Abstract: Based on the experimental results that solute-depleted zone was observed in Cu-28Zn-4Al (mass fraction) at 523 K, △G is calculated as a positive according to the thermodynamic criteria for the spinodal decomposition of a ternary systems. So, the solute-depleted zone cannot be formed by spinodal decomposition. Dislocation density required by the formation of solute-depleted zone is estimated greater than 7.89×109 cm-2, according to the segregation of solute atoms around dislocations, which is not consistent with the practical situation for the alloy at 523 K. Associated with the internal friction experimental fact that internal friction peaks appear within the incubation for bainitic transformation in Cu-Zn-Al alloy, the equilibrium temperature, T0, is evaluated as 433 K for solute-depleted Cu-25Zn-3.4Al, which is lower than the experimental temperature 523 K. Thus, nucleation by shear mechanism is impossible in this circumstance. Therefore, it is concluded that, like bainite in steels and Ag-Cd, bainite in Cu-Zn-Al alloys nucleates by diffusional mechanism, just implied by the experimental existence of solute-depleted zone.

Key words: Bainitic transformation, Prebainitic phenomenon, Solute-depleted zone...