precipitation hardening , austenitic type superalloys are described . these alloys contain 0 . 5 to 1 . 5 weight percent silicon in combination with about 0 . 05 to 0 . 5 weight percent of a post irradiation ductility enhancing agent selected from the group of hafnium , yttrium , lanthanum and scandium , alone or in combination with each other . in addition , when hafnium or yttrium are selected , reductions in irradiation induced swelling have been noted .