A ternary alloy Nb-20W-10Cr (in weight percents) is explored for high temperature applications up to 1500°C. Oxidation resistance of this alloy is tested in a range of temperature from 700 to 1500°C. It is subjected to oxidation in air for a period of 24 hours (STO) and 168 hours (LTO). The oxidation experiment performed at 1500°C shows significant amount of melting of the alloy. Experiments conducted at 700 to 1400°C show that surface finish does not affect oxidation kinetics. Weight gain method has been used to determine the kinetics of oxidation in air. Metal oxides on the surface of the samples have been examined by X-ray diffraction, Optical Microscopy, Scanning Electron Microscopy, Auger Electron Spectroscopy, and X-ray Photoelectron Spectroscopy.
The STO in air at the temperatures between 700 and 1400°C indicates continuous weight gain up to 1400°C except for a drop observed from 950 to 1000°C. However, the alloy completely turns in to powder at 800, 850, 900, and 950°C temperatures of oxidation in air. XRD on these powders indicate presence of oxides Nb 2 O 5 , NbO 2 , Cr 2 O 5 , Cr 2 O 3 , WO 2 , WO 3 , and NbCrO 4 . The powder formation during oxidation in air is perhaps due to the formation of WO 3 with an extremely high linear expansion coefficient.
The oxidation curves of LTO of the alloy in air shows normal behavior of decreased oxidation resistance with an increase in temperature at 700 and 1100°C. However, improved oxidation resistance has been found with increase of temperature at and above 1200°C and up to 1400°C. This enhanced oxidation resistance at elevated temperatures can be attributed to NbCr 2 which is an intermetallic microconstituent in the alloy. Activation energy for oxidation has been calculated using the oxidation curves.
