MICROSTRUCTURAL EVOLUTION AND TRANSFORMATION PATHWAYS IN THE NEAR MONOTECTIC ZN RICH ZN-BI ALLOYS DURING RAPID SOLIDIFICATION

The possible pathways for microstructural development under nonequilib rium condition by the rapid solidification of alloys containing liquid miscibility gap have been studied using Zn-Bi binary alloys as a mode l system. The primary aim is to explore the possibility of the interme diate formation of a thermodynamically unstable solid solution as a pr ecursor phase which can spontaneously decompose to yield nanodispersio ns. It is shown that the information of crystallographic shape of the nanosized dispersions obtained by transmission electron microscopy and the derived point groups can be utilised to arrive at a definite conc lusion of this possibility. Our results establish the formation of suc h a solid solution owing to the kinetic process of nonequilibrium trap ping in spite of a very strong clustering tendency. The latter leads t o the spontaneous decomposition of the solid solution during quenching to yield nanodispersions.