Ductile-to-brittle-transition refers to observable change in fracture mode with decreasing temperature—from slow ductile crack growth to rapid cleavage. It is exhibited by body-centred cubic metals and presents a challenge for integrity assessment of structural components made of such metals. Local approaches to cleavage fracture, based on Weibull stress as a cleavage crack-driving force, have been shown to predict fracture toughness at very low temperatures. However, they are ineffective in the transition regime without the recalibration of Weibull stress parameters, which requires further testing and thus diminishes their predictive capability. We propose new Weibull stress formulation with thinning function based on obstacle hardening model, which modifies the number of cleavage-initiating features with temperature. Our model is implemented as a post-processor of finite element analysis results. It is applied to analyses of standard compact tension specimens of typical reactor pressure vessel steel, for which deformation and fracture toughness properties in the transition regime are available. It is shown that the new Weibull stress is independent of temperature, and of Weibull shape parameter, within the experimental error. It accurately predicts the fracture toughness at any temperature in the transition regime without relying upon empirical fits for the first time.

No abstract

Understanding corrosion mechanisms is of importance for reducing the global cost of corrosion. While the properties of engineering components are considered at a macroscopic scale, corrosion occurs at micro or nano scale and is influenced by local microstructural variations inherent to engineering alloys. However, studying such complex microstructures that involve multiple length scales requires a multitude of advanced experimental procedures. Here, we present a method using correlated electron microscopy techniques over a range of length scales, combined with crystallographic modelling, to provide understanding of the competing mechanisms that control the waterside corrosion of zirconium alloys. We present evidence for a competition between epitaxial strain and growth stress, which depends on the orientation of the substrate leading to local variations in oxide microstructure and thus protectiveness. This leads to the possibility of tailoring substrate crystallographic textures to promote stress driven, well-oriented protective oxides, and so to improving corrosion performance.

Ferritic steels, which are typically used for critical reactor components, including reactor pressure vessels (RPV), exhibit a temperature-dependent probability of cleavage fracture, termed ductile-to-brittle transition. The fracture process has been linked to the interaction between matrix plasticity and second phase particles. Under high-enough loads, a competition exists between cleavage and ductile fracture, which results from particles rupturing to form micro-cracks or particles decohering to form micro-voids, respectively. Currently, there is no sufficiently adequate model that can predict accurately the reduced probability of cleavage with increasing temperature and the associated increase of plastic deformation. In this work, failure probability has been estimated using a local approach to cleavage fracture incorporating the statistics of micro-cracks. It is shown that changes in the deformation material properties are not enough to capture the significant changes in fracture toughness. Instead, a correction to the fraction of particles converted to eligible for cleavage micro-cracks, with an exponential dependence on the plastic strains, is proposed. The proposed method is compared with previous corrections that incorporate the plastic strains, and its advantages are demonstrated. The method is developed for the RPV steel 22NiMoCr37 and using experimental data for a standard compact tension C(T) specimen. The proposed approach offers more accurate calculations of cleavage fracture toughness in the ductile-to-brittle transition regime using only a decoupled model, which is attractive for engineering practice.

Understanding the in-reactor corrosion behavior of zirconium alloys is essential for optimizing the lifetime of fuel assemblies. Recent advances in available experimental methods have enabled the characterization of oxide morphology, crystallography, and chemical heterogeneity with unprecedented detail for both autoclave and reactor formed oxides. Advanced high-resolution techniques have already improved the understanding of zirconium alloy corrosion performance. However, they are carried out on small volumes of material and require preparation of thin samples, which can lead to changes in the phase distribution in the oxide and often show varied results from different regions of a single bulk specimen. The present study utilizes high-spatial-resolution electron backscatter diffraction (EBSD) performed on bulk samples to produce spatially resolved microtexture data from nanograined zirconium oxide over a large area, which has not previously been possible. This advanced method of plan-view oxide texture analysis, alongside targeted focused ion beam cross-section measurements and substrate EBSD analysis, has revealed well-defined regions of monoclinic oxide grains that exhibit different textures depending on the orientation of the substrate grain on which they have formed. The observed variations in oxide texture have significant implications on any conclusions drawn solely from methods that are limited to the characterization of small areas—especially where sampling areas are smaller than the substrate grain size. Two competing mechanisms of oxide grain growth and nucleation are discussed, and detailed EBSD analysis illustrates a correlation between local oxide texture and corrosion rate. This analysis is performed on specimens of autoclave-tested Zircaloy-2 and ZIRLO and highlights differences in oxide texture development between the two alloys, indicating the significance of material composition and thermomechanical processing on corrosion behavior.