Extrinsic Size Effects Research Articles

Metal Nanocomposites Reinforced by Ceramic Nanoarchitecture: Exploiting Extrinsic Size Effects for High Mechanical Strength.

The pursuit of harnessing superior mechanical properties achieved through the size effect on a macroscopic scale has been a prominent focus in engineering, as size-induced strengthening is enabled only in the nanoscale regime. This study presents a metal/ceramic/metal (MCM) nanocomposite reinforced by ceramic nanoarchitectures. Through proximity-field nanopatterning, the inch-scale production of nanoarchitecture films is enabled in a single fabrication step. The developed three-dimensional (3D) Ni/Al2O3/Ni nanocomposite film exhibits significantly high compressive strength, corresponding to an increase of approximately 30% compared with that calculated using the upper limits of the conventional rule of mixtures. The exceptional strength of the 3D MCM nanocomposite can be attributed to the extrinsic size effect of the ceramic nanoarchitectures. By combining size-induced strengthening of ceramics with the strengthening law for composites, a new type of strengthening model is derived and experimentally validated using the 3D MCM nanocomposite.

Experimental investigation on extrinsic size effect in micro-orthogonal cutting of commercial aluminium alloy

The achievable precision in micro-machining is constrained by a phenomenon known as the size effect. The edge radius of the tool significantly influences the size effect leading to non-proportional scaling of process performance. The process becomes unscalable below a certain ratio of uncut chip thickness to edge radius termed as critical relative tool sharpness (RTS). Despite extensive studies on the size effect associated with non-linear process performance, very limited studies have focused on the critical RTS range. In the present study, orthogonal cutting experiments were conducted on a commercial aluminium alloy (Al6060) with different cutting-edge radiuses in the RTS range of 0.5 to 0.01. Extensive analysis of the morphology of the chips and tool-work material adherence has been carried out and correlated with the observed size effect on surface integrity. One of the key findings in this study was the size effect in the chips with respect to the chip curl radius and pitch. Between RTS 0.5 and 0.09, an increase in chip curl and pitch is observed, followed by a sharp fall up to RTS 0.07. In addition, another notable phenomenon was the formation of dual-layered chips with deformed layer on the rake side and lamellae like features on the free side. Furthermore, size effect was observed in surface roughness, surface hardness and flank adherence. In summary, the present investigation offers a comprehensive understanding of the size effect within the critical RTS range elucidating its multifaceted impact on micro-cutting.

Size effects in fatigue crack growth in confined volumes: A microbending case study on nanocrystalline nickel

Mechanical size effects are a well known phenomenon when the sample volume is reduced or the characteristic length of the microstructure is changed. While size effects in micropillar compression (smaller is stronger) or due to grain refinement (Hall-Petch) are well understood, this is less so in fracture mechanics. Given this lack of knowledge, the main question addressed in this work is: What happens to the fatigue crack growth properties when extrinsic size effects play a role? To answer this question, nanocrystalline nickel cantilevers, ranging in width from 5 to ▪, were subjected to fatigue crack growth. The crack growth rates and stress intensity factors were calculated and the Paris exponent in the stable crack growth regime was determined. It was found that the results scatter more strongly for the smaller cantilevers compared to the larger cantilevers. Results are interpreted in terms of plastic zone size and ligament size which are found to be critical for small cantilevers.

Extrinsic and Intrinsic Size Effects on Fracture Strength of Metallic Thin Films

Extrinsic and Intrinsic Size Effects on Fracture Strength of Metallic Thin Films

On the consequences of intrinsic and extrinsic size effects on the mechanical response of nanoporous Au

In this study, the consequence of intrinsic and extrinsic size effects on mechanical responses of nanoporous gold is investigated via microcompression testing. By varying the micropillar diameter (D) between 1 µm and 20 µm and the ligament size (L), 50 nm and 350 nm, a critical ratio (α = D/L = 20) was found, above which the test structure can be considered a representative volume element, resulting in identical mechanical response and uniform deformation. Below that value, both flow stress and elastic modulus decrease with decreasing pillar diameter, as evidenced for a measurement series with a fixed ligament size of 350 nm where the flow stress decreased by more than 50% (from approximately 5 to 2.5 MPa) and the elastic modulus was reduced from approximately 0.5 GPa to almost zero. Stochastic behavior along with non-uniform deformation and failure is observed for α < 10, suggesting that the size of the load-bearing units in this material is about 10 times the corresponding ligament size.

