Impact of permanent magnet stirring on dendrite growth and elastic properties of Sn–Bi alloys revealed by pulse echo overlap method

Abstract

Studying and understanding the dendritic growth process is a challenging topic related to liquid-solid phase transition, as it helps to predict the final microstructure controlling the solder properties. In a specific case of the design of Sn-Bi and Sn-Bi-Cu alloys, the solidification microstructures and corresponding electrical and elastic properties were studied with and without permanent magnetic stirring (PMS), as their influence on the growth morphology of dendrites is not yet fully assessed to date. We use pulse echo overlap (PEO) method for measuring the polycrystalline bulk modulus K, Young's modulus E, shear modulus G, Poisson ratio υ and hardness H. The PMS-driven flow caused a disruption of the columnar β-Sn dendrites and columnar-to-equiaxed transition (CET). Such behavior is believed to evolve from dendrite fragmentation, arises through complex hyper-branched morphologies at the origin of Lorentz force and Seebeck effect that acting on the melt. Both the hardness and elastic modulus are increased as the Poisson's ratio decreased. Moreover, the Pugh ratio clarified the ductility behavior of the alloy samples, while Poisson's ratio and electrical resistivity display slight decrease in the ionic contribution with applying PMS and/or Cu content. These results open new ways to predict the final microstructure controlling the dendritic growth in metallic alloys.