Dislocation interactions govern the mechanical response of materials and are influenced by local chemistry, temperature, and the rate at which stress is applied. This seminar explores static and dynamic microstructural responses in solder reliability and hydrogen embrittlement in steel to show how these factors affect localized plasticity in metallic systems, which is essential for predicting material behavior and preventing failure.

 

Solder reliability studies examine static microstructural interactions where dislocations encounter relatively immobile obstacles such as intermetallic particles and grain boundaries, while also presenting the complexity of testing at relatively high homologous temperatures. These features act as a framework that impedes dislocation motion, and the mechanical response can be governed by dislocation multiplication and pinning; at lower temperatures, these mechanisms are dominant. However, since room temperature represents a relatively high homologous temperature for solder alloys, time-dependent creep behavior also occurs, and dislocation mobility is enhanced not only by applied stress but also by thermally activated processes. Nanoindentation was employed to investigate pop-in events, providing insight into the critical stress for dislocation activation/nucleation, as well as hardness, modulus, strain-hardening/softening behavior, and the broader localized mechanical response. The study on SAC305 with 1–3 wt.% Bi showed that Bi additions increase hardness through solid-solution and precipitate strengthening but enhance strain-softening due to localized deformation associated with microstructural heterogeneity. To examine the transition with increasing homologous temperature, nanoindentation was performed from 0.6 to 0.9 Tm, capturing the shift from dislocation-controlled creep to deformation increasingly assisted by climb, recovery, and diffusion. Post-indentation microscopy revealed slip bands, grain boundary sliding, and intergranular shear associated with these mechanisms.

 

In contrast, the hydrogen embrittlement work focuses on dynamic dislocation–hydrogen interactions in shot-peened steel, where shot peening introduces a high dislocation density and compressive residual stress near the surface. Solute hydrogen, a mobile interstitial species in steel at room temperature, alters dislocation mobility and promotes dislocation rearrangement, consistent with hydrogen-enhanced localized plasticity mechanism. After hydrogen charging, the peened specimens exhibited relaxation of compressive residual stress and reduced defect density within the deformed surface layer. Microstructural characterization provided evidence of dislocation rearrangement after hydrogen exposure. Although hydrogen content in shot-peened specimens was nearly twice than unpeened samples, they showed less ductility loss, whereas unpeened specimens became increasingly brittle. These results suggest that HELP is more dominant in peened specimens, while hydrogen-enhanced decohesion (HEDE) is more prevalent in the unpeened condition, leading to the distinct fracture behavior.