Neutron irradiation damage in molybdenum

Abstract

Electron transmission microscopy has been used to study defect clusters in neutron-irradiated molybdenum. Bulk specimens having resistivity ratios, ρ₂₉₃°K/ρ_4·2°K, ∼ 175 and ∼ 7500 were irradiated to a dose of 4·65 × 10¹⁷ fission n. cm⁻² at 77°K. Diffraction contrast experiments at ∼ 300°K have shown that visible strain centres are present in both materials following the irradiation. Estimates of the apparent volume density, N _c, of clusters indicate that N _c > 5 × 10¹⁶ cm⁻³ in the lower resistivity ratio material and N _c ∼ 6 × 10¹⁵ cm⁻³ in the higher resistivity ratio material. Detailed studies on the latter material indicated that the observed image topographies are consistent with those expected from aggregates of point defects in the form of dislocation loops having Burgers vector directions of the types 〈110〉 and 〈111〉. Moreover, stereoscopic measurements of the depth dependence of these images, interpreting the resulting layer structure in terms of theoretical intensities appropriate when the black-white intensity distribution is determined by the edge component of a small dislocation loop, suggest that ∼ 80% of the observed clusters are interstitial in nature. A comparison between the defect structures of the two materials investigated has been interpreted as indicating that impurities play an important role in determining the scale of nucleation and hence the extent of growth of the dominant interstitial aggregate. Finally, as a consequence of these observations it has been concluded that long-range migration of an interstitial point defect has occurred at mean temperatures substantially below the stage III recovery peak (i.e. T=423−473°K) of induced electrical resistivity in neutron-irradiated molybdenum.