Defect characterization, mechanical and thermal property evaluation in CVD-W after low-dose neutron irradiation

Ultra-high purity (> 99.9999 wt%) chemical vapour deposited tungsten (CVD-W) samples were neutron irradiated in the BR2 reactor (Belgium) at T-irr < 210 degrees C to similar to 0.15 dpa, followed by isochronal annealing at 500, 800 and 1100 degrees C. Defect characterization showed that dislocation loops dominated the as-irradiated damage microstructure and were mostly <= 5 nm. Void formation was observed after post-irradiation annealing at 1100 degrees C. The mechanical and thermal properties of CVD-W were evaluated based on tensile tests, Vickers hardness and temperature wave analysis. Fractography study suggested that a transition from intergranular fracture to cleavage fracture took place in the material after neutron irradiation. Hardening was found similar to 23% after irradiation. Subsequent annealing below 800 degrees C saw further increase in hardness, featuring a maximum value of about Hv = 487. Softening occurred at 1100 degrees C. Thermal diffusivity dropped by similar to 65% after irradiation and similar to 40% of this degradation recovered at 1100 degrees C.