Molecular ion irradiations of molybdenum

Abstract

A series of TEM experiments was carried out to systematically investigate the formation of vacancy dislocation loops in displacement cascades in molybdenum. Single-crystal foils of high-purity molybdenum were irradiated with Sb⁺ single ions and and molecular ions to low doses (≤10¹⁶ ions m⁻²). Three different ion energies were employed (60, l20 and 180 keV) in order to systematically vary the total cascade energy and the energy per atom in the molecular ion. Dislocation loop sizes and defect yields were found to be larger for molecular ions than for single ions of the same energy. In (011) foils, most loops had Burgers vectors b = a/2⟨111⟩ lying in the plane of the foil. However, in molecular ion irradiations, a small fraction of loops with b = a⟨100⟩ was also found. This fraction was higher for than for ions and also increased with ion energy. In (001) foils, defect yields were much smaller because of the loss of glissile a/2⟨111⟩ loops to the surface, but a⟨100⟩ loops were still present in molecular ion irradiations. The habit planes of both a/2⟨111⟩ and a⟨100⟩ loops were consistent with nucleation on {110} planes by the Eyre–Bullough mechanism. These results are compared with recent molecular dynamics simulations of the effect of the mass of primary knock-on atoms on displacement cascades in iron.

Keywords:

• radiation damage
• displacement cascades
• cascade collapse

Acknowledgements

We acknowledge with thanks discussions with Dr. A.F. Calder, including discussions of unpublished MD simulations. We thank Dr. Zhongfu Zhou for assistance in the preparation of Figure 1.

Notes

Note

1. At these doses, little cascade overlap occurs so that we are looking essentially at individual cascade events. The doses correspond approximately to 0.001–0.01 dpa.