Experimental study on the as-cast solidification of the Si-rich alloys of the Nb–Si–Mo ternary system

This paper describes the as-cast solidification of the Nb–Si–Mo ternary alloys in the Si-rich region, which have attracted great attention for the use of the Nb–Si and the MoSi2 based alloys as ultra-high temperature structural materials. The microstructural observation and phase identification for the as-cast Nb–Si-Mo alloys in Si-rich region were studied using backscattered electron (BSE) imaging, electron beam microprobe analysis (EPMA) and X-ray diffraction (XRD) analysis. Based on the present experimental study along with the literature reported data, the liquidus surface projection of the Nb–Si–Mo system is determined. Four primary solidification regions have been involved in the Si-rich region of the system, which are Diamond-(Si), β(Nb,Mo)5Si3, Nb(Mo)Si2 and αMo(Nb)Si2. Two invariant reactions exist in the Si-rich side, which are L + αMo(Nb)Si2 = Nb(Mo)Si2 + β(Nb,Mo)5Si3 and L + αMo(Nb)Si2 = Nb(Mo)Si2 + Diamond-(Si). According to the as-cast microstructural morphologies as well as their chemical compositions and crystalline structures, the solidification paths for different kinds of alloys are analyzed.