Groupe de Physique des Matériaux

Conventional metals become harder with decreasing grain sizes, following the classical Hall-Petch relationship. However, this relationship fails and softening occurs at some grain sizes in the nanometer regime for some alloys. In this study, we discovered that plastic deformation mechanism of extremely fine nanograined metals and their hardness are adjustable through tailoring grain boundary (GB) stability. The electrodeposited nanograined nickel-molybdenum (Ni-Mo) samples become softened for grain sizes below 10 nanometers because of GB-mediated processes. With GB stabilization through relaxation and Mo segregation, ultrahigh hardness is achieved in the nanograined samples with a plastic deformation mechanism dominated by generation of extended partial dislocations. Grain boundary stability provides an alternative dimension, in addition to grain size, for producing novel nanograined metals with extraordinary properties.

It is well known that, at a macroscopic level, the boundary condition for a viscous fluid at a solid wall is one of ``no slip.'' The liquid velocity field vanishes at a fixed solid boundary. We consider the special case of a liquid that partially wets the solid (i.e., a drop of liquid, in equilibrium with its vapor on the solid substrate, has a finite contact angle). Using extensive molecular dynamics simulations, we show that when the contact angle is large enough, the boundary condition can drastically differ (at a microscopic level) from a no-slip condition. Slipping lengths exceeding 30 molecular diameters are obtained for a contact angle of 140\ifmmode^\circ\else\textdegree\fi{}, characteristic of mercury on glass. This finding may have important implications for the transport properties in nanoporous media under such ``nonwetting'' conditions.

This paper presents simple models useful in analyzing the growth of nanostructures obtained by cluster deposition. After a brief survey of applications and experimental methods, the author describes the Monte Carlo techniques for simulating nanostructure growth. Simulations of the first stages, the submonolayer regime, are reported for a wide variety of experimental situations: complete condensation, growth with reevaporation, nucleation on defects, and total or null cluster-cluster coalescence. [Note: Software for all these simulation programs, which are also useful for analyzing growth from atomic beams, is available on request from the author.] The aim of the paper is to help experimentalists, in analyzing their data, to determine which processes are important and to quantify them. Experiments on growth from cluster beams are discussed, as is the measurement of cluster mobility on the surface. Surprisingly high mobility values are found. An important issue for future technological applications of cluster deposition is the relation between the size of the incident clusters and the size of the islands obtained on the substrate, which is described by an approximate formula depending on the melting temperature of the deposited material. Finally, the author examines the atomic mechanisms that can explain the diffusion of clusters on a substrate and their mutual interaction, to aggregate keeping their integrity or to coalesce.

Severe plastic deformation (SPD) is effective in producing bulk ultrafine-grained and nanostructured materials with large densities of lattice defects. This field, also known as NanoSPD, experienced a significant progress within the past two decades. Beside classic SPD methods such as high-pressure torsion, equal-channel angular pressing, accumulative roll-bonding, twist extrusion, and multi-directional forging, various continuous techniques were introduced to produce upscaled samples. Moreover, numerous alloys, glasses, semiconductors, ceramics, polymers, and their composites were processed. The SPD methods were used to synthesize new materials or to stabilize metastable phases with advanced mechanical and functional properties. High strength combined with high ductility, low/room-temperature superplasticity, creep resistance, hydrogen storage, photocatalytic hydrogen production, photocatalytic CO2 conversion, superconductivity, thermoelectric performance, radiation resistance, corrosion resistance, and biocompatibility are some highlighted properties of SPD-processed materials. This article reviews recent advances in the NanoSPD field and provides a brief history regarding its progress from the ancient times to modernity. Abbreviations: ARB: Accumulative Roll-Bonding; BCC: Body-Centered Cubic; DAC: Diamond Anvil Cell; EBSD: Electron Backscatter Diffraction; ECAP: Equal-Channel Angular Pressing (Extrusion); FCC: Face-Centered Cubic; FEM: Finite Element Method; FSP: Friction Stir Processing; HCP: Hexagonal Close-Packed; HPT: High-Pressure Torsion; HPTT: High-Pressure Tube Twisting; MDF: Multi-Directional (-Axial) Forging; NanoSPD: Nanomaterials by Severe Plastic Deformation; SDAC: Shear (Rotational) Diamond Anvil Cell; SEM: Scanning Electron Microscopy; SMAT: Surface Mechanical Attrition Treatment; SPD: Severe Plastic Deformation; TE: Twist Extrusion; TEM: Transmission Electron Microscopy; UFG: Ultrafine Grained. © 2022 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group.

