Directional Solidification of Sn-Cu 6 Sn 5 In Situ Composites

Abstract

The Sn-Cu system presents an important interest from academic and technological point of view because it is part of the family ofalloys proposed as lead-free solder alloys for electronic components and also due to the mechanisms involved during the growth ofthe different phases. Sn-Cu system has two intermetallic phases, i.e., ε-Cu3Sn and η-Cu6Sn5, and η can be used as the negative(anode) electrode in Li-ion batteries, alone or as part of (Co,Ni)xCu6−xSn5-type composites. Obtaining this η phase from liquidwith the appropriate chemical composition is a very difficult task because it has a formation temperature lower than liquidus forsuch a composition. In this way, the η phase appears as a consequence of a solid-solid transformation from the ε phase However, itis possible to find the η phase as the primary or secondary phase after a eutectic reaction for lower concentrations of Cu. On theother side, the Cu6Sn 5 phase shows a hexagonal to monoclinic solid-solid transformation around 187°C, which could affect themechanical system stability when it is used as solder. In this work, directional solidification at different growth velocities ofhypereutectic Sn-Cu samples was performed. The resultant microstructure varies with the growth velocity, but it is formed for afibber-like primary phase Cu6Sn5 which is projected towards the liquid phase. Behind this region, these fibbers are rounded by atwo-phase Sn-Cu6Sn5 structure. In this way, three zones could to be defined in the sample during the directional growth: (i) anentirely solid two-phase region, formed by η rounded by β(Sn) + η eutectic-like structure, (ii) a two-phase solid (η) + liquid, and(iii) the remnant liquid in front of the interface.