Yannick Champion

Nanostructured Cu-Cr alloy with high strength and electrical conductivity

The influence of nanostructuring by high pressure torsion (HPT) on strength and electrical conductivity in the Cu-Cr alloy has been investigated. Microstructure of HPT samples was studied by transmission electron microscopy with special attention on precipitation of small chromium particles after various treatments. Effect of dynamic precipitation leading to enhancement of strength and electrical conductivity was observed. It is shown that nanostructuring leads to combination of high ultimate tensile strength of 790–840 MPa, enhanced electrical conductivity of 81%–85% IACS and thermal stability up to 500 °C. The contributions of grain refinement and precipitation to enhanced properties of nanostructured alloy are discussed.

HRTEM study of defects in twin boundaries of ultra-fine grained copper

Twin boundaries (TBs) in ultra-fine grained (UFG) copper prepared by powder metallurgy were investigated using high-resolution transmission electron microscopy (HRTEM) and geometric phase analysis (GPA). Specimens were analyzed both before and after mechanical deformation (compression of 40%) and emphasis placed on the study of TB defects. Twin boundaries in the as-processed specimens are mainly disoriented from the perfect Σ3 coincidence. They present a faceted structure with coherent {111} and incoherent {112} facets. The latter have a 9R structure and the {111}/{112} junctions are associated with sessile dislocations of Frank type . Shockley glissile dislocations with Burgers vector of type are also present. This microstructure is interpreted in terms of the absorption and decomposition at room temperature of lattice dislocations (60° type). After mechanical deformation, an enrichment of twins at dislocations and a decrease of step density and height is observed and quantified by statistical analysis. Deformation mechanisms of UFG copper are discussed in light of these observations.

Sintering of copper nanopowders under hydrogen: an in situ X-ray diffraction analysis

Abstract The reduction by hydrogen gas of the cuprite layer on copper nanocrystals and the subsequent sintering of the nano-particles were studied using in-situ X-ray diffraction and dilatometry. Spherical nanocrystals produced by evaporation and condensation have an average size of 35 nm, exhibiting a large surface curvature. Each nanoparticle is coated with a 3.5 nm layer of Cu 2 O, which is rough and disordered, as revealed by high-resolution electron microscopy. Reduction by hydrogen of this curved cuprite layer occurs at 363 K, which is ≈65 K lower than is observed on a layer supported by micrometer-sized or bulk copper with a flat surface. The reduction process and its effect on the sintering of nanopowders are analysed and discussed.

Preparation and characterization of nanocrystalline copper powders

Owing to potentially interesting properties linked to the high volume fraction of grain boundary atoms with respect to those with a bulk environment, nanocrystalline materials have attracted substantial interest. Metallic nanocrystals have been produced using a gas condensation technique. The authors have developed a different process (cryogenic melting) to produce such materials and this has been used to produce nanometric powders of Fe, Ni, Co, Cr and alloys thereof. The essential advantage is that moderate quantities (about 50 g per hour) are produced. Here, the authors report on the production of nanocrystalline Cu powders using the cryogenic melting technique. Copper has been chosen in order to subsequently compare its mechanical properties with those of the other metals prepared previously (Fe, Ni, Co, Cr) and also for its potential intrinsic mechanical properties and applications. The powders were characterized using X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy (XPS).

Synthesis and characterization of core-shell structure silica-coated Fe29.5Ni70.5 nanoparticles

In view of potential applications of magnetic particles in biomedicine and electromagnetic devices, we made use of the classical Stober method-base-catalyzed hydrolysis and condensation of tetraethoxysilane (TEOS)-to encapsulate FeNi nanoparticles within a silica shell. An original stirring system under high power ultrasound made it possible to disperse the otherwise agglomerated particles. Sonication guaranteed particles remained dispersed during the Stober synthesis and also improved the efficiency of the method. The coated particles are characterized by electron microscopy (TEM) and spectroscopy (EDX) showing a core-shell structure with a uniform layer of silica. Silica coating does not affect the core magnetic properties. Indeed, all samples are ferromagnetic at 77 K and room temperature and the Curie point remains unchanged. Only the coercive force shows an unexpected non-monotonic dependence on silica layer thickness.

