Probing the ultimate limits of metal plasticity
Probing the ultimate limits of metal plasticity
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DOI：
10.1038/nature23472
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发表时间：
2016-04
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通讯作者：
L. Zepeda-Ruiz;A. Stukowski;T. Oppelstrup;V. Bulatov
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作者：
L. Zepeda-Ruiz;A. Stukowski;T. Oppelstrup;V. Bulatov
通常，金属的强度和塑性由晶格中的位错线缺陷来定义，位错线缺陷的运动导致材料沿沿着晶格面滑动。位错动力学模型通常被用作真实原子动力学的中尺度代理，这是计算昂贵的常规执行。然而，原子模拟准确地捕捉到了材料响应的每一种可能机制，解决了原子运动的每一个“抖动和摆动”，而位错动力学模型则没有。在这里，我们提出了体心立方金属钽体单晶塑性的全动态原子模拟。我们的目标是量化位错介导的塑性达到极限的条件，并了解超过任何这样的极限金属会发生什么。在我们的模拟中，在恒定的压力、温度和应变速率的条件下，金属沿其[001]晶轴以沿着的应变速率压缩。为了解决我们研究的长度尺度（85-340 nm）和时间尺度（1 ns-1μs）上晶体塑性过程的复杂性，我们使用最近开发的原位计算显微镜方法，将模拟中生成的大量瞬态轨迹数据重新转换为可以由人类分析的形式。我们的模拟预测，在达到一定的应变极限条件时，位错本身不再能够减轻机械载荷;相反，另一种机制，称为变形孪晶（晶格的突然重新取向），成为动态响应的主导模式。在此极限以下，金属呈现与应变路径无关的塑性流动稳定状态，其中只要此后的应变条件保持不变，流动应力和位错密度保持恒定。在这种独特的状态下，钽像粘性流体一样流动，同时保持其晶格并保持坚固和坚硬的金属。
Ordinarily, the strength and plasticity properties of a metal are defined by dislocations—line defects in the crystal lattice whose motion results in material slippage along lattice planes. Dislocation dynamics models are usually used as mesoscale proxies for true atomistic dynamics, which are computationally expensive to perform routinely. However, atomistic simulations accurately capture every possible mechanism of material response, resolving every “jiggle and wiggle” of atomic motion, whereas dislocation dynamics models do not. Here we present fully dynamic atomistic simulations of bulk single-crystal plasticity in the body-centred-cubic metal tantalum. Our goal is to quantify the conditions under which the limits of dislocation-mediated plasticity are reached and to understand what happens to the metal beyond any such limit. In our simulations, the metal is compressed at ultrahigh strain rates along its [001] crystal axis under conditions of constant pressure, temperature and strain rate. To address the complexity of crystal plasticity processes on the length scales (85–340 nm) and timescales (1 ns–1μs) that we examine, we use recently developed methods ofin situcomputational microscopy,to recast the enormous amount of transient trajectory data generated in our simulations into a form that can be analysed by a human. Our simulations predict that, on reaching certain limiting conditions of strain, dislocations alone can no longer relieve mechanical loads; instead, another mechanism, known as deformation twinning (the sudden re-orientation of the crystal lattice), takes over as the dominant mode of dynamic response. Below this limit, the metal assumes a strain-path-independent steady state of plastic flow in which the flow stress and the dislocation density remain constant as long as the conditions of straining thereafter remain unchanged. In this distinct state, tantalum flows like a viscous fluid while retaining its crystal lattice and remaining a strong and stiff metal.