Alloying and Hardness of Eutectics with Nbss and Nb5Si3 in Nb-silicide Based Alloys

Abstract

In Nb-silicide based alloys, eutectics can form that contain the Nbss and Nb5Si3 phases. The Nb5Si3 can be rich or poor in Ti, the Nb can be substituted with other transition and refractory metals, and the Si can be substituted with simple metal and metalloid elements. For the production of directionally solidified in situ composites of multi-element Nb-silicide based alloys, data about eutectics with Nbss and Nb5Si3 is essential. In this paper, the alloying behaviour of eutectics observed in Nb-silicide based alloys was studied using the parameters ΔHmix, ΔSmix, VEC (valence electron concentration), δ (related to atomic size), Δχ (related to electronegativity), and Ω (= Tm ΔSmix/|ΔHmix|). The values of these parameters were in the ranges −41.9 < ΔHmix <−25.5 kJ/mol, 4.7 < ΔSmix < 15 J/molK, 4.33 < VEC < 4.89, 6.23 < δ < 9.44, 0.38 < Ω < 1.35, and 0.118 < Δχ < 0.248, with a gap in Δχ values between 0.164 and 0.181. Correlations between ΔSmix, Ω, ΔSmix, and VEC were found for all of the eutectics. The correlation between ΔHmix and δ for the eutectics was the same as that of the Nbss, with more negative ΔHmix for the former. The δ versus Δχ map separated the Ti-rich eutectics from the Ti-poor eutectics, with a gap in Δχ values between 0.164 and 0.181, which is within the Δχ gap of the Nbss. Eutectics were separated according to alloying additions in the Δχ versus VEC, Δχ versus <Si>, δ versus <Si>, and VEC versus <Si> maps, where <Si> = Al + Ge + Si + Sn. Convergence of data in maps occurred at δ ≈ 9.25, VEC ≈ 4.35, Δχ in the range ≈ 0.155 to 0.162, and <Si> in the range ≈ 21.6 at.% to ≈ 24.3 at.%. The convergence of data also indicated that the minimum concentration of Ti and maximum concentrations of Al and Si in the eutectic were about 8.7 at.% Ti, 6.3 at.% Al, and 21.6 at.% Si, respectively, and that the minimum concentration of Si in the eutectic was in the range 8 < Si < 10 at.%.

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 design methodology NICE [14] utilizes relationships between the parameters δ, ∆χ, and VEC that describe the alloying behaviour of (1) Nb-silicide based alloys and (2) the phases that can be present in their microstructures [15][16][17][18][19]. NICE makes use of the relationships between the aforementioned parameters and (i) the concentrations of solutes in (a) alloy and (b) phases and (ii) the creep rate at different temperatures and stresses, (iv) the weight change in isothermal oxidation at 800 and 1200 • C, and (v) the Si macrosegregation to design alloy compositions, predict properties, and assist the selection of alloys for alloy development research.…”

“…There was macrosegregation in all three as cast alloys (Table 2). Phenomena linked with the macrosegregation of elements in Nb-silicide based alloys were discussed in [20] where the macrosegregation of Si was linked with the partitioning of other solutes between the key phases in the microstructure of Nb-silicide based alloys, namely the Nb ss , Nb 5 Si 3 , C14-NbCr 2 Laves and the Nb ss + Nb 5 Si 3 eutectic [16][17][18][19]. Tin is one of the alloying additions that have a strong effect on macrosegregation [20].…”

“…The parameters δ (related to atomic size), ∆χ (related to electronegativity), and number of valence electrons per atom filled into the valence band (VEC) describe well the alloying behaviour (a) of Nb-silicide based alloys [15] and (b) of the most important phases in their microstructures [16][17][18][19]. Also, there are relationships between the properties of these alloys (creep, weight gain in isothermal oxidation) and their phases and the alloy or phase parameters δ, ∆χ and VEC [17][18][19]. These relationships are used in the alloy design methodology NICE [14] to design new Nb-silicide based alloys.…”

On the Microstructure and Isothermal Oxidation at 800 and 1200 °C of the Nb-24Ti-18Si-5Al-5Cr-5Ge-5Sn (at.%) Silicide-Based Alloy

“…The Nb ss + βNb 5 Si 3 eutectic is metastable in the Nb-Si binary. In binary alloys, this eutectic is promoted by solidification at high cooling rates and/or high melt undercoolings (i.e., under RS (rapid solidification) conditions [55]), while, in ternary and higher-order alloys, it is also promoted by additions that can suppress the Nb 3 Si silicide [48]. Tin and Ge are such additions, as is Al, but not Cr when added to Nb-24Ti-18Si [32][33][34][40][41][42]52,53].…”

“…When Al and Cr were present simultaneously in Nb-24Ti-18Si-5Al-5Cr (alloy KZ5 in Reference [42]), the Nb ss + βNb 5 Si 3 was formed instead of the Nb ss + Nb 3 Si eutectic. In the Nb-Si binary, the Si content of the metastable eutectic was suggested to be about 20 at.% [55] (although this value depends on which version of the Nb-Si binary phase diagram is used [48]). In ternary and higher-order alloys, the concentration of Si plus other element(s) (like Al, Sn, Ge) in the eutectic is about 21 at.% [48].…”