We determined the structure of the hydrated Cu(II) complex by both neutron diffraction and first-principles molecular dynamics. In contrast with the generally accepted picture, which assumes an octahedrally solvated Cu(II) ion, our experimental and theoretical results favor fivefold coordination. The simulation reveals that the solvated complex undergoes frequent transformations between square pyramidal and trigonal bipyramidal configurations. We argue that this picture is also consistent with experimental data obtained previously by visible near-infrared absorption, x-ray absorption near-edge structure, and nuclear magnetic resonance methods. The preference of the Cu(II) ion for fivefold instead of sixfold coordination, which occurs for other cations of comparable charge and size, results from a Jahn-Teller destabilization of the octahedral complex.

Insoluble Langmuir monolayers are investigated in the presence of dipolar forces which can have two origins: permanent dipoles in neutral monolayers and induced dipoles in charged monolayers. The main effect of the additional long-range repulsive interactions is to stabilize undulating phases at thermodynamic equilibrium. Phase diagrams are obtained in two limits: close to the liquid–gas critical point via a Ginzburg–Landau expansion of the free energy (mainly within a mean-field approximation), and at low temperatures by free energy minimization. Possible applications of this theory to experiments at the liquid–gas, liquid expanded–liquid condensed, and solid–liquid transitions are discussed.

Clouds of impurity atoms near line defects are believed to affect the plastic deformation of alloys. Three-dimensional atom probe techniques were used to image these so-called Cottrell atmospheres directly. Ordered iron-aluminum alloys (40 atomic percent aluminum) doped with boron (400 atomic parts per million) were investigated on an atomic scale along the <001> direction. A boron enrichment was observed in the vicinity of an <001> edge dislocation. The enriched region appeared as a three-dimensional pipe 5 nanometers in diameter, tangent to the dislocation line. The dislocation was found to be boron-enriched by a factor of 50 (2 atomic percent) relative to the bulk. The local boron enrichment is accompanied by a strong aluminum depletion of 20 atomic percent.

Abstract Intercrystalline molecular connections in semicrystalline polymers have been the subject of numerous discussions and controversies. Nevertheless, there is one point of agreement: such intercrystalline tie molecules have a prime role in the mechanical and use properties of the materials, notably the resistance to slow crack growth. This article is a critical review of the mechanisms of generation of the tie molecules during the stage of crystallization and of the experimental and theoretical assessment of their concentration. Polyethylene and related materials are mainly studied. The contribution of chain entanglements is also discussed in parallel with tie molecules. Particular attention is paid to Huang and Brown's statistical approach, which appears to be the most appropriate one for predictive purposes and has aroused much interest from various authors. Attempts are made to provide solutions to the shortcomings of this model. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1729–1748, 2005

The low-energy cluster beam deposition technique (LECBD) is applied to produce cluster assembled films with hitherto unknown nanostructured morphologies and properties. Neutral clusters having the very low energy gained in the supersonic expansion at the exit of the inert gas condensation-type source are deposited without fragmentation upon impact on the substrate. Depending on the deposition conditions (nature, size and flux of incident clusters, nature and temperature of the substrate, vacuum conditions), granular nanostructures resulting from the diffusion and coalescence of supported clusters are obtained with materials of any type (covalent or metallic). A critical size for coalescence limits the supported grain size and, finally, highly porous thick films growing by random stacking of nanoparticles are obtained. A recent model developed by combining several dynamical processes simultaneously occurring on the substrate (deposition - diffusion - aggregation, DDA) is used to simulate the cluster assembled film morphology in good agreement with the experimental observations. Examples of novel materials obtained by LECBD are presented to illustrate the interesting potentialities of the technique. In the case of covalent materials such as carbon and silicon, 'amorphon'-type disordered structures, different from the conventional amorphous structures (a-C and a-Si), are obtained with some unique properties. With transition metal (Fe, Co and Ni) cluster assembled films, a specific magnetic behaviour, resulting from the competition between the intrinsic properties of the grains (magnetocrystalline anisotropy) and the interactions between grains, is observed. Also, films of clusters embedded in various co-deposited matrices are produced in order to control the interactions between grains via the matrix materials (insulating, conducting ...). Interesting optical properties (from metallic clusters in ) or giant magnetoresistance effects (from Co clusters in silver) are reported for such systems, emphasizing the future role of LECBD in various fields of applications such as optical and optoelectronic nanostructures, magnetic and magneto-optic nanostructures and quantum devices.

