On the Improvement of the High Temperature Oxidation Performance of Niobium Silicide-based Alloy Systems Produced by Laser Additive Manufacture
thesisposted on 23.07.2020, 20:13 by Andrew C. Douglas
There has been significant research into improving the efficiency of aerospace engines. This can be done by increasing the operating temperature of the turbine section of the engine. Turbine blades can be subjected to temperatures <1350°C. Nickel-based superalloys are the current material, but they are at the limits of their temperature capability.
Niobium silicide alloys offer a potential route to higher engine efficiencies. Binary Nb-Si alloys possess a solidus temperature near 1900°C. However, to operate in such temperatures the alloy needs to be able to resist high temperature oxidative attack. Nb-Si alloys have not proven to be resistant to oxidation thus far. If they are to replace Ni-based superalloy they will need improved oxidation performance. Processing these alloys is also challenging as the high temperature melt reacts with traditional mould materials.
This work aimed to improve the oxidation resistance of a MASC-based alloy by using uncommon elemental additions. Manufacture of the alloys in this study was done using direct metal deposition (DMD), a form of laser additive manufacture. This eliminates the need for a mould. DMD is relatively untested, so this study investigated the properties of an MASC-based Nb-Si alloy after creation by DMD. An assessment of macro-segregation was performed, which found no evidence this phenomena.
The alloy then underwent oxidation tests, comparing the results to literature source where arc vacuum melting was used. DMD marginally improved oxidation properties and changed the mechanism at 800°C. This study found a parabolic regime, followed by a linear regime. With the viability of DMD confirmed, additions of Zr (with Y) and Ta were added. At 1 at%, Zr showed minor improvements at 800°C in the linear regime. 2 at% Ta resulted in markedly improved parabolic rate constants at 1200°C. Additions of Ta show great potential in improving the oxidation resistance of Nb-Si alloys.