Thermal He-desorption spectrometry was used to investigate He-vacancy clustering in V-4Cr-4Ti alloy at various implantation temperatures. Samples were implanted with 1keV He to a dose of 1014/cm2 at implantation temperatures ranging from 300 to 700K. Trapping of He within the sample was almost completely suppressed at implantation temperatures above 700K. The occurrence of He-vacancy clustering at gas impurities was not observed at temperatures above 500K. This was consistent with a proposed model according to which the He-vacancy-O complex was unstable above 550K. The trapping of He at 500 to 700K was attributed to pre-existing traps such as fine precipitates. In some experiments, implantation at high temperatures was preceded by room-temperature pre-implantation to doses which ranged from 1.8 x 1013 to 1014/cm2. It was observed that clusters which were created during room-temperature implantation were stable enough to provide further cluster growth during subsequent implantation at 700K.

Helium-Vacancy Clustering in V-4Cr-4Ti at Elevated Temperatures. A.V.Fedorov, A.Van Veen, A.I.Ryazanov: Journal of Nuclear Materials, 1998, 258-263, 1396-9