As-cast microstructures and solidification paths of the Nb–Si–Ti ternary alloys in Nb5Si3–Ti5Si3 region

The as-cast microstructures and solidification paths of the Nb–Si–Ti ternary alloys in the Nb5Si3–Ti5Si3 region were investigated. Since there exist some isomorphous compounds in the Nb5Si3–Ti5Si3 region, such as αNb5Si3 with B3Cr5 prototype, βNb5Si3 with Si3W5 prototype, γNb5Si3 with Mn5Si3 prototype, and Ti5Si3 with Mn5Si3 prototype, the primary solidification areas of these compounds were not typically indentified in previous experiments. In the present paper, the microstructure observation, the phase identification, and the composition measurement were performed using scanning electron microscopy (SEM), X-ray diffraction (XRD), and electron probe microanalysis (EPMA), respectively. No ternary compound is found. There exist three primary solidification areas, βNb5−x(Ti)xSi3, αNb5−x(Ti)xSi3, and Ti5−x(Nb)xSi3 in the Nb5Si3–Ti5Si3 region. Together with the literature-reported experimental data and optimization results, the liquidus projection of the whole Nb–Si–Ti ternary system is constructed, and totally ten primary solidification areas—diamond-Si, Nb1−x(Ti)xSi2, Ti1−x(Nb)xSi2, Ti1−x(Nb)xSi, Ti5−x(Nb)xSi4, βNb5−x(Ti)xSi3, αNb5−x(Ti)xSi3, Ti5−x(Nb)xSi3, (Nb,Ti)3Si, and BCC—and nine transitional invariant reactions—L + Nb1−x(Ti)xSi2 → Ti1−x(Nb)xSi2 + Si, L + Nb1−x(Ti)xSi2 → Ti1−x(Nb)xSi2 + Ti5−x(Nb)xSi4, L + Ti5−x(Nb)xSi4 → Ti1−x(Nb)xSi2 + Ti1−x(Nb)xSi, L + βNb5−x(Ti)5Si3 → Nb1−x(Ti)xSi2 + Ti5−x(Nb)xSi4, L + βNb5−x(Ti)xSi3 → αNb5−x(Ti)xSi3 + Ti5−x(Nb)xSi4, L + αNb5−x(Ti)xSi3 → Ti5−x(Nb)xSi3 + Ti5−x(Nb)xSi4, L + αNb5−x(Ti)xSi3 → βNb5−x(Ti)xSi3 + Ti5−x(Nb)xSi3, L + βNb5−x(Ti)xSi3 → Ti5−x(Nb)xSi3 + (Nb,Ti)3Si, and L + (Nb,Ti)3Si → Ti5−x(Nb)xSi3 + BCC—are confirmed.