Phase Stabilisation of Nb-Si-Ti Alloys via Elemental Additions and Post-Processing
thesisposted on 30.07.2019, 11:21 by Adam J. S. Allen
Efficiency is a primary concern of the commercial aerospace sector, both in terms of environmental factors and financial incentive. One way to improve engine efficiency is to increase the turbine entry temperature of the engine, but this is currently limited by the material of choice, nickel superalloys. Nb-Si alloys have the potential to replace current turbine materials with a combination of a Nb5Si3 silicide phase providing strength and a Nbss solid solution matrix increasing overall toughness. There are, however, problems with this alloy system that must be overcome if they are to become a viable replacement. One such difficulty observed is in promoting the optimal α-Nb5Si3, with long (100 h), energy consuming (1500°C) heat treatments often being required. This work explored ways to promote the formation of α-Nb5Si3 and suppressing β/γ-Nb5Si3 phases using elemental additions and novel processing techniques such as additive manufacturing and hot isostatic pressing. Additive manufacturing was implemented due to its flexible nature and lack of mould requirement. Hot isostatic pressing was used to successfully consolidate porosity after formation of Nb-Si alloys. Comparison of rapid solidification from additive manufacturing to other manufacturing techniques showed the formation of alternate phases to the literature, suggesting that novel processing techniques influence promoted phases. Elemental addition of Cr, Hf, Ti, Zr, Y and Ta were used to control phase evolution. Additions of Hf displayed Hafnia inclusions at all stages of production. Addition of Cr in high concentrations promoted α-Nb5Si3, but also produced the unexpected cubic C15 Laves phase over the hexagonal C14 phase. Tiss was noted in some alloys when phase transitions occurred during post-processing, although after long heat treatments these were reduced. High levels of Zr showed promotion of γ-Nb5Si3, whilst low concentrations allowed for α-Nb5Si3 formation. Additions of Ta helped to promote α-Nb5Si3 during the formation stage and helped reduce β-Nb5Si3 after short postprocessing heat treatments (5 h), potentially opening faster heat treatment routes.