Smaller is stronger: The effect of strain hardening

Single-crystal face-centered cubic metal pillars synthesized using a focused ion beam are reported to be stronger when compressed in smaller volumes. Using in situ Laue diffraction and crystal plasticity simulations it is shown that plastic deformation is initially controlled by the boundary constraints of the microcompression tests, followed by classical crystal plasticity for uniaxial compression. Taking the stress at which the change between the two modes occurs as strength of the pillar instead of the flow stress at a fixed amount of strain, the "smaller is stronger" trend is considerably reduced, if not eliminated, and what remains is a size dependence in strain hardening. The size-dependent increase in flow stress is a result of the early activation of multiple slip systems and thus the evolution of the microstructure during compression. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.