Reactive Element Addition for Cr2O3- and Al2O3-Forming Heat-Resisting Alloys

ABSTRACT

High-temperature oxidation behaviour of Cr₂O₃-forming NiCr(Si, Y, Lu) and NiCrSi(Y, Lu) alloys and Al₂O₃-forming FeCrAl(S, Pd, Pt, Y, Hf) alloys were studied in flowing oxygen (100cc/min) and/or oxygen-water vapor (dew point; 353K) for 18ks at high temperatures, by mass gain measurements, amount of spalled oxide, observation of surface appearance, X-ray diffraction (XRD), scanning electron microscopy (SEM), electron probe microanalysis (EPMA) and field emission-transmission electron microscopy/energy dispersive spectroscopy (FE-TEM/EDS). For Cr₂O₃-forming alloys, both silicon and rare earth additions markedly decreased mass gain and improved oxide adherence after oxidation at 1573K for 18ks in oxygen. FeCrAlS alloys were exposed to oxygen for 18ks at 1273, 1373, 1473, 1573 and 1673K. Alumina scales on these alloys spalled easily during cooling, and markedly spalled after oxidation at 1573K. Alumina spallation increased by water-vapor after oxidation at higher temperatures more than 1473K. The scale adherence was improved by appropriate palladium, platinum, yttrium and hafnium additions. However, the appropriate addition depended on oxidation temperature, oxidation time, additional element and oxidizing atmospheres. Cyclic oxidation of FeCrAl(0-0.5)Y alloys was performed up to five cycles (1 cycle­­ = 18ks) in oxygen-water vapor at 1573K. Amount of spalled oxide of FeCrAl alloy decreased with increasing number of cycle. On the other hand, FeCrAl0.5Y alloy showed good oxide adherence after any number of cycle.