Revealing the ductile-to-brittle transition mechanism in polycrystalline body-centered tetragonal tin (Sn) for cryogenic electronics

The inherent ductile-to-brittle transition (DBT) of body-centered tetragonal Sn at cryogenic temperatures restricts the use of Sn-based solders in the interconnection of cryogenic electronics, but little is known about the deformation behaviors accompanying with the transition and the underlying transition mechanism. In this work, the deformation features before cryogenic brittle fracture and the DBT mechanism in polycrystalline Sn were studied through uniaxial tensile experiments at different temperatures. Compared to the softening process stimulated by dynamic recovery and dynamic recrystallization before ductile fracture at room temperature (similar to 293 K), a high strain hardening rate (similar to 5% of the shear modulus) is maintained during the linear hardening period preceding brittle fracture at the liquid nitrogen temperature (similar to 77 K) due to the pronounced intersecting of {301} deformation twins. But the irreconcilable velocity difference between dislocation glide (similar to 3 mu m/s) and twin thickening ( 10 mu m/s) at 77 K leads to a premature brittle fracture in the midst of the linear hardening, and indeed the DBT. The suggested specific DBT mechanism is substantiated by the fact that a significant increase in the velocity (similar to 1500 mu m/s) with the increasing temperature (123 K) allows the dislocation slip to readily accommodate the shear strains due to {301} twin thickening at the grain boundaries, thereby resulting in ductile fracture rather than brittle fracture. This deep understanding about the DBT in polycrystalline Sn may help forge a new path to design ductile and strong Sn-based solders and solder joints for cryogenic electronics by deformation twinning. (C) 2022 Elsevier B.V. All rights reserved.