Omega phase transformation: a transformation due to instability in the BCC phase

The omega phase was initially thought to be a phase encountered only in group IV elements (Ti, Zr, Hf) and its based alloys. Recent results, however, have shown that the formation of this phase is not confined to group IV but it is an in built transformation into any metallic system containing body centre cubic crystal structure. Many recent finding about the formation of the ω phase on one hand have settled the age old issues but on the other hand raised new question about the phase transformation; in particular, the discovery of the ordered derivatives of the ω phase, which could be described in terms of a superimposition of replacive and displacive ordering waves in the β phase. The present talk deals with several such issues related with this phase transformation. Several controversies, possible mechanisms, pretransition effects and the possible applications related to the formation of this phase transformation will be addressed. It will also be shown that the symmetry changes associated with the transformation can predict the sequence of ω related transformations. (author)