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.