[en] An iron-nickel-chromium age-hardenable alloy suitable for use in fast breeder reactor ducts and cladding which utilizes the γ'' strengthening phase and characterized in having a morphology of the γ'' phase enveloping the γ' phase and delta phase distributed at or near the grain boundaries. The alloy consists essentially of about 40-50% nickel, 7.5-14% chromium, 1.5-4% niobium, 0.25-0.75% silicon, 1-3% titanium, 0.1-0.5% aluminum, 0.02-0.1% carbon, 0.002-0.015% boron, and the balance iron. Up to 2% manganese and up to 0.01% magnesium may be added to inhibit trace element effects; up to 1% zirconium may be added to increase radiation swelling resistance; and up to 3% molybdenum may be added to increase strength