Papers by Frederic Sansoz
Nano letters, Jan 7, 2015
Pushing the limits of elastic deformation in nanowires subjected to stress is important for the d... more Pushing the limits of elastic deformation in nanowires subjected to stress is important for the design and performance of nanoscale devices from elastic strain engineering. Particularly, introducing nanoscale twins has proved effective in rising the tensile strength of metals. However, attaining ideal elastic strains in nanotwinned materials remains challenging, because non-uniform twin sizes locally affect the yielding behavior. Here, using in-situ high-resolution transmission electron microscopy tensile testing of nanotwinned [111]-oriented gold nanowires, we report direct lattice-strain measurements that demonstrate a strong Hall-Petch type relationship in the elastic strain limit up to 5.3%, or near the ideal theoretical limit, as the twin size is decreased below 3 nm. It is found that the largest twin in nanowires with irregular twin sizes controls the slip nucleation and yielding processes in pure tension, in agreement with earlier atomistic simulations. Continuous hardening b...
This study presents the development and validation of a two-scale numerical method aimed at predi... more This study presents the development and validation of a two-scale numerical method aimed at predicting the mechanical behavior and the inter-granular fracture of nanocrystalline (NC) metals under deformation. The material description is based on two constitutive elements, the grains (or bulk crystals) and the grain-boundaries (GBs). Their behaviors are determined atomistically using the quasicontinuum (QC) method by simulating the plastic deformation of ½1 1 0 tilt crystalline interfaces undergoing simple shear, tension and nano-indentation. Unlike our previous work (Péron-Lührs et al., 2013) however, the GB thickness is here calibrated in the model, providing more accurate insight into the GB widths according to the interface misorientation angle. In this contribution, the new two-scale model is also validated against fully-atomistic NC simulation tests for two low-angle and high-angle textures and two grain sizes. A simplified strategy aimed at predicting the mechanical behavior of more general textures without the need to run more QC simulations is also proposed, demonstrating significant reductions in the computational cost compared to full atomistic simulations. Finally, by studying the response of dogbone samples made of NC copper, we show in this paper that such a two-scale model is able to quantitatively capture the differences in mechanical behavior of NC metals as a function of the texture and grain size, as well as to accurately predict the processes of inter-granular fracture for different GB character distributions. This two-scale method is found to be an effective alternative to other atomistic methods for the prediction of plasticity and fracture in NC materials with a substantial number of 2-D grains such as columnargrained thin films for micro-scale electro-mechanical devices.
Atomistic simulations using the quasicontinuum method are performed to examine the mechanical beh... more Atomistic simulations using the quasicontinuum method are performed to examine the mechanical behavior and underlying mechanisms of surface plasticity in nanocrystalline aluminum with a grain diameter of 7 nm deformed under wedge-like cylindrical contact. Two embedded-atom method potentials for Al, which mostly differ in their prediction of the generalized stacking and planar fault energies, and grain boundary (GB) energies, are used and characterized. The simulations are conducted on a randomly oriented microstructure with h1 1 0i-tilt GBs. The contact pressure-displacement curves are found to display significant flow serration. We show that this effect is associated with highly localized shear deformation resulting from one of three possible mechanisms: (1) the emission of partial dislocations and twins emanating from the contact interface and GBs, along with their propagation and intersection through intragranular slip, (2) GB sliding and grain rotation and (3) stress-driven GB migration coupled to shear deformation. Marked differences in mechanical behavior are observed, however, as a function of the interatomic potential. We find that the propensity to localize the plastic deformation at GBs via interface sliding and coupled GB migration is greater in the Al material presenting the lowest predicted stacking fault energy and GB energy. This finding is qualitatively interpreted on the basis of impurity effects on plastic flow and GB-mediated deformation processes in Al.
MRS Proceedings, 2003
We have examined the micromechanical behavior of in situ formed metallic glass composites by perf... more We have examined the micromechanical behavior of in situ formed metallic glass composites by performing in situ high-energy synchrotron X-ray scattering during uniaxial compression. The load partitioning between the amorphous matrix and the reinforcing particles was examined by measuring the lattice strains in the crystalline particles during compressive loading. The crystalline particles yield in compression during loading followed by tensile yielding during unloading. The large elastic mismatch between the two phases leads to large residual strains after each loading cycle. The load partitioning was also examined with finite element modeling (FEM). The predicted von Mises effective stress in the crystalline particles from the FEM calculations compares well with the experimentally determined von Mises effective stress so long as the deformation is elastic in both particles and matrix. After the particles yield, the model predicts strain hardening of the particles that is not observed experimentally.
