In the present study the Hu-Cocks micromechanical model [1,2] for dislocation-obstacle interactions, implemented in a crystal plasticity self-consistent model, is employed to simulate thermo-mechanical histories typical for AGR nuclear plants in order to assess the implications of creep-fatigue interactions in 316H stainless steel. Their physical model is enhanced by including the effect of dynamic recovery, which introduces a new material parameter -the annihilated segment length . The full model contains five independent material parameters; other parameters are prescribed by the fundamental physics of inelastic deformation processes. Having calibrated the model, we explore its ability to predict material response under complex loading histories to provide insight into the physical phenomena controlling cyclic-creep interactions. Introduction of strain dwells during cyclic loading results in an increase of the extent of relaxation with increasing number of cycles, but histories with dwells at different strain levels indicate that relaxation is strongly dependent on initial stress and level of constant strain. Predictions of history-dependent relaxation demonstrate that the least stress relaxation results after creep into the secondary regime and the largest stress drop results during hold-dwells after monotonic elastic-plastic loading, with the cyclic-dwell history behaviour laying in between these two. Both prior cycling and the generated residual stress field are found to affect the primary creep regime under hold-stress dwells. These results are consistent with experimental observations; this demonstrates that deformation response is dependent on both the evolution of microstructural state and redistribution of stress between the grains of the polycrystalline aggregate.

The present article examines the predictive capabilities of a crystal plasticity model for inelastic deformation which captures the evolution of dislocation structure, precipitates and solute atom distributions at the microscale, recently developed by Hu and Cocks [1,2]. The model is implemented within a self-consistent framework and a crystal plasticity finite element (CPFE) scheme. Through direct comparison between the two CP schemes and with an extensive material database for Type 316H stainless steel, the different types of information and the degree to which the models are consistent with experimental observations are assessed. The study demonstrates an agreement between the SCM and the CPFE schemes, providing confidence in the micromechanical deformation model employed. The multi-scale approach also allows the effects of micro-scale deformation processes, related to dislocation-obstacle interactions, on the global deformation response to be captured. Modelling results from this study and their comparison to experimental observations show that deformation of polycrystalline materials, such as 316H stainless steel, is controlled by the evolution of microstructural state of the material and the redistribution of stress between individual grains. The study suggests that the SCM is a feasible tool to simulate and explain the deformation behaviour of complex alloys under industrially-relevant thermo-mechanical operating histories. The CPFE framework captures the effects of the variation in grain geometry and provides more detailed information about the variation of stress and strain within the individual grains, particularly their distribution near grain boundaries and triple pointswhich are important to understand in the context of damage development and failure. The SCM predicts a -stiffer‖, more creep-resistant response than a CPFE model for a given set of material parameters due to the more highly-constrained deformation modes allowed in the model. As a result, material parameters calibrated using one modelling approach are not necessarily suitable for use in another approachalthough parameters obtained when fitting the different models should not vary significantly.

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