Ordered omega structures in Ti and Zr based intermetallics

With advances made in the understanding of phase transformations occurring in various intermetallic phases it is becoming clear that the ω related phases have much wider temperature and compositional phase fields and have frequent occurrence than the initial assumptions where it was assumed that the occurrence of this phase is confined only to Group 4 elements. The ω -phase, which is an equilibrium phase in Group 4 metals at high pressure, forms in several alloys and intermetallics as a stable/metastable phase. The β → ω phase transformation is essentially an instability driven phase transformation where a systematic collapse of atomic planes is induced in the lattice either by pressure, temperature or by irradiation to initiate the phase transformation. It is because of this reason the progress of the phase transformation is expressed in terms of amplification of a sinosidal displacement wave. The formation of ordered ω phases can be viewed in terms of a superimposition of displacive and replacive ordering transformations where along with the lattice collapse of the β phase, a replacive transformation involving diffusion or random jumps of atoms is required for decorating different sub-lattice sites by different atomic species. The extent of overlapping of these transformations in the formation of ordered ω phases is not completely understood. However, attempts have been made to explore this aspect by examining the sequential formation of several intermediate stable/ metastable phases. In the present talk the formation of various ω-phases will be discussed. Specific examples from various intermetallics, particularly Zr and Ti based intermetallics, will be taken to show that these phases can be generated by the superimposition of replacive and displacive ordering of atoms in the bcc lattice. The role of vacancies in the formation of few of the phases will be discussed. It will also be shown that based on the symmetry changes involved in the formation of these phases, it is possible to establish a structural relationship between various phases and using the crystallographic information a symmetry tree can be constructed which could be utilized in predicting the sequence of phase transformation. (author)