BETA-3-ALPHA-1 BAINITIC TRANSFORMATION IN A CU-ZN-AL ALLOY

The antiphase domain (APD) structure and the chemical composition of alpha-1 bainite plates which form in a Cu-26.7 mass% Zn-4.0 mass% Al alloy during isothermal aging at 423 K have been studied by TEM dark field imaging and thin foil energy dispersive X-ray (EDX) microanalysis, respectively. At early stages of the transformation, both nearest-neighbor (NN) and next-nearest-neighbor (NNN) APD structures of the beta-3 parent phase are inherited by the alpha-1 plate having an M18R structure. The EDX microanalysis indicates that a composition difference exists between the alpha-1 plate and the beta-3 parent phase. With increasing aging time, the alpha-1 plate thickens in a manner that only the NN APD structure remains inherited in the thickened layers which become NNN disordered while both NN and NNN order and their APD structures remain inherited in the plate tip. In addition, solute rich composition profiles are present in the parent phase adjacent to the interface. Local solute depletion is also observed at dislocations in the parent phase. It is suggested that solute depleted defects act as nucleation sites for the formation of the alpha-1 bainite plate which lengthens through a shear mechanism. Subsequent plate thickening is then controlled by a diffusional process. This model is consistent with the transformation kinetics, crystallography and microchemistry of the alpha-1 plates.