The origin of the large resistance to void swelling observed in Fe-35.5Ni-7.5Cr

While it is well known that increases in the nickel content of austenitic steels delay swelling, less attention has been paid to the fact that decreases in the chromium level from the usual 15-18% range also suppress swelling. Neutron irradiation studies show the longest postponement of swelling in Fe-Ni-Cr alloys occurring at ≅35 wt% nickel and 7.5% chromium. The causes of the suppression and the subsequent loss of swelling resistance have been sought using electron microscopy and quantitative x-ray microanalysis. Swelling in this alloy is preceded by a substantial densification arising from large radiation-induced spinodal-like micro-oscillations in composition which appear to destroy the resistance to void nucleation, such that void nucleation is favored in the nickel-poor chromium-rich regions. Alloy optimization efforts now concentrate on solute additions which are designed to delay the microsegregation process. The simple homogeneous austenite region shown on phase diagrams for Fe-Ni-Cr alloys near the Invar compositional range appears not be the true equilibrium state. When the results of this investigation are compared with those of other studies it appears that radiation seems to accelerate the evolution toward stability, particularly if the irradiation occurs below the critical order-disorder temperature. Even above the critical temperature a spinodal-like decomposition occurs during irradiation. This produces compositional micro-oscillations with periods on the order of hundreds of nanometers in the range 550-6250C