Effect of alloying, rapid solidification, and surface kinetics on the high-temperature environmental resistance of niobium. Annual technical report, November 1985-January 1986

Factors affecting the formation of protective alumina scales on niobium-base alloys by selective oxidation have been investigated. Alumina cannot be formed in air at l atm. on binary Nb-A1 alloys at any N(Al). Theoretical knowledge of selective oxidation was applied to Nb-A1 alloys to alter behavior. The effects of Al content, temperature, atmosphere, third element additions, and microstructure on the transition from internal to external oxidation of aluminum was evaluated, and conditions under which protective alumina scales can form on Nb-A1 alloys were defined. Third element additions are required to form protective alumina. The most effective additions are those that can reduce the solubility and diffusivity of oxygen, enhance diffusion of A1, and limit transient oxidation. Additions of Ti, Cr, V and Si were identified as most promising for providing oxidation resistance in Nb-A1 alloys. The feasibility of forming compact, adherent alumina scales on Nb alloys at a minimum N(A1)=0.32 in air at 1100-1600 C was demonstrated. Alumina scale could not be formed below 1100 C. Preoxidation above 1100 C can be used to preform alumina scales that wil protect at lower temperatures, but is effective only if the alloy is not cooled to room temperature prior to exposure atmore » lower temperatures. Rapid-solidification processing does not appear to offer any significant benefit.« less