A detailed pulse field evaporation study was made of the field evaporation characteristics of atoms from the first five ledges of (011) planes of high purity (≤1·5 × 10–6at.fr. impurity level) tungsten of well-annealed specimens and specimens irradiated with 35 or 40 keV W+ ions at room temperature (middle of Stage II) in vacua of ~10–6 torr. The detailed examination of >1·8 × 104 frames of ciné film of the well-annealed control specimens and >2·6 × 104 frames of film of the irradiated specimens showed that the ‘ bright spots ’ identified by Buswell as vacancies decorated by surface contamination, were in reality normal tungsten atoms associated with the ledges of (Oil) planes. The identification of these ‘ bright spots ’ as tungsten ledge atoms was achieved by employing a field evaporation increment of ~ 2·5 × 10–3 of an (011) plane per evaporation pulse. The concentrations of ledge ‘ bright spots ’ in the control and in the irradiated specimens were almost identical. The vacancy concentrations, as measured in seven different families of high-index planes [(111), (233), (122), (133), (433), (342), and (543)] changed by a factor of 102 as a result of the irradiation. Hence, it was concluded that Buswell’s model for the ‘ bright spots ’ in the ledges of (Oil) plane is incorrect. An example was given of the field evaporation of a small vacancy cluster in an (011) terrace plane which gave rise to a double ledge ‘ bright spot’ contrast effect. A hard sphere ball model of a cluster of three vacancies in an (Oil) terrace showed that these ledge ‘ bright spots ’ were the byproduct of the field evaporation process. Thus, this special type of contrast effect must be taken into account when analysing ciné film of irradiated specimens. The existence of the phenomenon of ledge ‘ bright spots ’ does not affect our prior interpretation of self-interstitial atoms as also causing ‘ bright spot ’ contrast, as the two effects are readily distinguishable from one another.
ASJC Scopus subject areas
- Physics and Astronomy(all)