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

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.

On the Microstructure and Properties of Nb-12Ti-18Si-6Ta-2.5W-1Hf (at.%) Silicide-Based Alloys with Ge and Sn Additions

“…In synergy with Al and/or Cr, Sn or Ge improve the oxidation resistance of Nb-silicide-based alloys in the pest oxidation regime when added individually [3,13,[24][25][26] and in the pest regime and at higher temperatures when added simultaneously [3,27]. There is no data about (i) the microstructure and (ii) the oxidation of Nb-silicide-based alloys in which Ge and Sn are added simultaneously with TMs and RMs but without Al and Cr additions.…”

“…As regards the above constraints, the (a), (d), and (e) were related to the creep target and data about creep in [4,13] and references within, the (b), (d), (e), and (g) were linked with (c) and the aim to have specific strengths higher than Ni-based superalloys, and the (b), (f), and (g) were linked with oxidation resistance. The choice of Sn concentrations, and thus constraints (f) and (g), was also "guided" by literature about the effect of Sn or Ge on the oxidation behavior of Nb-silicide-based alloys [3,13,[24][25][26][27].…”

“…The latter is defined as MACSi = C max Si − C min Si where C max Si and C min Si , respectively are the maximum and minimum Si concentrations in the cast alloy [26,27]. The MACSi values were 5.6 and 4.9 at.% for the alloys JZ1 and KZ2, respectively.…”

“…Pest oxidation at intermediate temperatures (600-900 • C) is suppressed by the synergy of Al, Cr, Si, and Ti with B, Ge, or Sn [3,13]. Spallation of oxide scales formed at high temperatures is a common phenomenon [22,[24][25][26]. However, scale spallation was prevented in a Nb-silicide-based alloy when Ge and Sn were added simultaneously with Al, Cr, Hf, Si, and Ti [27].…”

“…Elements in RCCAs are the group IV, V, and VI TMs and RMs and sometimes there are additions of Al and/or Si (and Co or Ni). The same elements are used as additions in Nb-silicide-based alloys [13] with the exception of Ta, V, or Zr that have been used less frequently and Co and Ni that have not been used at all.Toughness at room temperature, oxidation in the pest regime (600-900 • C) and at high temperatures (≥1000 • C), and creep at T ≥ 1050 • C and stress (σ) in the range 50 to 300 MPa have been evaluated for Nb-silicide-based alloys [2][3][4][5]13,[18][19][20][21][22][23][24][25][26][27][28][29] and references within. Properties of the key phases in the latter alloys have also been measured [13] and references within and the effects of alloying on properties of Nb 5 Si 3 have been studied [13] and references within.…”