Experimental study on phase relationships in the Si-rich portion of the Nb–Si–W ternary system

This paper describes the experimental study on liquidus projection and the isothermal section at 1873 K of the Nb-Si-W ternary system in the Si-rich portion. The microstructures and solidification paths of the as-cast alloys were analysed. The constituent phases and their equilibrium compositions of the as-cast heat-treated alloys were determined. The microstructure observation, the phase identification and the composition measurement were performed using scanning electron microscopy (SEM), X-ray diffraction (XRD) and electron probe microanalysis (EPMA), respectively. No ternary compound was found. For the liquidus projection in the investigated Si-rich portion, there exist four primary solidification regions, Diamond-Si, beta(Nb,W)(5)Si-3, Nb(W)Si-2 and W(Nb)Si-2, and two eutectic invariant reactions, L ->beta(Nb,W)(5)Si-3 +W(Nb)Si-2+ Nb(W)Si2 and L -> W(Nb)Si-2+Nb(W)Si-2+ Diamond-Si. The type of the latter invariant reaction was determined with the help of the thermodynamic assessment of the system. Together with literature reported investigations for the Nb-W-rich portion, the whole liquidus projection of the Nb-Si-W ternary system was constructed. For the isothermal section at 1873 K in the investigated Si-rich portion, there are three three-phase regions, liquid + Nb(W)Si-2+W(Nb)Si-2, Nb(W)Si-2+alpha Nb(W)(5)Si-3+beta(Nb,W)(5)Si-3 and Nb(W)Si-2+W(Nb)Si-2+beta(Nb,W)(5)Si-3, and seven two-phase regions, liquid Nb(W)Si-2, Nb(W)Si-2+ alpha Nb(W)(5)Si-3, alpha Nb(W)(5)Si-3 + beta(Nb,W)(5)Si-3, Nb(W)Si-2+beta(Nb,W)(5)Si-3, liquid +W(Nb)Si-2, Nb(W)Si-2+W(Nb)Si-2 and W(Nb)Si-2+beta(Nb,W)(5)Si-3. (C) 2013 Elsevier Ltd. All rights reserved.