Grain splitting is a mechanism for grain coarsening in colloidal polycrystals

TL;DR

This paper introduces a novel grain coarsening mechanism called grain splitting in colloidal polycrystals, supported by experiments and a new hard sphere model, highlighting its significance especially in small-grain regimes.

Contribution

The study reveals grain splitting as a new coarsening mechanism and develops a hard sphere model to analyze its energetics, extending understanding beyond traditional theories.

Findings

Grain splitting occurs via rotating granules with shapes determined by lattice structure.

The free energy barrier for splitting decreases with smaller grain size.

Grain splitting becomes more relevant in nanocrystalline regimes.

Abstract

In established theories of grain coarsening, grains disappear either by shrinking or by rotating as a rigid object to coalesce with an adjacent grain. Here we report a third mechanism for grain coarsening, in which a grain splits apart into two regions that rotate in opposite directions to match two adjacent grains' orientations. We experimentally observe both conventional grain rotation and grain splitting in 2D colloidal polycrystals. We find that grain splitting occurs via independently rotating "granules" whose shapes are determined by the underlying triangular lattices of the two merging crystal grains. These granules are so small that existing continuum theories of grain boundary energy are inapplicable, so we introduce a hard sphere model for the free energy of a colloidal polycrystal. We find that during splitting, the system overcomes a free energy barrier before ultimately reaching a lower free energy when splitting is complete. Using simulated splitting events and a simple scaling prediction, we find that the barrier to grain splitting decreases as grain size decreases. Consequently, grain splitting is likely to play an important role in polycrystals with small grains. This discovery suggests that mesoscale models of grain coarsening may offer better predictions in the nanocrystalline regime by including grain splitting.