%0 Artikel
%@ 1359-6454
%A Ovri, H.
%A Markmann, J.
%A Barthel, J.
%A Kruth, M.
%A Dieringa, H.
%A Lilleodden, E.
%D 2023
%J Acta Materialia
%N 1006
%P 118550
%R doi:10.1016/j.actamat.2022.118550
%T Mechanistic origin of the enhanced strength and ductility in Mg-rare earth alloys
%U https://dx.doi.org/10.1016/j.actamat.2022.118550
%X Magnesium (Mg) alloys with low concentrations of rare earth additions are known to exhibit strengths and ductility that are significantly higher than those obtained in traditional Mg alloys. However, the mechanisms that underlie these improvements are still open to debate. We assessed these mechanism(s) by carrying out in-depth analysis of the deformation behavior in single crystals of pure Mg and a homogenized Mg-0.75 at.% Gd alloy oriented for twinning, pyramidal- and basal-slip. We observed a fivefold increase in basal CRSS, an eightfold increase in twinning CRSS and a fourfold decrease of the pyramidal/basal CRSS (P/B) ratio due to Gd addition. We also observed that while twinning and pyramidal slip activities were similar in the two material systems, basal slip was radically different. Specifically, basal slip was planar in the alloy but wavy in pure Mg. Our work reveals that these observations are a consequence of Gd-rich short-range ordered (SRO) clusters in the alloy. We show that interactions between dislocations and the SRO clusters would lead to significant increases in strength and slip activity, and consequently, ductility improvements in homogenized polycrystalline Mg-Gd alloys.
%0 Artikel
%@ 0002-7820
%A Gomez-Gomez, A.
%A Winhard, B.
%A Lilleodden, E.
%A Huber, N.
%A Furlan, K.P.
%D 2023
%J Journal of the American Ceramic Society
%N 1252
%P 1273 - 1286
%R doi:10.1111/jace.18811
%T Unraveling the role of shell thickness and pore size on the mechanical properties of ceramic-based macroporous structures
%U https://dx.doi.org/10.1111/jace.18811
2
%X Macroporous structures are of interest for several technological applications such as catalysis, sensors, filters, membranes, batteries, energy conversion devices, structural colors, and reflective thermal barrier coatings. Ceramic-based inverse opal macroporous structures are especially interesting for high-temperature applications. However, the interrelation between the structural parameters, mechanical properties, and thermal stability of such structures is not yet clarified. In this work, we analyzed the mechanical properties as well as the thermal stability of aluminum oxide inverse opal three-dimensional macroporous structures with different macropore sizes and shell thicknesses produced by atomic layer deposition. Our results show that the structures’ thermal stability increased with increasing shell thickness and macropore size, however, their higher stability was not linked to their mechanical properties. To be able to explain this unexpected behavior, finite element modeling simulations were performed, showing that bending stresses became more pronounced with increasing shell thickness, potentially creating additional critical sites for crack initiation and consequent structural failure.
%0 Artikel
%@ 2475-9953
%A Brinker, M.
%A Thelen, M.
%A May, M.
%A Rings, D.
%A Krekeler, T.
%A Lakner, P.
%A Keller, T.
%A Bertram, F.
%A Huber, N.
%A Huber, P.
%D 2022
%J Physical Review Materials
%N 2794
%P 116002
%R doi:10.1103/PhysRevMaterials.6.116002
%T How nanoporous silicon-polypyrrole hybrids flex their muscles in aqueous electrolytes: In operando high-resolution x-ray diffraction and electron tomography-based micromechanical computer simulations
%U https://dx.doi.org/10.1103/PhysRevMaterials.6.116002
11
%X Macroscopic strain experiments have revealed that silicon crystals traversed by parallel, channel-like nanopores functionalized with the artificial muscle polymer polypyrrole (PPy) exhibit large and reversible electrochemomechanical actuation in aqueous electrolytes. On a macroscopic scale these actuation properties are well understood. However, on the microscopical level this system still bears open questions, as to how the electrochemical expansion and contraction of PPy acts on to np-Si pore walls and how the collective motorics of the pore array emerges from the single-nanopore behavior. Here we present synchrotron-based, in operando x-ray diffraction on the evolving electrostrains in epilayers of this material grown on bulk silicon. An analysis of these experiments with micromechanical finite-element simulations, that are based on a full three-dimensional reconstruction of the nanoporous medium by transmission electron microscopy (TEM) tomography, shows that the in-plane mechanical response is dominantly isotropic despite the anisotropic elasticity of the single-crystalline host matrix. However, the structural anisotropy originating from the parallel alignment of the nanopores led to significant differences between the in- and out-of-plane electromechanical response. This response is not describable by a simple two-dimensional arrangement of parallel cylindrical channels. Rather, the simulations highlight that the dendritic shape of the silicon pore walls, including pore connections between the main channels, causes complex, highly inhomogeneous stress-strain fields in the crystalline host. Time-dependent x-ray scattering experiments on the dynamics of the actuator properties hint towards the importance of diffusion limitations, plastic deformation, and creep in the nanoconfined polymer upon (counter)ion adsorption and desorption, the very pore-scale processes causing the macroscopic electroactuation. From a more general perspective, our study demonstrates that the combination of TEM tomography-based micromechanical modeling with high-resolution x-ray scattering experiments provides a powerful approach for in operando analysis of nanoporous composites from the single nanopore up to the porous-medium scale.
%0 Artikel
%@ 0010-938X
%A Yasakau, K.
%A Maltseva, A.
%A Lamaka, S.
%A Mei, D.
%A Ovri, H.
%A Volovitch, P.
%A Ferreira, M.
%A Zheludkevich, M.
%D 2022
%J Corrosion Science
%N 1917
%P 109937
%R doi:10.1016/j.corsci.2021.109937
%T The effect of carboxylate compounds on Volta potential and corrosion inhibition of Mg containing different levels of iron
%U https://dx.doi.org/10.1016/j.corsci.2021.109937
%X Corrosion protection and surface properties of Magnesium (Mg) containing 51 ppm Fe (HP-Mg) and 341 ppm Fe (CP-Mg) were assessed by EIS, AFM/SKPFM, and photoluminescence spectroscopy in NaCl solutions with/without fumarate (Fum), 2,5-pyridinedicarboxylate (PDCA) and salicylate (Sal) carboxylates. The PDCA was effective in corrosion inhibition of HP-Mg and CP-Mg, while the Sal was efficient in inhibition of CP-Mg only. Volta potential (VPD) changes on Mg were evaluated considering the contribution of molecular dipoles and chemical dipoles due to interaction of carboxylate groups present in carboxylates with Mg. VPD of CP-Mg increased due to adsorption of Fe(III)-salicylate complexes and salicylate molecules.
