Atomistic Mechanisms of Fatigue in Nanotwinned Copper
Xiaoling Zhou (Tsinghua University), Xiaoyan Li (Tsinghua University), Changqing Chen (Tsinghua University)
From Atomistics to Reality: Spanning Scales in Simulations and Experiments Symposium A
Wed 10:45 - 12:15
CIT 165
Nanotwinned (NT) metals, a new kind of hierarchical nanostructured material, have already attracted enormous research attention over past five years due to their unusual combination of ultra-high strength, good ductility, elevated strain hardening and large fracture toughness. Fatigue is one of the primary failure modes and is of both scientific and technological interests. So far, only limited studies regarding the fatigue of NT metals are available and the underlying mechanism currently remains unknown. Among them, existing experimental results are somewhat contradictory, with opposite effects of the nanotwins on fatigue reported. To develop a fundamental understanding of the fatigue mechanism in NT metals, we investigate the cyclic deformation of NT Cu using a combination of molecular statics and dynamics simulations. The presence of nanoscale twins is found to induce significantly improved resistance to fatigue crack growth. For a fatigue crack in twin-free nanocrystalline samples, it propagates by linking nano-voids formed ahead of the crack tip whist in nanotwinned samples it advances as the crack tip alternately blunts and re-sharpens, due to dislocation emission and slip. Both detwinning and crack closure are observed in the path of fatigue crack in nanotwinned samples. With the twin density increasing, the deformation mechanisms of detwinning and crack closer increase the dissipated energy of fatigue cracking, eventually leading to an enhancement in fatigue resistance. The atomistic simulations show that the fatigue crack growth in nanotwinned Cu is in the Paris’ law regime. In conjunction with experimental results (having the same twin densities as simulations), a quantitative estimation of the Paris’ law exponent (~ 4.0) is obtained, which is in excellent agreement with previous theoretical predictions.