Researchers from City University of Hong Kong (CityUHK), in collaboration with experts from local and overseas universities, have reshaped scientists’ fundamental understanding on the kinetic processes in crystalline materials, throwing lights on new approaches for materials processing and microstructure tailoring.

Crystals are characterised by well-arranged atoms in a lattice structure. Grain boundaries are planar defects where crystals with different orientations meet. Most crystalline materials are polycrystals, aggregates of many polyhedral crystallite grains with different orientations. Hence, grain boundaries are common defects in materials, which profoundly influence the mechanical and physical properties of materials. Manipulating the population of grain boundaries emerges as a potent strategy for tailoring material properties.

Traditionally, it is believed that the migration rate of a grain boundary is proportional to the driving force, with the proportionality coefficient, known as mobility, assumed to be constant. However, this research challenges that viewpoint by revealing that grain-boundary mobility is actually migration-direction-dependent, rather than a constant value.

“Consequently, grain boundaries can migrate unidirectionally without a net driving force. Interestingly, such non-driven grain-boundary migration resembles the unidirectional rotation of a Brownian ratchet,” said Professor Han Jian of the Department of Materials Science and Engineering (MSE), CityUHK.

This research is published in the top-tier journal Science, titled “Grain boundaries are Brownian ratchets”.

The research reveals that the mobility of most grain boundaries depends on the direction of grain-boundary migration. By conducting atomistic simulations on numerous grain boundaries under various conditions, the researchers have solidified the notion that grain-boundary mobility exhibits directionality as long as the two grains adjacent to the grain boundary are not symmetry-related, which is a common scenario.

As a grain boundary moves faster in one direction than in the opposite direction, a driving force oscillating about zero can induce grain-boundary migration in one direction. The researchers explained this behaviour with the Brownian ratchet model.

“Brownian ratchet is a device with a ratchet rotating unidirectionally as the paddle wheel undergoes rotation randomly in either direction due to the random kick of the atoms. Similarly, a grain boundary migrates unidirectionally when subjected to oscillatory driving forces or temperature,” said Qiu Caihao, a PhD student in MSE and first author of the paper.

This research reshapes the current understanding of grain boundary kinetics held by most researchers and textbooks, and implies a novel approach to controlling the microstructural evolution of materials.

The corresponding authors of this paper are Professor Han, Professor David Srolovitz of the University of Hong Kong, Professor Marco Salvalaglio of TU Dresden, and Professor Pan Xiaoqing of the University of California, Irvine.