Grain Boundary Sliding In Bicrystals: Experiments and Atomistic Simulations

Abstract

This Ph.D. dissertation investigates the influence of tension and compression loading on grain boundary sliding (GBS) using creep experiments and atomistic simulations.

Grain boundary sliding (GBS) is defined as the relative rigid-body displacement of two grains along their common boundary plane. It is a key mechanism during high-temperature deformation, alongside dislocation creep and diffusion creep. Depending on conditions, GBS acts either as:

Lifshitz sliding - accommodating diffusion creep.

Rachinger sliding - functioning as an independent mechanism, contributing significantly to superplastic deformation.

GBS can also lead to intergranular fracture by cavity nucleation if stress concentrations are not relieved by dislocation motion or diffusion flow. Factors influencing GBS include grain boundary structure, segregation, and dislocation-boundary interactions.

Experimental Study Creep experiments were performed on Al bicrystals with two orientations at 3 MPa and 608 K.

GBS components were measured using digital image correlation (DIC).

Results showed higher GBS contribution in tension compared to compression during early deformation, though differences diminished with strain.

EBSD analysis revealed more pronounced substructure evolution near grain boundaries in tension, explaining faster slide-hardening rates.

Atomistic Simulations Motivation: to understand the role of the normal stress component at grain boundaries in shear and diffusion-assisted matter transport.

-surfaces were calculated using molecular statics and nudged elastic band (NEB) methods.

Bicrystals with symmetric tilt boundaries (<100> 5 and <100> 37) were generated using LAMMPS with embedded atom potentials.

Simulations revealed asymmetry in -surfaces under tension vs compression, suggesting differences in sliding resistance.

Vacancy formation and migration energies were evaluated:

Formation energy was highest at the grain boundary plane and lowest in adjacent planes.

37 boundaries showed lower formation energy than 5 boundaries.

Loading direction had no effect on vacancy formation energy but significantly influenced vacancy migration energy.

Conclusion This study demonstrates:

GBS contributes more under tension than compression in early deformation stages.

Substructure evolution near grain boundaries is more pronounced in tension.

Atomistic simulations reveal tension-compression asymmetry in -surfaces and vacancy migration energies.

These findings provide mechanistic insights into tension-compression asymmetry observed in nanocrystalline materials and high-temperature deformation.