Strengthening mechanisms in nanocrystalline alloys

The mechanisms for strengthening nanocrystalline metals by alloy additions are reviewed and a new model for nano-particle strengthening by Orowan bypassing in nano-grains is proposed. Recent experimental results for three different nanocrystalline alloy systems, Fe–Pb, Fe–Al2O3 and Al–Pb are presented and analyzed in terms of non-equilibrium solid solution strengthening, nano-composite strengthening and Orowan particle strengthening, respectively. Conflicting alloy hardening and softening effects observed in Al–Pb appear to be the result of interplay between Orowan particle hardening and a softening mechanism due to grain-boundary segregation. Preliminary MD simulations support the latter suggestion.