Thermodynamic Basis of Non-equilibrium Phase Transformations of bcc β-Phase in Ti-Mo System

This study reports the experimental identification of transformation products of high temperature bcc beta phase in Ti-xMo (x = 1, 7, 15, 25 wt%) alloys and aims to understand the transformations by thermodynamic modelling. The high temperature bcc beta phase had undergone martensitic transformation in Ti-1Mo and Ti-7Mo alloys, resulting in acicular martensitic structure within large beta grains. X-ray diffraction (XRD) analysis confirmed the martensite to be hcp (alpha') and orthorhombic (alpha aEuro(3)) in Ti-1Mo and Ti-7Mo alloys respectively. Combined analysis of XRD and transmission electron microscopy (TEM) suggested the formation of fine plates of alpha aEuro(3), omega (omega) and bcc beta phases in Ti-15 and Ti-25 Mo alloys. Calculation of enthalpy of formation supported the stability of solid solution phase over the amorphous phase in the entire concentration range of Mo.