On Nb silicide based alloys: Part II

Abstract

The paper examined alloying behaviour in cast and heat treated Nb-silicide based alloys where alloying additions were transition metals (Cr, Hf, Mo, Ta, Ti, W), and simple metal or metalloid elements (Al, B, Ge, Sn). The microstructures of the alloys consisted of bcc Nbss solid solution (which could be of three types, namely normal, Ti rich and with no Si) and intermetallic(s). The study used the parameters entropy (ΔSmix) and enthalpy (ΔHmix) of mixing, atomic size difference (δ), electronegativity difference (Δχ), valence electron concentration (VEC), Ω = Tm ΔSmix/| ΔHmix | and the ratio sd/sp of sd over sp electronic configuration elements. The values of the parameters were in the ranges - 44.8 < ΔHmix < - 32.7 kJ/mol, 8.3 < ΔSmix < 14.7 J/molK, 4.4 < VEC <4.9, 8.1 < δ < 14.3, 0.12 < Δχ < 0.24, 0.57 < Ω < 0.95 and 2.12 < sd/sp < 4.03. Plots of Δχ versus δ and Δχ versus ΔHmix separated the alloys in three groups according to alloying additions. In these plots the B containing alloys formed their own group. The data for Nb solid solutions from Part I (J Alloys Compd. 708 (2017) 961–971) was used to compare the parameters of the alloys and solid solutions. Plots of Δχ versus ΔHmix and Δχ versus Ω separated the alloys and their bcc Nb solid solutions according to whether they contained B or not. The parameters ΔHmix, ΔSmix, δ and Δχ of some of the Nb-silicide based alloys were in the ranges reported for bcc solid solution + intermetallic(s) high entropy alloys. The values of the parameters ΔHmix, ΔSmix, VEC, δ and Δχ were in better agreement with those reported for amorphous alloys.