An energy based modeling for the acoustic softening effect on the Hall-Petch behavior of pure titanium in ultrasonic vibration assisted micro-tension

Abstract

The acoustic softening effect in metals during plastic deformation has been widely investigated in the past decades. However, the mechanism of such an acoustic plasticity remains controversial. As a result, several models were proposed to understand the acoustic softening effect in terms of stress superposition, thermal activation theory, crystal plastic theory and other mechanisms associated with dislocation evolution. In this study, we proposed a mechanism that the athermal dislocation dynamics may heterogeneously change at microstructure level during ultrasonic vibration assisted (UVA) deformation. Specifically, the work required to eject dislocations from grain boundaries may be altered by the acoustic energy absorption difference of a dislocation in the grain interior and one in the grain boundary. As a result, the acoustic softening effect on the Hall-Petch behavior was modeled by incorporating a power function of acoustic energy density into the dislocation ejection work. To validate the developed model, UVA micro-tension tests were conducted on pure titanium specimens. Results showed that the Hall-Petch slope decreased due to ultrasonic vibration, and the ultrasound-induced decrease of the Hall-Petch slope increased with plastic deformation. Note that our model predictions matched well with the experimental results at the lower strains, providing an alternative insight into the acoustic softening effect on the Hall-Petch behavior. Microstructure examinations showed that the superimposed ultrasonic vibration in micro-tension could lead to the retardation of texture evolution, the decreases in kernel average misorientation (KAM) value, low-angle grain boundary (LAGB) fraction and dislocation density in titanium foils, which somewhat supported our model assumptions in terms of the acoustic energy dependent reduction in dislocation density and the enhanced dislocation ejection causing the improvement of plastic compatibility.