Size and passivation effects in the torsion of thin metallic wires

On elastic gaps in strain gradient plasticity: 3D discrete dislocation dynamics investigation

“…As depicted in Fig. 5a, the apparent yield stress increases with decreasing the grain size d, which is in accordance with the strengthening effect observed in numerous small scale experiments [52,53]. Moreover, linear regression analysis indicates that the apparent yield stress evolves approximately linearly with d −1/2 as predicted by the Hall-Petch (HP) relation (Fig.…”

“…1 for illustration): (i) elastic gap at initial yield, which can be described as a delay in the plastic flow at the classical yield point, and (ii) elastic gap after an infinitesimal change in the boundary conditions, which causes an interruption of the initiated plastic flow until a further yield point is reached. The first type of elastic gaps is somewhat accepted in the literature [13,28,49] and is typically explained by the strengthening effect, i.e., increase of the apparent yield stress with decreasing size, which has been observed in many small scale experiments [4,52,53]. It is worth mentioning that the strengthening effect, as observed experimentally, is more likely associated with a micro-plastic regime with a high hardening rate at the beginning of plasticity, rather than a delay in the plastic flow in the strict sense as predicted by SGP theories.…”

Toward the development of plasticity theories for application to small-scale metal structures

“…The present set of experiments on the L-beam specimens systematically reveals the potential of very thin surface coatings to appreciably enhance the strength of micron-scale structures. However, we should call attention to an early experimental study by Vlassak et al ( 53 ), which demonstrated that surface passivated Cu thin films in bending have higher strength than their unpassivated counterparts, and a very recent study by Xie et al ( 54 ) on torsion of polycrystalline Cu wires with diameters ranging from 50 μm to 25 μm, which revealed a small, but clear, strengthening effect of a thin Ti surface coating.…”