Abstract
The recrystallization assisted microstructural changes during cyclic heat exposure are surmised detrimental to the thermal fatigue resistance of the tungsten monoblocks that cover the surface of the divertor components of a nuclear fusion reactor. A numerical framework to predict recrystallization in tungsten monoblocks during cyclic heat exposure is presented in this work. The framework is based on a thermal model coupled to a Johnson-Mehl-Avrami-Kolomogorov (JMAK) type recrystallization model. The influence of the initial (starting) microstructural state, i.e. the degree of deformation, on the extent of recrystallization, is considered through the activation energy for recrystallization. For a highly deformed microstructure, significantly more recrystallization is observed compared to self-diffusion driven recrystallization. The activation energy for recrystallization within the range 322 to 350 kJmol−1 during cyclic heat exposure of a monoblock is determined from the simulated recrystallization extent in relation to the experimental observations of differently processed monoblocks. The influence of the heating time per cycle for different activation energies on the recrystallization extent is also investigated. Instead of bulk recrystallization, only localized surface recrystallization is observed for a combination of a shorter heating time per cycle and a moderately deformed initial microstructure, which may contribute to improving the thermal fatigue resistance of the monoblocks.
Highlights
The viability of fusion reactors as a sustainable and commercial source of energy necessitates a prolonged lifetime of their components
Tungsten monoblocks manufactured via rolling by AT&M, China and exposed to cyclic high heat flux (HHF) loads achieved via an electron beam at IDTF (ITER divertor testing facility, Russia) were delivered by Research In struments (RI) GmbH
A novel modelling framework, coupling transient heat flow to the JMAK model to predict the dynamic evolution of the temperature dependent recrystallized fraction in tungsten monoblocks during cyclic heat exposure is presented
Summary
The viability of fusion reactors as a sustainable and commercial source of energy necessitates a prolonged lifetime of their components. The macro-crack formation during HHF exposure has been linked to tungsten’s brittleness in the recrystallized state, thereby leading to crack-nucleation at the surface, followed by crack-growth in the bulk due to tensile stresses during cooling [25] This brittle behaviour of recrystallized tungsten, implying a higher brittle-to-ductile transition temperature (BDTT) is attributed to segregation of impurities and the associated weakening of the grain boundaries (predominantly high angle grain boundaries) [26,27]. A novel numerical approach to account for the evolution of the recrystallized fraction during cyclic HHF loading is proposed, which allows for assessment of the influence of the dynami cally changing microstructural state on the thermo-mechanical behav iour (not addressed here) This approach is based on a thermal heat conduction model coupled with a Johnson-Mehl-Avrami-Kolmogorov (JMAK) based recrystallization model.
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