Engineer metals with internal interfaces for enhanced mechanical performance

Gradient microstructures with internal interfaces exist ubiquitously in nature and are increasingly being introduced in next generation engineering materials with unprecedented mechanical properties. Here we discuss some recent studies on engineering metals with nature-inspired internal interfaces and gradients. First, metals typically suffer from cumulative, irreversible damage to microstructure during cyclic deformation, leading to limited fatigue life along with cyclic responses that are unstable and history-dependent. Through atomistic simulations and variable-strain-amplitude cyclic loading experiments at stress amplitudes lower than the tensile strength of the metal, we report a history-independent and stable cyclic response in bulk copper samples with microstructures mimicking the highly oriented nanoscale twin boundaries in conch shells. We demonstrate that this unusual cyclic behaviour is governed by an unusual type of dislocations called correlated ‘necklace’ dislocations (CNDs). Furthermore, we show that introducing gradient nanotwinned structure in metals results in extra strengthening that defies the classical rule of mixture theory. This phenomenon is attributed to another new type of dislocations called bundles of concentrated dislocations (BCDs).