Atomistic simulations of spherical indentations in nanocrystalline gold (original) (raw)
Monotonic and cyclic plastic deformation behavior of nanocrystalline gold: atomistic simulations
Journal of Molecular Modeling, 2019
Monotonic and cyclic plastic deformation behavior of nanocrystalline gold was investigated at room temperature using molecular dynamics simulation. Yielding starts in nanocrystalline gold by initiation of 1/6 〈1 1 2〉 Shockley partial dislocation from the grain boundary. A stacking fault is created between the grain boundary and moved Shockley partial dislocation. With the progression of further deformation, increase in dislocation density inside the grain, the formation of dislocation locks such as Stair-rod 1/6 〈1 1 0〉 and Hirth 1/3 〈1 0 0〉 dislocations, generation and removal of stacking fault tetrahedron are observed. Grain coarsening and a rise in the number of dislocation inside the grains were noted with an increasing number of cycles.
Grain Boundary Structure Evolution in Nanocrystalline Al by Nanoindentation Simulations
2005
The nanoindentation of a columnar grain boundary (GB) network in nanocrystalline Al has been examined by atomistic simulation. The goal of this study was to gain fundamental understanding on the relationship between structure evolution at GBs and incipient plasticity for indenter tips significantly larger than the average grain size. The nanoindentation simulations were performed by quasicontinuum method at zero temperature. A GB network made of vicinal and high-angle <110> tilt GBs was produced by generating randomly-oriented 5-nm grains at the surface of a 200 nm-thick film of Al. The major findings of this investigation are that (1) nanocrystalline GB networks profoundly impact on the nanoindentation response and cause significant softening effects at the tip/surface interface; (2) GB movement and deformation twins are found to be the predominant deformation modes in columnar Al, in association with shear band formation by GB sliding and intragranular slip, and crystal growth by grain rotation and coalescence; and (3) the cooperative processes during plastic deformation are dictated by the atomic-level redistribution of principal shear stresses in the material.
Dislocation nucleation during nanoindentation of aluminum
Journal of Applied Physics, 2008
Through multiscale simulations, we explore the influence of both smooth and atomically rough indenter tips on the nucleation of dislocations during nanoindentation of single-crystal aluminum. We model the long-range strain with finite element analysis using anisotropic linear elasticity. We then model a region near the indenter atomistically and perform molecular dynamics with an embedded atom method interatomic potential. We find that smooth indenters nucleate dislocations below the surface but rough indenters can nucleate dislocations both at the surface and below. Increasing temperature from 0 to 300 K creates prenucleation defects in the region of high stress and decreases the critical depth.
One-to-one spatially matched experiment and atomistic simulations of nanometre-scale indentation
Nanotechnology, 2013
We have carried out nanoindentation studies of gold in which the indenter is atomically characterized by field-ion microscopy and the scale of deformation is sufficiently small to be directly compared with atomistic simulations. We find that many features of the experiment are correctly reproduced by molecular dynamics simulations, in some cases only when an atomically rough indenter rather than a smooth repulsive-potential indenter is used. Heterogeneous nucleation of dislocations is found to take place at surface defect sites. Using input from atomistic simulations, a model of indentation based on stochastic transitions between continuum elastic-plastic states is developed, which accurately predicts the size distributions of plastic 'pop-in' events and their dependence on tip geometry.
Analysis at atomic level of dislocation emission and motion around nanoindentations in gold
Surface Science, 2004
Scanning tunnelling microscopy of a reconstructed Au(0 0 1) surface shows that two types of dislocation configuration are created by nanoindentation: ÔmesaÕ-shaped dislocation loops and ÔscrewÕ dislocation loops. We analyse the generation of these loops at the initial stages of plastic deformation of the surface, their glide and cross-slip and their interaction with existing steps on the surface. We also show that the standard dislocation theory for an elastic continuum adequately explains the observed dislocation configurations, their spatial distribution, motion and interaction with other defects.
