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Title: The effect of Sn on the phase stability and oxidation behaviour of Nb silicide based alloys
Author: Xu, Zhen
ISNI:       0000 0004 5992 1533
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2016
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The manufacturers of future aero engines would need new ultra-high temperature alloys for blade applications with capabilities beyond those of Ni based superalloys to meet performance and environmental targets. The new alloys should meet property goals set by industry and would be expected to operate under conditions where turbine entry temperatures could be as high as 1800 °C and alloy surface temperatures  1300 °C. Ni based alloys cannot be used because of the melting temperature of Ni. Nb silicide based alloys are strong candidates for blade applications owing to their lower density and potential to offer a balance of properties that can satisfy property goals. The oxidation of the new alloys in service must be comparable with that of single crystal Ni based superalloys, such as CMSX4, at 1050°C. The oxidation of the new alloys is significantly improved by additions of Sn. However, to date it is not understood how Sn affects the oxidation of these alloys in the pest regime and high temperatures. The research described in this thesis attempts to understand how Sn on its own and in synergy with Al and Cr individually and simultaneously contributes towards improving the oxidation behaviour, how the stability of Nb3Sn, Nbss, Nb5Si3 and Nb5Si3 and Laves phase in Nb silicide based alloys is affected by the addition of Sn in the alloys and what the role played by Sn is in the isothermal oxidation behaviour of alloys at 800 °C (in the pest regime) and 1200 °C. Model Nb-Si ternary alloys with low and high concentrations of Sn and higher order model Nb-24Ti-18Si based alloys with 5 at.% of each Al and Cr added individually and simultaneously were studied in the cast and heat treated conditions (100 h at 1200 °C, 1500 °C or 1450 °C depending on alloy). The nominal compositions (at.%) of the alloys were Nb-14Si-3Sn (ZX1), Nb-12.5Si-7.5Sn (ZX2), Nb-24Ti-18Si-5Cr-2Sn (ZX3), Nb-24Ti-18Si-5Cr-5Sn (ZX4), Nb-24Ti-18Si-5Al-2Sn (ZX5), Nb-24Ti-18Si-5Al-5Sn (ZX6), Nb-24Ti-18Si-5Cr-5Al-2Sn (ZX7) and Nb-24Ti-18Si-5Cr-5Al-5Sn (ZX8). The thesis presents a brief review of relevant literature on Nb silicide based alloys and outlines a method for making Sn containing alloys with control of Sn losses during clean melting using arc melting. The microstructures of the alloys were studied using XRD, SEM, SEM-EDS, EPMA-WDS and TGA. Tin suppressed the Nb3Si. The Nbss + Nb5Si3 metastable eutectic formed in all alloys except the alloy ZX8. Depending on alloy composition and solidification conditions transitions from anomalous to anomalous + normal to normal eutectics were observed from the bottom towards the bulk of ingots. Tin on its own increased the stability of the lamellar microstructure of the eutectic up to 1500 °C but the additions of Al and/or Cr had the opposite effect. Increasing the Sn content in the ternary alloys accelerated the Nb5Si3  Nb5Si3 transformation during heat treatment. The latter was also enhanced during the solidification of the alloys with only Cr (ZX3, ZX4). The Nb5Si3 was stable in all the heat treated higher order alloys. Increasing the Sn content led to formation of Nb3Sn in the cast microstructure. With the exception of alloy ZX3 (low Sn, only Cr) the A15 phase was stable at 1500 °C in all alloys. Aluminium on its own destabilised the Nbss in the heat treated alloy ZX6. Only in the alloy ZX3 (low Sn, only Cr) the Laves phase was not stable at 1500 °C. Tin with Al and/or Cr suppressed pest oxidation. The scales formed on all alloys at 1200 °C spalled off. The “best” alloys at 800 °C and 1200 °C respectively were the alloys ZX4 and ZX6. All alloys at 1200 °C followed linear oxidation kinetics. Cr and the presence of Laves phase and Al and the absence of Laves phase were important for oxidation at 800 °C and 1200 °C, respectively. The oxide scales formed at 800 °C consisted of Nb rich and Nb and Si rich oxides. The same oxides plus Ti rich oxide were in the scales formed at 1200 °C in all alloys with the exception of the alloy ZX5. Ti rich oxide was also observed in the scale of the alloy ZX4 at 800 °C. The improved oxidation behaviour of Sn containing alloys was attributed to Sn rich area(s) that formed below the scale/substrate interface in all alloys at 800 °C and 1200 °C, with the exception of the alloy ZX7 at 800 °C. The thickness of the Sn rich area increased with Sn concentration in the alloy and with temperature. In the Sn rich areas the Sn rich intermetallics Nb3Sn, Nb5Sn2Si, NbSn2 were formed. The latter was absent in the alloys ZX5 (“best” low Sn alloy at 1200 °C) and ZX6 (“best” high Sn alloy at 1200 °C) and in the alloy ZX8. In all the alloys all phases in the diffusion zone and bulk were contaminated by oxygen. In the diffusion zone the effect was very severe for the Nbss. In general the contamination of the Nbss by oxygen was worse than Nb5Si3. The oxygen concentration in the A-15 phase in the bulk did not change ( 6 at.%) with increasing Sn in the alloy, but its Sn content increased. At 800 °C the contamination of the bulk of ZX4 and ZX8 by oxygen shifted the phase equilibria and in the former alloy made the Laves phase unstable and in the latter caused the formation of Nbss and Nbss + Nb5Si3 lamellar microstructure (the Nbss was absent in the cast alloy). The contamination of the bulk of the alloy ZX8 by oxygen at 1200 °C caused a shift in phase equilibria and led to the destabilisation of the Nbss in the microstructure.
Supervisor: Tsakiropoulos, Panos Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available