The Effect of Al, Mo, and B on the Oxidation Behavior of Three Nb-based Alloys

Extensive work has been undertaken to develop niobium based structural alloys for high temperature applications. These developments have led to the increase in engine operating temperatures and better performance efficiency. New materials including alloys based on metals with higher melting points such as molybdenum and niobium with silicon are now being examined as a better alternative to nickel base superalloys. Materials with a niobium silicide based composites have the potential for higher temperature capabilities. The oxidation behavior of Nb-20Cr-10Si and Nb-20Cr-10Si-5Al has been studied in a range of temperature from 700 to 1400°C in static air. Isothermal oxidation experiments indicate that addition of 5 atomic percent Al is beneficial in reducing the weight gain per unit area. The phases present include Nb solid solution, NbCr2, Nb5Si3 and Nb9Si2Cr3. However, low temperature pesting appears to be a problem especially at 800°C. High temperature oxidation is characterized mainly by spalling. Internal oxidation has been observed by the formation of Al2O3 along the interfaces of (a)Nbss and NbCr2 and (b)Nbss and Nb5Si3. The oxidation behavior of the Nb-25Cr-15Si-20Mo-10B and Nb-25Cr-15Si-20Mo-15B has also been studied in a range of temperature from 700 to 1400°C in static air. Isothermal and cyclic oxidation testing revealed that higher boron content is beneficial for the increase of oxidation resistance. The phases present include NbCr2, Nb5Si3 with the formation of Nbss being suppressed by the increase of Mo and B. The addition of boron is seen to suppress low temperature pesting and the presence of spallalling is eliminated from the boron alloys exhibiting a continuous well adhered oxide layer. All alloys were characterized by SEM and XRD including BSE, EDS and x-ray mapping modes in SEM