The approach of the elastic continuum limit in small amorphous bodies formed by weakly polydisperse Lennard-Jones beads is investigated in a systematic finite-size study. We show that classical continuum elasticity breaks down when the wavelength of the solicitation is smaller than a characteristic length of approximately 30 molecular sizes. Due to this surprisingly large effect ensembles containing up to $N=40000$ particles have been required in two dimensions to yield a convincing match with the classical continuum predictions for the eigenfrequency spectrum of disk-shaped aggregates and periodic bulk systems. The existence of an effective length scale $\ensuremath{\xi}$ is confirmed by the analysis of the (non-Gaussian) noisy part of the low frequency vibrational eigenmodes. Moreover, we relate it to the nonaffine part of the displacement fields under imposed elongation and shear. Similar correlations (vortices) are indeed observed on distances up to $\ensuremath{\xi}\ensuremath{\approx}30$ particle sizes.

The nonequilibrium dynamics of a binary Lennard-Jones mixture in a simple shear flow is investigated by means of molecular dynamics simulations. The range of temperature T investigated covers both the liquid, supercooled, and glassy states, while the shear rate γ covers both the linear and nonlinear regimes of rheology. The results can be interpreted in the context of a nonequilibrium, schematic mode-coupling theory developed recently, which makes the theory applicable to a wide range of soft glassy materials. The behavior of the viscosity η(T,γ) is first investigated. In the nonlinear regime, strong shear-thinning is obtained, η∼γ−α(T), with α(T)≃23 in the supercooled regime. Scaling properties of the intermediate scattering functions are studied. Standard “mode-coupling properties” of factorization and time superposition hold in this nonequilibrium situation. The fluctuation-dissipation relation is violated in the shear flow in a way very similar to that predicted theoretically, allowing for the definition of an effective temperature Teff for the slow modes of the fluid. Temperature and shear rate dependencies of Teff are studied using density fluctuations as an observable. The observable dependence of Teff is also investigated. Many different observables are found to lead to the same value of Teff, suggesting several experimental procedures to access Teff. It is proposed that a tracer particle of large mass mtr may play the role of an “effective thermometer.” When the Einstein frequency of the tracers becomes smaller than the inverse relaxation time of the fluid, a nonequilibrium equipartition theorem holds with 〈mtrvz2〉=kBTeff, where vz is the velocity in the direction transverse to the flow. This last result gives strong support to the thermodynamic interpretation of Teff and makes it experimentally accessible in a very direct way.

Liquid foams can behave like solids or liquids, depending on the applied stress and on the experimental timescale. Understanding the origin of this complex rheology which gives rise to many applications and which resembles that of many other forms of soft condensed matter made of closely packed soft units requires challenging theoretical questions to be solved. We briefly recall the basic physics and physicochemistry of foams and review the experiments, numerical simulations and theoretical models concerning foam rheology published in recent years.

Using molecular dynamics simulations we study the out of equilibrium dynamic correlations in a model glass-forming liquid. The system is quenched from a high temperature to a temperature below its glass transition temperature and the decay of the two-time intermediate scattering function $C({t}_{w}{,t+t}_{w})$ is monitored for several values of the waiting time ${t}_{w}$ after the quench. We find that $C({t}_{w}{,t+t}_{w})$ shows a strong dependence on the waiting time, i.e., aging, depends on the temperature before the quench, and, similar to the case of spin glasses, can be scaled onto a master curve.

Magnetic interactions involving ferromagnetic layers separated by an insulating barrier have been studied experimentally on a fully epitaxial hard-soft magnetic tunnel junction: $\mathrm{F}\mathrm{e}/\mathrm{M}\mathrm{g}\mathrm{O}/\mathrm{F}\mathrm{e}/\mathrm{C}\mathrm{o}$. For a barrier thickness below 1 nm, a clear antiferromagnetic interaction is observed. Moreover, when reducing the MgO thickness from 1 to 0.5 nm, the coupling strength increases up to $J=\ensuremath{-}0.26\text{ }{\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{\ifmmode\cdot\else\textperiodcentered\fi{}}\mathrm{c}\mathrm{m}}^{\ensuremath{-}2}$. This behavior, well fitted by theoretical models, provides an unambiguous signature of the interlayer exchange coupling by spin-polarized quantum tunneling.

