Microstructures and Isothermal Oxidation of the Alumina Scale Forming Nb1.45Si2.7Ti2.25Al3.25Hf0.35 and Nb1.35Si2.3Ti2.3Al3.7Hf0.35 Alloys

Mohammad Ghadyani; Claire Utton; Panos Tsakiropoulos

doi:10.3390/ma12050759

Microstructures and Isothermal Oxidation of the Alumina Scale Forming Nb_1.45Si_2.7Ti_2.25Al_3.25Hf_0.35 and Nb_1.35Si_2.3Ti_2.3Al_3.7Hf_0.35 Alloys

Materials (Basel). 2019 Mar 5;12(5):759. doi: 10.3390/ma12050759.

Authors

Mohammad Ghadyani¹, Claire Utton², Panos Tsakiropoulos³

Affiliations

¹ Department of Materials Science and Engineering, Sir Robert Hadfield Building, University of Sheffield, Mappin Street, Sheffield, S1 3JD, UK. m.ghadyani@sheffield.ac.uk.
² Department of Materials Science and Engineering, Sir Robert Hadfield Building, University of Sheffield, Mappin Street, Sheffield, S1 3JD, UK. c.utton@sheffield.ac.uk.
³ Department of Materials Science and Engineering, Sir Robert Hadfield Building, University of Sheffield, Mappin Street, Sheffield, S1 3JD, UK. p.tsakiropoulos@sheffield.ac.uk.

Abstract

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_1.45Si_2.7Ti_2.25Al_3.25Hf_0.35 (MG5) and Nb_1.35Si_2.3Ti_2.3Al_3.7Hf_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 αAl₂O₃ 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 αAl₂O₃ scale at 1200 °C, (iii) to have no solid solution, (iv) to form only hexagonal Nb₅Si₃ and (v) to have microstructures consisting of hexagonal Nb₅Si₃, Ti₅Si₃, Ti₅Si₄, 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 Nb₅Si₃ core was surrounded by the Ti₅Si₄ 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,  and  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.

Keywords: Nb-silicide based alloys; coatings; complex concentrated alloys; high entropy alloys; high temperature oxidation; intermetallics; multi-principle element alloys; pest oxidation.

Abstract

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