Defect evolution of neutron irradiated ITER grade tungsten after annealing

Abstract

The microstructural evolution of neutron irradiated tungsten (W) after annealing and its correlation with the corresponding mechanical properties provides valuable insight on the defect interactions and their annihilation processes. This would result in the identification of recovery mechanisms, leading to the design of healing processes and thus, enabling the lifetime extension of fusion reactor components. Within this framework, samples from ITER specification forged W bar were neutron irradiated to a dose of 0.2 displacements per atom (dpa) at 600 °C in the Belgian Material Test Reactor (BR2) and subsequently annealed at 800 and 1000 °C. The evolution of the irradiation induced defects after annealing has been assessed by transmission electron microscopy (TEM), positron annihilation lifetime spectroscopy and electrical resistivity and its effect on the mechanical properties are discussed in terms of Vickers hardness. Neutron irradiation results in the formation of dislocation loops and voids. TEM observations show an increase in the size of both defect types after post irradiation (PI) annealing at both temperatures, accompanied by an initial increase in their density after PI annealing at 800 °C, followed by a subsequent decrease after PI annealing at 1000 °C. The presence of TEM-irresolvable defects of both types is revealed in the as-irradiated state, which is evidenced by the evolution of Vickers hardness, resistivity and positron lifetimes and their correlation after the PI annealing. In the as-irradiated state, as well as after PI annealing at both temperatures, the hardening is dominated by voids.