Correlation of size, structure and thermodynamics of alloying additions on <i>β</i> -phase stabilization in Ti alloys

Abstract Novel β -Ti alloy development through advanced manufacturing technologies like additive manufacturing (AM) requires a robust understanding of the effect of alloying additions, especially on the martensite transformation behavior. Avoiding martensite is the other side of the same coin as stabilizing β -phase in rapid cooling is associated with such processes. In this study, for the individual binary combinations of the alloying elements with Ti, the calculated T 0 and experimental martensite start temperatures ( M S ) are comprehensively analyzed for the atomic radii and crystal structure effects, in the light of latest Calphad (Calculation of Phase Diagram) data. Wherever not available, thermodynamic database files were compiled in-house. Important trends are revealed with regard to the atomic radii within a type of crystal structure of alloying additions on martensite transformation. For a specific crystal structure, a trend of decreasing solute composition limit for martensitic transformation with increasing atomic size difference between the alloying element and Ti is seen. Effect of atomic size difference between solute and solvent on distortion in the product phase, vis-à-vis on the strain energy barrier for martensitic transformation, and β -phase stabilization are discussed. The study provides useful insights for the selection of alloying elements while designing Ti alloy for processes such as AM. The results are validated using experiments on one selected alloy, i.e. Mo addition to Ti, since Mo is identified as non-toxic and allergy free β -phase stabilizer to Titanium for biological application. The microstructural and phase analysis using optical microscopy, energy-dispersive x-ray spectroscopy, and x-ray diffraction of as-cast and homogenized Ti-Mo alloy match with the revealed theoretical trends.