Development of Niobium / Niobium-Silicide Based Alloys for Ultra High Temperature Applications
R. P. Dicks, IRC in Materials Processing, University of Birmingham, Birmingham, United Kingdom; X. Wu, University of Birmingham, Birmingham, United Kingdom
Materials based on niobium / niobium-silicides have been identified as candidates for use in gas-turbine engines. This is due to their high temperature capability and low density, around 6.6 g/cm3, in comparison to nickel-base superalloys. There are, however, a number of undesirable properties that must be addressed if their implementation within the industry is to be achieved. These are primarily a low ductility at room temperature and poor oxidation behaviour at intermediate and high temperatures. In this study, a large range of compositions based on the Nb / Nb-silicide system are screened in order to identify those that offer the best combination of oxidation resistance at both intermediate and high temperatures as well as ductility at room temperature. Samples, 20mm x 20mm x 3mm, were manufactured using the Direct Laser Fabrication (DLF) technique, by feeding a mixture of pre-alloyed and elemental powders into the laser focal point, and controlling its movement via a CAD file to build the desired geometry layer by layer. This method allows a large range of compositions to be produced more efficiently than using button melting. Nb-25Ti-16Si-8Hf-2Cr-1.9Al at% was used as a base composition and other elements, such as Cr and Si, were added in different amounts to individual samples. The laser fabricated Nb-silicide based samples were tested for oxidation resistance at 800 and 1200°C. The oxidised samples were sectioned to understand the oxidation mechanisms and the influence of added elements. The compositions of phases present in the material and those formed during oxidation were measured using EDX. The hardness and fracture surface topography of the samples tested at room temperature have also been assessed.
Summary: Niobium / Niobium-silicide based materials have been identified as candidates for use within the gas-turbine engine industry. This work studies these materials using the Direct Laser Fabrication (DLF) technique with a particular emphasis on oxidation behaviour.