%0 Artikel
%@ 2059-8521
%A Richert, C.
%A Wu, Y.
%A Hablitzel, M.
%A Lilleodden, E.
%A Huber, N.
%D 2021
%J MRS Advances
%N 2682
%P 519 - 523
%R doi:10.1557/s43580-021-00099-w
%T Image segmentation and analysis for densification mapping of nanoporous gold after nanoindentation
%U https://dx.doi.org/10.1557/s43580-021-00099-w
20
%X Segmentation of scanning electron microscopy (SEM) images of focused ion beam (FIB) cross-sections through indented regions in nanoporous gold (np-Au) is carried out. A key challenge for image analysis of open porous materials is the appropriate binarization of the pore and gold ligament regions while excluding material lying below the cross-sectional plane. Here, a manual approach to thresholding is compared to global and local approaches. The global thresholding resulted in excessive deviations from the nominal solid fraction, due to a strong gray-scale gradient caused by the tilt angle during imaging and material shadowing. In contrast, the local thresholding approach delivered local solid fractions that were free of global gradients, and delivered a quality comparable to the manual segmentation. The extracted densification profiles vertically below the indenter as well as parallel to the surface showed an exponential-type decay from the indenter tip towards the nominal value of 1 far from the indenter.
%0 Artikel
%@ 0027-8424
%A Elder, K.L.M.
%A Beck Andrews, W.
%A Ziehmer, M.
%A Mameka, N.
%A Kirchlechner, C.
%A Davydok, A.
%A Micha, J.-S.
%A Chadwick, A.F.
%A Lilleodden, E.T.
%A Thornton, K.
%A Voorhees, P.W.
%D 2021
%J Proceedings of the National Academy of Sciences of the United States of America: PNAS
%N 1376
%P e2104132118
%R doi:10.1073/pnas.2104132118
%T Grain Boundary Formation Through Particle Detachment During Coarsening of Nanoporous Metals
%U https://dx.doi.org/10.1073/pnas.2104132118
30
%X Grain boundary formation during coarsening of nanoporous gold (NPG) is investigated wherein a nanocrystalline structure can form by particles detaching and reattaching to the structure. MicroLaue and electron backscatter diffraction measurements demonstrate that an in-grain orientation spread develops as NPG is coarsened. The volume fraction of the NPG sample is near the limit of bicontinuity, at which simulations predict that a bicontinuous structure begins to fragment into independent particles during coarsening. Phase-field simulations of coarsening using a computationally generated structure with a volume fraction near the limit of bicontinuity are used to model particle detachment rates. This model is tested by using the measured NPG structure as an initial condition in the phase-field simulations. We predict that up to ∼5% of the NPG structure detaches as a dealloyed Ag75Au25 sample is annealed at 300 °C for 420 min. The quantity of volume detached is found to be highly dependent on the volume fraction and volume fraction homogeneity of the nanostructure. As the void phase in the experiments cannot support independent particles, they must fall and reattach to the structure, a process that results in the formation of new grain boundaries. This particle reattachment process, along with other classic processes, leads to the formation of grain boundaries during coarsening in nanoporous metals. The formation of grain boundaries can impact a variety of applications, including mechanical strengthening; thus, the consideration and understanding of particle detachment phenomena are essential when studying nanoporous metals.
%0 Artikel
%@ 0883-7694
%A Zhang, X.
%A Lilleodden, E.
%A Wang, J.
%D 2021
%J MRS Bulletin
%N 2296
%P 217 - 224
%R doi:10.1557/s43577-021-00069-5
%T Recent trends on studies of nanostructured metals
%U https://dx.doi.org/10.1557/s43577-021-00069-5
3
%X Nanostructured metals have been intensively investigated as these materials have abundant interfaces (grain boundaries and phase boundaries) that can drastically impact the mechanical and physical properties of materials. Significant research has been performed to understand the deformation mechanisms of nanostructured metals, and many of the major findings have been adopted by industry for the manufacturing of advanced materials for various applications. In this article, we highlight several recent breakthroughs and briefly introduce the focus of the articles in this issue that provide a more in-depth understanding of the forefronts of the field. Finally, we point out some of the research directions that may warrant further investigations.
%0 Artikel
%@ 0921-5093
%A Maghsoudi, M.
%A Ziehmer, M.
%A Lilleodden, E.
%D 2020
%J Materials Science and Engineering A
%N 1297
%P 138747
%R doi:10.1016/j.msea.2019.138747
%T Detwinning-mediated hardening in Mg: A microcompression study of a single twin boundary
%U https://dx.doi.org/10.1016/j.msea.2019.138747
%X We present the first experimental measurement of the hardening arising from the detwinning of a single twin boundary in Mg. Microcompression tests were performed on two sets of microcolumns: (i) single crystals having a “parent” grain orientation of nearly (0 0 0 1) along the compression axis, and (ii) bicrystals involving the parent grain and a single twin boundary. A comparison of the stress-strain data shows significant differences in the deformation characteristics of the deformed microcolumns. The bicrystalline microcolumns undergo detwinning, as indicated by a stress plateau, which leads to a nominally single crystalline microcolumn of the orientation of the parent grain. Microcompression beyond the plateau shows that the detwinned microcolumns exhibit a considerably higher yield stress and strain hardening rate than the single crystalline “parent” microcolumns. Electron back-scattered diffraction analyses on the cross-sections of the deformed microcolumns reveal higher misorientations in the detwinned region indicative of a high content of unpaired dislocations (so-called GNDs), compared to the deformed parent microcolumns. This discrepancy in misorientation distribution between the samples is consistent with a detwinning-mediated hardening response, as observed, and points to the creation of dislocation debris as a consequence of the detwinning process.
%0 Artikel
%@ 0884-2914
%A Huber, N.
%A Richert, C.
%D 2020
%J Journal of Materials Research
%N 1219
%P 2831 - 2834
%R doi:10.1557/jmr.2020.257
%T Comment to “Skeletonization-based beam finite element models for stochastic bicontinuous materials: Application to simulations of nanoporous gold” by C. Soyarslan et al. [J. Mater. Res. 33(20), 3371 (2018)]
%U https://dx.doi.org/10.1557/jmr.2020.257
20
%X Soyarslan et al. [J. Mater. Res. 33(20), 3371 (2018)] proposed a beam-finite element model for the computation of effective elastic properties of nanoporous materials, where the ligament diameter along the skeleton is determined with the biggest sphere algorithm. Although this algorithm is often used in the literature, it is known that it systematically overestimates the diameter in network structures. Thus, the need for further stiffening of the junction zones as proposed by the authors is in contradiction to the literature. Furthermore, the factor 40 appears to be one order of magnitude too high. We show that the 3D microstructures generated from random Gaussian fields contain features that are violating the assumption of circular cross-sections and, therefore, cannot be captured by the biggest sphere algorithm. Consequently, the authors required an unphysically high value of 40 to compensate this hidden effect.
