Liquid Phase Separation And Core-Shell Structure Of Al-Bi-Sn-(Cu) Immiscible Alloys

Immiscible alloys are well suited as functional materials, such as bearings, electrical contacts,switches, and superconducts, etc. They usually suffer from heavy segregation under ordinarycasting, which is resulted from the decomposition within the miscibility gap of a homogeneousliquid into two immiscible liquids generally with distinct density difference. But this characteristicprovides an opportunity to in-situ fabricate composites with core-shell morphology. If the Bicontent in the shell of Al/Sn-Bi core-shelled particles is high, this will result in the decrease ofelectrical and thermal conductivities, and grains coarsening. In this case, the mechanical propertyand welding performance decrease. Besides, the Bi is kind of costly and has limited resources.Therefore, it is very necessary to investigate the formation of Al-Bi-Sn immiscible alloys withhigher Al content and lower Bi content within the the miscibility gap. The Cu addition is takeninto consideration due to the Sn-Bi microstructure coarsening. In this paper, Al/Sn-Bi-(Cu)core-shelled particles have been successfully prepared, via liquid phase separation. Themorphology, composition, microstructure, and phase transformation of the core-shelled particlesare investigated, and the formation mechanism of the core-shell morphology has been discussed indetail.First, the Al/Sn-Bi core-shell structures with higher Al content and low Bi content aresuccessfully produced, via liquid phase separation. The core-shell microstructure is clearlyobserved in Al70Bi11Sn19and Al75Bi9Sn16immiscible alloys, combining the high melting point,superior electrical and thermal conductivities of the Al-rich core with a low-temperature Sn-Bilead-free solder of the shell. With the increasing superheat or the decreasing free fall distance ofdroplets, the core-shell Al75Bi9Sn16morphology turns from a single-core type into a crescent multi-core type. With increasing the particle size from0.5mm to0.9mm, the core-shellmorphology of Al70Bi11Sn19alloy turns from a crescent multi-core type into concentric oreccentric single-core types. Based on the simulation of temperature field of Al-Bi-Sn alloydroplets during solidification, the formation mechanism of the core-shell morphology has beendiscussed in detail, which is attributed to an outcome of the competition among the surfacesegregation, Marangoni and Stokes motions, Ostwald ripening and cooling rate.Second, the core-shell morphology of (Al34.5Bi65.5)90Cu10alloy turns from a single-corecrescent type into eccentric multi-core types with the increasing superheat. With the Cu addition toAl75Bi9Sn16alloy, the (Al75Bi9Sn16)95Cu5core-shell structures are produced, combining the Al-richcore and Sn-Bi hypoeutectic alloy. The morphology, microstructure, composition and phasetransformation of the core-shelled particles were investigated by means of scanning electronicmicroscopy, energy dispersive spectrometer and differential scanning calorimetry. It reveals thatthe particle comprises a shell of Sn-Bi alloy and a core of Al alloy with better electrical andthermal properties. Cu mainly exists in the Al-rich core with the Cu addition. The size of Bi phasein the shell of Cu containing-alloy particles is about only one third of that in the alloy without Cu,showing the significantly refining effect of Cu. The temperatures of eutectic and monotecticreactions decreased while the temperature of liquid phase separation increased with the Cuaddition.