Effect of sample size and cooling rate on the plastic deformation behavior of bulk metallic glasses: A comparative study

Understanding the differences between the cooling-rate and size effects on the plastic deformation behavior of bulk metallic glasses (BMGs) is important not only for uncovering the plastic deformation mechanisms but also for achieving better plasticity of BMGs. In the present work, both the cooling-rate and size effects of a Zr-based BMG were investigated by quasi-static compressive experiments. The findings show that the macroscopic plasticity of BMGs can be remarkably enhanced by increasing the casting cooling rate, but moderately improved by decreasing the sample size. The intrinsic atomic arrangement stemming from cooling rate is the primary factor which affects the plastic deformation behavior rather than the extrinsic size effect, and the size effect is also dependent on the intrinsic cooling rate. The findings provide more explanations for the different size effects of BMG communities at different conditions, and are also of significance for achieving better plasticity in BMGs and BMG structures.

The influence of thickness/grain size ratio in microforming through crystal plasticity

Microscale forming operations have become popular due to miniaturisation which focuses on micro scale design and manufacturing of devices, components and parts, e.g. in RF-MEMS. In such processes when the component dimension becomes comparable with the grain size, considerable size effect is observed. Recent experimental studies on micron sized sheet specimens have shown that the ratio between thickness (t) and grain size (d) has a significant influence on the mechanical behavior of materials which cannot be explained merely with the intrinsic (grain) size effect. Even though the grain sizes are similar, remarkable differences in mechanical response is obtained for cases with different thicknesses. In this context the aim of the current study is to address this extrinsic type size effect through crystal plasticity simulations. A series of local crystal plasticity finite element simulations are conducted for different values of t/d ratio in tensile specimens. Different granular morphologies of polycrystalline samples are generated using Voronoi tessellation and tested under axial loading conditions. The macroscopic influence of varying t/d ratio is studied in detail and compared with the experimental findings in the literature.

Effect of Strain Rate and Extrinsic SIZE Effect on Micro-Mechanical Properties of Zr-Based Bulk Metallic Glass

In this study, the mechanical properties and deformation features of Zr-based bulk metallic glass (BMG) are investigated at micro-scale via in situ micro-pillar compression. Furthermore, the effects of the strain rate and micro-pillar diameter on respective stress–strain curves are investigated. Together with the mechanical properties, such unique in situ micro-pillar compression techniques provide physical status to the micro-pillars, referring to the instances of stress–strain curves. It is noted that the effect of the strain rate on the stress–strain behaviour of the BMG diminishes with increasing micro-pillar diameter. In contrast, yield and ultimate compressive strength increase with increasing micro-pillar diameter, up to 4 µm. The deformation details after compression, as a result of conformed mechanical loading, are analysed by SEM and TEM. As evident from electron microscopy investigation, the plastic deformation is evidenced by the presence of multiple slip/shear bands, acting as load accommodation mechanisms in the course of mechanical loading together and resemble local plastic flow (ductile in nature) between two shear plans.

Effects of radiation damage and precipitate distribution on micro-pillar compression testing of irradiated CuCrZr

The results of small-scale mechanical tests are convoluted by the so-called size effect, whereby materials appear stronger when the scale of the test is reduced to the order of microns or less. The dimensional range over which this occurs has been shown to be linked to a change in sample microstructure, such as the addition of defects induced by irradiation. To investigate this response, a CuCrZr alloy was subjected to proton irradiation and mechanically tested using micro compression of pillars with a range in size. It was found that irradiation defects dominate over the extrinsic size effect and the sensitivity to differences in precipitate microstructure was also somewhat reduced, suggesting that size-independent results could be obtained from much smaller test volumes in irradiated material compared to their non-irradiated counterparts. Finally, comparison was made between the increase in yield strength predicted by models and the experimentally measured values to establish the key parameters driving the strengthening behaviour.

Effects of Sample Geometry and Grain Size on Mechanical Property of Electrodeposited Gold Evaluated By Micro-Bending Test

