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.

纳米晶粒（ng）和纳米孪晶（nt）金属的强度分别随着晶粒尺寸和孪晶厚度的减小而增加，直到达到临界值，在该临界值以下强度降低。这种行为称为反向霍尔-Petch效应（RHPE），也已在纳米晶立方氮化硼（cBN）和金刚石中观察到。然而，令人惊讶的是，nt-cBN和nt-金刚石的硬度随着孪晶厚度降低至几纳米而连续增加，这表明在这些共价材料中不存在RHPE。造成这种行为的机制仍存在争议。在这里，我们使用分子动力学和第一性原理计算研究ng和nt金刚石的增强机理。对于ng金刚石，混晶位错（SSD）与晶界原子运动之间的竞争导致了RHPE。对于nt金刚石，SSD仍然占主导地位，但其沿双边界的滑动在动力方面未显示出优于其他滑动平面的优势。双畴被锁定且机械稳定，可抵抗SSD传播并抑制RHPE。这些发现为纳米双价共价材料的硬化机理提供了新的见解。