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Register a new userIntergranular stress distributions in polycrystalline aggregates of irradiated stainless steel
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In order to predict InterGranular Stress Corrosion Cracking (IGSCC) of post-irradiated austenitic stainless steel in Light Water Reactor (LWR) environment, reliable predictions of intergranular stresses are required. Finite elements simulations have been performed on realistic polycrystalline aggregate with a recently proposed physically-based crystal plasticity constitutive equations validated for neutron-irradiated austenitic stainless steel. Intergranular normal stress probability density functions are found with respect to plastic strain and irradiation level, for uniaxial loading conditions. In addition, plastic slip activity jumps at grain boundaries are also presented. Intergranular normal stress distributions describe, from a statistical point of view, the potential increase of intergranular stress with respect to the macroscopic stress due to grain-grain interactions. The distributions are shown to be well described by a master curve once rescaled by the macroscopic stress, in the range of irradiation level and strain considered in this study. The upper tail of this master curve is shown to be insensitive to free surface effect, which is relevant for IGSCC
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Irradiation Assisted Stress Corrosion Cracking (IASCC) is a material degradation phenomenon affecting austenitic stainless steels used in nuclear Pressurized Water Reactors (PWR), leading to the initiation and propagation of intergranular cracks. Such phenomenon belongs to the broader class of InterGranular Stress Corrosion Cracking (IGSCC). A micromechanical analysis of IGSCC of an irradiated austenitic stainless steel is performed in this study to assess local cracking conditions. A 304L proton irradiated sample tested in PWR environment and showing intergranular cracking is investigated. Serial sectioning, Electron BackScatter Diffraction (EBSD) and a two-step misalignment procedure are performed to reconstruct the 3D microstructure over an extended volume, to assess statistically cracking criteria. A methodology is also developed to compute Grain Boundary (GB) normal orientations based on the EBSD measurements. The statistical analysis shows that cracking occurs preferentially for GB normals aligned with the mechanical loading axis, but also for low values of the Luster-Morris slip transmission parameter. Micromechanical simulations based on the reconstructed 3D microstructure, FFT-based solver and crystal plasticity constitutive equations modified to account for slip transmission at grain boundaries are finally performed. These simulations rationalize the correlation obtained experimentally into a single stress-based criterion. The actual strengths and weaknesses of such micromechanical approach are discussed.
We elaborate the recently introduced theory of flow stress, including yield strength, in polycrystalline materials under quasi-static plastic deformations, thereby extending the case of single-mode aggregates to multimodal ones in the framework of a two-phase model which is characterized by the presence of crystalline and grain-boundary phases. Both analytic and graphic forms of the generalized Hall-Petch relations are obtained for multimodal samples with BCC ($alpha$-phase Fe), FCC (Cu, Al, Ni) and HCP (Cu, Al, Ni) and HCP ($alpha$-Ti, Zr) crystalline lattices at $T=300K$ with different values of the grain-boundary (second) phase. The case of dispersion hardening due to a natural incorporation into the model of a third phase including additional particles of doping materials is considered. The maximum of yield strength and the respective extremal grain size of samples are shifted by changing both the input from different grain modes and the values at the second and third phases. We study the influence of multimodality and dispersion hardening on the temperature-dimensional effect for yield strength within the range of $150-350K$.
The elastoplastic behavior of a two-phase stainless steel alloy is explored at the crystal scale for five levels of stress biaxiality. The crystal lattice (elastic) strains were measured with neutron diffraction using tubular samples subjected to different combinations of axial load and internal pressure to achieve a range of biaxial stress ratios. Finite element simulations were conducted on virtual polycrystals using loading histories that mimicked the experimental protocols. For this, two-phase microstructures were instantiated based on microscopy images of the grain and phase topologies and on crystallographic orientation distributions from neutron diffraction. Detailed comparisons were made between the measured and computed lattice strains for several crystal reflections in both phases for scattering vectors in the axial, radial and hoop directions that confirm the models ability to accurate predict the evolving local stress states. A strength-to-stiffness parameter for multiaxial stress states was applied to explain the initiation of yielding across the polycrystalline samples across the five levels of stress biaxiality. Finally, building off the multiaxial strength-to-stiffness, the propagation of yielding over the volume of a polycrystal was estimated in terms of an equation that provides the local stress necessary to initiate within crystals in terms of the macroscopic stress.
The mechanism of the evolution of the deformed microstructure at the earliest stage of annealing where the existence of the lowest length scale substructure paves the way to the formation of the so-called subgrains, has been studied for the first time. The study has been performed at high temperature on heavily deformed Ti-modified austenitic stainless steel using X-ray diffraction technique. Significant changes were observed in the values of the domain size, both with time and temperature. Two different types of mechanism have been proposed to be involved during the microstructural evolution at the earliest stages of annealing. The nature of the growth of domains with time at different temperatures has been modelled using these mechanisms. High-resolution transmission electron microscopy has been used to view the microstructure of the deformed and annealed sample and the results have been corroborated successfully with those found from the X-ray diffraction techniques.
The galling behaviour of 316L stainless steel was investigated in both the unoxidised and oxidised states, after exposure in simulated PWR water for 850 hours. Galling testing was performed according to ASTM G196 in ambient conditions. 316L was found to gall by the wedge growth and flow mechanism in both conditions. This resulted in folds ahead of the prow and adhesive junction, forming a heavily sheared multilayered prow. The galling trough was seen to have failed through successive shear failure during wedge flow. Immediately beneath the surface a highly sheared nanocrystalline layer was seen, termed the tribologically affected zone (TAZ). It was observed that strain-induced martensite formed within the TAZ. Galling damage was quantified using Rt (maximum height - maximum depth) and galling area (the proportion of the sample which is considered galled), and it was shown that both damage measures decreased significantly on the oxidised samples. At an applied normal stress of 4.2 MPa the galled area was 14 % vs. 1.2 % and the Rt was 780 um vs. 26 um for the unoxidised and oxidised sample respectively. This trend was present at higher applied normal stresses, although less prominent. This difference in galling behaviour is likely to be a result of a reduction in adhesion in the case of the oxidised surface.
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