Influence of Microstructure Length Scale on the Tensile Properties and Superplasticity of Cu-Doped Sn-34Bi TIM Alloy

Sn-Bi alloys are candidates for use as thermal interface materials, TIM. Such materials are often used to conduct heat away from a temperature sensitive device. Sn-Bi alloys are considered competitive in cost and cost volatility, being particularly interesting with doping of ternary elements. Among various attended pre-requisites, high manufacturability when utilizing rolling or pressing is a particular benefit. To take advantage of this property, Sn-Bi alloys should be designed to ensure high ductility, maximizing manufacturability. Suitable microstructures capable of improving the alloy's plasticity are very desirable. The present investigation aims to evaluate the effects of starting as-cast microstructures on the strength and ductility of the Sn-34 wt.%Bi-0.1 wt.%Cu alloy. Specimens with very different length scales of the dendritic array were subjected to tensile tests at three temperatures: − 50°C, 25°C and 60°C. It was demonstrated that the samples with more refined microstructure are related to slightly higher tensile properties at room temperature if compared to the results observed for the coarser microstructure specimens. On the other hand, much higher ductility was observed for specimens having more refined microstructure tested at 60°C. Strain was three times higher than those characterizing coarse related specimens. This disparity in ductility has been investigated. The principal mechanism of flow in superplasticity was found to be the grain boundary displacement. Various contributing factors were recognized for the Sn-Bi-Cu alloy sample with initial fine-dendritic form, which are: grain size lower than 10 μm, complete destruction of the dendritic array, significant rotation of grains during loading at 60°C and incidence of cavities.