Grain boundary effect on nanoindentation: A multiscale discrete dislocation dynamics model
Journal of The Mechanics and Physics of Solids, 2019
Nanoindentation is a convenient method to investigate the mechanical properties of materials on small scales by utilizing low loads and small indentation depths. However, the effect of grain boundaries (GB) on the nanoindentation response remains unclear and needs to be studied by investigating in detail the interactions between dislocations and GBs during nanoindentation. In the present work, we employ a threedimensional multiscale modeling framework, which couples three-dimensional discrete dislocation dynamics (DDD) with the Finite Element method (FEM) to investigate GB effects on the nanoindentation behavior of an aluminum bicrystal. The interaction between dislocations and GB is physically modeled in terms of a penetrable GB, where piled-up dislocations can penetrate through the GB and dislocation debris at GBs can emit full dislocations into grains. In the simulation, we confirmed two experimentally observed phenomena, namely, pop-in events and the dependence of indentation hardness on the distance from GB. Two pop-in events were observed, of which the initial pop-in event is correlated with the activation and multiplication of dislocations, while the GB pop-in event results from dislocation transmission through the GB. By changing the distance between the indenter and GB, the simulation shows that the 2 indentation hardness increases with decreasing GB-indenter distance. A quantitative model has been formulated which relates the dependency of indentation hardness on indentation depth and on GB-indenter distance to the back stress created by piled-up geometrically necessary dislocations in the plastic zone and to the additional constraint imposed by the GB on the plastic zone size.
Dislocation cross slip and formation of terraces around nanoindentations in Au(001)
PHYSICAL REVIEW B, 2003
Dislocation configurations associated to the first stages of plastic deformation around a nanoindentation in a Au͑001͒ surface are recognized and characterized. Dislocations with a screw component are shown to glide across ͕111͖ planes and by a cross-slip mechanism give rise to revolving terraces in the neighborhood of the nanoindentation trace with their edges parallel to compact ͗110͘ directions. The standard dislocation theory for an elastic continuum is shown to adequately describe the dislocation configuration observed and to provide an explanation of the generation of the terraces.
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 twolamellae-thickness on the mechanical properties of copper under nanoindentation, but also shed lights onto the guideline of understanding other FCC nanocrystalline materials.
Atomistic simulations of incipient plasticity under Al (111) nanoindentation
Mechanics of materials, 2005
Atomistic simulations are performed for the study of defect nucleation and evolution in Al single crystal under nanoindentation. Methodologies employed include the molecular dynamics and molecular mechanics simulations with embedded-atom potentials. Simulated is the indenting process on Al(1 1 1) surface with the spherical tip of indenter. Using the visualization technique of centrosymmetry parameters, homogeneous nucleations and early evolutions of dislocations are investigated for deepening our understanding of incipient plasticity at the atomic scale. We have shown that the nucleation sites of initial dislocation loops vary with the empirical potentials chosen for the simulation. Identifications are also made for the continuously changing structures of dislocation locks underneath the indenter tip and for the glide of prismatic partial dislocation loops far away from the contact surface.
Physical Review B, 2005
The strength of nanocrystalline aluminum has been studied using molecular dynamics simulation. Nanocrystalline models consisting of hexagonal grains with grain size d between 5 nm and 80 nm are deformed by the application of tension. A transition from grain-size hardening to grain-size softening can be observed in the region where d Ϸ 30 nm, which is the optimum grain size for strength. In the grain-size hardening region, nanocrystalline models primarily deform by intragranular deformation. Consequently, a pile-up of dislocations can be observed. When the grain size becomes less than 30 nm, where the thickness of the grain boundaries cannot be neglected in comparison to the grain sizes, the dominant deformation mechanism of nanocrystalline metals is intergranular deformation by grain boundary sliding. Further, geometrical misfits by grain boundary sliding are accommodated by the grain rotation mechanism. Moreover, cooperative grain boundary sliding occurs in the 5 nm model. The optimum grain size is controlled by the relationship between resistance to intergranular deformation by grain boundary processes and intragranular deformation resisted by the grain boundary. Therefore, the primary role of the grain boundary changes in the region where the optimum grain size is observed.
