Stacking fault energy modulates softening mechanisms in FCC metals during ultrasonic normal vibration

Abstract

Stacking fault energy (SFE) is shown to strongly correlate with, though not solely govern, the plastic deformation mechanisms of face-centered cubic (FCC) metals, yet its role during ultrasonic normal vibration (UNV)-assisted forming remains elusive. Through UNV-assisted tensile experiments and EBSD analysis of three FCC metals with distinct SFEs (low-SFE stainless steel, medium-SFE copper, and high-SFE aluminum alloy), this study reveals two distinct softening phenomena that correlate with SFE. The acoustic softening effect, most pronounced in medium-SFE copper, is interpreted as arising from a high-frequency periodic elastic strain field that reduces the activation barriers‌ for dislocation glide and cross-slip. For acoustic residual softening, the magnitude exhibits a systematic trend of increasing with decreasing SFE, being strongest in low-SFE stainless steel. This is associated with enhanced dynamic recovery and, in low- and medium-SFE materials, a UNV-associated reduction in twin boundary fraction that weakens the twin strengthening contribution. These findings suggest that SFE modulates the relative efficiency of distinct restoration mechanisms under UNV.