Quenched. omega. phase in a Ti-V alloy

A study has been made of the ..omega.. phase in a quenched Ti--23 at. % V alloy at room temperature using single-crystal x-ray-diffraction techniques. The ..omega.. phase appears as coherent particles with an average volume of approx.2500 unit cells. An observed slow time dependence of the diffraction patterns shows that the ..omega..-phase transformation continues to progress at room temperature until finally, 16 months after the quench, the alloy has almost completely transformed. The various ..omega..-phase peaks display significant differences in width, and so both peak widths and peak intensities are used as data to be matched by structural models. The five acceptable models found have three common characteristics: (1) The ..omega..-phase displacements are position dependent, not uniform, varying steeply from a large value at the particle center to zero at the boundary; (2) the displaced atoms have very anisotropic, anharmonic ''temperature factors''; and (3) the particle is shaped as a thick, rectangular platelet, with its plane parallel to )112) and its long dimension along (111), supporting a Hatt-Roberts model of the transformation. We suggest that position-dependent displacement distributions and anisotropic, anharmonic temperature factors should be a characteristic of all quenched, imperfect ..omega.. phases.