Investigating Nanoscale Contact Using AFM-Based Indentation and Molecular Dynamics Simulations
Metals, 2022
In this work we study nanocontact plasticity in Au thin films using an atomic force microscope based indentation method with the goal of relating the changes in surface morphology to the dislocations created by deformation. This provides a rigorous test of our understanding of deformation and dislocation mechanisms in small volumes. A series of indentation experiments with increasing maximum load was performed. Distinct elastic and plastic regimes were identified in the force-displacement curves, and the corresponding residual imprints were measured. Transmission electron microscope based measured dislocation densities appear to be smaller than the densities expected from the measured residual indents. With the help of molecular dynamics simulations we show that dislocation nucleation and glide alone fail to explain the low dislocation density. Increasing the temperature of the simulations accelerates the rate of thermally activated processes and promotes motion and annihilation of ...
Grain growth behavior at absolute zero during nanocrystalline metal indentation
Applied Physics Letters, 2006
The authors show using atomistic simulations that stress-driven grain growth can be obtained in the athermal limit during nanocrystalline aluminum indentation. They find that the grain growth results from rotation of nanograins and propagation of shear bands. Together, these mechanisms are shown to lead to the unstable migration of grain boundaries via process of coupled motion. An analytical model is used to explain this behavior based on the atomic-level shear stress acting on the interfaces during the shear band propagation. This study sheds light on the atomic mechanism at play during the abnormal grain coarsening observed at low temperature in nanocrystalline metals.
Atomistic simulation of the deformation mechanism during nanoindentation of gamma titanium aluminide
Computational Materials Science, 2012
In this paper we present a large-scale molecular dynamics simulation that describes the deformation mechanism of an ordered intermetallic compound (TiAl) during a nanoindentation procedure. Using a totally rigid spherical indenter we were able to address the question on which mechanism underlies its plastic deformation, namely homogeneous defect nucleation followed by the expansion of dislocation loops. By means of the calculated local pressure, local shear stress and spatial rearrangements of atoms beneath the indenter, it was possible to quantify the indentation damage on the crystalline structure. Our results show that both emission and interaction of dislocations are mediated by expansion of glide loops on the {1 1 1} planes resulting in the formation of prismatic loops. Moreover, through the load-penetration depth response we estimated the elastic modulus and hardness values of the system, which are in good agreement with experimental results.
Journal of theoretical and applied mechanics, 2014
Some specially designed metallic alloys crystallize during process of rapid quenching which aims their amorphization. Nevertheless, change in their mechanical properties could be seen compared to these obtained during conventional technological regimes of cooling. That attracts the attention in this elaboration. Full 3-D numerical simulations of nanoindentation process of two material models are performed. The models reflect equivalent elastic and different plastic material properties. The plastic behaviour of the first one is subjected to yield criterion of Dracker-Prager and this of the second one to yield criterion of Mises. The reported numerical results depending on the nanoindentation scale length of 1000 nanometers, suggest different adequacy of the two yield criteria to the data obtained experimentally with a Zr-Al-Cu-Ni-Mo alloy. It could be speculated that the different effects developed depending on the indenter travel of 1000 nanometers and taken into account in the two yield criteria stand behind this fact and determinate three structural levels of plastic deformation.
Review of Nanoindentation Size Effect: Experiments and Atomistic Simulation
Crystals, 2017
Nanoindentation is a well-stablished experiment to study the mechanical properties of materials at the small length scales of micro and nano. Unlike the conventional indentation experiments, the nanoindentation response of the material depends on the corresponding length scales, such as indentation depth, which is commonly termed the size effect. In the current work, first, the conventional experimental observations and theoretical models of the size effect during nanoindentation are reviewed in the case of crystalline metals, which are the focus of the current work. Next, the recent advancements in the visualization of the dislocation structure during the nanoindentation experiment is discussed, and the observed underlying mechanisms of the size effect are addressed. Finally, the recent computer simulations using molecular dynamics are reviewed as a powerful tool to investigate the nanoindentation experiment and its governing mechanisms of the size effect.