MRS Proceedings, 2007
Parallel molecular dynamics simulations were used to study the influence of pre-existing growth t... more Parallel molecular dynamics simulations were used to study the influence of pre-existing growth twin boundaries on the slip activity of bulk gold under uniaxial compression. The simulations were performed on a 3D, fully periodic simulation box at 300 K with a constant strain rate of 4×10 7 s -1 . Different twin boundary interspacings from 2 nm to 16 nm were investigated. The strength of bulk nano-twinned gold was found to increase as the twin interspacing was decreased. However, strengthening effects related to the twin size were less significant in bulk gold than in gold nanopillars. The atomic analysis of deformation modes at the twin boundary/slip intersection suggested that the mechanisms of interfacial plasticity in nano-twinned gold were different between bulk and nanopillar geometries.
MRS Proceedings, 2003
The deformation of copper with grain size less than 10 nm is investigated using a 2D continuum mo... more The deformation of copper with grain size less than 10 nm is investigated using a 2D continuum model incorporating atomistically-based constitutive relations. The local constitutive response of a series of symmetric and asymmetric tilt grain boundaries is obtained using an atomistic quasicontinuum method under tension and shear. The atomistic results show that it is possible to associate a constant maximum stress with each deformation mechanism triggered in the GB vicinity, i.e. GB sliding and decohesion, atom shuffling and partial dislocation emission. The GB strength is always found weaker in shear than in tension. This information is incorporated into a continuum polycrystalline model tested under compression. This model provides useful insights, in the absence of intragranular plasticity, into the onset of macroscopic quasi-plasticity, which results from GB sliding and collective grain rotation mechanisms.
Scripta Materialia, 2010
We examine the processes of spherical indentation and tension in Ni nanowires and thin films cont... more We examine the processes of spherical indentation and tension in Ni nanowires and thin films containing random distributions of nanoscale grains by molecular dynamics simulations. It is shown that the resistance to nanoindentation of nanocrystalline Ni nanowires with diameters of 12 and 30 nm tends not to depend on the wire diameter and free surfaces, contrary to nanoindentation in single-crystalline nanowires. Accommodation of plastic deformation by grain boundary sliding suggests a mitigation strategy for sample boundary effects in nanoscale plasticity.
Trends in Computational Nanomechanics, 2009
Predicting the integrity of metallic thin films deposited on semiconductors for microelectromecha... more Predicting the integrity of metallic thin films deposited on semiconductors for microelectromechanical systems (MEMS) applications requires a precise understanding of surface effects on plasticity in materials with nano-sized grains. Experimentally, the use of nanoscale contact probes has been very successful to characterize the dependence of flow stress on mean grain size in nanocrystalline metals. From atomistic simulations, several models of plastic yielding for metal indentation have also been proposed based on the nucleation and propagation of lattice dislocations, and their interaction with grain boundaries beneath penetrating tips. However, model refinement is needed to include the characteristics of materials whose grain size is much smaller than the typical plastic zones found in contact experiments. Particularly, cooperative deformation processes mediated by grain boundaries, such as grain rotation, deformation twinning, and stress-driven grain coarsening, can simultaneously emerge for very small grain sizes (< 20 nm), thus making a predictive understanding of plastic yielding elusive. This chapter summarizes our recent progress in using multiscale modeling to gain fundamental insight into the underlying mechanisms of surface plasticity in nanocrystalline face-centered cubic metals deformed by nanoscale contact probes. Two numerical approaches to model contact-induced plasticity in nanocrystalline materials, the quasicontinuum method and parallel molecular dynamics simulation, are reviewed. Using these techniques, we discuss the role of a grain boundary network on the incipient plasticity of nanocrystalline Al films deformed by wedge-like cylindrical tips, as well as the processes of stress-driven grain growth in nanocrystalline films subjected to nanoindentation
Thin Solid Films, 2007
The effects of structure and size on the deformation of b110N tilt bicrystals in copper are inves... more The effects of structure and size on the deformation of b110N tilt bicrystals in copper are investigated by concurrent multiscale simulations at zero temperature. In the simulation of eleven grain boundary (GB) structures, a direct relation is shown between structural units and sliding at GBs. We find that GB sliding operates by atom shuffling events localized on one particular type of structural units, which are present in the GB period. When this type of unit is absent, the GB deformation process occurs by migration, or GB-mediated nucleation of partial dislocations with no sliding, depending on the initial GB configuration. The elastic limit causing sliding is found to vary slightly at zero temperature, but no correlation was obtained with the GB energy at equilibrium. Additionally, both modulus of rigidity, and elastic limit remain constant as the bicrystal size varies from 1 nm up to 25 nm. However, differences in the stress relaxation after sliding are observed with respect to the size.
