The omega phase in pure Zr formed a~ 300 K under high pressure of 6.0GPa was examined using high resolution dark-field electron microscopy and selected area diffraction. Pressurized Zr has a two phase structure consisting of small elongated ellipsoidal (1) particles of d : 40-5OA and I = 100-120 A which are precipitated in an a matrix. There is a strong resemblance between morphologies of omega originated in either 01 or @. The principally new orientation relationship between z and tr) phase is established There-are three possible orientations (v~ants) of w in an a matrix. The, basal (OUOI) pkme of a is par&e! to {IZlO), and_tl~e [UOtH),direction coincides with everyone ofthc three dimctions of a: [2110&t), [I 12O]&a) and [ 1210]&) with deviations _ *S-7". The atomistic model explaining the data obtained is proposed. The a + ru transformation is regardqd as a displacivc type. According to the model, every three ncighbouri_ng close-packed atomic rows [lZlO], are displaced in a basal&ne (OOUl), on a distance a,/4 along (1210], direction and the next three along the opposite direction (1210],
and so on. Such displacements accompanied by small shut&s form : IIIO;,, planes. The cominon traits in a 4 w and #J -+ eu tr~ormatio~ are also discussed together with possible mechanism of the appearance of linear defects along ciosepacked rows necessary for displacivc transformation in Zr (Ti) and its alloys.