The structure of orthorhombic (α″) martensitic phase of Ti–8Nb, Ti–12Nb, and Ti–16Nb alloys has been investigated using Rietveld refinement of x-ray diffraction data. The chemical analysis data have been used to determine the site occupancy of initial models. A limit of the Wyckoff positions has been proposed based on the symmetry of the Cmcm space group, which allows the movement of atoms without breaking the space group symmetry. This has also been incorporated in the initial models of experimental alloys, and accordingly Wyckoff positions have been refined. It has been observed that the atoms move along the Y-axis (parallel to the b-axis) and the movement of atoms increases with increase in Nb concentration. The formation of orthorhombic (α″) phase has been explained based on the movement of atoms along the Y-axis. This in turn breaks the hexagonal symmetry and forms an orthorhombic phase.

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The structure of the B2 phase has been investigated in Ti-26Al-20Mo and Ti-37.5Al-12.5Mo alloys using Rietveld refinement of x-ray diffraction data in homogenized conditions. Different initial structure models have been used for the refinement. The site occupancy of the B2 phase has been calculated and compared to those of earlier experimental and theoretical investigations.

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This paper deals with the effect of thermomechanical processing on microstructural evolution of three alloys, viz. Ti-8Nb, Ti-12Nb and Ti-16Nb. The alloys were hot rolled at 800°C and then subjected to various heat treatments. Samples from hot-rolled alloys were given solution-treatment in β and α + β phase fields, respectively followed by water quenching and furnace cooling. The solution-treated alloys were subsequently aged at different temperatures for 24 h. Phases evolved after various heat treatments were studied using X-ray diffractometer, optical, scanning and transmission electron microscopes. The alloy Ti-8Nb exhibits α and β phases while the alloys Ti-12Nb and Ti-16Nb show the presence of α ″, β and ω phases in the as-cast and hot-rolled conditions. The β solution treated and water quenched specimen of the alloy Ti-8Nb displays α″ phase while the alloys Ti-12Nb and Ti-16Nb exhibit α″, β and ω phases. The alloy Ti-8Nb shows the presence of α, β and ω phases while those of Ti-12Nb and Ti-16Nb display the presence of α, α ″, β and ω in α + β solution treated and water quenched condition. The observation of ω phase in solution treated condition depends on the cooling rate and the Nb content while in the aged specimens, it is governed by aging temperature as well as the Nb content.

The structure of the B2 phase has been investigated in Ti-25Al-25Mo alloy using Rietveld refinement of X-ray and neutron diffraction data in as-cast and solution-treated conditions. Different initial structure models have been used for the refinement. The site occupancy of the various chemical constituents in the B2 phase has been calculated and compared with earlier investigations. The relative merits of neutron diffraction over X-ray diffraction for structural refinement of the B2 phase in Ti-25Al-25Mo alloy have been demonstrated.

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