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Strain rate effects on mechanical behavior and microstructure evolution with the sequential strains of TWIP steel
Materials Science and Engineering: A  (IF5.234),  Pub Date : 2022-01-14, DOI: 10.1016/j.msea.2022.142673
Chunhui Wang, Wang Cai, Chaoyang Sun, Xintong Li, Lingyun Qian, Jun Jiang

Dynamic and quasi-static mechanical behaviors and microstructural mechanisms of the TWIP steel were investigated at strain rates from 0.001 to 3000 s-1 by using a split Hopkinson tensile bar paired with interrupted strain fixtures and electron backscattered diffraction technology. The positive strain rate sensitivity of dynamic yield stress is significantly different from that of quasi-static stress at a transition strain rate of about 100 s-1 for the different yield mechanisms. Owing to the high stress from dynamic loading, the smaller twin onset strain and the higher twin fraction as well as the more intense twin boundary-dislocation interaction are responsible for the enhanced strain hardening behavior in the early stage of strain. Nevertheless, the rapid decrease of the strain hardening rate in the later stage and the resulting negative strain hardening rate sensitivity are due to the decreasing growth rate of the twin boundary fraction. It is closely associated with the softening effect of adiabatic temperature rise (∼117 °C) by raising the stacking fault energy and inhibiting twinning activity. In addition, the fuzzy regions caused by high-density dislocations within interfaces between twin and grain boundaries may lead to the lower elongation of the dynamic tensile specimen. The grains with a high Schmid factor are conducive to slip rather than twinning, which enables the twin region orientation for slip-mediated plastic deformation. This study thus advances the understanding of the strain and strain rate dependence of the mechanical behaviors and twinning mechanisms of TWIP steels for the safety performance of automobiles.