Microstructural characterization of transformation products of bcc β in Ti-15 Mo alloy

The paper presents the Rietveld analysis of XRD patterns using synchrotron and Cu-Kα sources and phase contrast microscopy of product phases of high temperature bcc β phase in Ti-15wt%Mo alloy, after quenching the β phase at different cooling rates and isothermal exposure at 748 K. XRD studies confirmed the unambiguous presence of ω in bcc β matrix, quenched at different rates and co-existence of hcp α, bcc β and hexagonal ω, on isothermal exposure. The unit cell dimensions, electron diffraction patterns, morphology of ω and the degree of displacement of atoms, suggest that ωiso has reached the ideal structure, while ωath is far away from ideality. A careful observation of correlations between certain interatomic distances in ω, α and β, measured based on XRD studies, suggests that bonding in the (00.2)ω plane is stronger than either (00.1)ω or (−12.0)ω, which is also confirmed using charge distribution maps. Present studies provided direct evidence for the well-known “graphitic-like” nature of bonding in (00.2)ω plane. The strengthening of the alloy in the presence of ω, confirmed by the 3-D distribution of strain in Ti-15Mo alloys, is attributed to the coherency strains of β lattice, demonstrated using atomic column contrast images.