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.