Electron Microscopy of Irradiated and Mechanically Tested Tungsten

As part of a program investigating the effects of neutron irradiation on the physical and mechanical properties of body centered cubic refractory metals, transmission electron microscopy has been carried out on irradiated tungsten after tensile and creep-rupture testing. These observations have shown the existence of a fluence threshold region between 5.9 × 1018 and 3.8 × 1019 nvt (E < 1 MeV) over which both microstructure and mechanical properties undergo abrupt changes. A series of specimens irradiated at pile ambient temperature (∼ 70°C) to various fast neutron fluences and subsequently tensile tested at 400°C showed dramatic evidence of the build-up of defect clusters with increasing exposure. The starting microstructure present in undeformed button heads of unirradiated control specimens consisted of large tungsten grains divided into many small subgrains by hexagonal dislocation networks, as shown in Figure 1. Irradiation to neutron fluences of 4.0 and 5.9 × 1018 nvt produced tiny dot clusters in the matrix, as shown in Figure 2.