[en] Fe₅₀Cu₅₀ metastable solid solution has been prepared using high-energy ball milling. The kinetics of the transformation has been studied by means of in situ neutron thermo-diffraction in the temperature range from 300 to 1200 K. Two different phases, disordered FCC-CuFe and a small amount of maghemite, are present in the as-milled alloy. When the temperature is increased a BCC-Fe phase segregates, and the maghemite transforms into wustite. The thermal evolution of the structural parameters and the percentage of the different crystalline phases are presented and discussed

[en] The whole crystallisation process of three Fe-rich FeZr metallic glasses has been studied using in situ neutron thermo-diffraction. While the beginning of the transformation occurs at the same temperature, the further evolution is strongly compositional dependent. Metastable BCC-Fe phase appears at intermediate temperatures only in Fe₇₅Zr₂₅ alloy. From this study a clear evidence of the existence of an FCC-Fe_3+xZr is shown

[en] We report on the synthesis of Co₃₀Ni₇₀ nanoparticles using hydrothermal method at low temperatures and short times (100 °C, 2 h) without any surfactant or external magnetic field. The effect of NaOH concentration on the crystal structure, microstructure, and magnetic properties of CoNi samples has been investigated by x-ray diffraction (XRD), scanning (SEM) and high-resolution transmission (HRTEM) electron microscopy, and vibrating sample magnetometry (VSM). From the Rietveld refinement of x-ray powder diffraction patterns, we have evidenced the coexistence of two phases with face-centered cubic (FCC) and hexagonal (HCP) crystal structures, being 12 nm and 3 nm, respectively, the values for the mean grain size of both phases. SEM images show that the basic microstructure is composed of quasi-spheres and a chain-like morphology appears with increasing the amount of NaOH. HRTEM images evidence the formation of such chains, and confirm the coexistence of FCC and HCP phases. The magnetic hysteresis loops show a clear dependence of the coercivity on the particle morphology indicating the role played by the magnetic shape anisotropy.

[en] A detailed microstructural study of nanocrystalline Fe powder samples, obtained by mechanical attrition, was carried out by X-ray diffraction experiments as a function of the milling time. During the process, the exchange energy between the vessels, balls and Fe powders leads to a change of the Fe microstructure: in particular, a significant decrease in the size of the coherent diffraction domains and the creation of a large number of linear defects which induces microstrains. This microstructure seems to be responsible for the distinct magnetic behaviour of the samples. In addition, on heating, the sample produces the relaxation of the nanostructure, recovering the initial magnetic properties; this process has been followed on the sample with larger milling time by neutron thermo-diffraction from room temperature up to 1200 K. The analysis of the diffraction patterns by the Rietveld method allows determining the microstructure at each stage

[en] This paper gives an overview of what kind of information and how deep can we go into the structure determination of materials, in metastable states, using neutron powder thermo-diffraction, whether temperature induced crystallisation and phase segregation processes have to be completely understood. We have obtained, by means of high energy ball milling technique, four different Fe-based compounds, showing unique characteristics (nanostructured Fe, FeNi alloys, FeCu solid solutions and FeZr amorphous alloys). In situ neutron diffraction experiments have been carried out in the temperature range between 300 and 1220 K in order to study the great variety of structural changes that takes place in these compounds. (copyright 2004 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

[en] In situ neutron powder thermo-diffraction experiments in the temperature range from 300 K to 1170 K have been carried out in nanostructured and metastable Fe₁₅Cu₈₅ and Fe₈₅Cu₁₅ solid solutions, which were synthesized by means of a mechanical alloying technique. We report on the microstructural changes and the phase transformations that take place during controlled heating and cooling processes. The average crystalline grain size is similar for both samples in the as-milled state (∼16-20 nm) while the induced strain is 2.5 times higher in the Fe-rich powders, reaching 1%. Moreover, the α-γ transformation for Fe₁₅Cu₈₅ starts at temperatures lower (∼900 K) than that expected for pure Fe (1183 K) due likely to the existence of local inhomogeneities in the composition of the ball milled material.

