Omega phase transformation - morphologies and mechanisms

Various morphologies of the omega phase are observed in the a and the beta phases of group-IV elements and their alloys where formation of this phase is induced by various means, namely, quenching from the beta-phase field, isothermal annealing, under high static as well as dynamic pressure, and irradiation. The presence of various morphologies, such as ellipsoidal, cuboidal, granular or plate shapes, under different conditions is demonstrated. It is shown that various factors, including shuffling of atoms, shearing of atomic planes, long-range diffusion of solute atoms, change in volume during transformation, etc., influence the formation of different morphologies of the omega phase. The underlying mechanisms in each case are discussed. Ordered omega phases, which exist as equilibrium phases in various alloy systems, are shown as a case of superimposition of the concentration wave on the displacement wave.