Fracture-mechanical properties of neutron irradiated ITER specification tungsten

Abstract The work focuses on the investigation of the influence of neutron irradiation on the fracture mechanical properties of ITER specification-conform, stress-relieved tungsten bar. The irradiation of miniaturized Three-Point Bend specimens was performed inside the fuel channel of the Material Test High Flux BR2 reactor of SCK•CEN in Mol. An irradiation damage dose close to 1 dpa (in tungsten) was achieved at 800°C with active temperature cooling and constant on-line temperature monitoring. Thick-wall stainless steel capsules were implemented for shielding the thermal neutrons in order to reduce Re transmutation down to ~2 at.% (and 0.2 at.% Os). The quasi-static fracture mechanical experiments were carried out following ASTM E399 aiming at the determination of plane strain fracture toughness KIc. The post irradiation examination of the specimens irradiated at 800°C reveals severe material embrittlement. At Ttest = Tirr = 800°C, a deformation free brittle fracture is observed. The fractographic investigations show a mixture of brittle transgranular cleavage and intergranular fracture. With increasing the test temperature above 1000°C, the fracture mode is changed from the brittle to a ductile one. Blunting of the notch tip leads to a suppression of the crack initiation and propagation and consequently no failure of the specimens was observed up to severe deflection levels. In some cases, however, a ductile crack growth with a characteristic dimple formation was identified, in addition. Based on the obtained results, the Ductile to Brittle Transition Temperature shift induced by the neutron irradiation at 800°C is evaluated to be 600-625°C.