Decomposition process in a FeAuPd alloy nanostructured by severe plastic deformation

The decomposition process mechanisms have been investigated in a Fe50Au25Pd25 (at.%) alloy processed by severe plastic deformation. Phases were characterized by X-ray diffraction and microstructures were observed using transmission electron microscopy. In the coarse grain alloy homogenized and aged at $450 ^{circ}\mathrm{C}$, the bcc \alpha-Fe and fcc AuPd phases nucleate in the fcc supersaturated solid solution and grow by a discontinuous precipitation process resulting in a typical lamellar structure. The grain size of the homogenized FeAuPd alloy was reduced in a range of 50 to 100nm by high pressure torsion. Aging at $450 ^{circ}\mathrm{C}$ this nanostructure leads to the decomposition of the solid solution into an equi-axed microstructure. The grain growth is very limited during aging and the grain size remains under 100nm. The combination of two phases with different crystallographic structures (bcc \alpha-Fe and fcc AuPd) and of the nanoscaled grain size gives rise to a significant hardening of the allo

Complementary use of TEM and APT for the investigation of steels nanostructured by severe plastic deformation

The properties of bulk nanostructured materials are often controlled by atomic scale features like segregation along defects or composition gradients. Here we discuss about the complimentary use of TEM and APT to obtain a full description of nanostructures. The advantages and limitations of both techniques are highlighted on the basis of experimental data collected in severely deformed steels with a special emphasis on carbon spatial distribution

Nanostructure and properties of a Cu-Cr composite processed by severe plastic deformation

A Cu-Cr composite was processed by severe plastic deformation to investigate the role of interphase boundaries on the grain size reduction mechanisms. The as-deformed material exhibits a grain size of only 20nm. This gives rise to a dramatic increase of the hardness. Some deformation induced Cu super saturated solid solutions were clearly exhibited and it is shown that they decrease the hardness. The formation of such supersaturated solid solution and their influence on the mechanical properties are discussed

Shear-induced α → γ transformation in nanoscale Fe-C composite

High-resolution transmission electron microscopy and three-dimensional atom probe observations show clearly that a reverse transformation of body-centred cubic ferrite to face-centred cubic austenite occurs during severe plastic deformation of a pearlitic steel resulting in a nanocrystalline structure, something that never occurs in conventional deformation of coarse-grained iron and steels. The driving force and the mechanisms of this reverse transformation are discussed. It is shown that nanostructure and shear stresses are essential for this process, and the results confirm molecular dynamics predictions of such transformations in nanocrystalline iron

Homogeneous Cu-Fe super saturated solid solutions prepared by severe plastic deformation

A Cu-Fe nanocomposite containing 50 nm thick iron filaments dispersed in a copper matrix was processed by torsion under high pressure at various strain rates and temperatures. The resulting nanostructures were characterized by transmission electron microscopy, atom probe tomography and M\"ossbauer spectrometry. It is shown that alpha-Fe filaments are dissolved during severe plastic deformation leading to the formation of a homogeneous supersaturated solid solution of about 12 at.% Fe in fcc Cu. The dissolution rate is proportional to the total plastic strain but is not very sensitive to the strain rate. Similar results were found for samples processed at liquid nitrogen temperature. APT data revealed asymmetric composition gradients resulting from the deformation induced intermixing. On the basis of these experimental data, the formation of the supersaturated solid solutions is discusse

Dust in an extremely metal-poor galaxy: mid-infrared observations of SBS 0335-052

The metal deficient (Z = Z_sun/41) Blue Compact Dwarf Galaxy (BCD) SBS 0335-052 was observed with ISOCAM between 5 and 17 mic. With a L_12mic/L_B ratio of 2.15, the galaxy is unexpectedly bright in the mid-infrared for such a low-metallicity object. The mid-infrared spectrum shows no sign of the Unidentified Infrared Bands, which we interpret as an effect of the destruction of their carriers by the very high UV energy density in SBS 0335-052. The spectral energy distribution (SED) is dominated by a very strong continuum which makes the ionic lines of [SIV] and [NeIII] very weak. From 5 to 17 mic, the SED can be fitted with a grey-body spectrum, modified by an extinction law similar to that observed toward the Galactic Center, with an optical depth of A_V~19-21 mag. Such a large optical depth implies that a large fraction (as much as ~ 75%) of the current star-formation activity in SBS 0335-052 is hidden by dust with a mass between 3x10^3 M_sun and 5x10^5 M_sun. Silicate grains are present as silicate extinction bands at 9.7 and 18 mic can account for the unusual shape of the MIR spectrum of SBS 0335-052. It is remarkable that such a nearly primordial environment contains as much dust as galaxies which are 10 times more metal-rich. If the hidden star formation in SBS 0335-052 is typical of young galaxies at high redshifts, then the cosmic star formation rate derived from UV/optical fluxes would be underestimated.Comment: 13 pages, 4 figures, requires aaspp4.sty, accepted in Ap

