The effects of silicon and titanium on void swelling and phase stability in 12Cr-15Ni austenitic alloys irradiated with 46 MeV nickel ions

Abstract

The structure of 20% cold-worked 12 wt%Cr-15 wt%Ni austenitic alloys with Si and Ti contents in the ranges 0.14-1.42 and <0.02-0.27 wt%, respectively, has been investigated by TEM following irradiation with 46 MeV Ni 6+ ions to 60 dpa at 525, 575 or 625° C in the Harwell VEC. Increasing the Si content progressively reduced the void concentrations and swelling to zero at all three temperatures and the addition of Ti further reduced the void concentration and swelling. The higher Si alloys were structurally unstable during irradiations at 525 and 575°C, when γ'(Ni 3Si) particles, M 23X 6 type precipitates and grains of a bcc phase were formed. Small amounts of the γ' and bcc phases were observed following irradiation at 625° C, the principal precipitates being of the M 23X 6 type. Voids were present in both austenite and bcc phases in the lower Si alloys and segregation of an unidentified fcc phase was observed around voids. It was concluded that all the void nucleation occurred in the austenite phase and that the bcc phase resulted from the partial transformation of the austenite to α'-martensite during cooling from the irradiation temperatures. This occurred as a consequence of the M s ( α') transformation temperature being raised to well above ambient owing to elemental depletions of the alloy matrices resulting from precipitation and segregation.