Adherence of oxide scale formed on Ni20Cr1Si alloys with small additions of rare earth elements

Abstract

The isothermal and cyclic oxidation behaviour of Ni20Cr1Si (where the composition is in approximate mass per cent) alloys with small additions of rare earth elements has been studied in air over the temperature range 1273–1523 K. The rate of oxidation was reduced by the addition of cerium. In most alloys except the alloy with the addition of 0.34 mass% Ce, the oxide scales began to spall off at about 1000 K during cooling; this was caused by the difference between the thermal contraction of the alloy and that of the oxides. The scales severely spalled again at around 500 K in the alloys oxidized above 1473 K. In these alloys a continuous layer of cristobalite was formed beneath the oxide scales. The spalling is probably due to the stress caused by the displacive transformation of cristobalite which involves a substantial volume change. The best improvement in the scale adherence was attained by the addition of 0.34 mass% Ce even in the cyclic oxidation. In this alloy, SiO 2 is formed as cristobalite in a spherical shape beneath the protective Cr 2O 3 layer. The stress generated by the transpormation of cristobalite is relaxed by the deformation of the alloy adjacent to the spherical SiO 2. The scale adherence was also improved by the superficial application of CeO 2 powder. The suppression of spalling was also observed in the alloy with the addition of yttrium, lanthanum or erbium. The good adherence of the scales is due to the prevention of void formation at the alloy-oxide interface and to the keying-on structure of scales.