Mircomechanical insights into unconstrained grain boundary sliding

TL;DR

This study investigates the intrinsic behavior of grain boundary sliding (GBS) in Ni bicrystal micropillars, revealing that GBS is primarily driven by dislocation-mediated mechanisms and is less affected by strain-rate sensitivity.

Contribution

It isolates the intrinsic GBS behavior by comparing bicrystal and single-crystal responses, providing new insights into the fundamental mechanisms at high temperatures.

Findings

Intrinsic GBS has low strain-rate sensitivity (~0.034).

Activation energy for GBS is 234 kJ/mol, indicating dislocation-mediated diffusion.

High strain-rate sensitivity in polycrystals is mainly due to accommodation processes.

Abstract

Grain boundary sliding (GBS) is a key deformation mechanism at high homologous temperatures in polycrystalline materials, however, its intrinsic behavior is often obscured by additional strain accommodation processes. In this study, dislocation-mediated unconstrained GBS was investigated using Ni bicrystal micropillars containing a single high-angle grain boundary. Micropillar compression tests were conducted over a temperature range from room temperature to $600\,^{\circ}\mathrm{C}$ and strain rates between $5\times10^{-4}$ and $10^{-1}\,\mathrm{s}^{-1}$. By comparing bicrystal and single-crystal responses, the intrinsic contribution of GBS was isolated. The strain-rate sensitivity remained low (SRS $\approx 0.034 \pm 0.017$), comparable to room temperature values, indicating the absence of diffusion-controlled accommodation mechanisms. The activation energy for GBS was determined to be $234\,\mathrm{kJ\,mol^{-1}}$, consistent with grain boundary diffusion-assisted glide of grain boundary dislocations. These results demonstrate that the high strain-rate sensitivity commonly associated with GBS in polycrystals originates primarily from accommodation processes rather than the intrinsic sliding mechanism.