Contribution to the Ti–Co–Sn system

The ternary phase diagram Ti–Co–Sn was studied using X-ray diffraction (XRD), scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). Isothermal sections at 1000 and 1200 °C have been determined experimentally for the first time in the entire range of compositions. Thirteen and ten three-phase regions were found at 1000 °C and 1200 °C, respectively. Vertical sections at 10, 20 and 30 at.% Sn were plotted. The most striking feature of the ternary Ti–Co–Sn phase diagram is formation of a ternary compound TiCo₂Sn (Heusler phase, τ), which was found at both investigated temperatures. The TiCoSn compound (half-Heusler phase) was not found. Among binary compounds, Ti₅Sn₃ and TiCo have the widest homogeneity regions. The Ti₅Sn₃ phase dissolves 10.0 and 9.8 at.% Co at 1200 and 1000 °C, respectively, forming an interstitial solid solution. The solubility of Sn in TiCo is more than 10 at.% at both 1200 and 1000 °C. The remaining binary intermetallic phases hardly dissolved the third component. The liquid phase at 1000 °C mainly exists in the Sn-rich corner, while at 1200 °C it stretches along Co–Sn side spreading from the Sn corner and is also present on the Ti-rich side. In addition, two four-phase invariant transition type reactions TiCo₂ (h) + (αCo) ⇄ τ + TiCo₃ and TiCo + TiCo₂ (h) ⇄ τ + TiCo₂ (c) were deduced.