The Embedded Super Star Cluster of SBS0335-052

We analyze the infrared (6-100 micron) spectral energy distribution of the blue compact dwarf and metal-poor (Z=Z_solar/41) galaxy SBS0335-052. With the help of DUSTY (Ivezic et al. 1999), a program that solves the radiation transfer equations in a spherical environment, we evaluate that the infrared (IR) emission of SBS0335-052 is produced by an embedded super-star cluster (SSC) hidden under 10^5 M_solar of dust, causing 30 mag of visual extinction. This implies that one cannot detect any stellar emission from the 2x10^6 M_solar stellar cluster even at near-infrared (NIR) wavelengths. The derived grain size distribution departs markedly from the widely accepted size distribution inferred for dust in our galaxy (the so-called MRN distribution, Mathis et al. 1977), but resembles what is seen around AGNs, namely an absence of PAH and smaller grains, and grains that grow to larger sizes (around 1 micron). The fact that a significant amount of dust is present in such a low-metallicity galaxy, hiding from UV and optical view most of the star formation activity in the galaxy, and that the dust size distribution cannot be reproduced by a standard galactic law, should be borne in mind when interpreting the spectrum of primeval galaxies.Comment: 32 pages, 3 figures,accepted for publication in A

Atomic scale characterization of deformation induced interfacial mixing in a Cu/V nanocomposite wire

The microstructure of a Cu/V nanocomposite wire processed by cold drawing was investigated by high resolution transmission electron microscopy and atom probe tomography. The experimental data clearly reveal some deformation induced interfacial mixing where the vanadium filaments are nanoscaled. The mixed layer is a 2nm wide vanadium gradient in the fcc Cu phase. This mechanical mixing leads to the local fragmentation and dissolution of the filaments and to the formation of vanadium super saturated solid solutions in fcc Cu

La utilización de marfil de cachalote en el Calcolítico de Portugal

Scientific analysis were undertaken within a research project concerning ivory objects from the Chalcolithic to Early Bronze Age of the Iberian Peninsula. In several of the analyses of objects from Portuguese Estremadura, especially V-perforated buttons, we could detect for the first time the presence of sperm whale ivory. This highlights the advantage and necessity of scientific analysis of ivory. It also clearly demonstrates that not all ivory used was ivory from African or Asian elephants, but we also did find ivory from the extinct Elephas antiquus, the hippopotamus and in this case sperm-whale. Thus, already in the Chalcolithic the raw material provenience was highly diverse, which in the absence of scientific analysis might lead to an erroneous interpretation of prehistoric exchange networks. Different methods, including optical microscopy, measurement of hardness and specific gravity, Micro-Raman Spectroscopy, elemental analysis and Isotopic Ratio Mass Spectrometry have been applied. In this paper we present these methods and the results, and we discuss about the implications of these results for the reconstruction of prehistoric economy and life in this region. Finally, taking into account the natural conditions as well as prehistoric and historic data of whale hunting and scavenging of beached animals, we conclude that the most plausible explanation for the presence of sperm whale ivory in Chalcolithic Portugal is the use of the teeth obtained from stranded animals. This interpretation is possible because of the human populations using this ivory are those living close to the sea and exploiting – among others– marine resources.En el marco de un proyecto de investigación sobre objetos de marfil del Calcolítico al Bronce Antiguo en la Península Ibérica efectuamos análisis científicos. En varios de los objetos de la Estremadura portuguesa, en especial en los botones con perforación en V, detectamos por primera vez la presencia de marfil de cachalote. Así se demuestra claramente la ventaja y la necesidad de efectuar análisis científicos del marfil. No todo el marfil utilizado provenía de los elefantes africanos y asiáticos. Además encontramos marfil del desaparecido Elephas antiquus, de hipopótamo, y en este caso de cachalote. Así ya en el Calcolítico el origen de la materia prima era muy diverso, lo que en ausencia de análisis científicos puede conducir a una errónea interpretación de las redes prehistóricas de intercambio. Los métodos empleados fueron la microscopia óptica, la medición de la dureza y del peso específico, la espectroscopía Micro-Raman, el análisis elemental y la espectrometría isotópica de masas. En este trabajo presentamos estos métodos y los resultados. Además discutimos sus consecuencias para la reconstrucción de la economía y la vida de las sociedades prehistóricas de la región. Teniendo en cuenta las condiciones naturales y los datos prehistóricos e históricos sobre la caza de ballenas y el aprovechamiento de animales varados consideramos que la explicación más plausible de la presencia de marfil de cachalote en el caso del Calcolítico portugués es el uso de dientes de cachalotes varados. Cabe esta interpretación dado que las poblaciones que emplean este marfil son las que viven cerca del mar explotando, entre otros, los recursos marinos