High-temperature oxidation behavior of Nb–Si–Cr alloys with Hf additions

The oxidation behavior of two alloys from the Nb–Si–Cr system containing hafnium has been investigated under isothermal and cyclic conditions. Nb–20Si–20Cr–(5,10)Hf alloys (composition in atomic percent) were exposed to air for 24 and 168 h over a range of temperatures from 700 °C to 1400 °C. A gravimetric method was used to determine the oxidation kinetics; weight gain per unit area as a function of temperature or time. Computed isothermal sections of the quaternary Nb–Si–Cr–Hf phase diagrams were used for alloy selection. XRD, SEM and EDS were used to characterize the phases present in the oxidation products and the alloys. Oxidation experiments revealed extremely good oxidation resistance at 700 °C and 800 °C and above 1200 °C under isothermal conditions for both alloys. Partial pesting was observed when the samples were exposed to 800 °C. Complete oxide formation was observed above 1000 °C for 5Hf and above 900 °C for 10Hf up to 1200 °C. Beneficial effects have been observed with the addition of 10Hf to the alloy compared to 5Hf at 700 °C, 1200 °C and 1300 °C resulting in a reduction of weight gain per unit area.

► Oxidation behavior of two alloys from the Nb–Cr–Si system containing hafnium has been investigated under isothermal and cyclic conditions. ► A gravimetric method was used to determine oxidation kinetics; weight gain per unit area as a function of temperature or time. ► Oxidation experiments revealed good oxidation resistance for both alloys at 700 °C and 800 °C and above 1200 °C under isothermal conditions, and at 700 °C for cyclic conditions.