Anisotropy of hardness and impression morphology in body-centered tetragonal tin (Sn) at cryogenic temperature and room temperature

Tin (Sn), as the matrix of most lead-free solders, plays a primary role in stress absorption during the service of small-scale solder joints under cryogenic environment. The Sn grain orientation would produce a strong effect on the cryogenic mechanical stability of solder joints due to the anisotropy of Sn with a body-centered tetragonal structure. Here, a clear dependence of grain orientation on the strain-hardening behaviors of Sn at the liquid nitrogen temperature (similar to 77 K) was revealed through a convenient and off-line Vickers micro-indentation test within various individual Sn grains. As a comparison, the same micro-indentation tests were also performed at room temperature (similar to 293 K). By analyzing the hardness and impression morphology of different indented grains, it is found that Sn grain exhibits a hardening effect with a higher hardness value and a sink-in indentation morphology when the indented crystallographic direction deviates from c axis at 77 and 293 K. The tendency to become harder is attributed to reduction in the resolved shear stress on {100}[001] slip system with the deviation, which renders dislocation slip more difficult to be activated. This orientation-dependent hardening effect becomes more pronounced at 77 K because of the occurrence of {301} deformation twinning and the same decrease of resolved shear stress on {301} deformation twinning with the deviation.