The precipitation behavior of ion irradiated V–4Cr–4Ti alloys at various oxygen and nitrogen levels

NIFS HEAT-1 and nine types of the V–4Cr–4Ti model alloys with different oxygen and nitrogen concentrations were irradiated with 2.4 MeV Cu2+ ions at 973 K up to doses of 3 dpa. Oxygen concentrations were 44–921 wppm and nitrogen concentrations were 11–1070 wppm, respectively. Fine Ti(O,N,C) precipitates were observed in all specimens. In low oxygen content specimens (<50 wppm), the number density of precipitates was lower than that of high content specimens. This was because the nucleation of precipitates, which resulted in irradiation hardening, was suppressed, especially on low damage level. On the other hand, nitrogen enhanced the hardening especially at low concentration. The total amounts of oxygen in precipitates, which were estimated from the density and size of precipitates at 0.75 and 3 dpa, were higher than that of matrix level before irradiation. The result suggests that oxygen diffuses from vacuum and other regions of the specimen to the irradiated region.