Mechanics of amorphous solids-identification and constitutive modelling

Both polymers and metals can be in an organised crystalline or amorphous glassy state, where for polymers usually at least a part of the structure is amorphous and metals are in a glassy state only when processed under special conditions. At the 15th European Mechanics of Materials Conference in September 2016 in Brussels, Belgium, a session focussing on the mechanical properties of amorphous or partly amorphous solid materials was organised, attempting to bridge descriptions found for metallic glasses and polymers, which share some common features, such as a rate- and temperature-dependent response, being prone to strain localisation in the form of shear bands, the occurrence of damage by cavitation, etc

Multi-scale microstructure evolution of tungsten under neutron and plasma loads

Tungsten (W) owing to its excellent high temperature properties, is the candidate material for plasma facing components in fusion reactors such as ITER and DEMO. However, the lifetime of the tungsten based plasma facing component and thereby the lifetime of the reactor, is dictated by the extreme particle (neutron and ions) and heat loads, and is not very well understood. The fast neutrons result in the generation of point and clustered lattice defects, which further interact with the plasma based helium ions, leading to nucleation and growth of helium bubbles. Additionally, these interactions in combination with high temperatures influence the microstructural evolutionby grain growth and recrystallization process, ultimately affecting the mechanical and thermal properties. Thus, an in-depth understanding on the role of helium ions in conjunction with heat and neutron loads is crucial for predicting the microstructure evolution under fusion conditions accurately.In the present work, a multi-scale model describing the simultaneous effect of the defect generation by neutron irradiation and helium implantation from the plasma, considering irradiation time scales of hours and component length scales is developed. At atomic length scales, the generation of defects such as the vacancies, self-interstitial atoms, their clustering and the trapping of helium at defects and their clusters are modelledusing a kinetic rate theory approach. Additionally, these microstructurallevel interactions are linked to mesoscopic length scales by considering the diffusion of mobile defects along the tungsten monoblock depth (ITER specifications). The spatially varying defect concentrations from the model are also used to obtain a measure of the spatially varying lattice stored energy, thereby allowing to link the effect of helium with mechanisms such as recrystallization and grain growth. The influence of the helium resolution from existing bubbles and microstructural sinks on the helium diffusionlength scales is investigated. Furthermore, the effect of helium cluster mobility on the overall helium retention in tungsten is found to be less-pronounced

Controlled irradiation hardening of tungsten by cyclic recrystallization

\u3cp\u3eThe economical lifetime of the divertor is a key concern for realizing nuclear fusion reactors that may solve the world's energy problem. A main risk is thermo-mechanical failure of the plasma-facing tungsten monoblocks, as a consequence of irradiation hardening induced by neutron displacement cascades. Lifetime extensions that could be carried out without prolonged maintenance periods are desired. In this work, the effects of potential treatments for extending the lifetime of an operational reactor are explored. The proposed treatments make use of cyclic recrystallization processes that can occur in neutron-irradiated tungsten. Evolution of the microstructure under non-isothermal conditions is investigated, employing a multi-scale model that includes a physically-based mean-field recrystallization model and a cluster dynamics model for neutron irradiation effects. The model takes into account microstructural properties such as grain size and displacement-induced defect concentrations. The evolution of a hardness indicator under neutron irradiation was studied. The results reveal that, for the given microstructure and under the assumed model behaviour, periodical extra heating can have a significant positive influence on controlling the irradiation hardening. For example, at 800 °C, if extra annealing at 1200 °C was applied after every 100 h for the duration of 1 h, then the hardness indicator reduces from maximum 48 to below 24.\u3c/p\u3

Modelling recrystallization and grain growth of tungsten induced by neutron displacement defects

A multi-scale model is developed for the long-term microstructural evolution of tungsten under cascade damage conditions and for different irradiation temperatures as they may occur in a divertor of a nuclear fusion reactor. In particular the competition between damage accumulation and recovery processes, including grain growth and recrystallization are captured. A mean-field model for recrystallization and grain growth is coupled to a cluster dynamics model for the evolution of the neutron damage. The displacement damage is produced in the form of defect clusters, for which a parameterized temperature-dependent scaling law is implemented. A physically-based grain nucleation model is implemented in the mean-field recrystallization model. The competition between the various temperature dependent mechanisms and their effects on the evolving microstructure is studied in the range of T= 1000 °C - 1400 °

Microstructural evolution and regeneration in neutron-irradiated tungsten monoblocks

The lifetime of the divertor in tokamak nuclear fusion reactors is uncertain, due to the severe heat, ion and neutron loads that are imposed on the plasma-facing monoblocks, which are made of tungsten. In this work, the microstructural evolution throughout the monoblock is modelled, using a multi-scale model that spans from displacement damage evolution to macroscopic material properties and temperature profiles. The evolution of the hardness and the thermal conductivity as a function of monoblock depth are studied, under a combination of heat and neutron loading, based on the concentrations of the radiation-induced defects. An increase of the temperature gradient over the monoblock is predicted, which entails serious consequences for the magnitude of the thermal stresses and the accompanying surface temperature. For the selected parameter set, the high surface temperature leads to recrystallization of a small layer of material near the surface, locally reducing the amount of irradiation hardening. Interim heat treatments of 1 h are simulated, which evoke recrystallization in the monoblock and which either reset the accumulated irradiation hardening, or keep it low, throughout the monoblock, thereby increasing the ductility again, and proving that such treatments could be a valuable tool on the way to prolongation of the lifetime of these monoblocks