Research on Internal Structural Evolution driven by Collective Defects Field Dynamics for Multiscale Modeling

本研究旨在发展新的方法来捕捉由于集体缺陷引起的内部结构变化的多尺度演化。在外加载荷作用下内部缺陷的生长过程中，作为宏观塑性现象的开端，理解纳米尺度的不可逆力学行为，即纳米塑性，是最重要的。由于纳米压痕的微米级压痕载荷和纳米级压痕深度可以精确控制，纳米压痕问题引起了人们的极大兴趣，我们发现了新的事实，即第一次位移突跃时的临界压痕载荷（即保持载荷的不稳定压痕深度突跃）与突跃宽度成线性关系。为了了解位移爆发过程中发生了什么，我们进行了大规模的分子动力学模拟。在一系列的这些分析中，有用的机械性能，如理想的解决在位错发射的剪切应力已被获得，然后转移到介观的方法，如离散位错方法。我们了解了压痕下最有可能发生的位错环的形成机制和位错集体发射机制，并发展了在细观尺度上进行无损观测的新技术。扫描电子感应声学显微镜（SEAM）可以在不破坏样品的情况下检测到位于亚表面的几个微米级缺陷。我们有很大的期望，应用我们自己的方法来捕捉内部的集体缺陷，但它被发现需要更高的斩波频率的电子束和电荷放大器电路的更高的频率。

This research aims at developing the new methodology to catch the multiscale evolution of the internal structural change due to the collective defects. In the process of growing the internal defects under the applied loads, it is the most important to understand the nanoscale irreversible mechanical behaviors, so called as the nanoplasticity, as the commencement of the macroscopic plastic phenomena. We have taken much interests on the nanoindentation problem because the micron-scale indent load and the nano-scale indent depth can be controlled precisely.We have found the new facts that the critical indent load at the first displacement burst, which has been recognized as the unstable indent depth jump keeping the load, and the burst width have the linear relation. To understand what is happening during the displacement burst, we have performed the large-scale molecular dynamics simulations. In the series of these analyses, the useful mechanical properties like the ideal resolved shearing stress at the dislocation emission have been obtained and then transferred to the mesoscopic methodology like the discrete dislocation method. We understand the formation mechanics of the prismatic dislocation loop which is the most potential events under the indentation and the collective dislocation emission which is the key to the burst mechanics.We have also developed the new technique of the nondestructive observation at the mesoscale. The scanning electron-indued acoustic microscopy(SEAM) can detect a few micron-scale defects placed at the subsurface without destruction of the sample. We had the big expectations to apply the our own method to catch the internal collective defects, however it is found to need the more high chopping frequency of the electron beam and the charge amplifier circuit for the higher frequency.