Microstructure and lattice parameters of suction-cast Ti-Nb alloys in a wide range of Nb concentrations

Metastable alloys based on the Ti-Nb system are considered promising candidates to replace the biomaterials currently used in medicine. Several very promising alloys like TNTM, TNZT, etc., were recently developed by adding Zr, Ta, Mo, Fe, or other elements to the Ti-Nb system. However, only a few fundamental studies were devoted to the structural characterisation of Ti-Nb alloys in the as-cast state. In this study, we analyze the microstructure, the phase transformations, the lattice parameters, the interaction of different phases, and Young's modulus of a wide range of Ti-Nb alloys produced by suction casting. The structure of experimental samples was investigated using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX) and synchrotron X-ray diffraction (SXRD) analysis. The samples with a Nb content of less than 17.5 wt % consisted of alpha ' phase. The pure alpha phase was observed only in the alloy with 17.5 wt % Nb. The alloys with an intermediate content of Nb (20-30 wt %) contained a mixture of alpha, beta and omega phases. The samples with 30-35 wt % Nb consisted of a mix of beta and omega phases. The samples with 37.5 and 45 wt % Nb included two types of beta phase. In addition to the regular beta phase, a little detectable amount of bcc phase was observed with a larger lattice parameter named beta minor. The mechanical properties strongly depended on the phase composition. We found that for alpha-dominated alloys, there is a correlation between Young's modulus and orthorhombicity and c/a ratio of alpha phase. The lowest Young's modulus (47 GPa) was observed in the alloy with 17.5 wt % Nb, which has orthorhombicity close to unity and c/a ratio close to 1.58.

Automated summary

The study on suction-cast Ti-Nb alloys shows that different Nb contents have a significant impact on the microstructure and phase transformations of the alloys, resulting in different phase compositions. The mechanical properties, such as Young's modulus, are directly influenced by the phase composition of the alloys.