Abstract
We performed molecular dynamics simulation of nanoindentation on Cu/Ni nanotwinned multilayer films using a spherical indenter, aimed to investigate the effects of hetero-twin interface and twin thickness on hardness. We found that both twinning partial slip (TPS) and partial slip parallel with twin boundary (PSPTB) can reduce hardness and therefore should not be ignored when evaluating mechanical properties at nanoscale. There is a critical range of twin thickness λ (~25 Å < λ < ~31 Å), in which hardness of the multilayer films is maximized. At a smaller λ, TPSs appear due to the reaction between partial dislocations and twin boundary accounts for the softening-dominated mechanism. We also found that the combination of the lowered strengthening due to confined layer slips and the softening due to TPSs and PSPTBs results in lower hardness at a larger λ.
Highlights
We performed molecular dynamics simulation of nanoindentation on Cu/Ni nanotwinned multilayer films using a spherical indenter, aimed to investigate the effects of hetero-twin interface and twin thickness on hardness
Before performing the nanoindentation on a Cu/Ni multilayer film, we demonstrated the validity of the potential and the nanoindentation model by comparing the fundamental physical properties predicted with molecular dynamics (MD) simulation with that obtained with first principles calculation/experiment (Tables S1 and S2 in Supplementary Materials), and comparing the simulation result of the indentations on pure Cu and Ni films with the result by Hertz solution (Figure S1 in Supplementary Materials)
By comparing the indentation force-depth (P-h) curve obtained with MD simulation with that with Hertz theory, we can find that they matches each other with an acceptable error (Figure S3 in Supplementary Materials), especially, the P-h curves obtained using an perfectly smooth sphere realized by a repulsive potential well matches the Hertz theory (Figure S3 in Supplementary Materials)
Summary
We performed molecular dynamics simulation of nanoindentation on Cu/Ni nanotwinned multilayer films using a spherical indenter, aimed to investigate the effects of hetero-twin interface and twin thickness on hardness. Li et al performed a large-scale molecular dynamics (MD) simulation of uniaxial tension of nanotwinned polycrystalline Cu14, and suggested a dislocation-nucleation-controlled softening mechanism in nanotwinned metals, i.e. when the twin thickness is below a threshold value, the nucleation of dislocation may result in softening, governing the strength of a material It is not clear if this kind of mechanism could be extended to Cu/Ni nanotwinned multilayer films. We performed MD simulations of nanoindentation on Cu/Ni nanotwinned multilayer films with various twin thicknesses using a spherical indenter, aimed to investigate the reaction between the activated dislocations and nanotwinned interfaces under three-dimensional stress states. The effects of twin thickness on the strengthening and hardening of the Cu/Ni multilayer films were analyzed in detail
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