%0 Artikel
%@ 2375-2548
%A Brinker, M.
%A Dittrich, G.
%A Richert, C.
%A Lakner, P.
%A Krekeler, T.
%A Keller, T.
%A Huber, N.
%A Huber, P.
%D 2020
%J Science Advances
%N 2650
%P eaba1483
%R doi:10.1126/sciadv.aba1483
%T Giant electrochemical actuation in a nanoporous silicon-polypyrrole hybrid material
%U https://dx.doi.org/10.1126/sciadv.aba1483
40
%X The absence of piezoelectricity in silicon makes direct electromechanical applications of this mainstream semiconductor impossible. Integrated electrical control of the silicon mechanics, however, would open up new perspectives for on-chip actuorics. Here, we combine wafer-scale nanoporosity in single-crystalline silicon with polymerization of an artificial muscle material inside pore space to synthesize a composite that shows macroscopic electrostrain in aqueous electrolyte. The voltage-strain coupling is three orders of magnitude larger than the best-performing ceramics in terms of piezoelectric actuation. We trace this huge electroactuation to the concerted action of 100 billions of nanopores per square centimeter cross section and to potential-dependent pressures of up to 150 atmospheres at the single-pore scale. The exceptionally small operation voltages (0.4 to 0.9 volts), along with the sustainable and biocompatible base materials, make this hybrid promising for bioactuator applications.
%0 Artikel
%@ 0921-5093
%A Odermatt, A.
%A Richert, C.
%A Huber, N.
%D 2020
%J Materials Science and Engineering A
%N 1297
%P 139700
%R doi:10.1016/j.msea.2020.139700
%T Prediction of elastic-plastic deformation of nanoporous metals by FEM beam modeling: A bottom-up approach from ligaments to real microstructures
%U https://dx.doi.org/10.1016/j.msea.2020.139700
%X For the prediction of elastic-plastic deformation behavior of nanoporous materials, a computationally efficient method is needed that integrates the complex 3D network structure and the large variation of ligament shapes in a representative volume element. Finite element simulations based on beam elements are most efficient, but for a quantitative prediction, a correction is required that accounts for the effects of the mass around the junctions. To this end, a nodal correction is presented that covers a wide range of parabolic-spherical ligament shapes. Smooth functions are provided that define the extension of the nodal corrected elements along the ligament axis, their radius and their yield stress as functions of the individual ligament shape. It is shown that the increase in radius of the nodal beam elements can be replaced by scaled material parameters. Simulations of randomized FEM beam networks revealed that, in relation to the randomization of the ligament axis, the distribution of the ligament shape has a stronger impact on the macroscopic stress–strain response and should thus receive particular attention during the characterization of nanoporous microstructures. This also emphasizes the importance of the thickness analysis via image processing. Combining a nodal corrected FEM beam model with geometry data derived from image multiplication of the skeleton and Euclidean distance transform of a nanoporous gold FIB-SEM tomography dataset significantly improves the prediction of the stress–strain curve. The remaining deviation is expected to stem from the known underestimation of the real ligament diameter by the Euclidean distance transform as well as local variations in the circularity of the ligaments.
%0 Artikel
%@ 1996-1944
%A Richert, C.
%A Huber, N.
%D 2020
%J Materials
%N 2513
%P 3307
%R doi:10.3390/ma13153307
%T A Review of Experimentally Informed Micromechanical Modeling of Nanoporous Metals: From Structural Descriptors to Predictive Structure–Property Relationships
%U https://dx.doi.org/10.3390/ma13153307
15
%X Nanoporous metals made by dealloying take the form of macroscopic (mm- or cm-sized) porous bodies with a solid fraction of around 30%. The material exhibits a network structure of “ligaments” with an average ligament diameter that can be adjusted between 5 and 500 nm. Current research explores the use of nanoporous metals as functional materials with respect to electrochemical conversion and storage, bioanalytical and biomedical applications, and actuation and sensing. The mechanical behavior of the network structure provides the scope for fundamental research, particularly because of the high complexity originating from the randomness of the structure and the challenges arising from the nanosized ligaments, which can be accessed through an experiment only indirectly via the testing of the macroscopic properties. The strength of nanoscale ligaments increases systematically with decreasing size, and owing to the high surface-to-volume ratio their elastic and plastic properties can be additionally tuned by applying an electric potential. Therefore, nanoporous metals offer themselves as suitable model systems for exploring the structure–property relationships of complex interconnected microstructures as well as the basic mechanisms of the chemo-electro-mechanical coupling at interfaces. The micromechanical modeling of nanoporous metals is a rapidly growing field that strongly benefits from developments in computational methods, high-performance computing, and visualization techniques; it also benefits at the same time through advances in characterization techniques, including nanotomography, 3D image processing, and algorithms for geometrical and topological analysis. The review article collects articles on the structural characterization and micromechanical modeling of nanoporous metals and discusses the acquired understanding in the context of advancements in the experimental discipline. The concluding remarks are given in the form of a summary and an outline of future perspectives.
%0 Artikel
%@ 1359-6454
%A Ziehmer, M.
%A Lilleodden, E.
%D 2020
%J Acta Materialia
%N 1006
%P 669 - 679
%R doi:10.1016/j.actamat.2020.08.026
%T The isothermal evolution of nanoporous gold from the ring perspective - an application of graph theory
%U https://dx.doi.org/10.1016/j.actamat.2020.08.026
%X The ring structures of five isothermally annealed nanoporous gold (npg) samples were analyzed explicitly by applying results and algorithms from graph theory to skeletonized 3D reconstructions from focused ion beam (FIB) tomography data. Simplified skeletons of the reconstructions were utilized, in which the real ligaments are reduced to straight edges between the branching points of the npg microstructure. So-called minimum weight cycle bases of each skeleton graph’s cycle vector space were calculated, assigning different weight functions to these straight edges: equal weights, Euclidean lengths, and the real ligament lengths from backmapping the Euclidean skeleton edges to the skeletonized real ligament sections. These cycle bases contain the maximum number of linearly independent rings that cannot be generated by smaller rings via the ring sum specified in the cycle vector space. Such a decomposition of the npg network structures into the fundamental ring building blocks served to provide a new perspective of the isothermal evolution of npg, since the coarsening of the npg network structure could be examined from analyzing the local ring topologies and the classification of the ring topological classes. Our results suggest an increasing relative dominance of ligament pinch-off events over ring collapse events, manifesting in a broadening of the distribution of topological classes, and leading to a small but steady increase of the average number of ring edges. Furthermore, self-similar evolution of the investigated sample series cannot be stated. The implications on the topological evolution of npg as a function of the solid volume fraction are discussed.
