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Title: Study of intermetallic alloys for high temperature applications 'beyond the Ni superalloys'
Author: Hernandez-Negrete, Ofelia
ISNI:       0000 0004 6058 6189
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2016
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Future aero-engines must run hotter than current ones to meet performance and environmental targets. This makes it necessary to search for new ultra-high temperature alloys with capabilities beyond those of Ni based superalloys (NBS). This thesis is about the former materials. Niobium silicide based alloys (NSBAs) have the potential to meet material property goals for future aero-engines. Significant improvements of their oxidation at low, intermediate and high temperatures have been achieved via alloying. In service, like the NBS, the NSBAs will need a coating system and should have acceptable oxidation resistance to "survive" in case of coating failure. The coating is envisaged to be thermal barrier (type) coating system (TBC) consisting of multi-alloy or compositionally gradient bond coat (BC), thermally grown oxide (TGO) and ceramic top coat (CTC). This thesis is about the search of intermetallic based alloys without "exotic" alloying additions (i.e., no Ru, Re, Pt) suitable for application in BC (alloys OHC1=Si-23Fe-15Cr-15Ti-1Nb,OHC2=Si-22Fe-12Cr-12Al-10Ti-5Nb, OHC3=Al-25.5Nb-8Cr-0.5Hf, OHC5=Si-25Nb-5Al-5Cr-5Ti) and the microstructure and oxidation of a model substrate alloy (OHS1=Nb-24Ti-18Si-5Al-5Cr-5Ge-5Sn) all nominal compositions in at .%. The alloys were prepared using arc melting. Their cast, heat treated and oxidised microstructures and their oxidation were studied using DSC, XRD, SEM, EDS and TG. The alloy OHS1 exhibited strong and weak macrosegregation respectively of Si and Ti and Al, Cr and Sn. The cast microstructure consisted of the A15-Nb₃Sn, (ßNb₅Si₃)_primary, Nb_ss, C14-NbCr₂ Laves phases and a ternary eutectic of the latter three. Of these only the Nb_ss was not stable after heat treatment at 1400 °C/100 h. The synergy of Sn and Ge with Al, Cr and Ti improved the oxidation at 800 and 1200 °C, there was no pest oxidation at 800 °C and no scale spallation at 1200 °C. The microstructure of the alloy OHC1 was composed of the [(TM)₆Si₅]_primary (T phase), τ₁ (FeSi₂Ti)and (Fe,Cr,Ti)Si phases and the eutectic of the latter two (T_eu = 1300 °C). The three phases were stable after heat treatment at 1200 °C/48 h and the latter two had coarsened. In the T phase the partition of Fe was opposite to that of Nb, Ti and Cr. A thin and compact oxide scale formed at 800 °C with linear kinetics. At 1200 °C the oxide scale formed with para-linear kinetics, consisted of a Cr₂₃O3 outermost layer and SiO₂ at the metal/scale interface and some parts of it had experienced melting at ~ 1155 °C. There was internal oxidation of the alloy. The microstructure of the alloy OHC2 was composed of the [(TM)₆Si₅]_primary, (TM)₅Si₃, FeSi₂Ti,Fe₃Al₂Si₃ and (Fe,Cr)(Si,Al) and an unidentified dark phase (DP). The latter two phases were not stable after heat treatment at 950 °C/48 h. The Al solubility in the T phase was very low. The alloy suffered liquation at 1200 °C. The oxidation at 800 °C was sub-parabolic. Owing to formation of low melting point phases, liquation at temperatures of interest and melting in the scale, the alloys OHC1 and OHC2 were not considered suitable for application in a BC for NSBAs. The alloy OHC5 was composed of the [(Nb,Ti)(Si,Al)₂]_primary, (Cr,Ti,Nb)₆Si₅, (Cr,Ti,Nb)(Si,Al)₂, and (Si) and (Al) solid solutions. The former two phases were stable after heat treatment at 1400 °C/100 h. At 800 °C there was no pesting, the oxidation was parabolic and a multiphase oxide scale was formed where the main oxides were Al₂O₃, SiO₂ and rutile type oxides. A critical Al content was required for external Al₂O₃ oxide formation on top of the (Nb,Ti)(Si,Al)₂ phase, this was ~ 3 at.% Al at 800 °C. At 1200°C an external α-Al₂O₃ scale was formed and the oxidation was parabolic at t > 21.5h. The alloy OHC3 was composed of the (Al₃Nb)_primary, C14-(Al,Cr)₂(Nb,Hf) Laves, a Hf-rich phase and (Cr,Al,Nb)_ss. The former three phases were stable after heat treatment at 1400 °C. The alloy followed parabolic oxidation kinetics at 800, 1200 and 1300 °C. The oxide scale was composed of an outer layer of (Al,Cr)NbO₄ and an inner oxide layer of α-Al₂O₃ at all three temperatures. However, some internal oxidation occurred at 800 °C and it was deeper along phase boundaries. At 1200 and 1300 °C the layer of α-Al₂O₃ was thicker and C14-(Al,Cr)₂(Nb,Hf) was formed at the metal/scale interface after oxidation of Al₃Nb. The presence of Hf in the alloy was beneficial for the adhesion of the α-Al₂O₃. At 1300 °C the (Al,Cr)NbO₄ top layer suffered partial spallation. The alloys OHC3 and OHC5 are considered suitable candidates for applications in BC for NSBAs after some improvements. Areas where future alloy development research should concentrate were recommended.
Supervisor: Tsakiropoulos, Panos Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available