A Study of the Effect of 5 at.% Sn on the Micro-Structure and Isothermal Oxidation at 800 and 1200 °C of Nb-24Ti-18Si Based Alloys with Al and/or Cr Additions.

Zhen Xu,Panos Tsakiropoulos,Claire Utton

doi:10.3390/ma13010245

Abstract

This paper presents the results of a systematic study of Nb-24Ti-18Si based alloys with 5 at.% Sn addition. Three alloys of nominal compositions (at.%), namely Nb-24Ti-18Si-5Cr-5Sn (ZX4), Nb-24Ti-18Si-5Al-5Sn (ZX6), and Nb-24Ti-18Si-5Al-5Cr-5Sn (ZX8), were studied to understand how the increased Sn concentration improved oxidation resistance. In all three alloys there was macrosegregation, which was most severe in ZX8 and the primary βNb5Si3 transformed completely to αNb5Si3 after heat treatment. The Nbss was not stable in ZX6, the Nb3Sn was stable in all three alloys, and the Nbss and C14-NbCr2 Laves phase were stable in ZX4 and ZX8. The 5 at.% Sn addition suppressed pest oxidation at 800 °C but not scale spallation at 1200 °C. At both temperatures, a Sn-rich area with Nb3Sn, Nb5Sn2Si, and NbSn2 compounds developed below the scale. This area was thicker and continuous after oxidation at 1200 °C and was contaminated by oxygen at both temperatures. The contamination of the Nbss by oxygen was most severe in the bulk of all three alloys. Nb-rich, Ti-rich and Nb and Si-rich oxides formed in the scales. The adhesion of the latter on ZX6 at 1200 °C was better, compared with the alloys ZX4 and ZX8. At both temperatures, the improved oxidation was accompanied by a decrease and increase respectively of the alloy parameters VEC (Valence Electron Concentration) and δ, in agreement with the alloy design methodology NICE (Niobium Intermetallic Composite Elaboration). Comparison with similar alloys with 2 at.% Sn addition showed (a) that a higher Sn concentration is essential for the suppression of pest oxidation of Nb-24Ti-18Si based alloys with Cr and no Al additions, but not for alloys where Al and Cr are in synergy with Sn, (b) that the stability of Nb3Sn in the alloy is “assured” with 5 at.% Sn addition, which improves oxidation with/out the presence of the Laves phase and (c) that the synergy of Sn with Al presents the “best” oxidation behaviour with improved scale adhesion at high temperature.

Highlights

Structural metallic materials for applications at high temperatures must have a balance of mechanical properties and oxidation resistance
Comparison with similar alloys with 2 at.% Sn addition showed (a) that a higher Sn concentration is essential for the suppression of pest oxidation of Nb-24Ti-18Si based alloys with Cr and no Al additions, but not for alloys where Al and Cr are in synergy with Sn, (b) that the stability of Nb3 Sn in the alloy is “assured” with 5 at.% Sn addition, which improves oxidation with/out the presence of the Laves phase and (c) that the synergy of Sn with Al presents the “best”
(ii) improved the adhesion of the scale that formed at 1200 ◦ C, which did not separate from the substrate, and linked the improved oxidation with Sn enrichment of the substrate below the scale where at 1200 ◦ C the Nb3 Sn and Nb5 Sn2 Si intermetallics were observed

Summary

Introduction

Structural metallic materials for applications at high temperatures must have a balance of mechanical properties and oxidation resistance. In 2007, Geng et al [5] reported that the addition of Sn in the Nb-24Ti-18Si-5Al-5Cr-5Hf-5Sn-2Mo alloy (i) suppressed pest oxidation at 800 ◦ C and (ii) improved the adhesion of the scale that formed at 1200 ◦ C, which did not separate from the substrate, and linked the improved oxidation with Sn enrichment of the substrate below the scale where at 1200 ◦ C the Nb3 Sn and Nb5 Sn2 Si intermetallics were observed The latter compound can be in equilibrium with Nb3 Sn and Nb5 Si3 at least up to 1200 ◦ C in the Nb-Si-Sn system [7], has the same crystal structure as βNb5 Si3 (prototype W5 Si3 ) as does the Nb5 Sn2 Al that can be in equilibrium with the A15-Nb3 Al compound [8].

Results

Discussion

Conclusion