%0 Artikel
%@ 0921-5093
%A Ovri, H.
%A Steglich, D.
%A Dieringa, H.
%A Lilleodden, E.T.
%D 2019
%J Materials Science and Engineering A
%N 1297
%P 226 - 234
%R doi:10.1016/j.msea.2018.10.099
%T Grain-scale investigation of the anisotropy of Portevin-Le Chatelier effect in Mg AZ91 alloy
%U https://dx.doi.org/10.1016/j.msea.2018.10.099
%X An aspect of Portevin-Le Chatelier (PLC) type plastic instability that is yet to be understood is its orientation dependence. Such knowledge is crucial in view of its implications for texture weakening and, by extension, improvement in formability in Mg–based alloys. In this work, insight into the micromechanisms that govern PLC and its orientation dependence in single grains of Mg AZ91 is achieved using a combination of spherical nanoindentation, local orientation image analysis and crystal plasticity based finite element simulations, which was specifically used to identify the anisotropy in slip activity for the investigated orientations. Moreover, a statistical thermal activation model that is based on the distribution of load jumps between consecutive displacement bursts in the load vs. displacement response is presented. The paper demonstrates the ability of the model to predict the thermal activation parameters for PLC effect. On the basis of the results, we propose a mechanistically sound model for PLC effect that explains the underlying micromechanisms, the role of Al and Zn atoms, and the origin of the orientation dependence of the phenomenon. We also highlight the influence of the PLC effect on formability in Mg–based alloys.
%0 Artikel
%@ 2296-8016
%A Richert, C.
%A Odermatt, A.
%A Huber, N.
%D 2019
%J Frontiers in Materials
%N 2827
%P 327
%R doi:10.3389/fmats.2019.00327
%T Computation of Thickness and Mechanical Properties of Interconnected Structures: Accuracy, Deviations, and Approaches for Correction
%U https://dx.doi.org/10.3389/fmats.2019.00327
%X Identifying local thickness information of fibrous or highly porous structures is challenging.
The analysis of tomography data calls for computationally fast, robust, and accurate
algorithms. This work systematically investigates systematic errors in the thickness
computation and the impact of observed deviations on the predicted mechanical
properties using a set of 16 model structures with varying ligament shape and solid
fraction. Strongly concave, cylindrical, and convex shaped ligaments organized in a
diamond structure are analyzed. The predicted macroscopic mechanical properties
represent a highly sensitive measure for systematic errors in the computed geometry.
Therefore, the quality of proposed correctionmethods is assessed via FEMbeammodels
that can be automatically generated from the measured data and allow an efficient
prediction of the mechanical properties. The results show that low voxel resolutions can
lead to an overprediction of up to 30% in the Young’s modulus. A model scanned with
a resolution of 200 voxels per unit cell edge (8M voxels) reaches an accuracy of a few
percent. Analyzing models of this resolution with the Euclidean distance transformation
showed an underprediction of up to 20% for highly concave shapes whereas cylindrical
and slightly convex shapes are determined at high accuracy. For the Thickness algorithm,
the Young’s modulus and yield strength are overpredicted by up to 100% for highly
concave ligament shapes. A proposed Smallest Ellipse approach corrects the Thickness
data and reduces this error to 20%. It can be used as input for a further robust
correction of the Thickness data using an artificial neural network. This approach is highly
accuratewith remnant errors in the predictedmechanical properties of only a few percent.
Furthermore, the data from the FEM beam models are compared to results from FEM
solidmodels providing deeper insights toward further developments on nodal corrections
for FEM beam models. As expected, the FEM beam models show an increasing
overprediction of the compliance with increasing solid fraction. As an unexpected result,
the mechanical strength can however be underpredicted or overpredicted, depending
on the ligament shape. Therefore, a nodal correction is needed that solves contradicting
tasks in terms of stiffness and strength.
%0 Artikel
%@ 1047-4838
%A Ovri, H.
%A Lilleodden, E.T.
%D 2019
%J JOM: Journal of the Minerals, Metals and Materials Society
%N 1226
%P 3343 - 3349
%R doi:10.1007/s11837-019-03697-0
%T On the Estimation of Thermal Activation Parameters for Portevin–Le Chatelier Effect from Nanoindentation Data
%U https://dx.doi.org/10.1007/s11837-019-03697-0
10
%X The development of methods to ascertain the activation enthalpy, ΔE a ΔEa, for Portevin–Le Chatelier (PLC) effect is of interest as it facilitates understanding of the underlying mechanisms and the identification of solute species that age dislocations during deformation. Currently, most models for estimating ΔE a ΔEa are based on the critical strain, ε c εc, for the onset of PLC during macroscopic uniaxial tests. However, an ε c εc is not always observed, and some of the models incorporate unverified dependences. In this work, we present a nanoindentation-based approach for estimating ΔE a ΔEa and the activation volume for the phenomenon. The approach is based on a more theoretically sound foundation and obviates the need for ε c εc. The derived parameters are in good agreement with reported values for the Al-Mg alloy studied herein. The results are discussed in terms of strain rate, indentation depth, and indenter geometry, and reveal the utility of the technique for investigations of PLC more generally.
%0 Artikel
%@ 1359-6462
%A Hablitzel, M.
%A Lilleodden, E.
%D 2019
%J Scripta Materialia
%N 1410
%P 67 - 70
%R doi:10.1016/j.scriptamat.2019.04.026
%T On measuring the independent mechanical response of the polymer phase from nanoporous gold polymer composites
%U https://dx.doi.org/10.1016/j.scriptamat.2019.04.026
%X This work aims to understand the mechanical properties and potential size effect in the polymeric phase of nanoporous gold composites. We take inspiration from the extraction metallurgy of gold to leach out the Au phase from the composites, fabricating a novel nanostructured polymeric material, which was subsequently investigated by micromechanical techniques. In the case of nanoporous epoxy, no dependency on the microstructural length-scale was observed in either the elastic or plastic response, and the modulus could be well predicted by an isostrain law applied to the properties measured on the bulk polymer.
%0 Artikel
%@ 0003-6951
%A Markmann, J.
%A Lilleodden, E.
