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A free-end adaptive nudged elastic band (FEA-NEB) method is presented for finding transition states on minimum energy paths, where the energy barrier is very narrow compared to the whole paths. The previously proposed free-end nudged elastic band method may suffer from convergence problems because of the kinks arising on the elastic band if the initial elastic band is far from the minimum energy path and weak springs are adopted. We analyze the origin of the formation of kinks and present an improved free-end algorithm to avoid the convergence problem. Moreover, by coupling the improved free-end algorithm and an adaptive strategy, we develop a FEA-NEB method to accurately locate the transition state with the elastic band cut off repeatedly and the density of images near the transition state increased. Several representative numerical examples, including the dislocation nucleation in a penta-twinned nanowire, the twin boundary migration under a shear stress, and the cross-slip of screw dislocation in face-centered cubic metals, are investigated by using the FEA-NEB method. Numerical results demonstrate both the stability and efficiency of the proposed method.

The effects of segregation of impurity molybdenum (Mo) atoms on the tensile mechanical properties of nanocrystalline nickel (Ni) are investigated with molecular dynamics simulation. The results show that the segregation of Mo atoms induces an obvious increase in the elastic modulus and strength of nanocrystalline Ni, and the strengthening effect is more significant with smaller grain size. When the grain size decreases below a critical value, at which the softening occurs in non-segregated Ni-Mo alloy, no evident softening phenomenon is observed in Mo-segregated systems. Furthermore, based on a bicrystal configuration, it is found that Mo atoms segregating to the grain boundary reduce the energy and mobility of the grain boundary, increasing the grain boundary stability and thus accommodating the strengthening. The present findings will shed light on the fabrication of high strength nanocrystalline materials by controlling the segregation of atoms.

Based on molecular dynamics simulations, tensile mechanical properties and plastic deformation mechanisms of nano-twinned Cu//Ag multilayered materials are investigated in this work. Simulation results show that, due to the stronger strengthening effect of the twin boundary than both the cube-on-cube and hetero-twin interfaces between Cu and Ag layers, the strength increases with the increase of layer thickness for nano-twinned Cu//Ag multilayered materials with a constant twin spacing, while it decreases with the increase of layer thickness for twin-free ones. The strength of hetero-twin multilayered materials is higher than that of the cube-on-cube samples due to the different hetero interfacial configurations. The confined layer slip of dislocation is found to be the dominant plastic deformation mechanism for twin-free hetero-twin multilayered materials and the strength versus twin spacing in nano-twinned samples follows the conventional Hall-Petch relationship. These findings will shed light on the understanding of the plastic deformation mechanisms and the fabrication of high strength nano-twinned multilayered metallic materials.

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Metals with nanoscale twins have shown ultrahigh strength and excellent ductility, attributed to the role of twin boundaries (TBs) as strong barriers for the motion of lattice dislocations. Though observed in both experiments and simulations, the barrier effect of TBs is rarely studied quantitatively. Here, with atomistic simulations and continuum based anisotropic bicrystal models, we find that the long-range interaction force between coherent TBs and screw dislocations is negligible. Further simulations of the pileup behavior of screw dislocations in front of TBs suggest that screw dislocations can be blocked kinematically by TBs due to the change of slip plane, leading to the pileup of subsequent dislocations with the elastic repulsion actually from the pinned dislocation in front of the TB. Our results well explain the experimental observations that the variation of yield strength with twin thickness for ultrafine-grained copper follows the Hall-Petch relationship.

Molecular dynamics simulations are performed to investigate the effects of grain size gradient and twin thickness gradient on uniaxial tensile deformation behaviors of gradient-structured polycrystalline copper. Simulation results reveal that there exist an inverse Hall-Petch effect and gradient structure regulated plastic deformation mechanisms. That is, the average flow stress of the gradient nanograined (GNG) model or the gradient nanotwinned (GNT) model decreases with decreasing the grain size or twin boundary (TB) spacing, exhibiting the breakdown in the Hall-Petch relationship when the grain size or TB spacing is smaller than a critical size. The dominant plastic deformation mechanism is found to transform from the grain boundary (GB)-mediated one to the dislocation-based counterpart with the increase of the applied strain in the GNG model. The TB migration and annihilation dominate the plastic deformation in the grains with small TB spacing; while in the grains with large TB spacing, the dislocation multiplication and cutting across TB are mainly responsible for accommodating the deformation in the GNT model. Furthermore, the gradient distribution of strain and incompatibility of deformation induced by GB or TB gradient distribution are observed by monitoring the microstructural evolution.

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