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Title: Development of bond coat alloy(s) for Ni based superalloys for low emission gas turbine technology for H2 rich Syngas
Author: Karagianni, Eleftheria
ISNI:       0000 0004 5369 2581
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2015
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This thesis describes the research performed as part of the materials package of the FP7 project H2-IGCC to design, develop and evaluate new bond coat (BC) alloys that form in situ alumina oxide scales. Literature on bond coat alloys and processing techniques used for Ni based superalloys is reviewed. The experimental techniques used to prepare and characterise the alloys of this research are described. Five NiCrCoAl based bond coat type alloys, namely Ni-23Co-20Cr-8.5Al (EK1), Ni-23Co-20Cr-8.5Al-4Ta (EK2), Ni-23Co-20Cr-8.5Al-4Ta-0.6Y (EK3), Ni-23Co-20Cr-8.5Al-4Ta-0.6Y-0.8Hf (EK4) and Ni-23Co-20Cr-8.5Al-4Ta-0.6Y-0.8Hf-2Si (EK5) (wt%), were studied in the as cast, heat treated (1200 °C) and oxidised (975 °C in air) conditions. The γ-Niss and β-NiAl phases were stable in all the alloys. The synergy of Ta and Y and of Ta, Y and Si enhanced the stability of the γ'-Ni3Al. The addition of Y stabilised Y containing intermetallics. The γ + β eutectic was not observed in the alloys EK3 and EK5, which would suggest that Y and Si suppress its formation. During solidification the formation of the γ-Niss and β-NiAl was accompanied respectively by partitioning of Al, Si and Ta to the melt, and Co, Cr, Si and Ta to the melt and Al to the β-NiAl. The partitioning of Al and Co between the γ-Niss and β-NiAl was not changed by the synergies of different alloying elements however the partitioning of Cr to the γ-Niss was increased by the synergy of Ta and Y and Ta, Y and Si. The alloying with Ta resulted to the highest Al + Cr and Cr contents in Niss and NiAl in the as cast condition and this was maintained after heat treatment. The Al + Cr content in the γ + β eutectic was essentially the same in the three alloys where the eutectic was formed but the Al/Cr ratio increased dramatically when the Ta was in synergy with Hf. Alloying with Ta had a dramatic effect on the volume fractions of γ-Niss and β-NiAl. In the as cast condition the vol% of the β-NiAl was higher than the vol% of the γ-Niss in the alloys EK2 and EK5, suggesting that alloying with Y and Hf tends to favour the γ-Niss rather than the β-NiAl. There were changes in the vol% of the aforementioned phases after the heat treatment, with the vol% of the β-NiAl increasing in all alloys. The higher vol% of β-NiAl was still observed in the alloys EK2 and EK5. The addition of Si enhanced chemical inhomogeneities in the microstructure and there was formation of Si and Y rich areas in the solidified microstructure where Ta and Hf had also partitioned. After heat treatment the formation of intermetallic phases in those areas was accompanied by severe micro-cracking in the microstructure and internal oxidation. Oxidation benefited from the reactive element addition(s). With the addition of reactive elements a significant amount of Al outward transport was reduced. Alloying with Si did not improve the oxidation rate and increased the depth below the scale where internal oxidation occurred. The "effectiveness" of reactive elements was compromised when the latter were simultaneously in synergy with Ta. The Niss and NiAl respectively were the main supplier of Al and Cr for the growth of the scale. Overall, the "best" oxidation behaviour was exhibited by the alloy that tended to form equal volume fractions of γ-Niss and β-NiAl, i.e., the alloy EK3. Alumina, chromia, spinels and nitrides were present in the scales formed on the alloys. GXRD confirmed the presence of transition aluminas and α-Al2O3. The morphologies of the aluminas were consistent with those reported in the literature. The spallation and cracking of scales was attributed to the formation and transformation(s) of transition aluminas to α-Al2O3 and the presence of reactive elements. Internal oxidation zones rich in Al formed below the scales. Diffusion zones rich in Cr formed below the internal oxidation zones. Nitrogen was analysed in all the diffusion zones. Ni rich phases were observed in the diffusion zones of the alloys EK2, EK3 and EK4 that contained Al, Co and Cr. In the substrate below the diffusion zone γ and γ'-Ni3Al was observed in the alloys EK2, EK3, EK4 and EK5. In all the alloys the contamination by nitrogen extended below the diffusion zones. In γ, γ'-Ni3Al and β-NiAl below the diffusion zone the concentration of nitrogen was around 5 to 10 at% but that of oxygen was < 1 at%. Below the diffusion zones a Cr rich phase was formed in EK2 and EK3 and a Ni rich phase was formed in EK4 and EK5. The latter phase had higher solubility for nitrogen compared with the former. New phase equilibria was established in the alloys EK2 to EK5 below the scale owing to the consumption of Al and Cr to form the scale and the contamination by oxygen and nitrogen.
Supervisor: Tsakiropoulos, Panos Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available