In a recent study published in Nature, Prof. LU Lei’s group at Shenyang National Laboratory for Materials Science (SYNL), Institute of Metal Research of Chinese Academy of Sciences, collaborating with Prof. GAO Huajian at Brown University, USA, revealed how nanotwinned metals are efficient to stabilize defects associated with repetitive strain and limits the accumulation of fatigued-related damage.

Researchers electroplated thin bulk Cu samples with highly oriented nanoscale twins to study the fatigue effects of nanotwined metals. Through a series of tension-compression experiments repeatedly on the sample with variable plastic strain amplitude, they found that increasing strain amplitude stepwise and then decreasing, and the stress amplitude of the sample reaches a steady state at a given strain amplitude and remains constant over the rest of the loading cycles, indicating a stable cyclic response.

The stress amplitude during the decreasing sequence is nearly identical to that during the increasing sequence at the same strain amplitude, implying that prior cyclic history has a negligible effect on the observed stress amplitude.

The history-independent cyclic response of nanotwinned metals is in sharp contrast to the history-dependent cyclic softening and the cyclic hardening of other conventional materials. Large scale molecular simulations demonstrated that this unusual behavior is governed by a type of single-slip, highly correlated necklace dislocations formed in the nanotwinned metal under cyclic loading.

The super-stable and reversible dislocations structure moves back and forth in a highly reversible manner during cyclic deformation, which does not destroy the coherency and stability of the twin structures. This unique fatigue mechanism is fundamentally distinct from traditional strain-localizing fatigue mechanisms associated with irreversible microstructural damage.

The unique cyclic behavior of nanotwinned metals not only advances the scientific understanding of the delocalized cyclic deformation mechanism but also sheds light on potential ways to tailor-design the microstructure of engineering materials with targeted fatigue properties.

This work was supported by the Ministry of Science and Technology of China, the National Natural Science Foundation and the Key Research Program of Frontier Science, Chinese Academy of Sciences.