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Microstructures and Isothermal Oxidation of the Alumina Scale Forming Nb(1.45)Si(2.7)Ti(2.25)Al(3.25)Hf(0.35) and Nb(1.35)Si(2.3)Ti(2.3)Al(3.7)Hf(0.35) Alloys

Coating system(s) will be required for Nb-silicide based alloys. Alumina forming alloys that are chemically compatible with the Nb-silicide based alloy substrate could be components of such systems. The intermetallic alloys Nb(1.45)Si(2.7)Ti(2.25)Al(3.25)Hf(0.35) (MG5) and Nb(1.35)Si(2.3)Ti(2.3)Al(3...

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Autores principales: Ghadyani, Mohammad, Utton, Claire, Tsakiropoulos, Panos
Formato: Online Artículo Texto
Lenguaje:English
Publicado: MDPI 2019
Materias:
Acceso en línea:https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6427440/
https://www.ncbi.nlm.nih.gov/pubmed/30841625
http://dx.doi.org/10.3390/ma12050759
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author Ghadyani, Mohammad
Utton, Claire
Tsakiropoulos, Panos
author_facet Ghadyani, Mohammad
Utton, Claire
Tsakiropoulos, Panos
author_sort Ghadyani, Mohammad
collection PubMed
description Coating system(s) will be required for Nb-silicide based alloys. Alumina forming alloys that are chemically compatible with the Nb-silicide based alloy substrate could be components of such systems. The intermetallic alloys Nb(1.45)Si(2.7)Ti(2.25)Al(3.25)Hf(0.35) (MG5) and Nb(1.35)Si(2.3)Ti(2.3)Al(3.7)Hf(0.35) (MG6) were studied in the cast, heat treated and isothermally oxidised conditions at 800 and 1200 °C to find out if they are αAl(2)O(3) scale formers. A (Al/Si)(alloy) versus Nb/(Ti + Hf)(alloy) map, which can be considered to be a map for Multi-Principle Element or Complex Concentrated Nb-Ti-Si-Al-Hf alloys, and a [Nb/(Ti + Hf)](Nb5Si3) versus [Nb/(Ti + Hf)](alloy) map were constructed making use of the alloy design methodology NICE and data from a previously studied alloy, and were used to select the alloys MG5 and MG6 that were expected (i) not to pest, (ii) to form αAl(2)O(3) scale at 1200 °C, (iii) to have no solid solution, (iv) to form only hexagonal Nb(5)Si(3) and (v) to have microstructures consisting of hexagonal Nb(5)Si(3), Ti(5)Si(3), Ti(5)Si(4), TiSi silicides, and tri-aluminides and Al rich TiAl. Both alloys met the requirements (i) to (v). The alumina scale was able to self-heal at 1200 °C. Liquation in the alloy MG6 at 1200 °C was linked with the formation of a eutectic like structure and the TiAl aluminide in the cast alloy. Key to the oxidation of the alloys was the formation (i) of “composite” silicide grains in which the γNb(5)Si(3) core was surrounded by the Ti(5)Si(4) and TiSi silicides, and (ii) of tri-aluminides with high Al/Si ratio, particularly at 1200 °C and very low Nb/Ti ratio forming in-between the “composite” silicide grains. Both alloys met the “standard definition” of high entropy alloys (HEAs). Compared with HEAs with bcc solid solution and intermetallics, the VEC values of both the alloys were outside the range of reported values. The parameters VEC, Δχ and δ of Nb-Ti-Si-Al-Hf coating alloys and non-pesting Nb-silicide based alloys were compared and trends were established. Selection of coating alloys with possible “layered” structures was discussed and alloy compositions were proposed.
