Comparison of the dispersion hardening of niobium alloys with carbides and nitrides

Abstract

1. The improved strength characteristics of binary niobium-carbon and niobium-nitrogen alloys are mainly due to solid-solution hardening, which remains operative up to temperatures of 1000–1100°C. The carbide and nitride phases in these alloys are precipitated in the form of large irregular particles whose strengthening effect is negligible. 2. In multicomponent niobium alloys, dispersion hardening is achieved by the use of up to 3 vol.% of carbides and nitrides of the Group IVA metals. The composition, production technology, and methods of processing of these alloys are determined by the solubility of carbon and nitrogen, respectively, in the matrix phases. 3. At any given volume concentration of the hardening phases in these alloys, carbide hardening is more effective at temperatures below 1000–1100°C. At temperatures above 1100°C and also where improved processing characteristics are required, nitride hardening is preferable. 4. Carbide dispersion hardening is achieved by subjecting niobium alloys to special thermomechanical treatment, as a result of which very fine carbide particles are precipitated out of the supersaturated solid solution of the alloys. Nitride hardening is a result of the formation of a disperse two-phase structure in the course of solidification. 5. Certain processing operations (welding, heat treatment) raise the ductile-to-brittle transition temperature of dispersion-hardened niobium alloys. This effect is due to the presence of an excess nitrogen content in the solid solution in the case of nitride hardening and to the appearance in the structure (mainly at me grain boundaries) of coarse carbide particles in the case of carbide hardening.