In recent years, MEMS (microelectromechanical systems) devices are required to have high functionality and small size, especially for MEMS acceleration sensors, and sensitivity of a MEMS acceleration sensor is highly dependent on the Brownian noise [1,2]. The Brown noise is known to be inversely proportional to mass of movable parts in the sensor, and the noise is lowered by increasing the mass [1,2], which implies a number of volume is needed to ensure high sensitivity. However, increasing size of the component is not preferable. Because of this, a design of gold-based MEMS acceleration sensor is proposed. The density of gold is 19.3 g/cm3, and gold has high corrosion resistance and chemical stability. Most importantly, gold electrodeposition process can be easily integrated with current MEMS fabrication process.Mechanical properties of the material are essential informtion for design of MEMS components. For instance, the yield stress is needed to guarantee elastic deformation of the material during operation of the MEMS device. Mechanical properties of metallic materials are known to change with size of the sample used in a mechanical property characterization becomes smaller than micrometer scale, which is known as the sample size effect [3]. Therefore, mechanical property characterization of materials toward MEMS should be conducted using samples having the same dimensions as those used in MEMS. Various micro-mechanical testing methods have been developed to characterize micro-specimens, such as micro-compression test, micro-tensile test, and micro-bending test. Among them, micro-bending test is suggested to be the most suitable method to evaluate materials for applications in MEMS when compared with micro-compression and micro-tensile tests since cantilever-like structure is commonly utilized in movable components, such as the micro-spring in MEMS accelerometers. On the other hand, micro-cantilevers are reported to have the sample geometry effect [4], which the sample size effect is only observed when varying thickness of the micro-cantilever. Also, it has been reported that the sample size effect is dependent on ratio of the sample diameter to the average grain size, and materials prepared by electrodeposition are usually polycrystal. Thus, relationship between the sample geometry effect and the average grain size is investigated in this study.Two gold films were electrodeposited at different current density to change the average grain size. The average grain size was determined by electron backscatter diffraction, and the average grain size of film A and B were 1.2~1.3 and 0.6 μm, respectively. Samples used in the micro-bending test were micro-cantilevers fabricated from the gold films by focus ion beam. Thicknesses of the micro-cantilever were ranged from 6.0 to 13 μm. The micro-bending tests are conducted using a test machine specially designed for micro-specimens developed in our group as shown in Fig. 1.The yield stresses were in a range of 428 to 553MPa, which were all higher than that of the bulk gold (55 to 220 MPa). Table 1 shows the detailed dimensions and yield stress of all samples. Yield stresses were plotted against the log scale of thickness shown in Fig. 2. At first, the sample geometry effect was confirmed in samples fabricated from both film A and B. Then, the yield stresses of samples fabricated from film B were all higher than those of film A, which was a result of the Hall-Petch relationship. In Fig. 2, the slop was steeper for film A indicating the sample geometry effect was more obvious in samples composed of larger grains. [1] D. Yamane, T. Konishi, T. Matsushima, K. Machida, H. Toshiyoshi, and K. Masu, Appl Phys Lett 104 (2014) 074102[2] K. Machida, T. Konishi, D. Yamane, H. Toshiyoshi, K. Masu, ECS Trans. 61 (2014) 21-39.[3] J.R. Greer and J.Th.M. De Hosson: Plasticity in small-sized metallic systems: Intrinsic versus extrinsic size effect. Prog. Mater. Sci. 56, 654 (2011).[4] K. Suzuki, “Sample Geometry Effect on Mechanical Property of Electrodeposited Gold Evaluated by Micro-Bending Test", 45th International Conference on Micro & Nano Engineering (MNE2019), Rhodes Greece, Sep. 2019 Figure 1

Dislocation-graphene interactions in Cu/graphene composites and the effect of boundary conditions: a molecular dynamics study

In this paper, systematic molecular dynamics simulations were employed to study dislocation-graphene interactions in Cu/graphene systems using two carefully selected configurations: (i) edge dislocations in the pileup interacting with graphene during nanoindentation and (ii) complex dislocations interacting with graphene in compression of layered nanopillars. The intrinsic and extrinsic size effects were investigated with respect to varying the Cu lamella thickness and pillar sizes, revealing an anomalous extrinsic size effect: the smaller, the weaker. To understand which boundary conditions are physically consistent, both free and periodic graphene were considered. It was seen that edge dislocations in the pileup continuously transmitted across the free graphene through Cu/graphene interfacial sliding and graphene reorientation, whereas transmission was very difficult for periodic graphene as its in-plane deformation was required. In compression of Cu/graphene nanopillars, the strengthening effect of free graphene was found to be less obvious than that of the periodic case, which could be attributed to free graphene edges acting as dislocation sources after the compressed Cu overflowed the graphene sheet from all directions. We therefore conclude that the strengthening effect of periodic graphene inclusions appears to be overestimated, such that free graphene should be more appropriate.

