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

Highlights: • High orthorhombicity and low c/a ratio of α'' phase result in low Young's modulus. • Small volume change in $\beta \to \omega$ transformation causes a high Young's modulus. • Two different $\beta$ metastable phases have been observed in Nb rich alloys. • Lattice strains in $\beta \to \omega$ transformations are smaller than in $\beta \to {\alpha }^{\prime }/{\alpha }^{″}$ transformations. 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 ${\alpha }^{\text{′}}$ phase. The pure α'' 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 α'', β and ω phases. The samples with 30–35 wt % Nb consisted of a mix of β and ω phases. The samples with 37.5 and 45 wt % Nb included two types of β phase. In addition to the regular β phase, a little detectable amount of bcc phase was observed with a larger lattice parameter named ${\beta }_{\text{minor}}$. The mechanical properties strongly depended on the phase composition. We found that for α''-dominated alloys, there is a correlation between Young's modulus and orthorhombicity and c/a ratio of α'' 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.