Nonisothermal and Isothermal Oxidation Behavior of Nb-Si-Mo Alloys

The nonisothermal and isothermal oxidation behavior of arc-melted, hypereutectic Nb-19Si-5Mo, Nb-18Si-26Mo, and hypoeutectic Nb-13Si-4Mo, and Nb-12Si-15Mo ternary alloys have been investigated vis-à-vis that of Nb and Nb-10Si alloy. The nonisothermal studies carried out using simultaneous thermogravimetric and differential thermal analysis in the temperature range of 50 °C to 1300 °C using heating rates of 5 °C, 15 °C/min, 25 °C/min, and 40 °C/min have shown oxidation initiation at lower temperatures for slower heating rates and higher Mo content. Isothermal oxidation experiments have been carried out in dry air at 800 °C, 1000 °C, and 1150 °C for 24 hours and the kinetics have been examined by the power-law equation. The studies show that the ternary Nb-Si-Mo alloys undergo less mass gain compared to the Nb and the Nb-10Si alloy. The hypo- or hypereutectic character of the alloys controls the oxidation behavior at 800 °C, while the effect of Mo content is predominant at temperatures of 1000 °C to 1150 °C. X-ray diffraction and scanning electron microscopy (SEM) accompanied by energy dispersive X-ray analysis have shown that the oxide scales of all the alloys are made of Nb2O5 and SiO2. The beneficial effect of alloying with Mo is attributed to the improved sinterability of the oxide scale and increase in the activity of Si, which assists in the formation of a stable and continuous layer of SiO2 at the alloy-oxide interface.