Experimental measurements of Ti–Sb binary phase diagram for half-Heusler thermoelectric alloys

The Ti–Sb binary system, a critical component of half-Heusler thermoelectric alloys, remains incompletely defined due to the significant difference in melting points between Sb (630°C) and Ti (1670°C). In this work, Ti–Sb diffusion couples were prepared using argon gas-protected seal welding to prevent Sb volatilization during long-term annealing. Seven groups of samples were annealed at temperatures ranging from 400–900°C for 14 days. Eight equilibrium phases—(αTi), (βTi), (Ti₃Sb), Ti₂Sb, Ti_1.35Sb, TiSb, TiSb₂ and (Sb)—were identified based on microstructural and compositional analyses using electron probe microanalysis (EPMA). The equilibrium conjugate compositions reveal that the solubility of Sb in the hexagonal close-packed (αTi) phase is 0.33 at.%, while it significantly increases to ~7 at.% in the body-centred cubic (βTi) phase at 850°C due to its structural transformation. The detected phases (Ti₃Sb), Ti₂Sb, Ti_1.35Sb (or Ti_11−xSb_8−γ), TiSb and TiSb₂ exhibit stoichiometric compositions. Notably, the (Ti₃Sb) phase shows a low Sb content (~22.5 at.%) below 600°C but transitions to a higher Sb content (~25.5 at.%) above 700°C, a change attributed to its structural modification from cubic-(Ti₃Sb) to tetragonal-(Ti₃Sb). These findings provide critical insights into the phase equilibria and structural transformations in the Ti–Sb system, enhancing its understanding for thermoelectric applications.