Development of a multi-scale approach for modeling cast duplex stainless behavior

Monitoring pressurized water reactors components is of interest in long-term operation of nuclear power plants. Components made out of cast duplex stainless steel are subjected to aging when held at their operating temperature. In the long run, thermal aging results in embrittlement due to a microstructural evolution of one of the two phases constituting duplex steels. Kinetics and impact of aging on fracture mechanics have been studied to a great extent using empirical approaches. However, deformation and damage mechanisms leading to fracture are not yet fully understood owing to the complexity of duplex steels microstructure. In this context, a microstructural characterization is performed jointly to in situ mechanical characterization to build a consistent micromechanical model taking the multi scale aspect of the microstructure into account. Eventually, the model must provide a fine description of the material mechanical behavior including the aged state. The microstructure complexity is apprehended by the means of Electron Backscattered Diffraction (EBSD) acquisitions. The material history and specific post-processings of crystallographic data allow microstructural scale properties that are decisive in describing the mechanical behavior to be unveiled. In order to relate crystal plasticity finite element computations to experimental observations, digital images correlation (DIC) is used to provide full-field measurements. They must be consistent with the deformation mechanisms of the studied steels and be accurate enough to capture small deformation mechanisms occurring in each phase (at a micrometre scale) and the resulting displacements at the larger scale of the microstructure (several millimetres). Accordingly, a rigorous approach has been developed to accurately capture large displacement fields and precisely superimpose them with crystallographic data obtained by EBSD. Such measurements highlight the heterogeneous mechanical behavior of the two phases according to their crystallographic orientation, morphology and thermal aging. Finally, a consistent simplified finite element model is proposed to reproduce fundamental features of the mechanical behavior and to further describe stresses and strains distribution in the two phases at the origin of damage nucleation.