Role of nitrogen on the oxidative stability of Ti5Si3 based alloys at elevated temperature

Ti5Si3 has been extensively studied as a candidate material for high temperature application due to its high melting point (2130 °C), low density (∼4.3 g/cm3) and excellent oxidation resistance in oxygen above 1000 °C. However, stoichiometric Ti5Si3 alloy experiences accelerated oxidation during exposure in air above 1000 °C. It was proposed that nitrogen was responsible for the increased oxidation in air. In the present study, the isothermal reaction kinetics of Ti5Si3 in nitrogen at 1000 °C was investigated. Compared to a slow parabolic oxidation rate in oxygen, a faster linear reaction rate was observed when Ti5Si3 is exposed to nitrogen. Further studies on the oxidation behavior for changing nitrogen/oxygen atmospheres showed that Ti5Si3 is stable for exposure up to 400 h at 1000 °C when the gas contained 50% N2. Breakaway oxidation occurs after short exposures when the gas contained at least 75% N2, and the reaction rate increased as the concentration of N2 increased. Furthermore, time to breakaway oxidation decreases with the increasing nitrogen partial pressure. Extensive analysis of the oxidation products with SEM and XRD revealed that the formation and fast growth of a nitride-containing subscale interferes with the establishment of the continuous protective silica scale and contributes to the breakaway oxidation.