THE OMEGA PHASE TRANSFORMATION

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Abstract
Different theoretical models for the -, o phase transformation in Ti, Zr and Hf alloys are reexamined and their predictions compared to experimental evidence. Although the fundamental reasons for the presence of the transformation are not fully understood at the present time, most of the features of the transition can be described quite accurately by phenomenological models. A brief discussion is presented of a recent Cluster Variation calculation which uses the bcc primitive unit cell as the basic configurational cluster. Such a calculation shows an increase in the concentration of activated complexes for diffusion as the P -t o transition is approached, thus explaining the enhancement in self-diffusion at low temperatures observed in Ti, Zr and Hf. The diffusion model based on the presence of o-like fluctuations in the 8-phase is, in addition, in excellent quantitative agreement with rather puzzling observations in P-stabilized alloys of the Group IV B transition metals.

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