Rapid Solidification Of Eutectic And Monotectic Alloy Under Space Simulation Condition

The rapid solidification of eutectic and monotectic alloys in Co-Ge, Al-Ge, Fe-Sn and Cu-Pb systems has been investigated under space simulation condition in drop tube. The main results are obtained as follows.The rapid eutectic and dendritic growth and the microstructural evolution in Co-29.7% Ge eutectic and Co-33% Ge hypereutectic alloys have been revealed. With the increase of undercooling, the morphologies of Co-29.7% Ge eutectic alloy display 'lamellar eutectic →anomalous eutectic' transformation. The primary β-Co₅Ge₃ phase in Co-33% Ge hypereutectic alloy transfers from columnar dendrite into equiaxed dendrite. When ΔT<172 K, the growth of the primary β-Co₅Ge₃ phase is controlled by solute diffusion, whereas the thermal diffusion becomes dominant when ΔT>172 K. The calculated coupled zone for Co-Ge eutectic alloy covers the composition and temperature ranges of 25.7³1.4% Ge and 1381¹158 K respectively.The relationships between the microstructure evolution and undercooling in Al-45% Ge hypoeutectic and Al-51.6% Ge eutectic alloys have been analyzed in details. The eutectic growth in Al-51.6%wt Ge alloy has shown a transition from lamellar eutectic to anomalous eutectic when undercooling becomes larger. The critical undercooling to form anomalous eutectic is 101 K, which is in agreement with the lower undercooling limit of eutectic coupled zone. The primary (Al) phase in Al-45% Ge hypoeutectic undergoes a microstructural morphology transition from columnar dendrite to equiaxed dendrite with the increase of undercooling. The growth of the primary (Al) phase is always controlled by solute diffusion. The composition and temperature ranges of the coupled zone for Al-Ge eutectic alloy are 25.7³1.4% Ge and 1381¹158 K respectively.The morphology transition and phase separation in Fe-Sn alloy during rapid solidification have been studied systematically. When undercooling is smaller, the microstructure for Fe-48.8% Sn monotectic alloy is characterized by coupled growth structure of (Fe) and (Sn) phases and/or spherical (Sn) particles in the matrix. When undercooling becomes larger, the microstructure shows only spherical Sn-rich particles homogenously distributed in the matrix. The solidification behavior of Fe-15.6% Sn and Fe-40% Sn hypomonotectic alloys is similar to hypoeutectic alloy, in which the dendrite growth is dominant and the dendrite becomes fine with the increase of undercooling. For Fe-58% Sn and Fe-85.1% Sn hypermonotectic alloys, a-Fe dendrite and some spherical particles composed of coupled growth structure are distributed in the Sn-rich matrix. The a-Fe dendrite becomes remelted dendrite when undercooling increases. Two layer and triple layer core-shell microstructures form in Fe-68% Sn hypermonotectic alloy whose temperature is the highest point of immiscibility gap.The rapid solidification microstructures of Cu-10% Pb hypomonotectic alloy, Cu-37.4% Pb monotectic alloy and Cu-64% Pb hypermonotectic alloy have been investigated. Theexperimental results show that the (Cu) phase always grows in dendrite manner, Pb-rich phase forms within the interdendritic region. The (Cu) dendrite transfers from coarse dendrite to equiaxed dendrite when undercooling increases. For Cu-37.4% Pb monotectic alloy, the numbers of single monotectic cell increase whereas the multiple monotectic cell increase at first and decrease later when undercooling becomes larger. The variation trend is from multiple monotectic cells to single monotectic cell. This indicates that single monotectic cell is the product of rapid solidification. Two layer and triple layer core-shell microstructures form in Cu-64% Pb hypermonotectic alloy.The rapid solidification microstructures for two eutectic alloys transform from lamellar to anomalous eutectic with the increase of undercooling. This transformation is a result of high undercooling by containerless state. The influence of the low gravity level is less conspicuous as far as microstructural evolution is concerned. The eutectic-like fibrous microstructure forms in two monotectic alloys. This phenomenon shows that the formation mechanism of fibrous microstructure is similar to eutectic growth, in which the two phases grow cooperatively. Two layer and triple layer core-shell microstructures appear in two hypermonotectic alloys whose liquidus temperature is the highest point of immiscibility gap. The Marangoni migration during liquid phase separation is the main reason for this kind of structure.