Abstract
The oxidation characteristics of eight Nb-Al alloys in a 10% oxygen atmosphere were studied in the temperature range of 700 to 1300 °C. Additions of aluminium generally improved the oxidation resistance of niobium. Dilute alloys, when oxidized at intermediate temperatures showed a considerable enrichment of aluminium in the oxide, due to the preferential oxidation of the latter. An alloy containing 6% Al, when oxidized, was found to have ~ 36% Al in the oxide layer, and this alloy had the minimum oxidation rate at 700°C. Alloys which had in excess of 9% Al did not show preferential oxidation in this temperature range, and this is attributed to the high stability of the compounds. At 900°C and above, the general oxidation kinetics of dilute Nb-Al alloys were found to obey a Lorriers type mechanism. Preferential oxidation of aluminium was insignificant in all alloys. Alloys rich in aluminium showed a substantial drop in the oxidation rate, due to the formation of a highly protective oxide of NbAlO4, which was identified as n-type semi-conductor. Most alloys suffered from internal contamination, which was particularly disadvantageous with the intermetallic phases, as they tended to crack readily on oxidation at intermediate temperatures. The maximum cracking susceptibility was shown by the sigma phase. The cracking phenomenon was caused by the formation of highly stressed regions at the grain boundaries, resulting in the separation of the grains. Above 1000°C, a dense oxide film was formed on NbAl3, which inhibited internal oxidation and cracking. Microhardness examination of the contaminated zones revealed the presence of high hardness gradients, especially after oxidation at intermediate temperatures, and from these, oxygen diffusion coefficients between 1.2 x 10-4 to 2.7 x 10-8 were calculated. Electron microscopy showed considerable particle precipitation in the contaminated zones, particularly at high temperatures, indicating phase dissociation.