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.