Fundamental Mechanisms for Irradiation-Hardening and Embrittlement: A Review

It has long been recognized that exposure to irradiation environments could dramatically degrade the mechanical properties of nuclear structural materials, i.e., irradiation-hardening and embrittlement. With the development of numerical simulation capability and advanced experimental equipment, the mysterious veil covering the fundamental mechanisms of irradiation-hardening and embrittlement has been gradually unveiled in recent years. This review intends to offer an overview of the fundamental mechanisms in this field at moderate irradiation conditions. After a general introduction of the phenomena of irradiation-hardening and embrittlement, the formation of irradiation-induced defects is discussed, covering the influence of both irradiation conditions and material properties. Then, the dislocation-defect interaction is addressed, which summarizes the interaction process and strength for various defect types and testing conditions. Moreover, the evolution mechanisms of defects and dislocations are focused on, involving the annihilation of irradiation defects, formation of defect-free channels, and generation of microvoids and cracks. Finally, this review closes with the current comprehension of irradiation-hardening and embrittlement, and aims to help design next-generation irradiation-resistant materials

Model for irradiation softening of nickel-based single crystal superalloys under ion irradiation

The phenomenon of irradiation softening has recently been noticed for nickel-based single crystal superalloys. However, corresponding theoretical analysis addressing the fundamental deformation mechanisms is limited so far. In this work, a novel irradiation softening model is developed to characterize the hardness-depth relations of ion-irradiated nickel-based single-crystal superalloys, which considers four deformation mechanisms including the irradiation softening effect, indentation size effect, damage gradient effect and soft matrix effect. Thereinto, irradiation softening can be mainly ascribed to irradiation-induced disordering of the γ′ precipitates that results in the loss of the ordered strengthening, and dissolution of the γ′ precipitated phase that mitigates the hardening behavior related to dislocation strain. In order to validate the developed model, experimental data of ion-irradiated nickel-based alloy Rene N4 is considered at different irradiation doses. A good agreement between the theoretical results and experimental data is obtained for the variation of indentation hardness as a function of the indentation depth. Further analysis of the deformation mechanisms indicates that: (1) when compared with the exiting dislocations, irradiation-induced defects have limited contribution to the hardness of ion-irradiated nickel-based alloy Rene N4. (2) It is the change of the precipitated phase dissolution that results in the variation of the extent of irradiation softening at different irradiation doses

Model for the hardness-depth relationships of ion-irradiated nanocrystalline metals

In this work, a mechanistic model is proposed for ion-irradiated nanocrystalline (NC) metals to characterize the evolution of hardness as a function of the indentation depth at room temperature and under quasi-static loading condition. At the grain level, grain interiors (GIs) and grain boundaries (GBs)-dominated hardening are addressed simultaneously in the developed model, which is able to effectively characterize the contribution of geometrically necessary dislocations (GNDs), statistically stored dislocations (SSDs), irradiation-induced defects, Hall-Petch effect and the intrinsic strength of GBs. Thereinto, the GIs-dominated hardening mechanisms are systematically analyzed by considering the evolution of microstructures, which include the average density of dislocations and irradiation-induced defects within the plastic zone, and are noticed to be affected by the high ratio of GBs. Main attentions are focused on the description of GBs influence that covers dislocation hardening and defect hardening. The rationality and accuracy of the proposed model are validated by comparing the theoretical results with corresponding experimental data under different irradiation conditions. The proposed model offers a promising way to analyze the irradiation hardening mechanisms of NC metals

High temperature nano-indentation of tungsten: Modelling and experimental validation

It is very well known that tungsten is intrinsically brittle at room temperature, and the characterization of its ductile properties by conventional mechanical tests is possible only above the ductile-to-brittle transition tem- perature (DBTT), i.e. above 500–700 K. However, the design of tungsten-based components often requires the knowledge of constitutive laws below the DBTT. Here, we carried out instrumented hardness measurements in the temperature range of 300–691 K by nano-indentation. The obtained results are used to extend a set of constitutive laws for the plastic deformation of tungsten, developed earlier on the basis of tensile data, which now covers the temperature range of 300–1273 K. The validation of the constitutive laws was realized by the crystal plasticity finite element method (CPFEM) model, which was applied to simulate the nano-indentation loading curves. The distribution of stress and strain under the indenter was also studied by the CPFEM to bring an insight on the extension of the plastic zone in the process of the indentation, which is of crucial importance when nano-indentation is used to resolve the microstructural features generated by e.g. irradiation by energetic particles, plasma exposure or thermo-mechanical treatment

Crystal plasticity finite element method simulation for the nano-indentation of plasma-exposed tungsten

In this work, the nano-indentation of plasma-exposed tungsten is simulated at room temperature and elevated temperature (300–700 K) by the recently developed crystal plasticity finite element model. A nonlinear function is applied to characterize the depth profile of plasma-induced dislocation density in the sub-surface region. The model parameters are calibrated by comparing the simulated results with corresponding experimental data at 300 K for both the force-depth and hardness-depth relationships. Furthermore, the mechanical responses of plasma-exposed tungsten are predicted at 500 K and 700 K in order to characterize the plasma effect at the fusion-relevant operational temperature. The dominant results and conclusions are that: (1) The heterogeneously distributed dislocations in the sub-surface region induced by the plasma exposure are responsible for the increase of hardness at 300 K. (2) The plasma-induced microstructural modification does not yield to considerable increase of hardness at operational temperature. (3) The expansion of the plastic zone in the sub-surface region is, to some extent, limited by the presence of plasma-induced dislocations. Whereas, the increase of temperature can effectively reduce this limitation

High temperature nano-indentation of tungsten: modelling and experimental validation

It is very well known that tungsten material is intrinsically brittle at room temperature, and characterization of its ductile properties by conventional mechanical tests is possible only above ductile to brittle transition temperature (DBTT) i.e. above 500-700K. However, the design of tungsten-based components often require the knowledge of constitutive laws below DBTT. Here, we carried out instrumented hardness measurements in the temperature range of 273-673K by nano-indentation. The obtained results are used to extend a set of constitutive laws for the plastic deformation of tungsten, developed earlier on the basis of tensile data, which now covers the temperature range of 273-1273K. The validation of the constitutive laws was realized by crystal plasticity finite element model (CPFEM), applied to simulate the nano-indentation loading curves. Distribution of stress and strain under indenter was also studied by CPFEM to bring an insight on the extension of the plastic zone in the process of the indentation, which is of crucial importance when nano-indentation is used to resolve microstructural features generated by e.g. irradiation by energetic particles, plasma exposure or thermo-mechanical treatment.JRC.G.I.4-Nuclear Reactor Safety and Emergency Preparednes