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

Abstract

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 Nb 9(Cr,Si) 5, isostructural with the Nb 9Co 3Ge 2 phase, was identified in the present study. Four primary solidification regions were identified in these alloys: (Nb), Nb 9(Cr,Si) 5, Nb 5Si 3, and Cr 2Nb(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 + Nb 3Si → Nb 5Si 3 + (Nb), Liquid + Nb 5Si 3 → Nb 9(Cr,Si) 5 + (Nb), Liquid + Nb 5Si 3 → Nb 9(Cr,Si) 5 + Cr 2Nb(C14), and Liquid → Nb 9(Cr,Si) 5 + (Nb) + Cr 2Nb(C14). At the Cr-rich side there are two invariant reactions: Liquid + Nb 5Si 3 → Cr 2Nb(C14) + Cr 3Si and Liquid → (Cr) + Cr 2Nb(C14) + Cr 3Si.