Origins of size effects in initially dislocation-free single-crystal metallic micro- and nanocubes

TL;DR

This study investigates the origins of size effects in dislocation-free silver micro- and nanocubes, revealing how surface and defect factors influence dislocation nucleation and yield strength at small scales.

Contribution

It provides new insights into dislocation nucleation mechanisms and the role of surface defects and shape in size-dependent strength in initially dislocation-free crystals.

Findings

Yield strengths up to 25% of theoretical in pristine samples.

Dislocation nucleation influenced by surface defects and shape.

Size effect persists regardless of initial defect state.

Abstract

We report phenomenal yield strengths, up to one fourth of the theoretical strength of silver, recorded in microcompression testing of initially dislocation free silver micro and nanocubes synthesized from a multistep seed growth process. These high strengths and the massive strain bursts that occur upon yield are results of the initially dislocation free single crystal structure of the pristine samples that yield through spontaneous nucleation of dislocations. When the pristine samples are exposed to a focused ion beam to fabricate pillars and then compressed, the dramatic strain burst does not occur, and they yield at a quarter of the strength of their pristine counterparts. Regardless of the defect state of the samples prior to testing, a size effect is apparent, where the yield strength increases as the sample size decreases. Since dislocation starvation and the single arm source mechanisms cannot explain a size effect on yield strength in dislocation free samples, we investigate the dislocation nucleation mechanisms controlling the size effect through careful experimental observations and molecular statics simulations. We find that intrinsic or extrinsic symmetry breakers such as surface defects, edge roundness, external sample shape, or a high vacancy concentration can influence dislocation nucleation, and thus contribute to the size effect on yield strength in initially dislocation-free samples.