Plastic flow in polycrystals

The plastic deformation of crystalline materials typically takes place via the elementary flow of topological defects such as dislocations.

Therefore, the dynamics of such defects under external stress is of central importance for understanding the mechanical behaviour of crystals. In contrast to their rapid propagation within single crystals, the motion of the defects is severely influenced by grain boundaries in polycrystals, leading to a mechanical reinforcement of polycrystalline materials which increases with the inverse average grain size. This empirically observed Hall-Petch relation has been explained with the Grain-boundary-assisted accumulation of defects which leads to an increasing yield strength. Evidence for this pile-up mechanism is provided by electron-microscopy experiments, where defects, which have been created by indentation of nanometer-sized tips, are observed to accumulate at the grain boundaries.

While the interactions of defects with grain boundaries (GB) have been intensively studied in atomic simulations, such detailed and particle-resolved investigations of the GB-defect interactions are limited from the experimental side. In particular, how the inhomogeneous atomic GB structure locally influences the incoming defects has not been thoroughly investigated in experiments. Such knowledge, however, is mandatory to provide quantitative relationships between the structure and the mechanical properties of polycrystalline materials.

Here, we experimentally and theoretically investigate with single-particle resolution how the atomic structure of GBs affects the dynamics of interstitial defects driven across monolayer colloidal polycrystals. Owing to the complex inherent GB structure, we observe that the motion of defects is more strongly hampered at specific positions of the GB than other positions, leading to their distortion and splitting upon crossing the GB. Below a critical driving force the defects are not able to cross GBs, which leads to their accumulation (pile-up) at these locations. A Hall–Petch-like relation is recovered by measuring the critical force as a function of the grain size. Remarkably, we observe that, under certain conditions, defects are reflected at GBs, leading to their enrichment at specific regions within polycrystals. Such channeling of defects within samples of specifically-designed GB structures opens up the possibility to design novel materials that are able to confine the spread of damage to certain regions.

Pile-up transmission and reflection of topological defects at grain boundaries in colloidal crystals
X. Cao, E. Panizon, A. Vanossi, N. Manini, E. Tosatti & C. Bechinger Nat. Commun, 11, 3079 (2020)