Intrinsic and extrinsic size effects in materials

Intrinsic and extrinsic size effects in materials

Analysis of local grain boundary strengthening utilizing the extrinsic indentation size effect

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Size effects on thermomechanical failure of layered structure with generalized particle dynamics multiscale methods

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Size Effects on the Mechanical Properties of Nanoporous Graphene Networks

AbstractIt is essential to understand the size scaling effects on the mechanical properties of graphene networks to realize the potential mechanical applications of graphene assemblies. Here, a “highly dense‐yet‐nanoporous graphene monolith (HPGM)” is used as a model material of graphene networks to investigate the dependence of mechanical properties on the intrinsic interplanar interactions and the extrinsic specimen size effects. The interactions between graphene sheets could be enhanced by heat treatment and the plastic HPGM is transformed into a highly elastic network. A strong size effect is revealed by in situ compression of micro‐ and nanopillars inside electron microscopes. Both the modulus and strength are drastically increased as the specimen size reduces to ≈100 nm, because of the reduced weak links in a small volume. Molecular dynamics simulations reveal the deformation mechanism involving slip‐stick sliding, bending, buckling of graphene sheets, collapsing, and densification of graphene cells. In addition, a size‐dependent brittle‐to‐ductile transition of the HPGM nanopillars is discovered and understood by the competition between volumetric deformation energy and critical dilation energy.

Intrinsic-extrinsic size effect relationship for micromechanical tests

Miniaturized mechanical tests are commonly utilized to evaluate properties of materials, including thin films, nanostructured, and irradiated materials. However, the specimen size effect occurs when miniaturized sample geometries contain too few dislocation sources, resulting in elevated yield stresses. The size effect is controlled by extrinsic (specimen dimensions) and intrinsic (microstructure) factors. Here, we summarize extrinsic and intrinsic size effects from micro-compression pillar, micro-cantilever bend, and flexure studies reported in the archival literature. We find an approximately linear relationship between intrinsic and extrinsic size effects. Meaningful mechanical properties can be measured when extrinsic size dominates the intrinsic size.

Tailoring strength and plasticity of Ag/Nb nanolaminates via intrinsic microstructure and extrinsic dimension

Nanolayered metallic composites usually deform via a transition from homogeneous deformation to major shear banding with decreasing layer thickness, and thus the improvement of strength often sacrifices the plasticity of materials. Here, we show two methods to promote brittle-to-ductile transition in nanolayered Ag/Nb pillars. Intrinsically, while keeping the pillar diameter constant, the reduction of layer thickness can increase the strength of multilayers and suppress shear induced failure. Extrinsically, for a constant layer thickness, decreasing the diameter of pillar suppresses shear bands and promotes more uniform plastic deformation. Furthermore, the critical layer thickness at peak strength of multilayers increases monotonically with decreasing pillar diameter. Interface structures evolve from amorphous layer to coherent interface with reduction of layer thickness. Homogeneous co-deformation mediated by heterogeneous interfaces and columnar grain boundaries promotes a unique work hardening behavior. This study indicates that a combination of intrinsic and extrinsic size effect may enable the accomplishment of high strength and uniform deformation simultaneously.

Size effects on plasticity in high-entropy alloys

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Concurrent atomistic-continuum simulations of uniaxial compression of gold nano/submicropillars

ABSTRACTIn this work, uniaxial compression of nano/submicropillars in Au with the initial diameter D between 26.05 and 158.53 nm was modelled by concurrent atomistic-continuum simulations. Two models with distinct surface facets were employed to explore the surface facets-dependent extrinsic size effects on the plastic deformation of pillars. It is found that (i) the yielding in pillars with smooth surfaces was controlled by dislocation nucleation from the two ends of the pillars, and (ii) in pillars with faceted surfaces, dislocations were initiated from the sharp edges on the surface. As a result of the differences in the plastic deformation mechanism between the two models, the yield stress decreased exponentially and increased nearly linearly with respect to an increasing D in pillars with smooth and faceted surfaces, respectively.

Size Effect and Deformation Mechanism in Twinned Copper Nanowires

Molecular dynamics simulations were performed to demonstrate the synergistic effects of the extrinsic size (nanowire length) and intrinsic size (twin boundary spacing) on the failure manner, yield strength, ductility and deformation mechanism of the twinned nanowires containing high density coherent twin boundaries CTBs paralleled to the nanowires’ axis. The twinned nanowires show an intense extrinsic size effect, i.e., shorter is stronger and more ductile, and an intense intrinsic size effect, i.e., thinner is stronger. Notably, the strengthening effect degradation of CTBs in the twinned nanowires is observed with an increase in nanowire length: remarkable strengthening effect can be obtained for the short nanowires, but the strengthening effect becomes less pronounced for the long nanowires. The twinned nanowires fail via a ductile manner or via a brittle manner depending on the synergistic effect of the nanowire length and twin boundary spacing. By atomic-level observation of the plastic deformation, we found that the emission of a trailing 30° partial from the free surface controls the yield behavior of the twinned nanowires. We also found that the special zigzag extended dislocations are formed by the dislocation–CTBs interactions, and propagate to sustain the plastic deformation.