A Study of the Effects of Al, Cr, Hf, and Ti Additions on the Microstructure and Oxidation of Nb-24Ti-18Si Silicide Based Alloys

Jack Nelson; Mohammad Ghadyani; Claire Utton; Panos Tsakiropoulos

doi:10.3390/ma11091579

A Study of the Effects of Al, Cr, Hf, and Ti Additions on the Microstructure and Oxidation of Nb-24Ti-18Si Silicide Based Alloys

Materials (Basel). 2018 Sep 1;11(9):1579. doi: 10.3390/ma11091579.

Authors

Jack Nelson¹, Mohammad Ghadyani², Claire Utton³, Panos Tsakiropoulos⁴

Affiliations

¹ Department of Materials Science and Engineering, Sir Robert Hadfield Building, The University of Sheffield, Mappin Street, Sheffield S1 3JD, UK. pha05jn@gmail.com.
² Department of Materials Science and Engineering, Sir Robert Hadfield Building, The University of Sheffield, Mappin Street, Sheffield S1 3JD, UK. m.ghadyani@sheffield.ac.uk.
³ Department of Materials Science and Engineering, Sir Robert Hadfield Building, The University of Sheffield, Mappin Street, Sheffield S1 3JD, UK. c.utton@sheffield.ac.uk.
⁴ Department of Materials Science and Engineering, Sir Robert Hadfield Building, The University of Sheffield, Mappin Street, Sheffield S1 3JD, UK. p.tsakiropoulos@sheffield.ac.uk.

Abstract

In Nb-silicide based alloys Al, Cr, Hf, and Ti additions are crucial for achieving balance of properties. It is not known how the simultaneous addition of Hf with Al and Ti, or Hf with Al, Cr, and Ti affects macrosegregation, and how the alloying affects hardness, Young's modulus and bulk alloy oxidation, and contamination of the solid solution Nb_ss and the Nb₅Si₃ compound by oxygen. Two alloys with nominal compositions (at.%) Nb-24Ti-18Si-5Al-5Hf (alloy NbSiTiHf-5Al) and Nb-24Ti-18Si-5Al-5Cr-5Hf (alloy NbSiTiHf-5Al-5Cr) were studied in the as-cast and heat-treated conditions and after isothermal oxidation at 800 and 1200 °C and were compared with similar alloys without Hf. In both alloys there was macrosegregation of Si and Ti, which was more severe in NbSiTiHf-5Al. Both alloys formed Nb_ss+βNb₅Si₃ eutectic. The Nb_ss was stable and its Al and Cr concentrations increased with increasing Ti concentration. In both conditions the βNb₅Si₃ was observed in the alloys NbSiTiHf-5Al and NbSiTiHf-5Al-5Cr, and the γNb₅Si₃ only in the alloy NbSiTiHf-5Al. In both heat-treated alloys, separate Hf-rich Nb₅Si₃ grains were formed. The Si and Al concentrations in Nb₅Si₃ respectively decreased and increased with increasing Ti concentration. Al and Cr had a stronger hardening effect in the Nb_ss than Al, Cr, and Hf. Al, Cr, and Ti had a stronger negative effect on the Young's modulus of the Nb_ss compared with Al, Cr, Hf, and Ti. When Nb was substituted by Ti, Cr, and Hf, and Si by Al in the βNb₅Si₃, the Young's modulus was reduced compared with the unalloyed silicide. At 800 °C both alloys did not exhibit catastrophic pest-oxidation after 100 h. The Nb_ss and Nb₅Si₃ were contaminated by oxygen in both alloys, the former more severely. At 1200 °C the scales spalled-off, more severely in the alloy NbSiTiHf-5Al, where substrate that was heavily contaminated by oxygen below the scale also spalled-off. In both alloys the contamination of Nb₅Si₃ and Nb_ss by oxygen was more severe compared with 800 °C, but the silicides were not contaminated by oxygen in their bulk. The Nb_ss was not contaminated by oxygen only in the bulk of the alloy NbSiTiHf-5Al-5Cr.

Keywords: Niobium alloys; Young’s modulus; hardness; intermetallics; oxidation; solid solution.

Abstract

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