Formation Mechanism Of Core-shell Structured Immiscible Alloys

Homogeneous immiscible alloys decompose into two immiscible liquids due tothe liquid-liquid decomposition reaction when being cooled into the immiscible zone,and form core-shell structures under certain conditions, which can be applied asmetallic composites in various industrial fields. A substantial research effort has beenundertaken to investigate the formation mechanism of core-shell structuredimmiscible alloys. However, the formation mechanism has not been clarified. Someeffects on the formation mechanism have been studied in this thesis. The followingconclusions have been made.(1) Core-shell immiscible alloys reported have usually been coated by a lay ofone phase with low surface energy, which restricted the potential application fields ofimmiscible alloys. Hypomonotectic and monotectic40Sn60Cu-Bi alloys have beenprepared by a casting method in this work. A core-shell structure which consists of ashell of CuSn-rich phase and a core of Sn-rich phase has been obtained. MonotecticCu-Pb alloy has been casted as well. The morphology of monotectic Cu-Pb alloy wasa network structure formed by Cu dendrites and Pb was distributed at the grainboundary or in the interdendritic regions. The reason for structure difference betweenmonotectic40Sn60Cu-Bi (wt.%) and Cu-Pb alloy was considered to be the differenceof wetting property between the liquid and solid produced by monotectic reaction inthe two alloys.(2) The effect of composition on the formation of40Sn60Cu-Bi hypermonotecticalloys has been studied by the casting method. With the content of Bi increasing, athree-layer core-shell structure, a two-layer core-shell structure and a dispersedstructure with CuSn-rich phase have been observed in40Sn60Cu-45%Bi,40Sn60Cu-62.5%Bi and40Sn60Cu-72.5%Bi alloys, respectively. The periphery of allthe alloys is composed of Bi-rich phase.(3) The effect of cooling rate on the formation of core-shell structure in40Sn60Cu-62.5%Bi alloy has been discussed. The Marangoni moving velocity VmofCuSn-rich droplets and the advancing velocity Viof Bi-rich phase solidification interface have been calculated. For three samples with different radi, the criticaldroplet radius has been got when Vm≥Vi. The least critical droplet radius was gainedand a largest core was obtained as well in the sample with a diameter of5mm.(4) The effect of Ce addition on the formation of core-shell structure in40Sn60Cu-60.0%Bi alloy has been investigated. The results show that the addition ofCe improved the temperature coefficient of liquid-liquid interfacial tension, andtherefore sped up the Marangoni velocity of CuSn-rich droplets. The core fraction hasbeen enlarged by20%with the addition of Ce.(5) Phase separation of Fe-(43±10x) at.%Cu (x=0、1、2、3) alloys underaerodynamic levitation has been studied. Structures of Fe-Cu alloys were determinedby the movement of minor-phase droplets and the rotation direction of alloy spheresin the state of levitated. Three typical kinds of structures have been obtained: adispersed structure, a core-shell structure and a parallel-layer structure. A core-shellstructure with a shell of Fe-rich phase and a core of Cu-rich phase formed inFe-23%Cu alloy, which can be used as metallic phase change capsules.