The uncatalyzed edge growth of carbon nanotubes was investigated by first-principles molecular dynamics simulations. At experimental temperatures the open end of single-walled nanotubes closed spontaneously into a graphitic dome, which may explain why these nanotubes do not grow in the absence of transition metal catalysts. On the other hand, chemical bonding between the edges of adjacent coaxial tubes ("lip-lip" interactions) trapped the end of a double-walled nanotube in a metastable energy minimum, thus preventing dome closure. These calculations show that this end geometry exhibits a high degree of chemical activity and easily accommodates incoming carbon fragments, supporting a model of growth by chemisorption from the vapor phase.

We present a quantitative study of the diffusion of spherical antimony clusters deposited on graphite surfaces. The experimental structures obtained during deposition are compared to predictions of recent computer models, and very good agreement is found. From this comparison we can obtain the diffusion coefficient of large antimony clusters containing around 2300 atoms moving on a graphite substrate: we find ${D\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}D}_{0}\mathrm{exp}({\ensuremath{-}E}_{a}/{k}_{B}T)$ with ${D}_{0}\ensuremath{\approx}1.6\ifmmode\times\else\texttimes\fi{}{10}^{4}{\mathrm{cm}}^{2}/\mathrm{s}$ and ${E}_{a}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.7\ifmmode\pm\else\textpm\fi{}0.1\mathrm{eV}$. This large value of ${D}_{0}$ suggests that the diffusion cannot be explained by a simple atomic activated process, but may rather involve collective motions of the atoms of the cluster.

The composition of β″ precipitates in an Al–Mg–Si alloy has been investigated by atom probe tomography, ab initio density functional calculations, and quantitative electron diffraction. Atom probe analysis of an Al-0.72% Si-0.58% Mg (at. %) alloy heat treated at 175 °C for 36 h shows that the β″ phase contains ∼20 at. % Al and has a Mg/Si-ratio of 1.1, after correcting for a local magnification effect and for the influence of uneven evaporation rates. The composition difference is explained by an exchange of some Si with Al relative to the published β″-Mg5Si6 structure. Ab initio calculations show that replacing the Si3-site by aluminum leads to energetically favorable compositions consistent with the other phases in the precipitation sequence. Quantitative electron nanodiffraction is relatively insensitive to this substitution of Al by Si in the β″-phase.

We investigate a general scenario for "glassy" or "jammed" systems driven by an external, nonconservative force, analogous to a shear force in a fluid. In this scenario, the drive results in the suppression of the usual aging process, and the correlation and response functions become time translation invariant. The relaxation time and the response functions are then dependent on the intensity of the drive and on temperature. We investigate this dependence within the framework of a dynamical closure approximation that becomes exact for disordered, fully connected models. The relaxation time is shown to be a decreasing function of the drive ("shear thinning" effect). The correlation functions below the glass transition temperature (Tc) display a two-time-scale relaxation pattern, similar to that observed at equilibrium slightly above Tc. We also study the violation of the fluctuation-dissipation relationship in the driven system. This violation is very reminiscent of the one that takes place in a system aging below Tc at zero drive. It involves, in particular the appearance of a two-temperature regime, in the sense of an effective fluctuation-dissipation temperature [L. F. Cugliandolo, J. Kurchan, and L. Peliti, Phys. Rev. E 55, 3898 (1997)]. Although our results are, in principle, limited to the closure relations that hold for mean-field models, we argue that a number of the salient features are not inherent to the approximation scheme, and may be tested in experiments and simulations.

A free-volume and friction viscosity model is presented versus pressure and temperature, valid for both gaseous and dense fluids. This model involves only three adjustable parameters for each pure compound. It is able to represent the gas-liquid transition and the behavior in the supercritical conditions. The model has been successfully applied to methane (885 data points for $0.01&lt;~P&lt;~200\mathrm{MPa}$ and $90.7&lt;~T&lt;~600\mathrm{K})$ and to propane (1085 data points for $0.01&lt;~P&lt;~200\mathrm{MPa}$ and $90&lt;~T&lt;~600\mathrm{K})$ in the gaseous and dense states (average absolute deviation is 2.59% for methane and 2.50% for propane, with maximum deviation of 14.8% for methane and 9.19% for propane). It has also been applied to hexane, octane, dodecane, benzene, trans-decaline, and 2,2-dimethylpropane (903 data points) in a large pressure range (up to 505.5 MPa). Considering these compounds the maximum deviation is 19.5% (for octane) and the average deviation is 3.51% in the worst case (dodecane, which has data points up to 501.6 MPa).

Abstract The innovation potential is high for bulk nanostructured materials (BNM) produced by methods of severe plastic deformation and accordingly this report focuses on very recent developments demonstrating the potential of using BNM for advanced and functional applications in engineering and medicine.