Al_(70)Bi_(11)Sn_(19)合金颗粒的核壳组织

THE CORE-SHELL STRUCTURE OF Al_(70)Bi_(11)Sn_(19) IMMISCIBLE ALLOY PARTICLES

利用Al_(70)Bi_(11)Sn_(19)难混溶合金,基于相分离制备了Al/Sn-Bi核壳型颗粒.利用SEM,EDS和DSC研究了颗粒的组织形貌、成分和相变行为,并结合温度场模拟,探讨了核壳形貌的形成机理.结果表明,合金颗粒以具有良好导电导热性的铝合金为内核,以Sn-Bi亚共晶无铅合金为壳层,表现出两阶段熔化特征;随粒径从0.5 mm增大到0.9 mm,核壳形貌从多核月偏食型向单核同心型及月偏食型转变,这是由于表面偏析、Marangoni和Stokes运动、Ostwald熟化以及冷速等多种因素相互竞争的综合作用.

Immiscible alloys are well suited as functional materials, such as bearings, electrical contacts, switches and superconductors, etc. They usually suffer from heavy segregation under ordinary casting, which is resulted from the decomposition within the miscibility gap of a homogeneous liquid into two immiscible liquids generally with distinct density difference. But this characteristic provides an opportunity to in situ fabricate composites with core-shell morphology. In this study, A1/Sn-Bi core-shelled particles have been successfully prepared by phase separation of A170BinSn19 alloy. The morphology, microstructure, composition and phase transformation of the core-shelled particles were investigated by means of SEM, EDS and DSC. It reveals that the particle comprises an Al-rich core with a Sn-Bi hypoeutectic shell, displaying a two-stage melting characteristic. The morphology of particles varies with size. With increasing the particle size from 0.5 mm to 0.9 mm, the core-shell morphology turns from a crescent multi-core type into concentric or eccentric single-core types. Based on the simulation of temperature field of A170Bi11Sn19 alloy droplets during solidification, the formation mechanism of the core-shell morphology has been discussed in detail, which is attributed to an outcome of the competition among the surface segregation, Marangoni and Stokes motions, Ostwald ripening and cooling rate.