Dendritic morphology observed in the solid-state precipitation in binary alloys

The precipitation of γ ₂ phase in Cu-Al β-phase alloys has been observed to occur in the dendritic morphology. Such morphology is rarely observed in the solid-state transformations. Earlier it was reported that the γ precipitates were formed in the dendritic shape when Cu-Zn β-phase alloys were cooled from high temperature. The characteristics of these two alloy systems have been examined to find the factors promoting the dendritic morphology in the solid-state transformations. Rapid bulk diffusion and fast interfacial reaction kinetics would promote such morphology. The kinetics of atom attachment to the growing interface is expected to be fast when crystallographic similarities exist between the parent phase and the precipitate. We have predicted the dendritic morphology in the solid-state precipitation in many binary alloy systems simply based on such crystallographic similarities. These alloys include, in addition to Cu-Al and Cu-Zn, the β-phase alloys in Ag-Li, Ag-Zn, Cu-Ga, Au-Zn, and Ni-Zn systems, γ-phase alloys in Cu-Sn and Ag-Cd systems, and δ-phase alloys in Au-Cd system. Of these, the alloys in Ag-Zn, Ni-Zn, Ag-Cd, and Cu-Sn systems were prepared and it was indeed found that the precipitates formed in the dendritic shape.