The on-going miniaturisation in many modern technological areas (e.g. micro/nanoelectronics, integrated circuits, micro-electrical-mechanical systems) requires the knowledge of mechanical properties at small scales. Metals are strong because of their microstructure. At small scales, constraints increase the stress at which plastic deformation can occur; these constraints can include physical boundaries, free surfaces, grain boundaries, or strain gradients. Since the geometric and microstructural dimensions of the components can range from microns to nanometers, the constraints of the dimensions on dislocation activities and the effects of surfaces and interfaces in the small-scale components result in some anomalous plastic properties, including the size effects, with smaller being stronger, plastic recovery, plastic recovery, with reverse plastic flows already occurring even when the stress is still under unloading, etc.
The outputs of this programme have a positive interaction with the EU 3-year project "StrengthABLE" funded by European Metrology Programme for Innovation and Research. Here, we focus on determining the length-scale dependence of strength and designing rules for materials strengthened by ‘length-scale engineering’. This project finally produced a novel micro-torsion instrument, which has been used for characterizing the mechanical properties of various fibrous materials, for example, Cu wire, spider silk etc. The experimental technique therefore has potential commercial value in the fields of textile, fibrous composite and medical devices, etc. Through this project, we contributed to the development of torsion tester at small scales. A long term goal is to exploit the size effect in novel high-strength, lightweight materials. The experimental programme, together with the theoretical interpretations on interpreting the data, is expected to lead to major advances in metallurgical engineering, with potentially trillion-dollar impact on a public good.
The key aim of this project is to develop a physical understanding of plasticity mechanisms operating in polycrystalline and single-crystalline solids, when material dimensions are reduced to the micrometer scale (or to the size of their instrinsic microstructure) subjected to nonuniform deformation at elevated temperatures. Special focus will be on the influence of microstructural properties (e.g. grain size, specimen dimension and texture) and loading conditions (e.g. with and without strain gradient, monotonic and cyclic torsion) on the mechanical behaviours of thin wires at different temperature. Torsion experiments for different specimen sizes and different materials have been conducted. The experimental results are used to be compared with the theoretical predictions. The results will give a deeper understanding of micro-scale mechanics.
