Surface chemistry and microstructure of FeCrAl alloys after rapid heating in steam environments and quenching

Iron-chromium-aluminum (FeCrAl) alloys are being investigated as prospective accident-tolerant fuel (ATF) cladding materials to replace zirconium (Zr)-based alloy cladding. Although extensive research has demonstrated the superior oxidation resistance of FeCrAl alloys, there is a lack of studies that integrate both high heating rates and water quenching, two critical factors in accident scenarios. The design and construction of a testing apparatus with induction heating is presented to simulate accident scenarios. Two FeCrAl alloys, APMT and C26M, were tested by subjecting sample tubes to high-temperature steam with heating rates of up to 93 °C/s, targeting temperatures of 800°C, 1000°C, and 1200°C, with a holding time of 10 min, followed by water quenching. APMT samples showed evolution of the oxide from three layers at 800°C to the formation of a protective Al₂O₃ oxide layer at 1200°C. C26M initiated with a Cr and Fe-rich alumina layer at 800°C, showing islands of iron oxides on the surface. At 1000°C, the alumina layer depleted in Fe and Cr grew ∼4 times thicker. The formation of a catastrophic oxidation layer rich in Fe, Cr, and Al was observed in the test at 1200°C, which may be attributed to material's grain size, and elemental diffusion at such heating rates. Both alloys exhibited grain growth, with C26M showing more pronounced growth from 52 up to 252 µm vs. 8.4 up to 14.2 µm for APMT, which may negatively impact mechanical properties such as strength and toughness, as evidenced by the decrease in hardness after testing. These observations emphasize the need for optimizing alloy composition and grain size to enhance ATF cladding performance under extreme reactor conditions.