| ABSTRACT:With the development of nanotechnology, it is found that the mechanical behavior of metallic materials with dimensions down to the nanometer scale is very different from that of the corresponding bulk materials. For a safe and reliable design of MEMS and NEMS, it is important to understand the mechanical behavior and deformation mechanism of the materials used. The plastic deformation mechanism of nano-materials is currently an active field of research essentially. At present, there’re difficulties for both traditional theory and mechanical experiments to characterize the mechanical properties of nano-materials. As an important method, molecular simulations can contribute to clarifying the competing deformation mechanism by providing explicit details on atomistic scale. In this work, molecular dynamics simulations are conducted to study some special mechanical properties and the related plastic deformation mechanisms of nanofilms and nanopillars. The main researches and results are listed as follow:1. Molecular dynamics simulations are employed to analyze the tension mechanical properties of single-crystalline nano-Cu films. Attention is directed to elucidate the microstructure evolution and deformation mechanisms. Computational results show that the plastic deformation mechanisms of [100] and [111]-oriented nano-Cu films are more likely due to the nucleation and gliding of partial dislocations and stacking faults. In particular, vacancy generation and migration in the film are carefully examined at the atomistic scale. It is found that the initial vacancy-type defects prefer to nucleate at the position of dislocation locks and the jogs of dislocation, while vacancy clusters and stacking-fault tetrahedrons are subsequently formed as the applied strain increasing. There is a size effect on the yield strength approximately obeying a power law σ∝H-m, the exponent value m here is mainly depend on whether the reorientation mechanism can be activated.2. The compression of Al nanopillars with different orientations and sizes are studied. For the four different orientations studied, different plastic deformation mechanisms are observed. In [100]-orientated nanopillars, stacking faults bounded by partial dislocations and micro-twins are the main plastic deformation mechanisms, whereas full dislocations are responsible for the deformation in [111],[112] and [265]-oriented nanopillars. The results show that whether the dislocation-starvation state can be achieved is a crucial factor governing the stress-strain response of small crystals. Both smaller diameter and fewer slip systems are expected to make the nucleated dislocation glide to the opposite free surface easily, thus inducing the serrated behavior. This is further confirmed by simulations on "coated" pillars, when dislocations produced are now trapped by the coating, and smooth rather than serrated deformation occurs corresponding to the frequent interactions in a mean-field manner.3. The deformation behaviors of square and circular Mg nanopillars are investigated. The results indicate that deformation mechanisms are different for different loading ways. Under c-axis tensile loading, it is found that {1012](π3) twin and pyramidal slips are the governing mechanisms of plastic deformation; while under c-axis compression, the nucleation and multiplication of <c+a> pyramidal slip is the main deformation mechanism. The different mechanisms under tension and compression are responsible for the so-called tension-compression asymmetry in magnesium and its alloys. Moreover, under compression-followed-by-tension loading, the pre-compression has no distinct influence on the deformation of the followed tension; while under tension-followed-by-compression loading, the followed compression deformation mechanism is changed to be detwinning, no longer the pyramidal slips. These results reveal the nature of the Bauschinger effect, which is closely related with the plastic deformation behaviors, such as slip, twinning and detwinning. |
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