Helium-vacancy clustering in V-4Cr-4Ti at elevated temperatures

Thermal helium desorption spectrometry was used to investigate the helium-vacancy clustering in V-4Cr-4Ti at various implantation temperatures. Samples were implanted with 1 keV helium to a dose of 1014 cm-2 at implantation temperatures ranging from 300 to 700 K. Helium trapping inside the sample was almost completely suppressed at implantation temperatures above 700 K. Helium-vacancy clustering at gas impurities was not observed at temperature above 500 K. This result is consistent with the earlier proposed model where the helium-vacancy-oxygen complex is unstable above 550 K. Helium trapping at 500-700 K is ascribed to pre-existing traps, e.g. fine-size precipitates. In a number of experiments the implantation at elevated temperature was preceded by room temperature (RT) pre-implantation with doses varying from 1.8 x 1013 to 1014 cm-2. It was observed that the clusters created during the RT implantation were stable enough to provide further cluster growth during the subsequent high temperature (700 K) implantation. (orig.)