Coarse Graining and Localized Plasticity between Sliding Nanocrystalline Metals

Tribological shearing of polycrystalline metals typically leads to grain refinement at the sliding interface. This study, however, shows that nanocrystalline metals exhibit qualitatively different behavior. Using large-scale atomistic simulations, we demonstrate that during sliding, contact interface nanocrystalline grains self-organize through extensive grain coarsening and lattice rotation until the optimal plastic slip orientation is established. Subsequently, plastic deformation is frequently confined to localized nanoshear bands aligned with the shearing direction and emanating from voids and other defects in the vicinity of the sliding interface.

Figure 1

Snapshots of the evolution of sliding contact between two nanocrystalline iron tribopartners. Each column presents the instantaneous state of the grain structure (top panel), the grain boundaries and dislocations (middle panel), and the corresponding sliding interface (bottom panel) at selected sliding times. The grain structure in the top row is exposed by performing a common neighbor analysis [38] using a 3.46 Å cutoff radius (with blue, white, and red standing for bcc, non-bcc, and twin boundary atoms). The middle row shows the simultaneous state of the extracted dislocations [39, 40, 41] and twin boundaries (with green and gray lines representing dislocations with $1/2⟨111⟩$ and non-$1/2⟨111⟩$ Burgers vectors and with gray and red zones marking the location of voids and twin boundaries). In order to correlate the structural dynamics with the way the system distributes the sliding induced velocity gradient over the system [42], the strain rates are visualized in the bottom row by coloring the atoms according to their change in displacement along the sliding $x$ direction after every 0.2 ns on a scale from 0 Å (blue) to 10 Å (red). As sliding progresses, the counterbodies cold weld and establish a coarsened grain layer where plasticity concentrates and the sliding interface localizes.

Figure 2

(a) Evolution of the bcc packed direction within the coarsened center grain. The graph shows the angle between the bcc $⟨111⟩$ packed direction and the sliding $x$ direction. The close-up insets show states of the crystal structure within the coarsened grain while looking in the direction of sliding. Clearly, the coarsened grain region strongly aligns its bcc packed direction along the direction of sliding. (b) Evolution of the shear stress [$\tau (t)$] showing the materials’ resistance along the direction of sliding and the progression of the ratio of non-bcc to bcc atoms [$R(t)$] indicating the general reduction in the number of grains and grain boundary volume. The arrows recall the instances corresponding to the snapshots in Fig. 1.

Figure 3

The zoomed-in snapshots in panels (a)–(d) show an example of grain boundary migration resulting in grain growth where the grain boundary around the grain containing the green dot migrates and is absorbed into the grain boundary of the coarsening middle grain with noticeable dislocation activity around the migrating boundary (see lower part of each panel). The close-up views in panels (e)–(h) show an example of the localization of the sliding shear at a vertically propagating twin boundary [red region in the middle of the lower part of panels (e)–(h)] within the coarsened grain. The coloring scheme is the same as in Fig. 1.

Figure 4

Snapshot of initial (0 ns) and final (11.8 ns) grain structure showing shear induced grain growth while sliding two nanocrystalline copper counterbodies at $10\mathrm{m}/\mathrm{s}$ under controlled pressure at 1 GPa. The plot shows the corresponding evolution of the shear resistance and the non-fcc to fcc atom ratio. The crystal structure is exposed through a common neighbor analysis where green, white, and red represent fcc, non-fcc, and stacking fault atoms.