Size effects in cobalt plastically strained in tension: impact on gliding and twinning work hardening mechanisms

Mechanical properties in tension of cobalt have been studied a wide range grain size d using thickness t samples from 125 to 700 ?m. The Hall and Petch (HP) relationship indicates that the yield stress strongly varies for t/d below critical value around 14, highlighting existence multicrystalline regime. behavior becomes almost single crystalline when is lower than 2.5, with very small HP slope. reduction ratio has also large influence on work hardening cobalt, especially beginning plasticity, which mainly corresponds stage basal slip mechanisms. However, these effects tend vanish twinning predominant. Nanoindentation measurements application simple composite model show appearance regime closely related surface develop through specimens. In addition, Kocks - Mecking predicts significant increase average free path dislocations stage. A quantitative analysis dislocation pile-ups during this supports hypothesis mechanical softening observed multicrystal higher activity dislocations, can escape more easily surfaces. When main deformation mechanism, glide restricted reduced, turn, effect. Consequently, gradually disappear hardening.