Thermal expansion of the Ti5Si3 and Ti6Si2B phases investigated by high-temperature X-ray diffraction

The thermal expansion coefficients (CTE) of the Ti5Si3 and Ti6Si2B phases were determined using high-temperature X-ray diffraction from 298 to 873 K. Alloys with nominal compositions Ti66Si34, Ti62.5Si37.5, Ti59Si41 and Ti66.7Si22.2B11.1 (at%) were prepared through arc-melting followed by heat-treatment at 1473 K during 60 h under argon atmosphere. The Ti5Si3 phase has a hexagonal structure, space group P63/mcm (No. 193) with room temperature lattice parameters a=7.4561 Å and c=5.1485 Å. This binary compound presents high thermal expansion anisotropy (αc/αa≅2.7), as a consequence of the large difference between the thermal expansion coefficients along the a (5.98×10−6 K−1) and c (16.64×10−6 K−1) axes. The Ti6Si2B phase crystallizes with a hexagonal unit cell, space group P  6¯2m (No. 189) with room temperature lattice parameters a=6.8026 Å and c=3.3374 Å. This phase exhibits a low thermal expansion anisotropy (αc/αa≅1), with the thermal expansion coefficients along the a and c axes equal to 9.7×10−6 K−1 and 9.6×10−6 K−1, respectively.