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, β(Nb,W)5Si3, Nb(W)Si2 and W(Nb)Si2, and two eutectic invariant reactions, L→β(Nb,W)5Si3+W(Nb)Si2+Nb(W)Si2 and L→W(Nb)Si2+Nb(W)Si2+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)Si2+W(Nb)Si2, Nb(W)Si2+αNb(W)5Si3+β(Nb,W)5Si3 and Nb(W)Si2+W(Nb)Si2+β(Nb,W)5Si3, and seven two-phase regions, liquid+Nb(W)Si2, Nb(W)Si2+αNb(W)5Si3, αNb(W)5Si3+β(Nb,W)5Si3, Nb(W)Si2+β(Nb,W)5Si3, liquid+W(Nb)Si2, Nb(W)Si2+W(Nb)Si2 and W(Nb)Si2+β(Nb,W)5Si3.