Abstract
Alloying with Al, Cr, Sn, and Ti significantly improves the oxidation of Nb silicide-based alloys at intermediate and high temperatures. There is no agreement about what the concentration of Sn in the alloys should be. It has been suggested that with Sn ≤ 3 at.% the oxidation is improved and formation of the brittle A15-Nb3Sn compound is suppressed. Definite improvements in oxidation behaviour have been observed with 5 at.% Sn or even higher concentrations, up to 8 at.% Sn. The research reported in this paper is about three model alloys with low Sn concentration and nominal compositions Nb-24Ti-18Si-5Cr-2Sn (ZX3), Nb-24Ti-18Si-5Al-2Sn (ZX5), and Nb-24Ti-18Si-5Al-5Cr-2Sn (ZX7) that were studied to understand the effect of the 2 at.% Sn addition on as-cast and heat-treated microstructures and isothermal oxidation in air at 800 and 1200 °C for 100 h. There was macrosegregation of Si and Ti in the alloys ZX3 and ZX5 and only of Si in the alloy ZX7. The Nbss was stable in all alloys. Tin and Ti exhibited opposite partitioning behaviour in the Nbss. The βNb5Si3 was the primary phase in all three cast alloys and had partially transformed to αNb5Si3 in the alloy ZX3. Aluminium in synergy with Sn increased the sluggishness of the βNb5Si3 to αNb5Si3 transformation during solidification. After the heat treatment the transformation of βNb5Si3 to αNb5Si3 had been completed in all three alloys. Fine precipitates were observed inside some αNb5Si3 grains in the alloys ZX5 and ZX7. In the latter alloys the A15-Nb3X (X = Al, Si, and Sn) formed after the heat treatment, i.e., the synergy of Al and Sn promoted the stability of A15-Nb3X intermetallic in these Nb-silicide-based alloys even at this low Sn concentration. A Nbss + Nb5Si3 eutectic formed in all three alloys and there was evidence of anomalous eutectic in the parts of the alloys ZX3 and ZX7 that had solidified under high cooling rate and/or high melt undercooling. A very fine ternary Nbss + Nb5Si3 + NbCr2 eutectic was also observed in parts of the alloy ZX3 that had solidified under high cooling rate. At 800 °C none of the alloys suffered from catastrophic pest oxidation; ZX7 had a smaller oxidation rate constant. A thin Sn-rich layer formed continuously between the scale and Nbss in the alloys ZX3 and ZX5. At 1200 °C the scales formed on all three alloys spalled off, the alloys exhibited parabolic oxidation in the early stages followed by linear oxidation; the alloy ZX5 gave the smallest rate constant values. A thicker continuous Sn-rich zone formed between the scale and substrate in all three alloys. This Sn-rich zone was noticeably thicker near the corners of the specimen of the alloy ZX7 and continuous around the whole specimen. The Nb3Sn, Nb5Sn2Si, and NbSn2 compounds were observed in the Sn-rich zone. At both temperatures the scales formed on all three alloys consisted of Nb-rich and Nb and Si-rich oxides, and Ti-rich oxide also was formed in the scales of the alloys ZX3 and ZX7 at 1200 °C. The formation of a Sn-rich layer/zone did not prevent the contamination of the bulk of the specimens by oxygen, as both Nbss and Nb5Si3 were contaminated by oxygen, the former more severely than the latter.
Highlights
Materials with capabilities beyond those of Ni-based superalloys would allow future aero-engines to meet stringent environmental targets recommended by regulatory bodies, for example ACARE (Advisory Council for Aircraft Innovation and Research in Europe)
At 1200 ◦ C the scales formed on all three alloys spalled off, the alloys exhibited parabolic oxidation in the early stages followed by linear oxidation; the alloy ZX5 gave the smallest rate constant values
Research towards the end of the last century demonstrated that Nb silicide-based alloys have the potential to replace Ni-based superalloys in aerofoil applications owing to their offering a balance of properties
Summary
Materials with capabilities beyond those of Ni-based superalloys would allow future aero-engines to meet stringent environmental targets recommended by regulatory bodies, for example ACARE (Advisory Council for Aircraft Innovation and Research in Europe). Progress on Nb silicide-based alloys has been reviewed by one of the present authors [2] It was shown [2] that the actual compositions of some Nb-silicide-based alloys, of some Nb with no Si solid solutions observed in Nb-silicide-based alloys, and of some eutectics with Nbss and Nb5 Si3 observed in Nb-silicide-based alloys satisfy the “standard definition” of the so-called “high-entropy alloys” (HEAs), “concentrated solid solution alloys” (CSSAs), “multi-principle element alloys” (MPEAs), and “complex concentrated alloys” (CCAs) (note that it is not suggested that all Nb-silicide-based alloys are HEAs). The motivation of the research presented in this paper was to understand how a low concentration of Sn in model Nb-24Ti-18Si silicide-based alloys with Al or Cr and Al and Cr addition(s) affects their microstructure and isothermal oxidation at 800 and 1200 ◦ C. The microstructures of the cast and heat-treated alloys Nb-24Ti-18Si-5Cr-2Sn (ZX3), Nb-24Ti-18Si-5Al-2Sn (ZX5), and Nb-24Ti-18Si-5Al-5Cr-2Sn (ZX7) (nominal compositions, at.%) are discussed, followed by the results and discussion for their oxidation at 800 ◦ C and 1200 ◦ C
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