Crystal structure and bonding characteristics of transformation products of bcc β in Ti-Mo alloys

Abstract

Abstract Rietveld analysis of X-ray diffraction patterns of α′, α″ and β phases of four alloys of Ti-Mo has been carried out to refine their crystal structures and understand their nature of bonding. The addition of molybdenum to titanium is found to shift the stability of crystal structure from hcp to orthorhombic to bcc, as expected. The unit cell dimensions and the atomic positions of α′, α″ and β phases were evaluated, with excellent reliability indices. The reduction in unit cell dimensions and its volume, bond length and co-ordination number with addition of molybdenum is consistent with the reduced size of solute atom. The orthorhombic α″ phase, a transition phase with a minor distortion of hcp α′, is found to be closer to α′ than β phase, based on the orthorhombicity (b/a) and the volume of equivalent orthorhombic unit cells of the phases and the atomic shuffle y in 4c Wyckoff positions along b axis of α″ phase. The Fourier electron density contour maps suggest that the bonding in all phases is predominantly metallic. However, addition of molybdenum delocalizes the spatial distribution of charge density, suggesting an enhancement of metallic character. Anisotropy in charge density was observed along 〈011〉 and 〈010〉 in (100), 〈101〉 and 〈001〉 in (010) and 〈110〉 and 〈010〉 in (001) plane. This offers direct evidence for the origin of anisotropy in the values of elastic constants, reported in literature.