Processing induced size effects in plastic yielding upon miniaturisation

Size effects in metals have received considerable attention in literature in the last decades. For preparing specimens dedicated processing techniques, such as laser-cutting, micro-milling, turning, etc., are used. Most of these processing methods intrinsically damage crystals just below the worked surface. In macroscopic applications, the effect on the overall mechanical behaviour can safely be neglected in most cases. Upon miniaturisation, however, the influence of the affected region becomes more important and may induce a processing induced size effect, which is far from negligible. Processing induced size effects are analysed by carefully characterising the plastic yielding in uniaxial tension of rectangular, 300μm-thick aluminium sheet specimens, with a well-defined homogeneous microstructure containing through-thickness grains. The specimens are processed to different widths by three independent machining techniques: (1) laser-cutting, (2) mechanical cutting, and (3) extensive grinding from a larger width. These independent techniques all result in a distinct processing induced size effect upon miniaturisation, i.e. an increase of up to 200% in yield stress for a decrease from about 12 to 3 grains over the specimen width. Using a simple Taylor averaging model, it is shown that the yield stress in the affected edge region increased to 210–350% of its initial (or bulk) value. In addition, it is found that even a prolonged anneal near the melting temperature can only partially remove the processing induced size effect. The results clearly demonstrate that processing induced size effects have to be considered in the design of miniaturised devices and parts as well as in scientific research relying on the testing of manufactured small-scale test specimens.