Influence of fast neutron fluence on the ductile-brittle transition temperature of tungsten, W-10Re, and W-3.4Ni-1.6Fe

Neutron irradiation experiments with samples (dimensions 2 x 2 x 50 mm3) of tungsten, W-10Re, and W-3.4Ni-1.6Fe (Densimet 18) were carried out in the temperature range 525-575 K in the material test reactors FRJ2 at Juelich and HFR at Petten up to fluences of 56 x 1024 neutrons m-2 (E > 0.1 MeV). Three-point bending tests at different test temperatures were performed in the hot cells of KFA-Juelich. All materials investigated, especially Densimet 18 and W-10Re, showed a strong increase in ductile-brittle transition temperature Tt with increasing neutron fluence. The Tt shift per 1024 neutrons m-2 in the low fluence range 0-5 x 1024 m-2 is 13.7 K for tungsten, 31.0 K for W-10Re, and more than 160 K for Densimet 18. Scanning electron microscopy studies and optical metallography revealed distinct radiation-induced microstructural variations, especially the formation of new subgrain boundaries with increasing fluence. The fracture morphology shifted from predominantly transcrystalline cleavage fracture to mixed fracture. (author)