Crystals E.P. Busso, N.P. O’Dowd, S. Dumoulin, and D. Allen† Department of Mechanical Engineering Imperial College, London, UK † ALSTOM Power Technology Centre, Whetstone, UK ABSTRACT A large proportion of the service life of high temperature single crystal components is taken up in initiating and growing surface cracks, which are driven by thermally induced stresses and environmental effects. In this work, the effect of a reduction in the volume fraction of the γ’ precipitate phase in Ni-base superalloys on the timedependent crack tip stresses is analysed numerically. Such variations can be related to the microstructural degradation caused by oxidation and diffusion processes around surface cracks in superalloy single crystals. The approach relies on a recently proposed multi-scale rate-dependent crystallographic theory to describe the macroscopic constitutive behaviour of the single crystal in terms of material parameters which depend explicitly on the characteristics of the precipitate phase at the microscale. Results of a finite element study on a typical compact tension specimen reveal a strong dependency of the local crack tip stresses and strains on the local volume fraction of the precipitate population. The implications of these findings in relation to the microstructural degradation caused by oxidation at crack faces are discussed. KEYWORDS Single crystals; Superalloys; Oxidation; Crack Growth; Fracture. INTRODUCTION In two-phase single crystals such as Ni-base superalloys, heterogeneities exist at both the microscopic and mesoscopic levels. At the microscale, they are introduced by the presence of the 0.5-1 µmγ’ precipitates and can become more pronounced due to the changing morphology of the precipitates during service and to inhomogeneous deformation patterns. At the mesoscale, e.g. in 10–100 µm sized regions, the presence of surface cracks also introduces a degree of heterogeneity in the local microstructure as a result of the local microstructural degradation caused by the oxidation of the crack faces [1][2]. Conventional single crystal models are unable to predict the effects of such heterogeneities as they generally treat the material as a homogeneous single crystal solid whose mechanical behaviour is uncoupled from interdiffusion and oxidation processes. It is well known, however, that local variations in precipitate volume fraction strongly affect the local material behaviour. Although much work has been done to explain such volume fraction effects, it was not until recently that they were quantified for a range of temperature and strain rate conditions using periodic unit-cell techniques [3][4]. The results have been incorporated into a state variable crystallographic formulation to account for experimentally observed precipitate size and volume fraction effects in a Ni-base superalloy [5]. 1
RkJQdWJsaXNoZXIy MjM0NDE=