The spallinlg behavior of the surface oxide formed on Ni-20Cr alloys with additions of 0.5%Ce or less and 1%Si was studied by means of the microscopic observation of oxidized surfaces and the weight-loss measurement during cooling after the isothermal oxidation in air over the temperature range of 1000 to 1260°C. The spalling initially occurred at the alloy grain boundaries and extended to the interior of the grains. The extent and mode of the oxide spalling depended on the microstructure and morphology of oxidized layers formed on the alloys, which varied with the composition of alloys as well as the time and temperature of oxidation.
In most alloys, the surface oxide began to spall off at about 700°C upon cooling, which was caused by the thermal stress arising from the thermal contraction difference between the alloy and the oxide. Furthermore, the surface oxide severely spalled off again at about 250°C in the Si-containing alloys which formed a continuous layer of cristobalite beneath the surface oxide. It was probably due to the stress resulting from the displacive transformation of high (β) cristobalite to low (α) cristobalite which involved a substantial volume change. The best improvement of oxide adherence in the alloy with additions of both 0.5%Ce and 1%Si could be explained by keying the protective surface oxide to the alloy substrate owing to the formation of the intergranular oxide pegs and the convoluted alloy/oxide interface.