Flow softening rules and mechanisms in Ti–6Al–4V alloy sheet during electrically assisted near-isothermal tension

Near-isothermal electrically assisted tensile tests were performed on Ti–6Al–4V alloy sheet by programmed current–time loading curve and real-time monitoring via infrared thermal imaging. Comparing the results of thermal-assisted tension (TAT) and electrically assisted tension (EAT) at equivalent tensile temperatures, it is found that the flow stress and elongation under EAT are generally lower than the ones under TAT. Moreover, significant flow softening of 14–88 MPa which increases with current density and strain is found in EAT, indicating the existence of electroplasticity. From microstructure observations, dislocation annihilation, formation of dislocation walls, sub-grains, and dynamic recrystallization were found under the EAT process at a temperature lower than the conventional recrystallization temperature range. In addition, electric pulse is provided to accelerate the α→β phase transformation by promoting local diffusion, resulting in the increased volume fraction of β phase from 23.02 to 31.42% and the formation of many acicular α phase during the rapid cooling process after EAT at 20 A/mm2. The enhanced flow softening in EAT is explained as the comprehensive influences of electrically induced α→β phase transformation, dynamic recovery, dynamic recrystallization, and related enhanced local Joule heating at newly born grain boundaries.