High temperature oxidation behavior of niobium-20silicon-20chromium alloys with hafnium additions
Advances in aircraft and gas turbine engine technology have steadily increased the operating temperatures, and even the state of the art materials can not meet the requirements for the new challenges. Efforts are under way to study the possibility of substituting nickel-based alloys with niobium-based alloys that offer higher melting points and excellent mechanical properties. Notwithstanding these good properties, the utilization of Nb-based alloys is limited since oxidation resistant is still a serious problem. Therefore understanding the corrosion behavior of Nb-based alloys is a crucial step to enhancing oxidation resistance. 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 hours 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 phase identification. 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 at 1000°C and 1100°C for 5Hf and above 900°C up to 1200°C for 10Hf. 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. Results indicate that Nb-based alloys are potential candidates for a new generation of refractory metals where microstructures and composition influence oxidation mechanisms.
Vazquez, Alma, "High temperature oxidation behavior of niobium-20silicon-20chromium alloys with hafnium additions" (2011). ETD Collection for University of Texas, El Paso. AAI1494377.