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spelling pubmed-64274402019-04-15 Microstructures and Isothermal Oxidation of the Alumina Scale Forming Nb(1.45)Si(2.7)Ti(2.25)Al(3.25)Hf(0.35) and Nb(1.35)Si(2.3)Ti(2.3)Al(3.7)Hf(0.35) Alloys Ghadyani, Mohammad Utton, Claire Tsakiropoulos, Panos Materials (Basel) Article Coating system(s) will be required for Nb-silicide based alloys. Alumina forming alloys that are chemically compatible with the Nb-silicide based alloy substrate could be components of such systems. The intermetallic alloys Nb(1.45)Si(2.7)Ti(2.25)Al(3.25)Hf(0.35) (MG5) and Nb(1.35)Si(2.3)Ti(2.3)Al(3.7)Hf(0.35) (MG6) were studied in the cast, heat treated and isothermally oxidised conditions at 800 and 1200 °C to find out if they are αAl(2)O(3) scale formers. A (Al/Si)(alloy) versus Nb/(Ti + Hf)(alloy) map, which can be considered to be a map for Multi-Principle Element or Complex Concentrated Nb-Ti-Si-Al-Hf alloys, and a [Nb/(Ti + Hf)](Nb5Si3) versus [Nb/(Ti + Hf)](alloy) map were constructed making use of the alloy design methodology NICE and data from a previously studied alloy, and were used to select the alloys MG5 and MG6 that were expected (i) not to pest, (ii) to form αAl(2)O(3) scale at 1200 °C, (iii) to have no solid solution, (iv) to form only hexagonal Nb(5)Si(3) and (v) to have microstructures consisting of hexagonal Nb(5)Si(3), Ti(5)Si(3), Ti(5)Si(4), TiSi silicides, and tri-aluminides and Al rich TiAl. Both alloys met the requirements (i) to (v). The alumina scale was able to self-heal at 1200 °C. Liquation in the alloy MG6 at 1200 °C was linked with the formation of a eutectic like structure and the TiAl aluminide in the cast alloy. Key to the oxidation of the alloys was the formation (i) of “composite” silicide grains in which the γNb(5)Si(3) core was surrounded by the Ti(5)Si(4) and TiSi silicides, and (ii) of tri-aluminides with high Al/Si ratio, particularly at 1200 °C and very low Nb/Ti ratio forming in-between the “composite” silicide grains. Both alloys met the “standard definition” of high entropy alloys (HEAs). Compared with HEAs with bcc solid solution and intermetallics, the VEC values of both the alloys were outside the range of reported values. The parameters VEC, Δχ and δ of Nb-Ti-Si-Al-Hf coating alloys and non-pesting Nb-silicide based alloys were compared and trends were established. Selection of coating alloys with possible “layered” structures was discussed and alloy compositions were proposed. MDPI 2019-03-05 /pmc/articles/PMC6427440/ /pubmed/30841625 http://dx.doi.org/10.3390/ma12050759 Text en © 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
spellingShingle Article
Ghadyani, Mohammad
Utton, Claire
Tsakiropoulos, Panos
Microstructures and Isothermal Oxidation of the Alumina Scale Forming Nb(1.45)Si(2.7)Ti(2.25)Al(3.25)Hf(0.35) and Nb(1.35)Si(2.3)Ti(2.3)Al(3.7)Hf(0.35) Alloys
title Microstructures and Isothermal Oxidation of the Alumina Scale Forming Nb(1.45)Si(2.7)Ti(2.25)Al(3.25)Hf(0.35) and Nb(1.35)Si(2.3)Ti(2.3)Al(3.7)Hf(0.35) Alloys
title_full Microstructures and Isothermal Oxidation of the Alumina Scale Forming Nb(1.45)Si(2.7)Ti(2.25)Al(3.25)Hf(0.35) and Nb(1.35)Si(2.3)Ti(2.3)Al(3.7)Hf(0.35) Alloys
title_fullStr Microstructures and Isothermal Oxidation of the Alumina Scale Forming Nb(1.45)Si(2.7)Ti(2.25)Al(3.25)Hf(0.35) and Nb(1.35)Si(2.3)Ti(2.3)Al(3.7)Hf(0.35) Alloys
title_full_unstemmed Microstructures and Isothermal Oxidation of the Alumina Scale Forming Nb(1.45)Si(2.7)Ti(2.25)Al(3.25)Hf(0.35) and Nb(1.35)Si(2.3)Ti(2.3)Al(3.7)Hf(0.35) Alloys
title_short Microstructures and Isothermal Oxidation of the Alumina Scale Forming Nb(1.45)Si(2.7)Ti(2.25)Al(3.25)Hf(0.35) and Nb(1.35)Si(2.3)Ti(2.3)Al(3.7)Hf(0.35) Alloys
title_sort microstructures and isothermal oxidation of the alumina scale forming nb(1.45)si(2.7)ti(2.25)al(3.25)hf(0.35) and nb(1.35)si(2.3)ti(2.3)al(3.7)hf(0.35) alloys
topic Article
url https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6427440/
https://www.ncbi.nlm.nih.gov/pubmed/30841625
http://dx.doi.org/10.3390/ma12050759
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