History-independent cyclic response of nanotwinned metals

Abstract

Nearly 90 per cent of service failures of metallic components and structures are caused by fatigue at cyclic stress amplitudes much lower than the tensile strength of the materials involved. Metals typically suffer from large amounts of cumulative, irreversible damage to microstructure during cyclic deformation, leading to cyclic responses that are unstable (hardening or softening) and history-dependent. Existing rules for fatigue life prediction, such as the linear cumulative damage rule, cannot account for the effect of loading history, and engineering components are often loaded by complex cyclic stresses with variable amplitudes, mean values and frequencies, such as aircraft wings in turbulent air. It is therefore usually extremely challenging to predict cyclic behaviour and fatigue life under a realistic load spectrum. Here, through both 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 that contain highly oriented nanoscale twins. We demonstrate that this unusual cyclic behaviour is governed by a type of correlated 'necklace' dislocation consisting of multiple short component dislocations in adjacent twins, connected like the links of a necklace. Such dislocations are formed in the highly oriented nanotwinned structure under cyclic loading and help to maintain the stability of twin boundaries and the reversible damage, provided that the nanotwins are tilted within about 15 degrees of the loading axis. This cyclic deformation mechanism is distinct from the conventional strain localizing mechanisms associated with irreversible microstructural damage in single-crystal, coarse-grained, ultrafine-grained and nanograined metals.

Tensile Properties and Thermal Stability of Unidirectionally <111>-Oriented Nanotwinned and <110>-Oriented Microtwinned Copper

“…Metallic materials with a high tensile strength might have a low-fatigue lifetime. However, the nanotwinned Cu films have been demonstrated to have high-fatigue resistance [8]. In addition, Li et al also reported nanotwinned Cu redistribution lines possess high-fatigue resistance to temperature cycling tests [20].…”

“…In recent years, nanotwinned copper (nt-Cu) was found to have high strength, ideal ductility, excellent thermal stability, low resistivity [4,5], high electromigration resistance [6], and history-independent cyclic response during fatigue tests [7,8]. Generally, magnetron sputtering and electroplating have been applied in fabricating highly <111>-oriented nt-Cu.…”

Effect of size of alpha phases on cyclic deformation and fatigue crack initiation during fatigue of an alpha-beta titanium alloy

“…Thus, due to the similar size and uniform distribution of α phase, FLM with nanoscale α phase possesses super resistance against fatigue crack initiation than BM. Moreover, Pan et al [36] has found that the dislocations move collectively back and forth along the nanotwins during cyclic loading, which indicated that nanotwins promotes the reversible dislocation slip and increases the fatigue life. Whereas, cyclic plastic deformation is produced at each cycle during low cycle fatigue.…”

In-situ nanoscale TEM observation of cooperative deformation mechanisms in nanocrystalline-Au films

“…As the extension of stacking faults, twin activities are highly related to partial dislocation activities. Moreover, studies revealed that the induced twins can enhance mechanical performances such as strength 88,89 , hardness 90,91 , fracture toughness 88,89 and anti-fatigue 92 .…”

“…However, in-situ TEM observation studies by Wang et al 94,95 revealed that full dislocation activities are still existent in the deformation processes of nc-Pt thin films, even in the d~10nm Pt grains. Figure 2.1 are high-resolution (HR) TEM images that show the full dislocations nucleation and annihilation processes during the tensile deformation processes 92 . Grain A (d~12 nm) was monitored during the whole deformation processes, as labelled by white dash lines in Figure 2.1a.…”