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A systematic study of grain refinement during impact of 4340 steel

机译：4340钢冲击过程中晶粒细化的系统研究

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Inhomogeneous plastic deformation at high strain rates and large strains results in the concentration of deformation and strain localization in adiabatic shear bands (ASBs). In the current study, AISI 4340 steel specimens were impacted at increasing strain rates/strains (impact momentums) to capture the formation of adiabatic shear bands. Advanced specimen preparation techniques using Focused Ion Beam (FIB) and extensive electron microscopy were used to identify the sequence of evolution of the shear bands. It was observed that the structure of the shear bands that evolve after strain localization starts out with elongation of the grains due to grain reorientation in the shear direction with the initiation of random and transverse dislocation boundaries along the elongated grains. For higher strain rates/strains during impact, the elongated grains break along the initiated dislocation boundaries resulting in the creation of smaller elongated broken grains and nanograins. Boundary refinement of the broken grains occurring through grain rotation and adiabatic heating results in the evolution of refined grains, subgrains and nanograins. The presence of elongated grains, broken grains, refined grains, subgrains and nanograins within the shear band structures demonstrates that the local deformation is inhomogeneous and that these mechanisms occur concurrently. It is concluded that the evolution of the shear band structure can be considered as a simultaneous layering of microstructures initially driven by dislocations which produce the final structures observed in the shear bands.
机译：高应变率和大应变下的不均匀塑性变形会导致绝热剪切带（ASBs）中的变形集中和应变局部化。在当前的研究中，AISI 4340钢试样受到应变率/应变（冲击动量）增加的影响，以捕获绝热剪切带的形成。使用聚焦离子束（FIB）和广泛的电子显微镜的先进标本制备技术可确定剪切带的演化顺序。观察到，在应变局部化之后产生的剪切带的结构开始于晶粒的伸长，这是由于晶粒在剪切方向上的重新取向，以及沿着伸长晶粒的随机和横向位错边界的开始。对于冲击过程中较高的应变率/应变，细长晶粒沿初始位错边界断裂，从而导致较小的细长断裂晶粒和纳米晶粒的产生。通过晶粒旋转和绝热加热发生的破碎晶粒的边界细化导致细化晶粒，亚晶粒和纳米晶粒的演化。剪切带结构内存在细长晶粒，破碎晶粒，细晶粒，亚晶粒和纳米晶粒，这表明局部变形是不均匀的，并且这些机制同时发生。结论是，剪切带结构的演变可以被认为是由位错驱动的微观结构的同时分层，这些位错产生了在剪切带中观察到的最终结构。

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