TEM studies of displacement cascades in Nb 3sn

Abstract

Displacement cascades in Nb 3Sn produced by bombardment with 20–60 keV Au ++ ions to doses ≲10 16 ions/m 2 have been investigated by transmission electron microscopy, using techniques of dark-field imaging in superlattice and fundamental reflections. Dark spots of rather low contrast were observed in superlattice reflections, showing little fine structure and visible into regions of foil of thickness at least 100 nm. The same defects imaged in fundamental reflections showed black-white images with the black-white vector l antiparallel to g in dark field, implying an outwardly directed strain field. Contrast calculations showed that the images observed in superlattice reflections are not inconsistent with the production at cascade sites of zones of reduced long-range order, although this possibility could not be confirmed completely unambiguously. The contrast in fundamental reflections can be explained by postulating that associated with defect regions is a spherically symmetric displacement field similar to that of an oversized coherent precipitate. Best agreement with experiment is found for a misfit parameter ϵ = 0.005, corresponding to a relative volume misfit of about 1.5%. The displacement field is consistent with displacements associated with anti-site exchange, as has been suggested in other work, and possibly resulting from a change from covalent to metallic bonding along 〈100〉 chains.