Size-dependent mechanical properties of crystalline nanoparticles

Defect-free crystalline nanostructures reach strengths which are close to their ultimate shear strength, since their deformation is controlled by dislocation nucleation from the surfaces. In this talk we examine how the size and shape of defect-free nanoparticles affect their mechanical properties. In the first part we discuss the indentation response. Earlier experiments on Au nanoparticles showed that they become easier to indent as they are smaller [1]. With large scale Molecular Dynamics Simulations, we show how the lateral dimensions give rise to size effect in indentation through the competition between dislocation storage and depletion on free surfaces. The latter mechanism is suppressed in BCC Fe nanoparticles due to strong pinning of dislocations to the indent, leading to a size-independent response to indentation [2]. Under compression, the size effect arises from a size-dependent dislocation nucleation threshold at the nanoparticle’s vertices [3]. A dislocation nucleation model is employed to study how stress at which FCC nanoparticles yield depends on material properties, such as the stacking fault energy and elastic constants. The size effect is shown to disappear in Ni3Al intermetallic nanocubes under compression, since the stress concentration vanishes in this geometry. An analysis of the dislocation evolution in Ni3Al nanoparticles shows that partial dislocations are nucleated at the vertices, shearing the nanoparticle with large complex stacking faults planes. This combined computational-experimental study provides us with insights on how to control dislocation-nucleation controlled deformation at the nanoscale

Nanoindentation of Au nanoparticles – A combined experimental/computational multiscale study

The idea of dimensionality and size effect the strength of metallic specimen as their typical size is pushed into the sub-micrometer scale is well established. The importance of the shape at the nanoscale was demonstrated on Au thin-films and nanoparticles in nanoindentation experiments. It was shown that nanoparticles are substantially softer than thin-films of the same height and the smallest nanoparticles are softer than the largest ones [1]. We propose that the size effect arises from the interaction between the lateral free surfaces on the plastic zone. However, experiments alone cannot provide the understanding on the governing microstructural dislocation mechanisms and we demonstrate here a combined experimental/computational study, by developing a multiscale frame to study nanoindentation of nanoparticles from the atomic- to the macro-scale. Please click Additional Files below to see the full abstract

Cross-Split of Dislocations: An Athermal and Rapid Plasticity Mechanism

The pathways by which dislocations, line defects within the lattice structure, overcome microstructural obstacles represent a key aspect in understanding the main mechanisms that control mechanical properties of ductile crystalline materials. While edge dislocations were believed to change their glide plane only by a slow, non-conservative, thermally activated motion, we suggest the existence of a rapid conservative athermal mechanism, by which the arrested edge dislocations split into two other edge dislocations that glide on two different crystallographic planes. This discovered mechanism, for which we coined a term “cross-split of edge dislocations”, is a unique and collective phenomenon, which is triggered by an interaction with another same-sign pre-existing edge dislocation. This mechanism is demonstrated for faceted α-Fe nanoparticles under compression, in which we propose that cross-split of arrested edge dislocations is resulting in a strain burst. The cross-split mechanism provides an efficient pathway for edge dislocations to overcome planar obstacles

Cross-slip in face-centered cubic metals: a general Escaig stress-dependent activation energy line tension model

<p>Cross-slip is a dislocation mechanism by which screw dislocations can change their glide plane. This thermally activated mechanism is an important mechanism in plasticity and understanding the energy barrier for cross-slip is essential to construct reliable cross-slip rules in dislocation models. In this work, we employ a line tension model for cross-slip of screw dislocations in face-centred cubic (FCC) metals in order to calculate the energy barrier under Escaig stresses. The analysis shows that the activation energy is proportional to the stacking fault energy, the unstressed dissociation width and a typical length for cross-slip along the dislocation line. Linearisation of the interaction forces between the partial dislocations yields that this typical length is related to the dislocation length that bows towards constriction during cross-slip. We show that the application of Escaig stresses on both the primary and the cross-slip planes varies the typical length for cross-slip and we propose a stress-dependent closed form expression for the activation energy for cross-slip in a large range of stresses. This analysis results in a stress-dependent activation volume, corresponding to the typical volume surrounding the stressed dislocation at constriction. The expression proposed here is shown to be in agreement with previous models, and to capture qualitatively the essentials found in atomistic simulations. The activation energy function can be easily implemented in dislocation dynamics simulations, owing to its simplicity and universality.</p

A multiscale study of the size-effect in nanoindentation of Au nanoparticles

International audienceThe mechanical response of nanoparticles is different than that of thin-films during nanoindentation tests. Moreover, it was shown experimentally that smaller nanoparticles are softer for nanoindentation. This size effect was attributed to the proximity of the free lateral surfaces to the indenter, which leads to dislocation-free surface interactions. We present here a multiscale study to show that the size effect is controlled by the interaction of the plastic zone formed beneath the indent and the lateral free surfaces. The detailed dislocation mechanisms and their interactions with the free surfaces are investigated using molecular dynamics (MD) and discrete dislocation dynamics (DDD) simulations. Au nanoparticles in the size range of 9–116 nm were indented with these two simulation techniques. The simulations show that shear dislocation loops are nucleated beneath the indent on all slip planes. Dislocations interactions facilitate their escape from beneath the indent, either by forming v- and u-shaped dislocations or prismatic loops that glide towards the lower part of the nanoparticles, or through glissile interactions that promote lateral dislocation motion. The effect of size on these dislocation mechanisms is discussed

Investigating Nanoscale Contact Using AFM-Based Indentation and Molecular Dynamics Simulations

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 dislocations under the indent while transferring material to the upper surface; dislocation density decreases in the plastic zone and material piles up around the indent. Finally, we discuss why a significant number of cross-slip events is expected beneath the indent under experimental conditions and the implications of this for work hardening during wear