%D 2019
%J Applied Physics Letters
%N 1044
%P 251602
%R doi:10.1063/1.5128049
%T Electro-chemo-mechanical coupling of nanoporous gold at the microscale
%U https://dx.doi.org/10.1063/1.5128049
25
%X The observation of reversible strengthening and stiffening of nanoporous gold (NPG) under electrochemical potential has opened opportunities to exploit this material for multifunctional applications. Yet the complex structural geometry and length-scales involved make a definitive understanding of structural correlations to the behaviors difficult at best. Achievement of coupled electro-chemo-mechanical testing at the micrometer scale is a key step toward this goal. Here, we introduce an experimental approach to investigate the elastic and plastic behaviors of NPG under electrochemical potential at the microscale using a modified nanoindentation setup and multiple load function. The in situ experiments in electrolyte show a significant increase by 32% in strength of pillars in a positive potential regime where oxygen adsorption occurred. This response was found to be reversible, which agrees with macroscopic results, while the elastic modulus was shown to be insensitive to the applied potential—an observation inconsistent with recent bulk dynamic mechanical analysis results.
%0 Artikel
%@ 0883-7694
%A Lilleodden, E.
%A Voorhees, P.
%D 2018
%J MRS Bulletin
%N 2296
%P 20 - 26
%R doi:10.1557/mrs.2017.303
%T On the topological; morphological; and microstructural characterization of nanoporous metals
%U https://dx.doi.org/10.1557/mrs.2017.303
%X The structural characterization of dealloyed nanoporous metals is a fundamental and active area of research, needed for the optimization of these structures for catalytic, electrosensing, biomedical, and mechanical functions. The prediction of properties requires identifying and quantifying salient structural characteristics, while insights into the relevant mechanisms of dealloying and coarsening can be achieved through in situ observations of structural evolution. Three-dimensional structural characterization techniques have advanced such that nanoscale quantification of topology, morphology, and crystallographic parameters are achievable, yet the field is new enough that the assessment and comparison of such parameters of different nanoporous metals are just beginning. Here, we explore the state of the art in structural characterization, focusing on nanoporous gold to exemplify the challenges, the achievements, and the potential associated with establishing an appropriate set of structural parameters for this unique class of materials.
%0 Artikel
%@ 1359-6454
%A Wang, L.
%A Huang, Z.
%A Wang, H.
%A Maldar, A.
%A Yi, S.
%A Park, J.
%A Kenesei, P.
%A Lilleodden, E.
%A Zeng, X.
%D 2018
%J Acta Materialia
%N 1006
%P 138 - 152
%R doi:10.1016/j.actamat.2018.05.065
%T Study of slip activity in a Mg-Y alloy by in situ high energy X-ray diffraction microscopy and elastic viscoplastic self-consistent modeling
%U https://dx.doi.org/10.1016/j.actamat.2018.05.065
%X Slip activity from various slip modes largely determines the yield strength and ductility of Mg alloys. Solid solution elements in Mg can change the slip activity dramatically. In this paper, far-field high energy X-ray diffraction microscopy (FF-HEDM) is employed to study slip activity in a Mg-3wt%Y alloy during an in situ tensile experiment. The specimen was incrementally loaded up to 3% engineering strain along the rolling direction. At each load step, FF-HEDM data were collected to track the crystallographic orientation, center of mass, and stress tensor changes of nearly 1000 grains in the probed volume. By analyzing the change in orientation and stress tensor of individual grains at different load steps, it is possible to identify the activated slip systems and measure their critical resolved shear stress (CRSS) values. Prismatic slip and pyramidal I slip are found to be very active in this alloy. The estimated CRSS values for basal slip, prismatic slip and pyramidal I slip are 12 MPa, 38 MPa, and 36 MPa, respectively. These CRSS values were applied in a dislocation-based elastic viscoplastic self-consistent (EVPSC) model that successfully simulated the tensile stress-strain curve from the FF-HEDM experiment. The model also qualitatively predicted the crystal rotation in most of the selected grains, though it underestimated the internal stress and the magnitude of crystal rotation in these grains. Influence of solute Y on the strength and ductility of Mg alloys is discussed.
%0 Artikel
%@ 1359-6454
%A Guglielmi, P.O.
%A Ziehmer, M.
%A Lilleodden, E.T.
%D 2018
%J Acta Materialia
%N 1006
%P 195 - 205
%R doi:10.1016/j.actamat.2018.03.009
%T On a novel strain indicator based on uncorrelated misorientation angles for correlating dislocation density to local strength
%U https://dx.doi.org/10.1016/j.actamat.2018.03.009
%X We present a new method based on uncorrelated misorientation measurements by Electron Backscattered Diffraction (EBSD) to characterize the dislocation density of site-specific areas selected on a bulk material. Gold samples submitted to different degrees of pre-straining are analyzed. A new scalar misorientation parameter called the Characteristic Misorientation Angle (CMA) is derived from uncorrelated misorientation data and compared to the more conventional parameters Grain Average Misorientation (GAM) and Grain Orientation Spread (GOS). We show that CMA is nearly independent of the scan step size and is more sensitive to plastic deformation than GAM and GOS. A coupled effect of local plastic strain and area size is observed on the measured values of CMA. Based on that, values of dislocation density are determined for site-specific areas whose strengths, as defined by the hardness at first pop-in, are subsequently measured by spherical nanoindentation. Results show that the site-specific strength of gold decreases with increasing initial dislocation density. While previous studies have suggested the same trend, the present work offers a new approach to more quantitatively correlate local dislocation densities to the onset of plasticity, without the need for destructive TEM investigations or micro-sample fabrication.
%0 Artikel
%@ 2075-4701
%A Richert, C.
%A Huber, N.
%D 2018
%J Metals
%N 2394
%P 282
%R doi:10.3390/met8040282
%T Skeletonization, Geometrical Analysis, and Finite Element Modeling of Nanoporous Gold Based on 3D Tomography Data
%U https://dx.doi.org/10.3390/met8040282
4
%X Various modeling approaches simplify and parametrize the complex network structure of nanoporous gold (NPG) for studying the structure–property relationship based on artificially generated structures. This paper presents a computational efficient and versatile finite element method (FEM) beam model that is based on skeletonization and diameter information derived from the original 3D focused ion beam-scanning electron microscope (FIB-SEM) tomography data of NPG. The geometrical skeleton network is thoroughly examined for a better understanding of the NPG structure. A skeleton FEM beam model is derived that can predict the macroscopic mechanical behavior of the material. Comparisons between the mechanical response of this skeleton beam model and a solid FEM model are conducted. Results showed that the biggest-sphere diameter algorithm implemented in the open-source software FIJI, commonly used for geometrical analysis of microstructural data, overestimates the diameter of the curved NPG ligaments. The larger diameters lead to a significant overestimation of macroscopic stiffness and strength by the skeleton FEM beam model. For a parabolic shaped ligament with only 20% variation in its diameter, a factor of more than two was found in stiffness. It is concluded that improved algorithms for image processing are needed that provide accurate diameter information along the ligament axis.
%0 Artikel
%@ 2352-4316
%A do Rosario, J.J.
