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, βNb_5-x (Ti) _x Si₃, αNb_5-x (Ti) _x Si₃, and Ti_5-x (Nb) _x Si₃ 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, Nb_1-x (Ti) _x Si₂, Ti_1-x (Nb) _x Si₂, Ti _1-x (Nb) _x Si, Ti_5-x (Nb) _x Si ₄, βNb_5-x (Ti) _x Si₃, αNb_5-x (Ti) _x Si₃, Ti_5-x (Nb) _x Si₃, (Nb,Ti)₃Si, and BCC - and nine transitional invariant reactions - L + Nb_1-x (Ti) _x Si₂ → Ti_1-x (Nb) _x Si₂ + Si, L + Nb_1-x (Ti) _x Si₂ → Ti_1-x (Nb) _x Si₂ + Ti_5-x (Nb) _x Si ₄, L + Ti_5-x (Nb) _x Si₄ → Ti_1-x (Nb) _x Si₂ + Ti_1-x (Nb) _x Si, L + βNb_5-x (Ti)₅Si₃ → Nb_1-x (Ti) _x Si₂ + Ti_5-x (Nb) _x Si₄, L + βNb_5-x (Ti) _x Si₃ → αNb_5-x (Ti) _x Si₃ + Ti_5-x (Nb) _x Si₄, L + αNb_5-x (Ti) _x Si₃ → Ti_5-x (Nb) _x Si₃ + Ti_5-x (Nb) _x Si₄, L + αNb_5-x (Ti) _x Si₃ → βNb _5-x (Ti) _x Si₃ + Ti_5-x (Nb) _x Si₃, L + βNb_5-x (Ti) _x Si₃ → Ti_5-x (Nb) _x Si₃ + (Nb,Ti)₃Si, and L + (Nb,Ti)₃Si → Ti_5-x (Nb) _x Si₃ + BCC - are confirmed.