Grain Boundary Diffusion Mechanisms in Metallic Alloys

Grain boundary diffusion is a form of short-circuit transport where atoms migrate preferentially along the interfaces between crystalline grains, often at rates orders of magnitude higher than through the lattice. This enhanced mobility arises from the structural disorder and excess free volume characteristic of grain boundaries, which lower activation barriers and provide high-density pathways for atomic jumps. Diffusion along these boundaries influences phenomena such as high-temperature creep, sintering, corrosion, precipitation and phase transformations. Temperature, impurity segregation and the presence of second-phase precipitates can modify boundary diffusivity by altering interfacial structure and energetics. Mechanistic regimes are typically categorised by the relative dominance of lattice and boundary diffusion, as described by classical formalisms such as those of Fisher and Coble. Advances in severe plastic deformation have enabled the production of ultrafine-grained and nanocrystalline alloys, where the increased volume fraction of grain boundaries allows tuning of transport processes through boundary character and morphology. Atomistic simulations reveal that short-circuit paths may involve collective string-like movements and that complexions formed by segregating elements or precipitating carbides can either accelerate or impede transport. Understanding these mechanisms is crucial for the design of alloys in energy, aerospace and microelectronic applications, where control of interfacial diffusion governs mechanical performance, stability under irradiation and high-temperature resistance.

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Grain Boundary Diffusion Mechanisms in Metallic Alloys publication trend

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Technical terms

Grain boundary: The interface between two crystalline grains with different orientations, characterised by structural disorder and enhanced atomic mobility.

Short-circuit diffusion: Rapid atomic transport along defect-rich paths such as grain boundaries or interfaces, bypassing slower lattice routes.

Segregation: The preferential accumulation of solute atoms at grain boundaries or interfaces, which alters interfacial chemistry and structure.

Precipitation: The formation of a second-phase particle at or near a grain boundary, which can modify boundary diffusivity and mechanical strength.

Vacancy: A point defect corresponding to a missing atom in the crystal lattice, enabling atomic migration through a jump mechanism.

Activation energy: The minimum energy barrier that atoms must overcome to migrate between lattice sites or along grain boundaries.

References

1. Grain boundary diffusion in a compositionally complex alloy: Interplay of segregation, precipitation and interface structures in a Ni–Cr–Mo alloy. Acta Materialia (2024).
2. Study on Diffusion Kinetics and Law of Chromium on the Surface of Low-Carbon Steel. Coatings (2023).
3. Kinetic Modeling of Grain Boundary Diffusion: Typical, Bi-Modal, and Semi-Lamellar Polycrystalline Coating Morphologies. Coatings (2022).
4. Atomic-level mechanisms of short-circuit diffusion in materials. International Journal of Materials Research (formerly Zeitschrift fuer Metallkunde) (2024).