Article

Multiscale simulation of onset plasticity during nanoindentation of Al (001) surface

School of Engineering and Materials Science, Queen Mary, University of London, London E1 4NS, UK
Acta Materialia (Impact Factor: 4.47). 09/2008; 56(16):4358-4368. DOI: 10.1016/j.actamat.2008.04.064

ABSTRACT

The onset of plasticity in crystalline materials is important to the fundamental understanding of plastic deformation and the development of precision devices. Dislocation nucleation and interactions at the onset of plasticity are investigated here using a multiscale quasi-continuum (QC) method for the nanoindentation of the (0 0 1) surface of a single crystal aluminium (Al) of 200 × 100 nm2 with infinite thickness. Deformation twinning was noted to occur during the nanoindentation of Al. We used unrelaxed and relaxed QC simulations with three embedded atom potentials of Al to evaluate the generalized planar fault (GPF) energies. The energy barrier for initial dislocation nucleation is much higher than that for subsequent nucleation events adjacent to the pre-existing defect. This mechanism promotes deformation twinning when some of the available slip systems are constrained. Dislocation initiation causes a minor load drop in the load–displacement curve, whereas major displacement excursion from experimental observations is the result of collective dislocation activities. (Some figures in this article are in color only in the on-line version.)

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Available from: Z. Xiao Guo, Nov 09, 2014
    • "Kim et al. [27] investigate dislocation interaction with a GB in nickel single crystal and a nickel bicrystal with a vertical GB, and show that the indentation nucleated dislocations in the shape of prismatic loops and they propagated along the slip system. Hereafter, much work was conducted on polycrystalline material [28] [29] [30]. With the improving technology and experimental method, more results from different studies can emerge and a better understanding is dispensable in the nanocrystalline materials to obtain more highly optimized materials. "
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    ABSTRACT: Molecular dynamics (MD) simulations are performed to study the nanoindentation onto three different crystal structures including the single crystalline, polycrystalline, and nanotwinned polycrystalline copper. To reveal the effects of crystal structure and twin-lamellae-thickness on the response of nanoindentation, we evaluate the evolution of crystalline structure, dislocation, strain, indentation force, temperature, hardness, and elastic recovery coefficient in the deformation zone. The results of MD simulations show that the hardness, elastic recovery ratio and temperature of those three nanocrystalline copper strongly depend on crystal structure and twin-lamellae-thickness. It is also revealed that as nanoindenter goes deeper, the extent of plastic zone becomes substantially larger. Initial dislocation always nucleates at the beneath of indenter, and the discrete drops of indentation force observed at certain indentation depths, indicates dislocation bursts during the indentation process. In particular, the twining and detwining are dominant over the dislocation nucleation in driving plasticity in nanotwinned polycrystalline during nanoindentation, which are in good agreement with the previous work. Furthermore, we find that plastic deformation has a strong dependence on crystal structure. The plastic deformation of the single crystalline copper relies on the generation, propagation and reaction of dislocations, that of the polycrystalline copper depends on the dislocation–grain boundary (GB) interactions, and that of the nanotwinned polycrystalline copper relies upon the dislocation–twin boundary (TB) interactions as well as twining/detwining. This work not only provides insights into the effects of crystal structure and two-lamellae-thickness on the mechanical properties of copper under nanoindentation, but also shed lights onto the guideline of understanding other FCC nanocrystalline materials.
    No preview · Article · Dec 2015 · Applied Surface Science
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    • "It can be seen from a critical analysis of the load-displacement curves that they found the initial displacement burst, often related to the initiation of dislocation activity, occurred at values close to the theoretical shear stress of the material. Jin et al. [17] investigated dislocation nucleation and interactions at the onset of plasticity using the multiscale quasicontinuum (QC) method for the nanoindentation of the single crystal aluminiurn. One important result in their research is that the energy barrier for initial dislocation nucleation is much higher than that for subsequent nucleation events adjacent to the pre-existing defect, which promoted deformation twinning when some of the available slip systems were constrained. "
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    ABSTRACT: Multiscale simulations of the tilted flat-ended nanoindentation with different tilted angles (from 5 degrees similar to 30 degrees) on the (-110) surface of nickel crystal were studied using the QC method. The model of the indentation is a flat-end indenter inclined by an angle epsilon driven into a halfplane vertically. Load-displacement responses, initial stages of the plasticity deformations and dislocation emissions for nickel film at different inclined angles were obtained and analyzed as well. An energy criterion was successfully proposed to analyze the critical load for the first dislocation emission beneath the edge of the indenter. Simulation results agree well with analytical ones. An elastic model based on the contact theory and the Peierls-Nabarro dislocation model were combined to analyze when and where the dislocation will be emitted beneath the lower surface of an inclined indenter. Results indicate that the key parameter is the ratio of the contact half-width to the position of the slip plane. This parameter shows the range in which a dislocation will probably be emitted. This mechanism explains the simulation results well. This work is of value for understanding the mechanism of dislocation emissions of FCC crystals under tilted flat-ended nanoindentation while providing approaches to predicting when the first dislocation will be emitted and where subsequent dislocations will probably be emitted.
    Full-text · Article · Oct 2015 · Acta Mechanica Solida Sinica
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    • "[2] For example, deformation twinning has been frequently observed even in NC face-centered cubic (fcc) materials that do not deform by twinning in their coarse-grained counterparts.[2] [3] Formation of twins in NC metals [2] [4] [5] plays a critical role in their physical and mechanical properties, such as good electrical conductivity, and excellent resistance to currentinduced diffusion.[6] [7] The interactions between twins and gliding dislocations at twin boundaries (TBs) have been observed both experimentally [7] [8] [9] [10] [11] [12] [13] [14] and by molecular dynamics (MD) simulations [15] [16] [17] [18] to result in an unusual combination of ultrahigh strength and high ductility.[7,9,19–24] In NC materials, MD simulations [25] [26] [27] have predicted that single or multiple deformation twins can be formed by emission of Shockley partial dislocations on adjacent {111} planes from grain boundaries . "
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    ABSTRACT: Most deformation twins in nanocrystalline face-centered cubic (NC fcc) metals are reported to produce zero-macrostrain, which is attributed to either random activation of partials (RAP) or cooperative slip of three partials (CSTP). Here, we report that when the RAP mechanism is suppressed, 44% twins in NC Cu produced zero-macrostrain via the CSTP mechanism. This indicates that both RAP and CSTP are major mechanisms to generate zero-macrostrain twins. In addition, our results also indicate that stress state affects the twinning mechanism in NC fcc metals, and monotonic activation of partials with the same Burgers vector dominates twin formation under monotonic stress.
    Full-text · Article · Nov 2013 · Research Letters in Materials Science
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