The formation mechanism of plate in beta Cu-Zn and Cu-Zn-Al alloys

Abstract

The initial formation and the subsequent early and late stage growth of the α₁ plates in Cu-Zn and Cu-Zn-AI alloys are studied by optical microscopy, transmission electron microscopy (TEM), and high-temperature TEMin situ. Single-phase α₁ plates (initial α₁ plates) can be obtained only at the early stage of isothermal transformation. Striations corresponding to stacking faults are observed during the nucleation of initial α₁ plates. A strong and discontinuous strain-stress field exists in front of the tips of initial α₁ plates and causes inducing nucleation and crossing growth. The displacement between stacking fault faces and the thickening along the direction of streaking contrast show that the growth of initial α₁ plate has the nature of shear transformation, and the shear direction along the streaking contrast corresponds to stacking faults. The superledges at the broad faces of the α₁ plates are also observed, but they move only along the direction of streaking contrast during plates thickening. The “overannealing” of initial α₁ plates during longer transformation time results in the disappearing of stacking faults, and corresponding initial α₁ plates are partially or completely “degenerated” into equilibrium α phase (here designated as late α₁ (α) plates). The thickening kinetics of late α₁ (α) plates seems to be compatible with the diffusional ledge mechanism, but the lateral movement of superledges along the broad faces of late α₁ (α) plates is not observed. According to the facts mentioned previously, a possible atomic mechanism for the formation of the α₁ plates is suggested.