Interfacial plasticity governs strain rate sensitivity and ductility in nanostructured metals

Abstract

Nano-twinned copper exhibits an unusual combination of ultrahigh strength and high ductility, along with increased strain-rate sensitivity. We develop a mechanistic framework for predicting the rate sensitivity and elucidating the origin of ductility in terms of the interactions of dislocations with interfaces. Using atomistic reaction pathway calculations, we show that slip transfer reactions mediated by twin boundary are the rate-controlling mechanisms of plastic flow. We attribute the relatively high ductility of nano-twinned copper to the hardening of twin boundaries as they gradually lose coherency during plastic deformation. These findings provide insights into the possible means of optimizing strength and ductility through interfacial engineering.

Fig. 1.

Atomistic modeling of interface-mediated slip transfer reactions. (a) A symmetric Cu bicrystal with a coherent TB in the middle, subjected to anti-plane shear. The bicrystal is cut to expose a pair of symmetric (111) slip planes inclined at θ = 70.5° with respect to the TB. (b) Schematic of two competing pathways of slip-transfer reaction discovered from CINEB calculations. The first pathway is a two-step process involving the absorption of an incoming screw into the TB, followed by desorption. The second involves direct transmission of the screw across the TB. (c) Atomic configurations of absorption, desorption and direct transmission; two views are shown for each state. Atoms are color coded by the central symmetry parameter, showing the stacking fault and TB.

Fig. 2.

Transmission electron microscopy image shows the accumulation of TB dislocations. Coarse-graining the discrete atomic-scale processes over time establishes an ADT kinetic equation. [Electron micrograph reproduced with permission from ref. (Copyright 2006, Elsevier).]

Fig. 3.

Screw dislocation absorption under the influence of preexisting TB dislocations. The preabsorption equilibrium state is shown in a and b; the transition state is shown in c. Atoms are color coded by Mises invariant of local strain (subtracting off the average strain) in a, showing elastic interaction of the incoming screw with two TB Shockley partials, and the central symmetry parameter (b and c). Note that the TB has migrated by one step after absorbing the two TB Shockley dislocations.