Comparison of microstructure with mechanical properties of irradiated tungsten

Abstract

Transmission electron microscopy has been used to study the microstructure of mechanically tested polycrystalline tungsten which had been irradiated to fast ( E > 1 MeV) neutron fluences ranging from 8.5 × 10 17 to 1.5 × 10 21 n/cm 2. A fluence threshold exists between 5.9 × 10 18 and 3.8 × 10 19 n/cm 2 ( E > 1 MeV), above which distinct defect cluster formation occurs. These clusters appear to contribute to the hardening of the material, leading to embrittlement in tensile specimens tested at 400° C. The microstructure of irradiated tungsten that was subsequently exposed to creep-rupture testing at 1100° C showed the presence of distinct dislocation loops, thought to result from interstitial aggregation. Although the microstructural features observed in irradiated tungsten appear to be related to fast ( E > 1 MeV) neutron fluence rather than thermal neutron fluence, the irradiation induced strengthening observed at 1100°C shows better correlation with thermal neutron fluence. The presence of rhenium and osmium atoms produced by thermal neutron transmutation reactions is believed to account for the property changes at 1100° C.