A comparison of the effect of Cr and Al additions on the oxidation behaviour of alloys from the Nb-Cr-Si system

Nb-30Cr-10Si and Nb-20Cr-10Si (at%) alloys with 5at% Al additions were subjected to isothermal conditions to compare the effects of Cr and Al on oxidation resistance. Oxidation in air was conducted for 24h at temperatures ranging between 700 and 1400 degrees C. The weight gain per unit area as a function of temperature was determined and samples were analysed using scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), X-ray diffraction (XRD), and x-ray mapping. The higher Cr content stabilizes the Nb9Si2Cr3 phase and suppresses the Nb5Si3 phase after oxidation. The addition of Al suppresses the amount of Nb-ss in the microstructure and enhances the adherence of the oxide scale to the metal. Internal oxidation was observed in alloys with Al addition and was most evident at 1100 degrees C. The Nb-30Cr-10Si alloy showed the best oxidation response in the low temperature range while the Nb-30Cr-10Si-5Al alloy showed best oxidation resistance at higher temperatures.