Atomistic processes of dislocation generation and plastic deformation during nanoindentation
To enable plastic deformation during nanoindentation of an initially defect-free crystal, it is necessary first to produce dislocations. While it is now widely accepted that the nucleation of the first dislocations occurs at the start of the pop-in event frequently observed in experiments, it is unclear how these initial dislocations multiply during the early stages of plastic deformation and produce pop-in displacements that are typically much larger than the magnitude of the Burgers vector. This uncertainty about the complex interplay between dislocation multiplication and strain hardening during nanoindentation makes a direct correlation between force–displacement curves and macroscopic material properties difficult. In this paper, we study the early phase of plastic deformation during nanoinden-tation with the help of large-scale molecular dynamics simulations. A skeletonization method to simplify defect structures in atomistic simulations enables the direct observation and quantitative analysis of dislocation nucleation and multiplication processes occurring in the bulk as well as at the surface.
Nature Materials, 2002
nature materials | VOL 1 | SEPTEMBER 2002 | www.nature.com/naturematerials 1 T he unusual mechanical behaviour of nanocrystalline materials 1 , showing either greatly enhanced ductility 2-4 or dramatically increased strength and hardness 5-7 is thought to arise from the intricate interplay between dislocation and grain-boundary processes (see, for example, ref. 8). The common low-temperature plasticdeformation mechanism in coarse-grained metals and ceramics involves the continuous nucleation of dislocations from Frank-Read sources and their glide, on well-defined slip systems, through the crystal. In a polycrystalline material, the size of these sources cannot exceed the grain size.Because the stress needed for their operation is inversely proportional to the size of the source, this deformation mechanism can operate only down to a grain size of typically about 1 µm. For a smaller grain size, mobile dislocations must be nucleated from other sources, such as the grain boundaries (GBs) or grain junctions.
Stress concentration around nanosized defects such as cavities always leads to plastic deformation and failure of solids. We investigate the effects of depth, size, and shape of a lotus-type nanocavity on onset plasticity of single crystal Al during nanoindentation on a (001) surface using a quasicontinuum method. The results show that the presence of a nanocavity can greatly affect the contact stiffness (S c) and yield stress (σ y) of the matrix during nanoindentation. For a circular cavity, the S c and σ y gradually increase with the cavity depth. A critical depth can be identified, over which the S c and σ y are insensitive to the cavity depth and it is firstly observed that the nucleated dislocations extend into the matrix and form a y-shaped structure. Moreover, the critical depth varies approximately linearly with the indenter size, regarding the same cavity. The S c almost linearly decreases with the cavity diameter, while the σ y is slightly affected. For an ellipsoidal cavity, the S c and σ y increase with the aspect ratio (AR), while they are less affected when the AR is over 1. Our results shed light in the mechanical behavior of metals with cavities and could also be helpful in designing porous materials and structures.
Atomic mechanism of shear localization during indentation of a nanostructured metal
2007
Shear localization is an important mode of deformation in nanocrystalline metals. However, it is very difficult to verify the existence of local shear planes in nanocrystalline metals experimentally. Sharp indentation techniques may provide novel opportunities to investigate the effect of shear localization at different length scales, but the relationship between indentation response and atomic-level shear band formation has not been fully addressed. This paper describes an effort to provide direct insight on the mechanism of shear localization during indentation of nanocrystalline metals from atomistic simulations. Molecular statics is performed with the quasi-continuum method to simulate the indentation of single crystal and nanocrystalline Al with a sharp cylindrical probe. In the nanocrystalline regime, two grain sizes are investigated, 5 nm and 10 nm. We find that the indentation of nanocrystalline metals is characterized by serrated plastic flow. This effect seems to be independent of the grain size. Serration in nanocrystalline metals is found to be associated with the formation of shear bands by sliding of aligned interfaces and intragranular slip, which results in deformation twinning.
An atomistic simulation methodology is presented in which well-defined dislocation loops can be introduced on arbitrary slip-systems of nanocrystalline (nc) grains. This approach allows one to study loop expansion and deposition of dislocation segments into the surrounding grain boundaries (GBs) at finite temperature. Such a dislocation loop creation method is intended to aid in the systematic study of the dislocation/GB interaction within a fully three-dimensional GB network geometry, and will also facilitate the atomistic study of the pile-up phenomenon as a function of GB misorientation.