Non-equilibrium primary crystallisation in silver-germanium alloy under microgravity conditions

The influence of convection on the morphology of primary dendrites and on t he distribution of germanium in primary crystals of hypoeutectic Ag-Ge allo y was studied. The experiment in microgravity provided conditions for the s uppression of melt flow. Both the morphology of the primary dendrites and t heir chemical composition profiles were found to vary considerably dependin g on the intensity of convection. Primary crystals in specimens solidified in the spare laboratory exhibited strong supersaturation with germanium. Th is supersaturation is associated with a decrease of solute in the eutectic, The overall segregation of germanium between crystals and the melt during the growth of primarily dendrites is strongly promoted by convection. The i nfluence of convection on the non-equilibrium primary crystallisation, part icularly on the solute trapping process, the occurrence of which is likely in the initial stage of primary crystalisation (solute rich core), is discu ssed. A higher solute gradient close to the core in the space sample implie s a higher melt stability below the liquidus when convection is suppressed. MST/4234.