On the generation of thixotropic structures during melting of Mg-9%Al-1%Zn alloy

The microstructural evolution of Mg-9%Al-1%Zn alloy in the form of mechanically comminuted chips and rapidly solidified granules under the sole influence of external heat was investigated. The mechanisms governing phase and morphological transformations in solid and semisolid states were assessed and verified by a comparison with the behaviour of both conventionally cast and cold-deformed ingots, subjected to similar thermal exposures. It was revealed that the cold deformation introduced during the mechanical fragmentation of chips and chemical microsegregation inherited from a rapid solidification by granules, represent key features which control the generation of equiaxed structures during the very initial stages of melting. Ostwald ripening and coalescence were operative in the semisolid mixture within the entire range of the liquid fraction. As a result, an increase in a volume fraction of the liquid was primarily accompanied by the reduction in the number of solid particles, while the reduction in the particle size was significantly lower than anticipated from a change in the solid/liquid ratio. Some implications for engineering the microstructure during semisolid processing are discussed.