Insights into the strengthening mechanisms of titanium alloy treated by electropulsing-assisted ultrasonic nanocrystal surface modification: Process, microstructure, and deformation behavior

Abstract

Ultrasonic nanocrystal surface modification (UNSM) properly improves fatigue resistance but shows limited processing efficiency on hard-to-deform alloys such as Ti6Al4V due to their high deformation resistance. This study employs an emerging electropulsing-assisted UNSM (EP-UNSM) approach that integrates pulsed current with conventional UNSM to improve strengthening efficiency and associated surface integrity. However, strain localization-induced nanocrystalline plastic instability may lead to fatigue deterioration under low cycle fatigue regimes. To clarify these aspects, systematic microstructure characterization and mechanical testing were conducted to determine the pathways by which EP-UNSM alters plastic deformation and fatigue strengthening mechanisms. Results reveal that, unlike the planar dislocation slip induced by limited plastic deformation in conventional UNSM, EP-UNSM activates pronounced non-basal dislocations and wavy slip patterns, thereby improving the plasticity and producing a deeper gradient nanostructure layer. Moreover, a dynamic electro-annealing mechanism is proposed that involves the synergistic reconfiguration of metastable dislocations and nanocrystals, forming periodic dislocation cells in EP-UNSM samples, in contrast to the sharp triple-junction grain boundaries observed in conventional UNSM. This microstructure evolution mitigates stress concentration at grain boundaries and enhances compressive residual stress stability, ultimately improving fatigue resistance across all stress regimes. These findings advance understanding of electropulsing-assisted deformation and guide anti-fatigue manufacturing strategies for titanium alloys and other hard-to-deform metals.