Experimental study of the liquid-solid phase equilibria at the metal-rich region of the Nb-Cr-Si system

There is presently limited information regarding liquid-solid phase equilibria at the metal-rich region of the Nb-Cr-Si system; these data are needed for the development of the Nb-silicide based in situ composites. In the present study a range of metal-rich Nb-Cr-Si alloys with Si concentrations up to 30 at.% was carefully selected using a phase diagram that was calculated from the prior developed thermodynamic description of the Nb-Cr-Si system. The as-cast microstructures were examined using scanning electron microscopy, X-ray diffraction, electron probe microanalysis, and electron backscatter diffraction. A new ternary phase Nb₉(Cr,Si)₅, isostructural with the Nb₉Co₃Ge₂ phase, was identified in the present study. Four primary solidification regions were identified in these alloys: (Nb), Nb₉(Cr,Si)₅, Nb₅Si₃, and Cr₂Nb(C14). Microstructural and microchemical evidence suggests six invariant reactions on the liquidus surface of the metal-rich region of the Nb-Cr-Si system. At the Nb-rich side there are four invariant reactions: Liquid + Nb₃Si → Nb₅Si₃ + (Nb), Liquid + Nb₅Si₃ → Nb₉(Cr,Si)₅ + (Nb), Liquid + Nb₅Si₃ → Nb₉(Cr,Si)₅ + Cr₂Nb(C14), and Liquid → Nb₉(Cr,Si)₅ + (Nb) + Cr₂Nb(C14). At the Cr-rich side there are two invariant reactions: Liquid + Nb₅Si₃ → Cr₂Nb(C14) + Cr₃Si and Liquid → (Cr) + Cr₂Nb(C14) + Cr₃Si.