Scripta Materialia, 2010
The effects of twin size and sample diameter on yield stress and surface dislocation emission in ... more The effects of twin size and sample diameter on yield stress and surface dislocation emission in twinned metal nanowires deformed uniformly were studied using classical dislocation theory and the concept of image force from twin boundaries. This theoretical study is shown to quantitatively capture the linear increase in yield stress as twin size decreases in periodically twinned Au nanowires predicted by atomistic simulations. The implication of this model as a yield criterion for realistic metal nanostructures with nanoscale growth twins is discussed.
Physical Review B, 2010
We use classical molecular-dynamics simulations to examine the strain-rate sensitivity of single-... more We use classical molecular-dynamics simulations to examine the strain-rate sensitivity of single-crystalline and twinned Au nanowires ͑NWs͒ with a diameter of 12.3 nm deformed in tension at temperatures between 10 K and 450 K. It is found that the strain-rate sensitivity above 100 K is significantly smaller in twinned Au NWs with perfectly circular cross-section than in similar NWs without twins, while the activation volume remains in the same range of 1b 3 -15b 3 with b the magnitude of Burgers vector. This behavior is markedly different from that generally observed in bulk face-centered cubic metals where addition of nanoscale twins increases both strength and strain-rate sensitivity. Furthermore, our simulations show a threefold decrease in strain-rate sensitivity in twinned Au NWs with zigzag morphology constructed by assembly of ͕111͖ surface facets in comparison to the different types of circular Au NWs. The rate-controlling deformation mechanisms related to surface dislocation emission and twin-slip interaction, and their dependence on temperature and surface morphology are analyzed in detail. The combination of ultrahigh strength and decreased sensitivity to strain-rate predicted above 100 K in twinned Au NWs with faceted surface morphology holds great promise for creating metallic nanostructures with increased failure resistance to extreme loading conditions.
Nature Materials, 2013
Coherent twin boundaries (CTBs) are widely described, both theoretically and experimentally, as p... more Coherent twin boundaries (CTBs) are widely described, both theoretically and experimentally, as perfect interfaces that play a significant role in a variety of materials. Although the ability of CTBs in strengthening, maintaining the ductility and minimizing the electron scattering is well documented, most of our understanding of the origin of these properties relies on perfect-interface assumptions. Here we report experiments and simulations demonstrating that as-grown CTBs in nanotwinned copper are inherently defective with kink-like steps and curvature, and that these imperfections consist of incoherent segments and partial dislocations. We further show that these defects play a crucial role in the deformation mechanisms and mechanical behaviour of nanotwinned copper. Our findings offer a view of the structure of CTBs that is largely different from that in the literature, and underscore the significance of imperfections in nanotwin-strengthened materials.
Nature Communications, 2013
Although nanoscale twinning is an effective means to enhance yield strength and tensile ductility... more Although nanoscale twinning is an effective means to enhance yield strength and tensile ductility in metals, nanotwinned metals generally fail well below their theoretical strength limit due to heterogeneous dislocation nucleation from boundaries or surface imperfections. Here we show that Au nanowires containing angstrom-scaled twins (0.7 nm in thickness) exhibit tensile strengths up to 3.12 GPa, near the ideal limit, with a remarkable ductile-tobrittle transition with decreasing twin size. This is opposite to the behaviour of metallic nanowires with lower-density twins reported thus far. Ultrahigh-density twins (twin thick-nesso2.8 nm) are shown to give rise to homogeneous dislocation nucleation and plastic shear localization, contrasting with the heterogeneous slip mechanism observed in singlecrystalline or low-density-twinned nanowires. The twin size dependent dislocation nucleation and deformation represent a new type of size effect distinct from the sample size effects described previously.
Nanoscale, 2012
Although coherent twin boundaries require little energy to form in nanoscale single crystals, the... more Although coherent twin boundaries require little energy to form in nanoscale single crystals, their influence on properties can be dramatic. In recent years, some important steps forward have been made in understanding and controlling twinning processes at the nanoscale, making possible the fabrication of nanoengineered twinning superlattices in crystalline nanowires. These advances have opened new possibilities for properties and functionalities at the atomic and quantum scales by modulating twin densities. This article presents a brief overview of recent theoretical and experimental progress in growth mechanisms and promising properties of coherent twinning superlattice nanowires with special emphasis toward cubic systems in semiconductor and metallic materials. In particular, we show how nanoscale growth twins can considerably enhance bandgap engineering and mechanical behaviour in quasi-one-dimensional materials. Opportunities for future research in this emerging area are also discussed.
