A Study of the Effect of 2 at.% Sn on the Microstructure and Isothermal Oxidation at 800 and 1200 °C of Nb-24Ti-18Si-Based Alloys with Al and/or Cr Additions

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 th...

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

“…The alloys ZX4, ZX6 and ZX8 of nominal compositions (at.%), respectively Nb-24Ti-18Si-5Cr-5Sn, Nb-24Ti-18Si-5Al-5Sn, and Nb-24Ti-18Si-5Al-5Cr-5Sn were prepared in the form of 20 g buttons using arc melting with a water cooled copper crucible in a Ti gettered Argon atmosphere and high purity (better than 99.99 wt.%) elements. In this paper, we refer to these alloys as the higher Sn content alloys, compared with the low Sn content alloys that were reported in [13], namely the alloys Nb-24Ti-18Si-5Cr-2Sn (ZX3), Nb-24Ti-18Si-5Al-2Sn (ZX7), and Nb-24Ti-18Si-5Al-5Cr-2Sn (ZX8). Specimens for heat treatments were wrapped in Ta foil and heat treated under a constant flow of Ti gettered Argon at 1500 • C (ZX4 and ZX6) or 1450 • C (ZX8) for 100 h [5,9,12].…”

“…The as cast, heat treated, and oxidised alloys were characterised using X ray diffraction (XRD) and scanning electron microscopy (SEM) equipped with energy dispersive spectrometers (EDS) and an electron probe micro analyser (EPMA) equipped with a wavelength dispersive spectrometer (WDS). Specimens were prepared as discussed in [13]. For the XRD experiments, a Siemens 5000 X-ray diffractometer (HiltonBrooks Ltd., Crew, UK) with monochromatic Cu-K α radiation was used and the specimens were scanned using 0.02 • step and two theta (2θ) from 20 to 100 degrees.…”

“…However, Nb-silicide based alloys must also have adequate creep, for which additions of Hf and refractory metals are essential [1,14] but unfortunately can mask the so-called "Sn-effect" (see previous section). Thus, our focus in [13] and in this paper was on Nb-silicide based alloys of the Nb-Ti-Si-Al-Cr-Sn system.…”

“…NICE also indicated that with increasing Sn concentration, (a) the creep rate and (b) the macrosegregation of Si will increase. Based on these results the nominal compositions of the alloys of this study were selected (see next section) as well as those of the low Sn content alloys that were studied in [13].…”

“…The motivation for the research presented in this paper was to make new contributions in the physical metallurgy of Nb-silicide based alloys that would help us answer some of the above questions. Recently, a systematic study which aspired to find out how Al and Cr individually or simultaneously, in synergy with 2 at.% Sn, improved oxidation of Nb-24Ti-18Si silicide based alloys, reported that the Nb 3 Sn compound was stable even at this low Sn concentration and confirmed the formation of Sn rich areas below the scale where Nb 5 Sn 2 Si was formed together with other Sn rich intermetallics, such as NbSn 2 and Nb 3 Sn [13]. In this paper, the systematic study is expanded to find out how a higher concentration of Sn in basically the same alloys (see next section) would affect their microstructures and isothermal oxidation at 800 and 1200 • C.…”