Rare earth application for heat-resisting alloys

High-temperature oxidation resistance of Al2O3- and Cr2O3-forming heat-resisting alloys with rare earths (yttrium-implanted FeCrAl, -added FeCrAl, -added FeCrAlPt alloys, Y2O3- or CeO2-coated NiCrSi, yttrium- or lutetium-added NiCr and NiCrSi) was studied in oxygen at high temperatures, by mass gain measurements, mass change measurements, amount of spalled oxide, observation of surface appearance, X-ray diffraction (XRD), scanning electron microscopy (SEM), electron probe X-ray microanalysis (EPMA) and transmission electron microscopy (TEM). After oxidation at 1573 K for 18 ks in oxygen, oxide scale on FeCrAl alloy spalled from the entire surface, however, yttrium-implanted FeCrAl alloys showed good oxide adherence. After oxidation at 1473 K for 18 ks in oxygen, mass gain of FeCrAlY alloys decreased with increasing yttrium of up to 0.1 wt.% follwed by an increase with the yttrium content, and the mass gain of FeCrAl0.005Pt0.05Y alloy with appropriate additions of platinum and yttrium was lower than that of FeCrAl0.1Y alloy. Yttrium-added FeCrAl alloys showed good oxide adherence. TEM analysis revealed that the alumina/alloy interface of FeCrAl0.005Pt0.05Y alloy showed good coherency. The scale surface of FeCrAl alloy was rough, however, those of FeCrAlY and FeCrAlPtY alloys were smooth. Cyclic oxidation of NiCrSi, Y2O3- or CeO2-coated NiCrSi alloys was studied up to 10 cycles (1 cycle: 300 s) at 1523 K in oxygen. Mass change of NiCrSi alloy increased up to 3 cycles and then decreased up to 10 cycles because of oxide spallation during cooling. On the other hand, mass change of Y2O3- or CeO2-coated NiCrSi alloy increased up to 10 cycles, and these alloys showed good oxide adherence. Granular Cr2O3 particles on Y2O3-coated NiCrSi alloy were in size smaller than these on CeO2-coated NiCrSi alloy. This result suggested that oxidation rate of Y2O3-coated NiCrSi alloy was lower than that of CeO2-coated NiCrSi alloy. After oxidation at 1473 and 1573 K for 18 ks in oxygen, mass gain of yttrium- or lutetium-added NiCr and NiCrSi alloys decreased. Oxide scales on NiCrSi alloy markedly spalled along with alloy grain boundaries during cooling. On the other hand, yttrium- or lutetium-added NiCrSi alloys showed good oxide adherence. Granular Cr2O3 particles on yttrium- or lutetium-added NiCr and NiCrSi alloys decreased in size with increasing yttrium or lutetium, and increased with increasing oxidation temperature.