The omega phase transformation in titanium alloys as an example of displacement controlled reactions

Abstract

The omega phase is shown experimentally to be formed by cooling certain metastable β-Ti alloys to cryogenic temperatures. The transformation is completely reversible and is characterized by a small temperature hysteresis. The diffusionless hexagonal ω phase so formed is of the same composition as the parent b.c.c. β phase, and the gradual progression from diffuse streaking in the diffraction patterns of the β phase to the sharp ω reflections is interpreted in terms of a displacive type reaction. Computer generated plots of the elastic energy of harmonic displacement waves in the b.c.c. lattice are shown to reproduce many of the features of the diffuse intensity in the electron diffraction patterns. The complex quasi-circular streaks observed in some zone patterns are shown to result from the planar intersection of spherical surfaces of intensity in reciprocal space. It is suggested that the progressive transition from circular to rectilinear streaks correspond to a decreased coupling between {111} displacement modes responsible for the diffuse intensity. At sufficiently low temperatures, the diffuse streaks, which lie close to traces of {111} planes, contain sharp intensity peaks at positions 1 3 〈112〉, characteristic of the ω phase.