Continuous strengthening in nanotwinned diamond

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

doi:10.1038/s41524-019-0256-2

Bin Wen, Bo Xu + Show 5 more

Open Access

https://doi.org/10.1038/s41524-019-0256-2

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Abstract

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.

Highlights

Nanostructuring can introduce immense amounts of interfaces such as grain boundaries (GBs) and twin boundaries (TBs) into a material, reducing dislocation pile-ups near the boundaries, thereby hindering dislocation transmission and increasing resistance to deformation.[1]
To clarify the different behaviors in ng- and nt-diamond, we investigated deformation modes in five representative samples, i.e., two ng-diamonds with average grain sizes of 3.11 nm and 16.23 nm, two nt-diamonds with an identical average d of 16.23 nm but different twin thicknesses of 0.618 nm and 5.562 nm, and one nt-diamond with average d of 5.08 nm and λ of 0.618
Distinct deformation modes were identified based on relative displacements of atoms with respect to the undeformed sample (Fig. 2 and Supplementary Figs. 11 and 12) and dislocation activities (Supplementary Videos 1−5)

Summary

Introduction

Nanostructuring can introduce immense amounts of interfaces such as grain boundaries (GBs) and twin boundaries (TBs) into a material, reducing dislocation pile-ups near the boundaries, thereby hindering dislocation transmission and increasing resistance to deformation.[1]. In nanotwinned (nt) diamond and cBN, hardness continuously increases with decreasing twin thickness to below 5 nm,[15,16] contrasting strikingly to that of nt-metals. In nt-diamond, shuffle-set dislocations (SSDs) dominate the plastic deformation but their slips along TBs do not have advantage energetically over those along other slip planes. TBs are mechanically stable and resist SSDs propagation across twin domains, leading to continuous operation of the Hall–Petch effect at the nanoscale level. This is in stark contrast to nt-metals, where detwinning becomes increasingly active as λ is reduced to below ca. This is in stark contrast to nt-metals, where detwinning becomes increasingly active as λ is reduced to below ca. 15 nm, due to collective slips of partial dislocations parallel to TBs.[6,9,10]

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Conclusion