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.
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