Next Article in Journal
Deformation Behavior and Microstructural Evolution during Hot Stamping of TA15 Sheets: Experimentation and Modelling
Previous Article in Journal
Solid-State Transformations in Inner Coordination Sphere of [Co(NH3)6][Fe(C2O4)3]∙3H2O as a Route to Access Catalytically Active Co-Fe Materials
Article Menu

Article Versions

Export Article

Open AccessArticle
Materials 2019, 12(2), 222; https://doi.org/10.3390/ma12020222

Microstructures and Isothermal Oxidation of the Alumina Scale Forming Nb1.7Si2.4Ti2.4Al3Hf0.5 and Nb1.3Si2.4Ti2.4Al3.5Hf0.4 Alloys

Department of Materials Science and Engineering, Sir Robert Hadfield Building, The University of Sheffield, Mappin Street, Sheffield S1 3JD, UK
*
Author to whom correspondence should be addressed.
Received: 29 October 2018 / Revised: 3 January 2019 / Accepted: 7 January 2019 / Published: 10 January 2019
PDF [4558 KB, uploaded 10 January 2019]

Abstract

Nb–silicide based alloy will require some kind of coating system. Alumina forming alloys that are chemically compatible with the Nb–silicide based alloy substrate could be components of such systems. The intermetallic alloys Nb1.7Si2.4Ti2.4Al3Hf0.5 and Nb1.3Si2.4Ti2.4Al3.5Hf0.4 were studied in the cast, heat treated and isothermally oxidised conditions at 800 and 1200 °C to find out if they are alumina scale formers. The alloys were designed using the alloy design methodology NICE and were required (i) not to have stable solid solution phase in their microstructures; (ii) not to pest and (iii) to form alumina scale. Their microstructures consisted of silicides and aluminides. Both alloys satisfied (i) and (ii) and formed thin scales at 800 °C. At 1200 °C the former alloy suffered from internal oxidation and formed alumina intermixed with Ti rich oxide beneath a thick “layered” scale of mixed oxides that contained Ti and/or Al and/or Si. There was no internal oxidation in the latter alloy that formed a thin continuous well adhering α-Al2O3 scale that was able to repair itself during oxidation at 1200 °C. In both alloys there was severe macrosegregation of Si, which in Nb1.3Si2.4Ti2.4Al3.5Hf0.4 was almost double that in Nb1.7Si2.4Ti2.4Al3Hf0.5. The severe macrosegregation of Si contributed to the formation of a “layered” structure in the former alloy that was retained at 800 and 1200 °C. Both alloys met the “standard definition” of High Entropy Alloys (HEAs). Compared with the range of values of the parameters valence band (VEC), δ and Δχ of bcc solid solution plus intermetallic(s) HEAs, only the Δχ of the alloy Nb1.7Si2.4Ti2.4Al3Hf0.5 was within the range and the parameters VEC and δ of both alloys respectively were outside and within the corresponding ranges. The alloy Nb1.3Si2.4Ti2.4Al3.5Hf0.4 exhibited strong correlations between the parameters Δχ, δ and VEC, and the range of values of each parameter was wider compared with the alloy Nb1.7Si2.4Ti2.4Al3Hf0.5. There was a strong correlation only between the parameters Δχ and δ of the latter alloy that was similar to that of the former alloy.
Keywords: high entropy alloys; intermetallics; pest oxidation; high temperature oxidation; Nb–silicide based alloys; coatings; complex concentrated alloys; multi-principle element alloys high entropy alloys; intermetallics; pest oxidation; high temperature oxidation; Nb–silicide based alloys; coatings; complex concentrated alloys; multi-principle element alloys
This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited (CC BY 4.0).
SciFeed

Share & Cite This Article

MDPI and ACS Style

Ghadyani, M.; Utton, C.; Tsakiropoulos, P. Microstructures and Isothermal Oxidation of the Alumina Scale Forming Nb1.7Si2.4Ti2.4Al3Hf0.5 and Nb1.3Si2.4Ti2.4Al3.5Hf0.4 Alloys. Materials 2019, 12, 222.

Show more citation formats Show less citations formats

Note that from the first issue of 2016, MDPI journals use article numbers instead of page numbers. See further details here.

Related Articles

Article Metrics

Article Access Statistics

1

Comments

[Return to top]
Materials EISSN 1996-1944 Published by MDPI AG, Basel, Switzerland RSS E-Mail Table of Contents Alert
Back to Top