On the Nb silicide based alloys: Part I – The bcc Nb solid solution

This paper is about the three types of bcc Nbss solid solution, namely normal Nbss, Nbss rich in Ti and Nbss with no Si that are observed in multi-component Nb silicide based alloys of Nb-Si-TM-RM-X (TM = Cr, Hf, Ti, V, RM = Mo, Ta, W, X = Al, B, Ge, Sn) systems. The entropy (ΔSmix) and enthalpy (ΔHmix) of mixing, atomic size difference (δ), electronegativity difference (Δχ), valence electron concentration (VEC) and the parameter Q = Tm ΔSmix/|ΔHmix| were calculated for fifty four solid solutions. The values of these parameters were −2 < ΔHmix < −15.9 kJ/mol, 5.8 < ΔSmix < 14.5 J/molK, 2.4 < δ < 9.7, 4.4 < VEC < 5.4, 0.039 < Δχ < 0.13 and 0.179 < Δχ < 0.0331 and 1.55 < Q < 8.9. The solid solutions with Δχ > 0.179 had no B, Ta and V and the solid solutions with no W had Δχ < 0.13. The atomic size difference parameter δ could separate the Nbss rich in Ti (δ > 5) and the Nbss with no Si (δ < 5). The formation of Ti rich Nbss and Nbss with no Si was attributed to the partitioning of Mo, Ti and W.