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)₅Si ₃, Nb(W)Si₂ and W(Nb)Si₂, and two eutectic invariant reactions, L→β(Nb,W)₅Si₃+W(Nb)Si ₂+Nb(W)Si₂ and L→W(Nb)Si₂+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)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)₅Si ₃, αNb(W)₅Si₃+β(Nb,W) ₅Si₃, Nb(W)Si₂+β(Nb,W)₅Si ₃, liquid+W(Nb)Si₂, Nb(W)Si₂+W(Nb)Si ₂ and W(Nb)Si₂+β(Nb,W)₅Si₃.