An atomic-crystallographic mechanism is proposed for the α —andgt;ω or transformation during irradiation. The mechanism is based on the propagation of a localized displacement wave of the [100]αclose-packed rows. From the phonon dispersion curves along (010)αthe displacement wave is decomposed into two transverse optical phonons and one transverse acoustic phonon. The proposed mechanism accounts for the ω morphology and can explain the presence of (2FORMULA PRESENTED0)αplanes of diffuse intensity in reciprocal space. The kinetic of the transformation was followed by in-situ length and electrical resistance measurements. An analytical model based on the proposed mechanism enabled us to fit the experimental curves. Finally the kinetic results were compared with those obtained previously for the transformation occurring under an applied static pressure. © 1999 Taylor andamp; Francis Group, LLC.