Effect of dispersed reactive metal oxides on the oxidation resistance of Ni--20 wt % chromium alloys

Nickel-20 wt percent chromium alloys containing ThO2, Y2O3, La2O3, Al2O3, and Li2O, as prepared by the mechanical alloying technique, were examined for isothermal and cyclic oxidation resistance in dry air at 1000, 1100, and 12000C. TDNiCr, a commercial electrical heating element alloy (Com Ni--20Cr) and a laboratory melted alloy (Lab Ni--20Cr) were also tested. It was found that Y2O3, La2O3, Al2O3, and ThO2 dispersoids markedly increased both isothermal and cyclic oxidation resistance compared with Lab Ni--20Cr at all temperatures; in contrast Li2O additions gave no improvement in protection. Com Ni--20C r was in between Lab Ni--20Cr and the Y2O3, Al2O3, and ThO2 containing alloys in both cyclic and isothermal oxidation performance. A mechanism based on alterations in the defect structure of Cr2O3 is proposed to explain these dispersed oxide effects on isothermal oxidation behavior. It is based on a reduction in cation transport rates which in turn alter the rate of oxide growth. ThO2-containing alloys fabricated by the mechanical alloying technique were found to have oxidation resistance fully equal to commercial TDNiCr. Com Ni--20Cr performed better than Lab Ni--20Cr, but not as well as TDNiCr