Analytical Electron Microscope Study on the Bainitic Transformation in β Phase Cu–Zn–Au Alloys

The bainitic transformation in β phase Cu–Zn–Au alloys has been investigated by means of analytical electron microscopy from crystal structural, morphological and compositional points of view. The crystal structure of bainite plates formed in Cu-40Zn-4Au (the 4% Au alloy) and Cu-40Zn-9Au (the 9% Au alloy) (at.%) alloys subjected to isothermal aging at 473 K for various periods was different, depending on their growth stage: The bainite plates thinner about 50 nm in the 4% Au alloy possessed a 9R type ordered structure similar to that of the martensites in subzerocooled β phase Cu–Zn binary alloy, but those plates thicker than about 100 nm lost the atomic order. On the other hand, the plates about 100 nm thick in the 9% Au alloy had a M18R type ordered structure, inheriting the L21 type order of the matrix, and still had an ordered structure corresponding at least to the nearest neighbor (nn) order of the matrix even when they grew about 300 nm thick. Even a very thin plate only about 10 nm thick observed in the 4% Au alloy was found to have a composition certainly different from that of the surrounding matrix. Furthermore, the matrix regions ahead not only of plate side but also of plate tip of bainite plates at early stages of formation in the 9% Au alloy was found to tend to be enriched a little with respect to Zn concentration. However, meaningful variations in composition were not found inside plates in the present study, contrary to the previous one by Doig and Flewitt. Base on these results obtained, a tentative model is proposed for the formation mechanism of bainite plates in the substitutional β phase Cu–Zn based alloys.