%A Berger, J.B.
%A Lilleodden, E.T.
%A McMeeking, R.M.
%A Schneider, G.A.
%D 2017
%J Extreme Mechanics Letters
%N 2677
%P 86 - 96
%R doi:10.1016/j.eml.2016.07.006
%T The stiffness and strength of metamaterials based on the inverse opal architecture
%U https://dx.doi.org/10.1016/j.eml.2016.07.006
%X The inverse opal architecture, a class of mechanical metamaterials recently shown to exhibit high specific strength and modulus, is further investigated here using carefully coupled experiments and finite element modeling. We demonstrate that this architecture can be exploited to achieve optimized specific strength and modulus, while simultaneously offering tunable optical bandgaps and large-area fabrication. Starting with a silica inverse opal structure and adding different thicknesses of titania (10–34 nm) the strength was gradually increased from 41 to 410 MPa and the elastic modulus from 1.7 to 8.3 GPa, within densities of 300–1000 kg m−3. Simulations confirmed that the inverse opal structure can outperform the state-of-the-art octet- and isotropic-truss designs in terms of Young’s, shear and bulk modulus, as well as in structural efficiency (total stiffness). Simulations also predict stresses in the titania coating and in the silica that are on the order of the theoretical tensile yield stresses at failure, indicating that size effects controlling defect population are responsible for the high strengths.
%0 Artikel
%@ 0020-7403
%A Jiao, J.
%A Huber, N.
%D 2017
%J International Journal of Mechanical Sciences
%N 2546
%P 234 - 243
%R doi:10.1016/j.ijmecsci.2017.10.011
%T Effect of nodal mass on macroscopic mechanical properties of nanoporous metals
%U https://dx.doi.org/10.1016/j.ijmecsci.2017.10.011
%X The current work investigates the effect of the nodal mass on the macroscopic mechanical behavior of nanoporous metals using the Finite Element Method. A nodal corrected beam modeling concept is introduced that allows local incorporation of the effective elastoplastic mechanical behavior of the nodal mass in the nodal area of a representative volume element (RVE). The calibration to the corresponding Finite Element solid model is achieved by integrating additional geometry and material parameters to the so-called nodal areas in the beam model. With this technique an excellent prediction can be achieved over a large range of deformation for different types of RVEs. From the results of the nodal corrected beam model, modified leading constants are determined in the scaling laws for Young's modulus and yield strength. The effect of the nodal correction is also studied with respect to various randomization levels. Finally, the ligament size dependent strength is analyzed by applying the proposed model to experimental data. It could be shown that the nodal correction improves the overall agreement with literature data, particularly for such data points that are related to samples with a high solid fraction.
%0 Artikel
%@ 0261-3069
%A Ovri, H.
%A Lilleodden, E.T.
%D 2017
%J Materials and Design
%N 2008
%P 69 - 75
%R doi:10.1016/j.matdes.2017.03.071
%T Temperature dependence of plastic instability in Al alloys: A nanoindentation study
%U https://dx.doi.org/10.1016/j.matdes.2017.03.071
%X An elevated temperature nanoindentation based method for characterizing the thermal dependence of plastic instability and assessing the activation energies associated with the phenomenon in Al alloys is presented in this work. The method exploits the nanoscale force–displacement resolution capabilities of the Nanoindenter, precludes the ambiguities inherent in the uniaxial testing based methods and offers increased reliability because of the statistical significance of the data achieved. The activation energies estimated for an Al—Mg and an Al—Li alloy with the proposed method were found to be 0.59 ± 0.07 eV and 0.72 ± 0.01 eV, respectively, and are consistent with values derived with other methods. The rate controlling mechanisms associated with these activation energies are described in terms of existing models for plastic instability in these alloy systems.
%0 Artikel
%@ 1359-6454
%A Ziehmer, M.
%A Hu, K.
%A Wang, K.
%A Lilleodden, E.T.
%D 2016
%J Acta Materialia
%N 1006
%P 24 - 31
%R doi:10.1016/j.actamat.2016.08.028
%T A principle curvatures analysis of the isothermal evolution of nanoporous gold: Quantifying the characteristic length-scales
%U http://dx.doi.org/10.1016/j.actamat.2016.08.028
%X A study of the isothermal evolution of a nanoporous gold (npg) microstructure after dealloying has been performed. In order to adequately characterize its complex three-dimensional bicontinuous ligament-ring structure, an analysis of the scaled principle curvatures κ1 and κ2 based on representative volumes of meshed 3D reconstructions was applied. Five npg samples, as obtained from an electrolytical dealloying process, with different mean ligament diameters ranging from ca. 25 nm (as-dealloyed) to ca. 420 nm (from annealing at 300° C) were analyzed. The results indicate that ligament surface flattening effects lead to small but distinct morphological changes during the investigated early and mid-stages of coarsening, visible in the scaled κ1- and κ2- marginal distributions. Thus, strictly speaking, self-similar evolution of npg cannot be confirmed, but dependent on the specific application, the evolution might be seen as “sufficiently” self-similar. Moreover, it is shown that the inverse mean principle curvatures from the marginal distributions can be used to identify the mean sizes of the two salient structural features, namely the ligaments and the rings. Both inverse mean principle curvatures scale linearly with the mean ligament diameter. Thus, for the material used in this study, one parameter is sufficient to characterize its microstructure. Finally, it is shown that rings resembling the ones from the real samples can be generated computationally by applying modified torus parameterizations. Surprisingly, a calculation of the curvature distribution of only one ”average” ring is sufficient to approximate the scaled kappa distributions accumulated from the ring distributions of the real samples.
%0 Artikel
%@ 1478-6435
%A Ziehmer, M.
%A Wang, K.
%A Lilleodden, E.T.
%D 2016
%J Philosophical Magazine
%N 2000
%P 3322 - 3335
%R doi:10.1080/14786435.2016.1222087
%T Nanoporous gold: 3D structural analyses of representative volumes and their implications on scaling relations of mechanical behaviour
%U http://dx.doi.org/10.1080/14786435.2016.1222087
32-34
%X We present a quantitative study of the salient structural parameters identified from so-called ‘representative volumes’ of the bicontinuous nanoporous gold (NPG) network, and examine the validity of self-similarity in describing its evolution. The approach is based on 3D-focused ion beam tomography applied to as-dealloyed and isothermally annealed NPG samples. After identifying sufficiently large representative volumes, we show that the ligament width distributions coarsen in a sufficiently self-similar, time-invariant manner, while the scaled connectivity density shows a self-similar ligament network topology. Using these critical parameters, namely mean ligament diameter and connectivity density, the Gibson–Ashby scaling laws for the mechanical response of cellular materials are revisited. The inappropriateness of directly applying the Gibson–Ashby model to NPG is demonstrated by comparing finite element method compression simulations of both the NPG reconstruction and that of the Gibson–Ashby solid model; rather than the solid volume fraction, we show that an effective load-bearing ring structure governs mechanical behaviour.
