Characterization of the formation of α₁plates from the β₃phase in a Cu- Zn- Au alloy

Abstract

The crystal structures and ordering structures of α₁ bainite plates and β₃ parent phase have been studied by means of electron microscopy in a Cu-40Zn-12Au (at. pct) alloy. The ordering transitions take place in the high-temperature β phase, and a stable L21 structure is reached even after direct quenching. The next-nearest-neighbor antiphase domains (NNNAPDs) are about 40 nm in size and have an island morphology surrounded by some nearly disordered regions. Wide isolated a, plates are present in 473 K aged microstructures, but at lower aging temperatures, thinner bainite plates tend to form in some coordinate modes. Reoriented second-stage thickening and stress-induced local thickening of thin a, plates have also been observed. Microanalysis of solute concentrations in thin bainite plates and in the matrix has shown the involvement of solute diffusion not only in the thickening process but also in the nucleating of the plate. Based on the results, an ordering facilitated shear mechanism is proposed for the a, bainite transformation, which can successfully explain its characteristics.