ABSTRACT

A number of recent Molecular Dynamics (MD) simulations have demonstrated that screw dislocations in face centred cubic (fcc) metals can achieve stable steady state motion above the lowest shear wave speed ($v_{\mathrm{s}\mathrm{h}\mathrm{e}\mathrm{a}\mathrm{r}}$) which is parallel to their direction of motion (often referred to as transonic motion). This is in direct contrast to classical continuum analyses which predict a divergence in the elastic energy of the host material at a crystal geometry dependent ‘critical’ velocity $v_{\mathrm{c}\mathrm{r}\mathrm{i}\mathrm{t}}$. Within this work, we first demonstrate through analytic analyses that the elastic energy of the host material diverges at a dislocation velocity ($v_{\mathrm{c}\mathrm{r}\mathrm{i}\mathrm{t}}$) which is greater than $v_{\mathrm{s}\mathrm{h}\mathrm{e}\mathrm{a}\mathrm{r}}$, i.e. $v_{\mathrm{c}\mathrm{r}\mathrm{i}\mathrm{t}}>v_{\mathrm{s}\mathrm{h}\mathrm{e}\mathrm{a}\mathrm{r}}$. We argue that it is this latter derived velocity ($v_{\mathrm{c}\mathrm{r}\mathrm{i}\mathrm{t}}$) which separates ‘subsonic’ and ‘supersonic’ regimes of dislocation motion in the analytic solution.

In addition to our analyses, we also present a comprehensive suite of MD simulation results of steady state screw dislocation motion for a range of stresses and several cubic metals at both cryogenic and room temperatures. At room temperature, both our independent MD simulations and the earlier works find stable screw dislocation motion only below our derived $v_{\mathrm{c}\mathrm{r}\mathrm{i}\mathrm{t}}$. Nonetheless, in real-world polycrystalline materials $v_{\mathrm{c}\mathrm{r}\mathrm{i}\mathrm{t}}$ cannot be interpreted as a hard limit for subsonic dislocation motion. In fact, at very low temperatures our MD simulations of Cu at 10 Kelvin confirm a recent claim in the literature that true ‘supersonic’ screw dislocations with dislocation velocities $v>v_{\mathrm{c}\mathrm{r}\mathrm{i}\mathrm{t}}$ are possible at very low temperatures.

摘要

最近的许多分子动力学（MD）模拟表明，面心立方（fcc）金属中的螺钉位错可以在最低剪切波速以上实现稳定的稳态运动（$v_{\mathrm{s}\mathrm{H}\mathrm{Ë}\mathrm{一种}\mathrm{[R}}$）平行于它们的运动方向（通常称为跨音速运动）。这与经典连续谱分析形成鲜明对比，经典连续谱分析预测了在取决于晶体几何形状的“临界”速度下基质材料的弹性能发散$v_{\mathrm{C}\mathrm{[R}\mathrm{一世}\mathrm{Ť}}$。在这项工作中，我们首先通过分析分析证明主体材料的弹性能以位错速度发散（$v_{\mathrm{C}\mathrm{[R}\mathrm{一世}\mathrm{Ť}}$）大于 $v_{\mathrm{s}\mathrm{H}\mathrm{Ë}\mathrm{一种}\mathrm{[R}}$， IE $v_{\mathrm{C}\mathrm{[R}\mathrm{一世}\mathrm{Ť}}>v_{\mathrm{s}\mathrm{H}\mathrm{Ë}\mathrm{一种}\mathrm{[R}}$。我们认为这是后者的后继速度（$v_{\mathrm{C}\mathrm{[R}\mathrm{一世}\mathrm{Ť}}$），将解析解中的位错运动的“亚音速”和“超音速”状态分开。

除了我们的分析外，我们还提供了一套综合的MD模拟结果，这些结果包括在低温和室温下对于一定范围的应力和几种立方金属的稳态螺杆错位运动。在室温下，我们独立的MD模拟和较早的工作都发现仅在我们推导的温度以下才存在稳定的螺钉错位运动$v_{\mathrm{C}\mathrm{[R}\mathrm{一世}\mathrm{Ť}}$。尽管如此，在现实世界中的多晶材料中$v_{\mathrm{C}\mathrm{[R}\mathrm{一世}\mathrm{Ť}}$不能解释为亚音速位错运动的硬限制。实际上，在非常低的温度下，我们在10开尔文温度下对Cu的MD模拟结果证实了文献中最近的说法，即真正的“超音速”螺旋位错和位错速度$v>v_{\mathrm{C}\mathrm{[R}\mathrm{一世}\mathrm{Ť}}$ 在非常低的温度下是可能的。