in order to demonstrate the advantages of the present invention , alloys having the nominal compositions shown in table i were melted . it will be noted upon review of table i that a gamma prime hardening austenitic base composition was selected and then additions of about 0 . 2 weight percent scandium , yttrium or hafnium were made to the base composition while varying the silicon content of the base composition between about 1 . 0 weight percent and about zero ( i . e ., impurity levels ). in this manner seven alloys having the nominal compositions shown in table i were melted into ingots . while it is desired to hold the levels of the other alloying elements constant from ingot to ingot , normal ingot to ingot variability in chemistry did occur . examples of the variability observed are indicated by the chemical analyses shown in table ii . this variability is not believed to have had a significant affect on the determination of the effect of additions of scandium , yttrium and hafnium , with and without silicon , on the swelling resistance and post irradiation ductility of the alloys studied . the ingots representing the alloys shown in table i were first hot worked to an intermediate size to improve the chemical homogeneity within the ingot and substantially remove the as cast microstructure of each ingot . after hot working , the intermediate size products were cold worked to final size in a series of steps having intermediate solution anneals between each cold working step . for example , the base , # 7 and # 8 alloy ingots were intially soaked for about 1 to 11 / 2 hours at about 1150 ° c . they were then press forged at about 1150 ° c . to a flat bar having a nominal thickness of about 5 / 8 inch . subsequently , each ingot received a homogenization treatment which entailed soaking the ingot at about 1225 ° c . for about one hour followed by about a 2 hour soak at about 1275 ° c . and then furnace cooling . intermediate product from each of these three ingots was then cold worked in steps to substantially final size . the reductions utilized in each step typically varied from about 25 to 45 percent . intermediate solution anneals at about 1150 ° c . for about 3 / 4 hour followed by furnace cooling were performed between each cold working step . the last cold working step comprised about a 25 % reduction . after the last cold working step , material from each of the heats shown in table i were solution treated and aged as follows : 1 . solution treating was performed by soaking at about 1050 ° c . for about 1 / 2 hour and was followed by air cooling . 2 . aging was then performed by soaking at about 800 ° c . for about 11 hours followed by air cooling . a secondary aging treatment was then performed by soaking at about 700 ° c . for about 8 hours followed by air cooling . samples of the fully fabricated and heat treated alloys were then irradiated in fast neutron fluxes to various fluences and at various temperatures . the addition of hafnium and yttrium to the base alloy were found to significantly improve swelling resistance as demonstrated in table iii . scandium , however , had no significant affect on swelling resistance . table iii______________________________________swelling characteristicsirrad - iation neutron fluence percent volume expansion * temp . × 10 . sup . 22 n ( e & gt ; 0 . 1 base ° c . mev )/ cm . sup . 2 alloy alloy # 7 alloy # 8______________________________________400 5 . 9 0 . 72 - 0 . 49 - 0 . 47427 6 . 8 1 . 29 - 0 . 32 - 0 . 33454 5 . 7 0 . 73 -- -- 482 6 . 7 0 . 86 -- -- 510 7 . 4 1 . 36 -- -- 538 7 . 4 2 . 23 0 . 91 -- 593 7 . 8 0 . 61 - 0 . 20 - 0 . 06650 7 . 7 0 . 41 - 0 . 37 - 0 . 44______________________________________ * negative values indicate a volume contraction additional samples irradiated at selected temperatures and fluences indicated in table iii were characterized as to their post irradiation ductility . these ductility results are shown in table iv . table iv__________________________________________________________________________post irradiation ductility as measured by disc bend testing percent strain , irradiation test base alloy # 2 alloy # 6 alloy # 3 alloy # 7 alloy # 4 alloy # 8temp . ° c . temp . ° c . alloy ( sc no si ) ( sc ) ( y no si ) ( y ) ( hf no si ) ( hf ) __________________________________________________________________________454 564 0 . 7 0 . 2 -- -- -- 0 . 7 -- 482 592 -- 1 . 0 * v . d . ** 0 . 1 v . d . ** -- v . d . ** 510 620 0 . 4 0 . 5 2 . 1 0 . 9 5 . 0 0 . 3 2 . 2538 648 0 . 3 0 . 5 1 . 2 0 . 3 1 . 1 0 . 5 0 . 8593 703 0 . 6 -- -- 0 . 3 -- -- -- __________________________________________________________________________ * tested at 564 ° c . rather than 592 ° c . ** v . d . = very ductile , i . e ., ductility greater than 5 % the disc bend ductility test used to test these alloys is a specially designed microductility test in which an indentor is pushed through a thin disc - shaped sample of the test material . the strain , ε , or measure of ductility provided by this test has been correlated with tensile test results . the correlation between these two tests is accurate for low ductility materials . the discs are typically about 1 / 8 inch or 3 mm . in diameter and approximately 0 . 009 - 0 . 014 inch thick . the ductility test results shown in table iv indicate that a significant improvement in the gamma prime hardened base alloy post irradiation ductility is obtained by the addition of scandium , yttrium or hafnium to the base alloy composition . however , it is also indicated that where the alloy contains no significant quantity of silicon , these additions did not enhance ductility . it is therefore believed that when about 0 . 05 to 0 . 5 weight percent of scandium , yttrium and / or hafnium is added to a gamma prime hardening austenitic alloy containing an effective amount of silicon the post irradiation ductility of the alloy should be enhanced . it is preferred that silicon be present at a level of about 0 . 5 to 1 . 5 weight percent . in addition it is believed that lanthanum may be substituted for all or part of the scandium , yttrium and hafnium . it is also believed that the benefits of the present invention are also applicable to gamma prime hardening austenitics which are placed in pile in a cold worked and aged condition or a cold worked condition . typical of the treatments that may be utilized are as follows : the present invention provides an improved precipitation strengthening austenitic superalloy for liquid metal fast breeder reactor ducts and fuel pin cladding . while the invention has been described in connection with specific embodiments , it will be readily apparent to those skilled in the art that various changes in compositional limits and heat treatments can be made to suit arrangements without departing from the spirit and scope of the invention .