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

本研究旨在发展一种新的方法来捕捉由集体缺陷引起的内部结构变化的多尺度演化。在内部缺陷在外加载荷作用下生长的过程中，最重要的是理解纳米尺度的不可逆力学行为，即纳米塑性，它是宏观塑性现象的开始。纳米压痕问题由于其微米级压痕载荷和纳米级压痕深度可以精确控制而引起了人们的广泛关注。我们发现了新的事实，即第一次位移爆破时的临界压痕荷载，即保持荷载的不稳定压痕深度跳跃，与爆破宽度呈线性关系。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。在这一系列的分析中，得到了位错发射时的理想剪切应力等有用的力学性能，然后将其转移到离散位错法等介观方法中。我们了解了压痕下最有可能发生的棱柱位错环的形成机制和爆裂力学的关键——集体位错发射。我们还开发了中尺度无损观测新技术。扫描电子诱导声显微镜（SEAM）可以在不破坏样品的情况下检测到少量微米级的亚表面缺陷。我们对用自己的方法捕获内部集体缺陷寄予了很大的期望，但发现它需要更高的电子束斩波频率和更高频率的电荷放大电路。

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.