[en] Highlights: • Ni₇₅Fe₂₅ powder were synthesized with a low cost hydrothermal method at low temperature. • The morphology depends on the amount of NaOH. • The coercive field depends on the morphology of samples. • Mössbauer spectroscopy reveals the existence of different local environments. We have successfully synthesised Ni₇₅Fe₂₅ nanostructured powders by means of low temperature (150 °C) hydrothermal method using sodium hydroxide (NaOH) as co-reduction agent. The powders are in fact large agglomerations of small nanoparticles with average diameters below 30 nm. The concentration of NaOH seems to play an important role in the final microstructure and morphology of the samples as deduced from scanning electron microscopy images. Although x-ray diffraction patterns reveal that the samples are single-phase with a face centered cubic crystal structure, the analysis of Mössbauer spectra suggests the existence of different local environments for the iron atoms. The room temperature saturation magnetization shows similar values for all the samples (≈90 Am²/kg), in contrast with the coercive field that is clearly influenced by the morphology of the samples, ranging from 60 Oe (spherical-shaped samples) to 150 Oe (denditric-shaped powders).

[en] New futures on the physical properties of ferromagnetic FeNi alloys have been found combining in situ neutron diffraction experiments and magnetic measurements in mechanical milled Fe-rich Fe-Ni metastable solid solutions. Apart from the well-known Invar effect, on heating these materials are characterised by the existence of a first-order martensite-austenite transformation that takes place at some system-dependent temperature. On cooling, the transformation occurs at a lower temperature than on heating; for Fe₈₀Ni₂₀ the size of the effect being larger than 100 deg C, much more than the values found in conventional FeNi alloys. These results are discussed considering intrinsic features as magnetovolume effects and/or extrinsic effects such as small grain size and the existence of defects

[en] The martensite-austenite (MA) transformation in Fe₈₀Ni₂₀ ball-milled powder has been studied through neutron thermo-diffraction experiments and magnetisation vs. temperature measurements between 300 and 1100 K. For the as-milled powder, the estimated size of mean coherent diffraction domains and lattice microstrains at room temperature (RT) are 11(2) nm and 0.8(2)%, respectively. The evolution of both parameters with temperature is presented and discussed. On heating the as-milled powder, the MA transformation begins above 630 K (200 K below the expected temperature for as-cast coarse-grained Fe₈₀Ni₂₀ conventional alloys) and finishes at 890 K. On cooling down from high temperature, the reverse transformation occurs below 400 K. These changes in the MA transformation temperatures are attributed to the initial nanostructure of the Fe₈₀Ni₂₀ material

[en] Research highlights: → Spin-glass-like behaviour o owing an re-entrant spin-glass transition. → No critical divergence observed in the maxima of the real and magnetic susceptibility on ball milled Fe₃₀Cr₇₀. → High temperature Curie-Weiss behavior of the magnetic susceptibility on ball milled Fe₃₀Cr₇₀. → Moderate shift of the freezing temperature per frequency in ball milled Fe₃₀Cr₇₀ obtained from the temperature dependence of the real magnetic susceptibility - Abstract: Nominal nanostructured Fe₃₀Cr₇₀ obtained from ball milling during 110 h has been investigated from dc magnetization and ac magnetic susceptibility. The as-milled sample is not monophasic and is formed of two phases, Fe_20±2Cr_80±2 (∼86 ± 2%) and iron (∼14 ± 2%). The ac susceptibility measurements show evidence of a re-entrant spin-glass-like transition for the Fe₂₀Cr₈₀ phase below 30 K. The shift of the freezing temperature per frequency decade is moderate when compared to that found in conventional spin-glass alloys. A Vogel-Fulcher activation process can be used to explain the frequency variation. The results are also analyzed in terms of Cole-Cole formalism for extracting information of relaxation time (τ ∼ 10^-5 to 10^-4 s).