%0 Artikel
%@ 1359-6454
%A Ovri, H.
%A Lilleodden, E.T.
%D 2015
%J Acta Materialia
%N 1006
%P 88 - 97
%R doi:10.1016/j.actamat.2015.01.065
%T New insights into plastic instability in precipitation strengthened Al–Li alloys
%U http://dx.doi.org/10.1016/j.actamat.2015.01.065
%X A mechanistic model that describes the microscopic mechanisms underlying plastic instability in precipitation strengthened Al–Li based alloy systems is proposed in this work. The model is based on experimental observations from high resolution nanoindentation tests and transmission electron microscopy (TEM) based methods, including in situ TEM tensile straining. These experiments show that dynamic strain aging (DSA), which is widely accepted as the underlying mechanism for plastic instability, cannot sufficiently account for the occurrence of plastic instability in Al–Li based alloy systems. It is proposed that an altogether different mechanism controls plastic instability, namely a diffusion-controlled pseudo-locking mechanism that accompanies order hardening. This mechanism does not require the concurrent operation of DSA by Li, which may be a nonviable mechanism given the low binding energy of Li to dislocation cores, for plastic instability to occur.
%0 Artikel
%@ 0921-5093
%A Ovri, H.
%A Jaegle, E.A.
%A Stark, A.
%A Lilleodden, E.T.
%D 2015
%J Materials Science and Engineering A
%N 1297
%P 162 - 169
%R doi:10.1016/j.msea.2015.04.039
%T Microstructural influences on strengthening in a naturally aged and overaged Al-Cu-Li-Mg based alloy
%U http://dx.doi.org/10.1016/j.msea.2015.04.039
%X A combination of transmission electron microscopy, atom probe tomography and high-energy X-ray diffraction was employed to investigate the influence of local microstructural changes on strengthening in a commercial Al-Li-Cu based alloy, AA2198, in the stretched and naturally aged, and overaged states. Strengthening in the stretched and naturally aged temper was shown to be governed by a combination of Cu-Cu clusters, δ′/β′ phase and solution strengthening. This is in contrast to another report which suggest that strength in this temper is only due to Cu-rich clusters [Decreus B et. al. Acta Mater 61 (2013) 2207]. On the other hand, although large volume fractions of equilibrium phases such as TB, and θ were present in the overaged temper, its strengthening was largely governed by order hardening, which is the strengthening mechanism associated with the δ′/β′ phase. The δ′/β′ phase remained in the matrix even after extensive overaging.
%0 Artikel
%@ 1751-6161
%A Guglielmi, P.O.
%A Herbert, E.G.
%A Tartivel, L.
%A Behl, M.
%A Lendlein, A.
%A Huber, N.
%A Lilleodden, E.T.
%D 2015
%J Journal of the Mechanical Behavior of Biomedical Materials
%N 2396
%P 1 - 10
%R doi:10.1016/j.jmbbm.2015.02.009
%T Mechanical characterization of oligo(ethylene glycol)-based hydrogels by dynamic nanoindentation experiments
%U http://dx.doi.org/10.1016/j.jmbbm.2015.02.009
%X Oligo(ethylene glycol)-based (OEG) hydrogel samples of varying cross-link densities and degrees of swelling were characterized through dynamic nanoindentation testing. Experiments were performed using a non-standard nanoindentation method, which was validated on a standard polystyrene sample. This method maximizes the capability of the instrument to measure the stiffness and damping of highly compliant, viscoelastic materials. Experiments were performed over the frequency range of 1 to 50 Hz, using a 1 mm diameter flat punch indenter. A hydration method was adopted to avoid sample dehydration during testing. Values of storage modulus (E′)(E′) ranged from 3.5 to 8.9 MPa for the different OEG-hydrogel samples investigated. Samples with higher OEG concentrations showed greater scatter in the modulus measurements and it is attributed to inhomogeneities in these materials. The (E′)(E′) values did not show a strong variation over frequency for any of the samples. Values of loss modulus (E″)(E″) were two orders of magnitude lower than the storage modulus, resulting in very low values of loss factor (E″/E′E″/E′<0.1). These are characteristics of strong gels, which present negligible viscous properties.
%0 Artikel
%@ 0921-5093
%A Cornec, A.
%A Kabir, M.R.
%A Huber, N.
%D 2015
%J Materials Science and Engineering A
%N 1297
%P 273 - 285
%R doi:10.1016/j.msea.2014.10.018
%T Numerical prediction of the stress–strain response of a lamellar GammaTiAl polycrystal using a two-scale modelling approach
%U http://dx.doi.org/10.1016/j.msea.2014.10.018
%X An advanced model incorporating a two-scale structural description with integrated constitutive formulations of crystal plasticity was adopted to describe the mechanical behaviour of a γTiAl polycrystal with grains of staggered (γ/α2)-phase lamellae. The numerical model assembles a polycrystalline compound of 64 lamellar grains generated from periodic unit cells (PUC) taking relevant phase configurations. The representative parameter set for the crystal plasticity are estimated by modelling the lamellar deformation and fitting the compression and tension test results in two steps: firstly, the fundamental parameters were identified for a poly-synthetically twinned single crystal (PST) under compression, and secondly, these PST parameters were adjusted to the γTiAl polycrystal consisting of fully lamellar grains. Numerical results show that the compression–tension anomaly in the stress–strain curves can be successfully described by a ‘high-grade’ PUC model including six domain variants of the γ-phase occurring in the lamellae. Using a PUC model with simplified mapping of lamellar microstructure, the prediction quality remains unsatisfactory with respect to the observed compression and tension anomaly and the crystal parameters are found to be inconsistent. Differently aligned lamellar grains in polycrystalline cubic model are predicted, which showed that the global stress–strain curves are weakly affected by different local alignments (or textures) of the grains, whereas, the single grain analyses show strong variations in local stress–strain curves. The simulated nature of local variations in grain scale stress–strain behaviour accords with the independent results from instrumented indentation testing of the same lamellar polycrystal.
%0 Artikel
%@ 1359-6454
%A Husser, E.
%A Lilleodden, E.T.
%A Bargmann, S.
