Nucleation of athermal omega phase in Cu–Zn system

Nucleation of the athermal omega (ω) phase transformation in the body-centered cubic parent phase is studied by evaluating the reciprocal free energy which includes the properties of coherent strain energy and the bulk free energy change. The droplet model of the omega formation is combined with the Landau-type of the free energy expression of phase transformation. The metastable ω cluster having a two dimensionally expanded dual phase structure is suggested to form in the heterophase fluctuation. Quantitative calculation of free energy has identified the critical transformation temperature T0 = 300 K in β-brass that is defined as the temperature at which the free energy of the metastable phase is the same as the free energy of the parent phase. The continuous transformation from the metastable ω cluster to a stable ω phase is shown to occur at the transformation temperature Tω that is well below T0.