MICROSTRUCTURES AND PHASE SELECTION IN RAPIDLY SOLIDIFIED ZN-MG ALLOYS

The Zn-Mg system has potential glass-forming ability, and therefore st udies were made of rapidly solidified zinc-based Zn-Mg alloys containi ng up to 6 wt % Mg. These alloys exhibited interesting eutectic phase selections and structural transitions across the ribbon thickness whic h are represented on a microstructure selection diagram for rapid soli dification conditions. Although rapid solidification is known in many cases to produce metastable phases, in this case the equilibrium eutec tic mixtures of Zn-Mg2Zn11 are observed after rapid solidification, wh ereas the metastable eutectic mixture Zn-MgZn2 forms under normal soli dification conditions. However, in the melt-spun Zn-Mg alloy which is exactly at eutectic composition, th ree different structures are obser ved across the ribbon thickness. These three structures do not exist s imultaneously in the same region, but structural transitions occur as the thickness increases from the wheel side to the free side. Eutectic and hypereutectic alloys show a tendency to form a metallic glass. In these alloys a critical growth velocity exists beyond which eutectic solidification is not possible, suggesting a possible transition from eutectic solidification to amorphous phase formation. The eutectic pha se selection and the extent to which a specific microstructure is pres ent depends on the variation in growth rate and solid-liquid interface stability during rapid solidification.