Shear direction induced transition mechanism from grain boundary migration to sliding in a cylindrical copper bicrystal
Graphical abstract
Introduction
Grain boundaries (GBs), act as obstacles to dislocation glide, are widely considered to be mechanically static structures. In contrast to their traditional description, a previous report outlined an experimental investigation of stress-driven GB migration in nanocrystalline aluminum thin films (Rupert et al., 2009). More studies on nanocrystalline metals have introduced the convincing evidence, which confirmed that GBs were not static structures as traditionally assumed (Gorkaya et al., 2009, 2011; Homer et al., 2013). In recent years, research has revealed that the dominant deformation mechanism of nanocrystalline metal materials switches from dislocation-mediated behavior to GB motion when the average grain size is less than 15 nm, resulting in the Hall-Petch relationship failure and material softening (Schiøtz and Jacobsen, 2003). To accurately describe GB motion in nanocrystalline metals, previous theories have predicted that cooperative GB sliding and without-shear-coupled migration were more energetically favorable than pure GB sliding and could enhance the ductility of nanocrystalline metals (Bobylev et al., 2010, 2011). Additional research has found that the cooperative GB motion could improve the critical stress intensity factor for crack growth, considerably enhancing the fracture toughness of such materials (Ovid'ko et al., 2011). In particular, the synergy of GB sliding and shear-coupled migration was shown to further enhance the ductility of nanocrystalline metals compared with these two deformation modes (Li and Soh, 2013). Subsequent experimental investigations into cooperative GB sliding and migration have revealed the plastic behavior of nanocrystalline metals, using well-designed in situ transmission electron microscopy (TEM) techniques (Li et al., 2020a; Wang et al., 2017).
To comfortably modulate the synergy of GB sliding and migration to meet different performance requirements in GB engineering, it is necessary and significant to understand the atomistic mechanism of individual GB sliding and migration. Control over GB dynamics may provide an effective mean for tailoring the mechanical and physical properties of nanocrystalline metals; thus, numerous forward-looking investigations have focused on some types of GB dynamics behavior, such as GB migration (Merkle et al., 2002; Rajabzadeh et al., 2013; Zhang et al., 2015) or GB sliding (Linne et al., 2020; Ovid'ko and Sheinerman, 2017; Son and Hyun, 2020; Son et al., 2020). Among these, the shear-coupled GB migration is considered to be a conventional mode for grain growth in metals (Thomas et al., 2017), with the prominent grain size (Zhou et al., 2019) and orientation (Hou et al., 2021), as well as temperature (Chen et al., 2020) dependence. However, the dominant mechanism of plasticity in fine-grained metal materials typically involves GB sliding (Borodin et al., 2020), which also plays a significant role in superplastic (Zekin et al., 1994) and creep behaviors (Sakane et al., 2021). GB sliding in aluminum tri-crystals was shown to exhibit dependence on the GB orientation and temperature (Weinberg, 1958). Atomistic simulations have been used to study the effect of applied shear on GB sliding in aluminum, indicating that the increase in applied shear provoked three sliding behaviors: no sliding, constant velocity sliding and parabolic sliding over time (Qi and Krajewski, 2007). Despite the fact that these factors affecting GB motion have been systematically investigated, few studies have been conducted to explore shear-induced transition from GB migration to sliding in nanocrystalline metals.
Generally, disconnections, topological line defects constrained to GB planes with both step and dislocation characteristics, effectively dominate the GB migration process (Han et al., 2018). In situ atomistic observations have revealed that shear-coupled GB migration can be realized through the lateral motion of GB disconnections (Zhu et al., 2019). A recent report has showed the extreme shear deformability (up to 364%) through twin boundary (TB) sliding, where TB sliding was significantly provoked by the formation or enlargement of the surface step at the TB-surface intersection (Zhu et al., 2021). This surface step, as a topological defect constrained to GB-surface intersections, could provoke GB sliding and was proposed to contrast with disconnections via GB migration. However, a comprehensive understanding of the nucleation mechanism of these defects at the atomic scale remains largely lacking.
Inspired by the success of GB engineering in nanocrystalline metals (Li et al., 2020b), we sought to control the transition from GB migration to sliding by external loading and to understand the transition mechanism. In this work, we found an effective approach using controllable shear directions (or shear angles) parallel to the GB plane to achieve the transition from GB migration to sliding in a cylindrical copper (Cu) bicrystal. Fully dynamic atomistic simulations indicated that the disconnection nucleation triggered GB migration, whereas the GB sliding was traced back to surface step nucleation from the free surface. The disconnection and surface step nucleation models were further developed and linked to reveal that the transition mechanism originated from the competition between the nucleation energies of the disconnection and surface step.
Section snippets
Simulation methods
The molecular dynamic (MD) simulations were carried out using large-scale atomic/molecular massively parallel simulator (LAMMPS) (Plimpton, 1995), and the inter-atomic interactions in the system were described by the embedded atom method (EAM) potential for Cu (Zhou et al., 2015). We elaborately constructed a nanopillar model, which can be conveniently used to assess a large number of models with different orientations by taking the shear direction as a variable. More importantly, the shear
Mechanical behavior of GB motion at different shear angles
Fig. 2 shows the stress-strain curves in the cylindrical Cu bicrystal at different shear angles of θ. A small and irregularly serrated profile of stress levels was identified for θ = 0°, 15°, 30° and 45°, possibly caused by atomic rearrangement in the boundary edges of the GB plane. In these cases, the dominant GB motion consisted of GB migration as shown in the atomic configurations in Fig. 3(a) and (b). The literature (Mishin et al., 2007) showed that GB migration followed the general
Discussion
For comparison with the other different GB structures, we also constructed two 〈110〉 symmetrical tilt GBs including the Σ27(115) GB with a misorientation angle of 31.6° and the Σ3(111) GB with a misorientation angle of 109.5° (or twin boundary), and two 〈110〉 asymmetric tilt GBs with misorientation angles of 80° and 130°, as shown in Supplementary Fig. 2. The atomistic modeling method is presented in Section 1 of the Supplementary materials. When the shear direction was along the y direction,
Conclusions
We studied the transition mechanism from individual GB migration to sliding in the cylindrical copper bicrystal by using controllable shear angles parallel to the GB plane using MD simulations. The conclusions are as follows:
(1) The individual GB showed simultaneous deformability of migration and sliding. The transition from GB migration to sliding was managed by the shear loading direction, indicating that GB motion was significantly anisotropic.
(2) From the atomistic observations, the
CRediT authorship contribution statement
Anping Hua: Conceptualization, Methodology, Software, Investigation, Formal analysis, Writing – original draft. Junhua Zhao: Conceptualization, Funding acquisition, Resources, Supervision, Writing – review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We gratefully acknowledge support from the National Natural Science Foundation of China (Grant Nos. 11972171, 11572140), the 111 project (Grant No. B18027), the Natural Science Foundation of Jiangsu Province (Grant No. BK20180031), Jiangsu Province “333” project, and the Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX21–2031).
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