Mechanistic Modeling of Intergranular Fatigue Crack Growth in P/M Nickel-Based Superalloys

摘要

The objective of this paper is the development of a multiscale, time-dependent crack growth model, which considers the role of creep, fatigue and environment interactions on both the bulk and grain boundary phases in ME3 disk material. The model is established by considering a moving crack tip along a grain boundary path in which damage events are described in terms of the grain boundary deformation and related accommodation processes. Modeling of these events was achieved by adapting an approach in which the grain boundary (GB) dislocation network is smeared into a Newtonian fluid element. The deformation behavior of this GB element is controlled by the crack tip both far and near deformation fields and the intrinsic GB viscosity. These concepts have been implemented using a multiscale model which utilizes the knowledge of the GB external and internal deformation fields. The external field is generated by coupling two continuum constitutive models including (ⅰ) a macroscopic internal state variable (ISV) model for the purpose of modeling the response of the far field region located several grains away from the crack path and (ⅱ) a microstructure explicit coarse scale crystal plasticity (XP) model in which the isotropic and kinematic hardening parameters are explicitly dependent on γ' precipitate size and volume fraction. This scale is appropriate for the representation of the continuum region at the immediate crack tip. The material parameters for the ISV and XP models are obtained from results of low cycle fatigue tests which were performed at three temperatures; 650, 704 and 760°C. The second requirement in the implementation of the cohesive zone model is a grain boundary deformation model which has been developed, as described above, on the basis of viscous flow rules of the boundary material. These rules correlate the rate of the grain boundary sliding displacement to material and load dependent parameters. This model is supported by dwell crack growth experiments carried out at the three temperatures mentioned in both air and vacuum environments. These tests consist of fatigue cycles with a frequency of 0.33 Hz having a dwell time superimposed at the maximum load level. The dwell time ranged from 0 seconds to 7200 seconds. Results of these tests have identified the transgranular/intergranular transitional frequency (0.01 Hz) below which the time-dependent crack growth model is applicable. Validation of this model has been carried out by comparing the numerically simulated crack growth data with that obtained experimentally. This comparison is used to optimize the different model components and to provide a route to assess the relative significance of each of these components in relation to the intergranular damage associated with dwell fatigue crack growth in ME3 superalloy.