Solute redistributions associated with nucleation and growth of α 1 plates in Cu–Zn and Au–Cu–Zn alloys

The formation of α₁ plates during isothermal aging of quenched, β′ phase, non-ferrous alloys has been taken to occur by a combined shear and diffusion regulated process. Cu–Zn alloy specimens (∼40 at.-%Zn) containing additions of Au up to ∼9 at.-% have been quenched to retain the β′ phase (B2) and isothermally aged in the temperature range 523–973 K to produce α₁ plates and α rods. These precipitates have been examined using optical and transmission electron microscopy, and their compositions studied using high spatial resolution, STEM–EDS X-ray microanalysis on thin foil samples. The initially formed α₁ plates have crystallographic characteristics similar to those of the martensitic product and are in agreement with the predictions of the phenomenological theory for martensite formation. The solute distributions between the parent β′ phase and the product α₁ plates have been measured and these are discussed in relation to the mechanism for growth of the α₁ plates.