Magnetic properties of nanocomposites containing Fe-Ni or Fe dispersed in a Mn-Zn ferrite matrix

Summary form only given. Nanocomposites have been synthesised, by mixing in a planetary miller, of nanosized Fe/sub 25/Ni/sub 75/ or Fe and MnZn ferrite powders. Structural properties, grain size and lattice distortion were estimated by X-ray diffraction. Dispersion of the metallic particles was examined by TEM and SEM. Hysteresis loops were measured using a VSM. The powder preparation process increases the anisotropy constant of the ferrite component through the residual lattice distortion. The temperature dependence of magnetization was recorded by Faraday balance measurement, and well-defined Curie points are visible.

Structural analysis of phases and heterophase interfaces in the zirconium–boron system

The αZr–ZrB2 eutectic, a model system for metal–boride interfaces, was prepared by r.f. induction melting from high-purity zirconium ingots and zirconium diboride powders. At the eutectic composition and depending on the cooling rate, the formation of either the ZrB phase or a Zr(B) solid solution has been observed in addition to the expected compound αZr and ZrB2. For slow cooling rates, the formation of the compound ZrB by a peritectoid reaction and most likely stabilized by light elements (carbon, nitrogen, oxygen) has been observed. After rapid quenching, TEM investigations revealed the formation of a zirconium-based metastable phase; this new phase, with a nearly fcc structure, has been found in thin foils and is directly related to hexagonal αZr by a Shoji–Nishiyama orientation relationship. The structure at interfaces with habit planes featured by trigonal symmetry ({0 0 0 1} for hexagonal and {1 1 1} for fcc), has been investigated using weak-beam diffraction contrast and high-resolution transmission electron microscopy. The interfaces with a small difference in lattice parameter are accommodated by a misfit dislocation network, whilst those with a large difference in lattice parameter exhibit a more complex structure with ledges and facets.

Surface adsorption effects on the lattice expansion of copper nanocrystals

Lattice expansion in nanocrystalline copper due to size and surface effect is reported. The lattice parameter is measured by in situ x-ray diffraction at various temperatures from −173°Cto150°C. The experiments are carried out on nanocrystalline copper powders having an average particle size of 40nm. The size effect on the lattice expansion is examined regarding a modified Laplace law, where a surface stress is considered instead of the usual scalar surface energy. The results are discussed taking into account oxidation state and the sorption of molecular species at the nanoparticles surface.

Microstructure and magnetic properties of uncoupled Sm2Co17-Cu nanocomposites

Nanocomposite Sm2Co17–Cu particles have been fabricated by low energy comilling of mechanically alloyed Sm2Co17 particles with Cu nanoparticles. The x-ray diffraction analyses show that the diffraction crystallite size (DCS) of Sm2Co17 decreases with increasing comilling time. Scanning and transmission electron microscopy observation demonstrates that the particle size is in the range of the DCS and the Sm2Co17 particles are separated by Cu particles. The coercivity as well as the remanence ratio decreases with increasing milling time due to the grain size reduction and grain separation. The nanocomposite Sm2Co17–Cu exhibits suitable magnetic and microstructure properties for high-density recording.

Activation Volume in Fine Grained Metals from Stress Relaxation and Nano-Indentation

Fine grained copper was studied using the stress relaxation technique and creep testing in nano-indentation, to determine the activation volume involved in the micro-mechanism of the deformation. This material exhibits a near-perfect elasto-plastic deformation, featured by a steep work-hardening, after the elastic domain, followed by flow at a constant stress. Measurements of the activation volumes in the various domains reveal the role of the dislocations and the variation in the dislocation density in the deformation mechanism. This emphasizes the importance, in the determination of the activation volume, of the deformation domain investigated as well as the testing technique used and whether in both cases, the measurement is carried out in a transient domain or condition where variation in dislocation density occurs.