Achieving microstructural enhancements at sub-zero temperatures

Conventional wisdom dictates that high temperatures are essential for atomic vibrations, which drive defect modifications and microstructural changes in materials. Even non-traditional processes, such as electropulsing, employ elevated temperatures. In this study, we challenge this understanding, hypothesizing that significant microstructural enhancements can be achieved even at sub-zero temperatures through an athermal annealing mechanism. To validate this, we leveraged the electron wind force (EWF) to anneal commercially pure silver specimens at −100 °C. Our observations revealed significant dislocation motion, grain boundary migration, and residual stress relaxation. Detailed analyses using electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) provided compelling evidence of (a) near-complete elimination of low-angle grain boundaries (LAGBs), (b) substantial grain growth and (c) marked reduction in local misorientation; all indicating effective dislocation annihilation and boundary migration. Furthermore, texture analysis demonstrated pronounced grain rotation and orientation alignment along specific crystallographic directions. TEM observations corroborated these findings, revealing the coalescence of sub-grains into larger domains and the formation of wider, more organized dislocation bands, consistent with dislocation rearrangement and stress relaxation within grains. These experimental results are supported by classical molecular dynamics simulations, which showed appreciable reduction in total dislocation density and a more pronounced decline in 1/2⟨110⟩ dislocations, confirming that EWF can facilitate defect annihilation, sub-grain coalescence, and lattice rotation in the absence of thermal activation. The fundamental mechanism underlying this phenomenon is proposed to be the reduced electron-phonon coupling, which enhances electron mobility and, consequently, the EWF magnitude at lower temperatures.