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The mechanical behaviour of tungsten under irradiation and heat loads
J.A.W. van Dommelen, V. Shah, M.A. Oude Vrielink, A. Mannheim, M.G.D. Geers

Department of Mechanical Engineering, Eindhoven University of Technology, Eindhoven, The Netherlands, J.A.W.v.Dommelen@tue.nl

Nuclear fusion is potentially an important future source of energy. Critical for the development of a fusion reactor that is economically viable are the integrity and lifetime of the so-called divertor. Through this component, energy in the form of heat is extracted from the fusion reactor and tungsten is the commonly chosen material for it. Although tungsten behaves relatively ductile at elevated temperatures, the failure mechanism of tungsten at low temperature is brittle fracture. The brittle-to-ductile transition temperature (BDTT) of tungsten is dependent on both the applied boundary conditions and the underlying microstructure. In addition to the heat load, the divertor is also subjected to large neutron and plasma loads. In particular, the neutron load will lead to the accumulation of lattice damage in the polycrystalline structure of the material, increasing the brittle-to-ductile transition temperature. The combination of a continuous source of lattice damage and a high temperature in some locations leads to an evolution of the microstructure in the form of recovery, grain growth and possibly recrystallization, also affecting the brittle-to-ductile transition temperature.

In this work, the evolution of the mechanical properties of tungsten under fusion conditions, and in particular the brittle-to-ductile transition is investigated. Experiments on divertor monoblocks subjected to electron beam heating demonstrate a reduction of the brittle-to-ductile transition temperature in the regions that have recrystallized. The dependence of this transition temperature on the microstructure of the material is modelled numerically using a full field approach, in combination with crystal plasticity and a probabilistic brittle failure model. A cluster dynamics model is used to describe the evolution of the lattice defect densities due to neutron irradiation. The effect of these lattice defects on the mechanical properties is captured using a dispersed barrier hardening model and this way the effect of neutron irradiation on the evolution of the brittle-to-ductile transition is modelled. The predicted increase of the brittle-to-ductile transition temperature is in line with experimental observations. On the other hand, the model shows a shift of the brittle-to-ductile transition to lower temperatures with increasing degree of recrystallization. A mean field model is used to describe the dynamic recrystallization process that is due to the competition between various damage and recovery mechanisms.