Abstract Nanotwinned metals are promising structural materials for resisting impact due to their excellent combination of strength and ductility. In this study, the microstructural evolution of a nanotwinned steel under extremely high-strain-rate ballistic impact was systematically investigated by nanoindentation as well as detailed electron microscopy characterization. It is found that the nanotwin structure remains similar after ballistic impact, while secondary twinning activates in a limited portion of grains. In contrast, dislocation gliding is the main plasticity mechanism in the nanotwinned steel during ballistic impact, which leads to substantial increase of hardness in the severely-deformed region close to the fracture surface. Dislocation multiplication is promoted during ballistic impact due to the phonon drag effect, resulting in a hardness increment that exceeds the maximum value achieved in quasi-static tension. In addition, recrystallization occurs in the nanotwinned steel during ballistic impact due to the significant temperature increase when the hot bullet contacted and transferred sufficient heat to the nanotwinned steel.

中文翻译：

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超高应变率变形下纳米孪晶钢的显微组织演变

摘要 纳米孪晶金属具有良好的强度和延展性，是一种很有前途的抗冲击结构材料。在这项研究中，通过纳米压痕和详细的电子显微镜表征系统地研究了纳米孪晶钢在极高应变率弹道冲击下的微观结构演变。发现纳米孪晶结构在弹道撞击后保持相似，而二次孪晶在有限部分的晶粒中激活。相比之下，位错滑动是纳米孪晶钢在弹道冲击过程中的主要塑性机制，导致靠近断裂面的严重变形区域的硬度大幅增加。由于声子拖曳效应，在弹道撞击过程中促进了位错倍增，导致硬度增量超过准静态张力达到的最大值。此外，在弹道冲击过程中，由于热子弹接触并将足够的热量传递给纳米孪晶钢时温度显着升高，纳米孪晶钢会发生再结晶。