The oxidation of oxide dispersion strengthened Ni-15Cr-5AI alloys

Abstract

The effects of variations in both the type and amount of oxide dispersoids (La₂O₃, Y₂O₃ and ThO₂) on the isothermal and cyclic oxidation behavior of Ni-15Cr-5Al have been investigated. In isothermal oxidation, the presence of the dispersoids had no marked effect on the total weight change. Oxidation did not appear to follow any simple rate law, although the rate of weight change decreased with time, indicating the formation of a protective oxide. A layered oxide was observed to be comprised of an inner layer of A1₂O₃ and an outer layer of nickel-chromium-aluminum spinel. It is proposed that the formation of these spinels is favored in dispersoid-containing alloys. The spinels in turn promote the formation of the protective equilibrium oxide, A1₂O₃. A mechanism, based on an increase in the number of oxide nucleation sites, is presented to explain the observed results. The dispersoids markedly improved the performance of the alloys in cyclic oxidation. It is proposed that the porosity observed in the outer spinels of dispersion-hardened alloys aids in the adjustment to thermally induced expansion and contraction. These pores lead to an improvement in protective oxide scale adhesion by providing sites for the accommodation of strain, thereby reducing the amount of stress at the oxide-metal interface.