The need for identifying or developing an alternate core structural material for commercial liquid metal fast breeder reactors (LMFBR) has been underscored by the large swellings that can be produced by high-dose ion bombardment of Types 304, 316, and 321, both in the annealed and cold-worked conditions. Hudson et al have already shown that the Nimonic alloy, PE-16, is very resistant to swelling and suggested that this property is due to the presence of a fine gamma prime precipitate. However, in view of the many criteria that must be met by a core material, it is essential that there be information on a variety of low-swelling alloys in order to provide some flexibility in optimizing the final combination of properties.

We have evaluated the swelling tendency in approximately 20 commercial alloys, mostly in the austenitic range of the iron-chromium-nickel system. The swelling has been produced with 5-MeV nickel ions in specimens that were preinjected with 15 ppm of helium. The “standard” bombardment conditions have been at 625°C with a dose of 10¹⁷ ions/cm², which produces ∼140 displacements per atom (dpa) at the damage peak.

The results on commercial alloys reveal negligible swelling in a ferritic alloy and show that the compositional range in the vicinity of 15 to 20Cr and 8 to 15Ni is particularly susceptible to large swellings. (This range includes Types 304, 316, 318, 321, and I2R72HV). There is a wide compositional range, which includes PE-16, in which the swelling is very low. The low swelling alloys include solid solution as well as precipitation hardened alloys showing that the presence of precipitates is not a necessary condition for low swelling. Special alloys containing only iron, chromium, and nickel have been used to confirm that major composition is a dominant factor governing the swelling behavior of iron-chromium-nickel alloys.

Information on how swelling depends upon major composition of simple iron-chromium-nickel alloys should provide the necessary background for evaluating the effects of minor additions, precipitates, and other structures in commercial alloys and can form the basis for determining the compositional range in which low swelling alloys should be selected or developed for reactor applications.