%D 2014
%J Acta Materialia
%N 1006
%P 206 - 219
%R doi:10.1016/j.actamat.2014.02.017
%T Computational modeling of intrinsically induced strain gradients during compression of c-axis-oriented magnesium single crystal
%U http://dx.doi.org/10.1016/j.actamat.2014.02.017
%X A finite-deformation strain gradient crystal plasticity model is implemented in a three-dimensional finite-element framework in order to analyze the deformation behavior and the stress–strain response of magnesium single crystals under c -axis orientation. The potential-based and thermodynamically consistent material model is formulated in a non-local and non-linear inelastic context in which dislocation densities are introduced via plastic strain gradients. Experiments have shown that the internal length scale of the microstructure starts to affect the overall stress–strain response when the sample size decreases to the micron scale. As a consequence, strain gradients develop, leading to an additional energetic-like hardening effect which results in an increase of the macroscopic strength with decreasing crystal size. In the case of uniaxial compression of c -axis-oriented single-crystal micropillars, the model is able to predict the discrete dislocation glide in terms of a band-shaped slip zone. Two different pillar sample sizes are taken into account in order to investigate the intrinsic size effect during plastic deformation where the crystallographic orientation leads to the activation of pyramidal {112¯2}〈112¯3〉 slip systems as reported in various experimental studies. The interaction of those slip systems is expressed in terms of latent hardening and excess dislocation development. A comparison between numerical results and corresponding experimental data is presented.
%0 Artikel
%@ 1359-6462
%A Huetsch, J.
%A Lilleodden, E.T.
%D 2014
%J Scripta Materialia
%N 1410
%P 49 - 51
%R doi:10.1016/j.scriptamat.2014.01.016
%T The influence of focused-ion beam preparation technique on microcompression investigations: Lathe vs. annular milling
%U http://dx.doi.org/10.1016/j.scriptamat.2014.01.016
%X Two commonly used focused ion beam (FIB) milling techniques were employed for Mg micropillar fabrication to investigate the influence of the individual FIB technique on the microcompression testing method. Results from lathe milled pillars show that the relatively high ion exposure of this technique relative to annular milling greatly affects both stress–strain response and deformation morphology; stresses reached are up to four times higher than for the annular milled pillars and cracking of a surface layer is observed.
%0 Artikel
%@ 0022-5096
%A Kupka, D.
%A Huber, N.
%A Lilleodden, E.T.
%D 2014
%J Journal of the Mechanics and Physics of Solids
%N 1259
%P 455 - 467
%R doi:10.1016/j.jmps.2013.12.004
%T A combined experimental-numerical approach for elasto-plastic fracture of individual grain boundaries
%U http://dx.doi.org/10.1016/j.jmps.2013.12.004
%X The parameters for a crystal plasticity finite element constitutive law were calibrated for the aluminum–lithium alloy 2198 using micro-column compression testing on single crystalline volumes. The calibrated material model was applied to simulations of micro-cantilever deflection tests designed for micro-fracture experiments on single grain boundaries. It was shown that the load–displacement response and the local deformation of the grains, which was measured by digital image correlation, were predicted by the simulations. The fracture properties of individual grain boundaries were then determined in terms of a traction–separation-law associated with a cohesive zone. This combination of experiments and crystal plasticity finite element simulations allows the investigation of the fracture behavior of individual grain boundaries in plastically deforming metals.
%0 Artikel
%@ 1895-1066
%A Kuntyi, O.
%A Okhremchuk, Y.
%A Bilan, O.
%A Hapke, J.
%A Saldan, I.
%D 2013
%J Central European Journal of Chemistry
%N 2374
%P 514 - 518
%R doi:10.2478/s11532-012-0189-9
%T Silver particles growth by pulse electrolysis in acetonitrile solutions
%U http://dx.doi.org/10.2478/s11532-012-0189-9
4
%X The morphology of silver particles deposited on ITO-glass surface by pulse electrolysis in acetonitrile solutions of AgNO3 has been analyzed. The influences of potential value (E) as well pulse duration (τon) and pause (τoff) on the size and geometry of the particles has been discussed. It has been shown that in the range of 0.0 ≤ E ≤ −1.5 V at τon = 6 ms and 90 ≤ τoff ≤ 490 ms formation of silver particles (∼20–50 nm) and their agglomeration (∼0.2–2 µm) take place. The tendency to increase size of the particles in 3D has been observed with the increase of cathode potential. Decreasing of duty cycle leads to more discrete deposited particles.
%0 Artikel
%@ 1359-6454
%A Knorr, I.
%A Cordero, N.M.
%A Lilleodden, E.T.
%A Volkert, C.A.
%D 2013
%J Acta Materialia
%N 1006
%P 4984 - 4995
%R doi:10.1016/j.actamat.2013.04.047
%T Mechanical behavior of nanoscale Cu/PdSi multilayers
%U https://dx.doi.org/10.1016/j.actamat.2013.04.047
13
%X Berkovich nanoindentation and uniaxial microcompression tests have been performed on sputter-deposited crystalline Cu/amorphous Pd0.77Si0.23 multilayered films with individual layer thicknesses ranging from 10 to 120 nm. Elastic moduli, strengths and deformation morphologies have been compared for all samples to identify trends with layer thicknesses and volume fractions. The multilayer films have strengths on the order of 2 GPa, from which Cu layer strengths on the order of 2 GPa can be inferred. The high strength is attributed to extraordinarily high strain hardening in the polycrystalline Cu layers through the inhibition of dislocation annihilation or transmission at the crystalline/amorphous interfaces. Cross-sectional microscopy shows uniform deformation within the layers, the absence of delamination at the interfaces, and folding and rotation of layers to form interlayer shear bands. Shear bands form where shear stresses are present parallel to the interfaces and involve tensile plastic strains as large as 85% without rupture of the layers. The homogeneous deformation and high strains to failure are attributed to load sharing between the amorphous and polycrystalline layers and the inhibition of strain localization within the layers.
%0 Artikel
%@ 0921-5093
%A Rao, D.
%A Huber, K.
%A Heerens, J.
%A dos Santos, J.F.
%A Huber, N.
%D 2013
%J Materials Science and Engineering A
%N 1297
%P 44 - 50
%R doi:10.1016/j.msea.2012.12.014
%T Asymmetric mechanical properties and tensile behaviour prediction of aluminium alloy 5083 friction stir welding joints
%U http://dx.doi.org/10.1016/j.msea.2012.12.014
%X The asymmetric material flow, severe plastic deformation and thermal cycle imposed on the base material during friction stir welding (FSW) result in unique microstructural development, which causes a gradient in local mechanical properties in the weld region. Micro-tensile and indentation testing were applied to determine the local mechanical properties in a friction stir welded joint. The local stress–strain curves exhibited a drastic change at the advancing side (AS) due to a steep gradient of mechanical properties. Finite Element Model (FEM) predictions of the tensile performance of the welded joints, based on the local mechanical properties measured by micro-tensile testing, were in very good agreement with the macro-tensile test data.