A long-standing obstacle to developing fatigue-resistant materials has been that metals typically suffer from large, accumulative, irreversible damages in microstructure during cyclic deformation, leading to a strong history-dependent and unstable cyclic response.

A recent study published in Nature shows how metals with nanoscale twins are efficient to stabilize defects associated with repetitive strain and limits the accumulation of fatigued-related damage.

Prof. LU Lei’s group at Shenyang National Laboratory for Materials Science (SYNL), Institute of Metal Research, Chinese Academy of Sciences (IMR, CAS), collaborated with Prof. Huajian Gao at Brown University, USA, focused on electroplated metals with nanotwins, tiny linear boundaries in a metal’s lattice that have identical crystalline structures on either side. To study the fatigue effects of nanotwined metals, they performed a series of tension-compression experiments repeatedly on the sample with variable plastic strain amplitude, increasing strain amplitude stepwise and then decreasing.

It is seen that 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 demonstrate that this unusual behavior is governed by a type of single-slip, highly correlated necklace dislocations (CND) formed in the nanotwinned metal under cyclic loading. The super-stable and reversible CND structure moves back and forth along the TBs 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 is 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.