CALPHAD assessment of bio-oriented Ti–Zr–Sn system and experimental validation in Ti/Zr-rich alloys

The development of CALPHAD-type thermodynamic database for Ti or Zr based biomedical alloys has been spurred by the increased interest in efficiently tailoring an alloy composition to obtain high stability of β_bcc, low Young's modulus, and free of detrimental phases. However, the thermodynamic prediction is not adequate to be performed without the information of key sub-ternary Ti–Zr–Sn system. In present work, the thermodynamic assessment of Ti–Zr–Sn system is performed via a critical evaluation of phase equilibria and microstructure development in this ternary system. The partial isothermal sections at 1323 K and 1473 K with Sn content below 40 at. % are obtained by analyzing chemical compositions and crystal structures of individual phases in the annealed alloys. The composition homogeneity range of most phases is validated to favor a ternary extension paralleling to the Ti–Zr axis. Particularly, β_bcc and η phases (with the chemical composition (Ti, Zr)₅Sn_3+x) show complete solubility of Ti and Zr from Ti–Sn edge to Zr–Sn edge. With the database, negligible ternary solubility of Zr₄Sn phase, microstructure development in the as-cast samples, and the controversial conclusions in literature are discussed. Most of the experimental findings, including equilibrium phase constitution, solidification sequence, DSC signals, projections of liquidus, are reproduced in a self-consistent way. The work moves towards the completeness of multi-component Ti/Zr thermodynamic database. It can be used for composition design of novel metastable β-type biomedical alloys.