npj: 田永君院士：纳米孪晶金刚石的持续强化

来源：知社学术圈

常规的材料强化方法在提高材料强度的同时通常会降低材料的韧性。最近的实验结果表明孪晶界可以同时提高金刚石的硬度和韧性。打破了传统的强度和韧性相互制约的关系，成为材料力学行为研究的热点问题。通过研究纳米晶和纳米孪晶金刚石的强韧化机制有望找到同时提高材料强度和韧性的普遍规律，但受研究方法的限制，目前仍存在许多争议。

来自燕山大学田永君院士领导的团队，与美国芝加哥大学的王雁宾教授合作,利用分子动力学和第一性原理计算方法，研究了纳米晶和纳米孪晶金刚石的强化机理。对于纳米金刚石，拖拉面位错运动与晶界原子相关的运动之间的竞争导致了反霍尔-佩奇效应。对于纳米孪晶金刚石，虽然拖拉面位错运动仍然占主导，但由于沿孪晶界面的滑移在能量上并不比沿其他滑移面的滑移更具优势，所以孪晶界面不容易迁移，结构比较稳定。因此，随孪晶厚度的减小，孪晶界面仍能起到阻碍位错运动的作用，从而抑制了反霍尔-佩奇效应的产生。特别地，该研究将材料化学健与位错运动模式建立了关联，初步揭示了化学健的性质（例如方向性等）对位错运动行为的影响规律。该研究结果为材料力学性能的微观机理研究提供了一个新的见解。

该文近期发表于npj Computational Materials 5: 117 (2019)，英文标题与摘要如下，点击左下角“阅读原文”可以自由获取论文PDF。

Continuous strengthening in nanotwinned diamond 

Bin Wen, Bo Xu, Yanbin Wang, Guoying Gao, Xiang-Feng Zhou, Zhisheng Zhao, and Yongjun Tian

Strengths of nanograined (ng) and nanotwinned (nt) metals increase with decreasing grain size and twin thickness, respectively, until reaching a critical value, below which strength decreases. This behavior is known as the reverse Hall–Petch effect (RHPE), which has also been observed in nanograined cubic boron nitride (cBN) and diamond. Surprisingly, however, hardness of nt-cBN and nt-diamond increases continuously with decreasing twin thickness down to several nanometers, suggesting the absence of RHPE in these covalent materials. The mechanism responsible for such a behavior remains controversial. Here we investigate the strengthening mechanisms in ng- and nt-diamond using molecular dynamics and first-principles calculations. For ng-diamond, the competition between shuffle-set dislocation (SSD) and grain boundary atom motions gives rise to RHPE. For nt-diamond, SSDs remain dominant but their slips along twin boundaries energetically show no advantage over those along other slip planes. Twin domains are locked and mechanically stable, resisting SSD propagation and inhibiting RHPE. These findings provide new insights into the hardening mechanism of nanotwinned covalent materials.