Revealing ductile-to-brittle transition mechanism and enhancing the cryogenic ductility of tin (Sn) for cryogenic electronics

The ductile-to-brittle-transition (DBT) of Sn occurring when the temperature decreases to a critical temperature poses a restriction on the Sn-based solders in the cryogenic electronics interconnection. Previous investigations about the DBT mainly adopt the Charpy impact test, however, the transient property of Charpy impact test makes it difficult to capture the deformation features accompanying DBT and clearly elucidate the DBT mechanism. In this paper, we studied the plastic deformation of Sn before cryogenic brittle fracture through in-situ cryogenic uniaxial tensile experiments under optical microscopy. It is found that {301} deformation twins were activated and thickened before the brittle fracture. Due to the occurrence of deformation twins, multiple twin-grain boundary intersections and twin-twin intersections formed within Sn grains, which could strongly prevent dislocation slip and thus lead to stress concentration. As a result, the brittle cracks initiate and propagate along these intersections. According to the suggested specific mechanism, we designed a green pre-treatment method of pre-twining-RT tempering to increase the cryogenic ductility of Sn-based solder alloys without any energy consumption. Eventually, the fracture elongation of Sn at LNT was significantly improved from ~6% to ~14%.