Crystallography and a model of the α → ω phase transformation in zirconium

The omega phase in pure Zr formed at 300 K under high pressure of 6.0 G Pa 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 ω particles of d ∼- 40-50 A ̊ and l = 100-120 A ̊ which are precipitated in an α matrix. There is a strong resemblance between morphologies of omega originated in either α or β. The principally new orientation relationship between α and ω phase is established. There are three possible orientations (variants) of ω in an α matrix. The basal (0001) plane of α is parallel to {12̄10}_∞ and the [0001]_∞ direction coincides with everyone of the three directions of α: [ 2 ̄110_∞(ω₁, [ 1 ̄ 1 ̄20]_∞(ω₂) and [ 1 ̄2 1 ̄0]_∞(ω₃) with deviations ~ ±5-7°. The atomistic model explaining the data obtained is proposed. The α → ω transformation is regarded as a displacive type. According to the model, every three neighbouring close-packed atomic rows [12̄10]_x are displaced in a basal plane (0001)_x on a distance a_x 4 along [12̄10]_x direction and the next three along the opposite direction [1̄21̄0]_x and so on. Such displacements accompanied by small shuffles form {12̄10}_∞ planes. The common traits in α → ω and β → ω transformations are also discussed together with possible mechanism of the appearance of linear defects along closepacked rows necessary for displacive transformations in Zr (Ti) and its alloys.