Kefu Gan1 Alfonso Ngan1

1, The University of Hong Kong, Hong Kong, , Hong Kong

Alloys with real microstructures involving second phase precipitates of duralumin and alternant lamellas from eutectic Sn/Pb system are investigated on mechanical properties during micron-scaled deformation. A series of unusual phenomena about size effect is reported as far from conventional notion, i.e., “smaller is stronger”: (i) precipitated duralumin much stronger than pure Al exhibits a prominent indentation creep with a shear viscosity approaching to pure Al within submicron depth but shear viscosity turns 4~5 times higher than pure Al in larger depth; (ii) precipitate duralumin micropillar exhibits a weakest strength at a critical size ~7µm meanwhile strain-hardening is also slowest and creep fastest at the same size. Moreover, the reduction of strength at the critical size is more significant in Peak-aged specimens than naturally aged ones; (iii) eutectic Sn/Pb micropillar presents an inverse size-dependent strength when the thickness of alternant lamellas approaches unit micron. Nevertheless, the scenario of inverse size effect will be suppressed with thickening the lamellar structure of Sn/Pb alloys. Theoretical modeling based on dislocation dynamics points out that those real microstructures are introducing an intrinsic length scale which will coact with external length scale coming from specimen size to control mechanical properties of micro-metals. The results indicate that the commonly known ‘smaller is stronger’ notion is not always right especially when intrinsic length scale is introduced by real microstructures to affect strength.