Nano Letters, 2007
The role of growth twin boundaries on the slip activity of gold nanopillars under uniaxial compre... more The role of growth twin boundaries on the slip activity of gold nanopillars under uniaxial compression was investigated by molecular dynamics simulation. A new type of size-dependent strengthening was found in twinned gold nanopillars. Strengthening resulted from slip arrests in the form of Lomer−Cottrell locks at the intersection of partial dislocations and twin boundaries. The significance of such phenomenon was found to depend on the twin size. These findings could help engineers design tunable mechanical properties in nanowires by interfacial plasticity.
Nano Letters, 2009
By using molecular dynamics simulations, we show that significant strain hardening and ultrahigh ... more By using molecular dynamics simulations, we show that significant strain hardening and ultrahigh flow stresses are enabled in gold nanowires containing coherent (111) growth twins when balancing nanowire diameter and twin boundary spacing at the nanoscale. A fundamental transition in mechanical behavior occurs when the ratio of diameter to twin boundary spacing is larger than 2.14. A model based on site-specific dislocation nucleation and cross-slip mechanisms is proposed to explain the size dependence of flow behavior in twinned nanowires under tensile loading.
Metallurgical and Materials Transactions A, 2006
We have examined the yielding and fracture behavior of Zr 57.4 Cu 16.4 Ni 8.2 Ta 8 Al 10 metallic... more We have examined the yielding and fracture behavior of Zr 57.4 Cu 16.4 Ni 8.2 Ta 8 Al 10 metallic-glassmatrix composites with a small volume fraction (;4 pct) of ductile crystalline particles under quasi-static uniaxial tension and compression and dynamic uniaxial compression. The yield stress of the composite is the same for quasi-static tension and compression, consistent with a von Mises yield criterion. The measured average angle between the shear bands and the loading axis in quasistatic compression is 47 6 2 deg, significantly larger than the value of ;42 deg typically reported for single-phase metallic glasses. Finite element modeling (FEM) shows that the measured value is consistent with both the von Mises criterion (48 6 4 deg) and the Mohr-Coulomb criterion (46 6 5 deg). The fracture surface angles, however, are 41 6 1 deg (compression) and 54 6 2 deg (tension), in good agreement with observations of single-phase metallic glasses. At low strain rates (,10 À1 s À1 ), the yield stress is independent of strain rate, while at higher strain rates (.10 0 s À1 ), the failure stress decreases with increasing strain rate, which again is similar to the behavior of single-phase glasses. These results indicate that while the presence of the particles has a significant effect on the yield behavior of the composites, the fracture behavior is largely governed by the properties and behavior of the amorphous matrix.
Metallurgical and Materials Transactions A, 2003
Fatigue crack growth experiments were carried out on Ti6242 alloy with large colony size. The all... more Fatigue crack growth experiments were carried out on Ti6242 alloy with large colony size. The alloy was heat treated to provide three different lamella size; fine, coarse, and extra coarse. Tests were conducted at two temperatures, 520°C and 595°C, using two loading frequencies, 10 and 0.05 Hz. The latter frequency was examined with and without a 300-second hold time. All tests were performed in air environment and at a stress ratio of 0.1. This study shows that at 520°C, the Fatigue crack growth rate (FCGR) is not significantly influenced by changes in the microstructure. For 0.05 Hz/low , however, the FCGR is higher in the fine lamellar microstructure and is accompanied by-the appearance of a plateau, which disappears in the extra large lamella microstructure. Furthermore, the addition of a 300-second hold time does not alter the crack growth rate. At 595°C, while the general level of the FCGR is higher than that at 520°C, the effects of loading frequency and hold time remain similar to those reported at the lower temperature. Unlike the results at 520°C, however, the FCGR at low is not influenced by variations in lamellar microstructure. Under all test conditions, the fatigue process is predominantly controlled by one single mechanism associated with transcolony fracture and formation of quasi-cleavage facets. The fatigue crack growth results and the associated fracture behavior as obtained in this study are correlated to the crack-tip shear activity and transmission at the interfaces. A general hypothesis accounting for the role of loading frequency, temperature, and microstructure on the observed cracking mechanisms is presented.
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Papers by Frederic Sansoz