Dynamic Recrystallization (DRX) as the Mechanism for Sn Whisker Development. Part II: Experimental Study

In Part I of this study, a dynamic recrystallization (DRX) model was proposed to describe the development of metal whiskers. A diffusion-assisted, dislocation-based mechanism would support the DRX steps of grain initiation (refinement) and grain growth. This, Part II, describes experiments investigating the time-dependent deformation (creep) of Sn under temperature conditions (0°C, 25°C, 50°C, 75°C, and 100°C) and stresses (1 MPa, 2 MPa, 5 MPa, and 10 MPa) that are commensurate with Sn whisker development, in order to parameterize the DRX process. The samples, which had columnar grains oriented perpendicular to the stress axis similar to their morphology in Sn coatings but of larger size, were tested in the as-fabricated condition as well as after 24 h annealing treatments at 150°C or 200°C. The steady-state creep behavior fell into two categories: low (<10⁻⁷ s⁻¹) and high strain rates (>10⁻⁷ s⁻¹). The apparent activation energy (ΔH) at low strain rates was 8 ± 9 kJ/mol for the as-fabricated condition, indicating that an anomalously or ultrafast diffusion mass transport mechanism assisted deformation. Under the high strain rates, the ΔH was 65 ± 6 kJ/mol (as-fabricated). The rate kinetics were not altered significantly by the annealing treatments. The critical strain (ε _c) and Zener–Hollomon parameter (Z) confirmed that these stresses and temperatures were nearly capable of causing cyclic DRX in the Sn creep samples, but would certainly do so in Sn coatings with the smaller grain size. The effects of the annealing treatments, coupled with the DRX model, indicate the need to maximize the creep strain rate during stress relaxation so as to avoid conditions that would favor whisker growth. This study provides a quantitative methodology for predicting the likelihood of whisker growth based upon the coating stress, grain size, temperature, and the similarity assumption of creep strain.