Deformation of micron-sized aluminium bi-crystal pillars

Abstract

The deformation of micron-sized single-crystals is jumpy and stochastic, and this may pose potential formability and reliability problems if components for future micro-machines are to be made from small metal volumes. In this work, micron-sized bi-crystal pillars were fabricated by focussed ion-beam milling from grain-boundary regions in coarse-grained polycrystalline aluminium. Each bi-crystal pillar contained a grain boundary intersecting its top surface, and was subjected to compression using a flat-ended nanoindenter tip. Their deformation was found to have smaller strain bursts, fewer periods of strain hardening at elastic-like rates, as well as greater work-hardening rate and flow stress, than single-crystal pillars of similar sizes. Transmission electron microscopy revealed severe dislocation accumulation in the deformed bi-crystal pillars, whereas the residual dislocation density remained low in single-crystal micro-pillars of similar dimensions after deformation to comparable strains. The results suggest that a grain boundary inside a micro-specimen can trap dislocations inside the specimen, leading to a significant rise in the strain-hardening rate as well as to smoother deformation.

Keywords:

• bi-crystal
• micro-crystals
• aluminium
• deformation
• dislocation interactions
• dislocation mechanics
• dislocations
• ion beams
• nanoindentation
• nanomechanics

Acknowledgements

We thank the Electron Microscope Unit of HKU for their assistance. The work described in this paper was supported by a grant from the Research Grants Council of the Hong Kong Special Administration Region, P.R. China (Project No. HKU7156/08E).