Romana Frattini

X-ray powder diffraction and Mossbauer study )of nanocrystalline Fe-Al prepared by mechanical alloying

Abstract Iron-aluminium alloys of composition Fe 50 Al 50 and Fe 75 Al 25 were produced by mechanical alloying (MA) of the pure elemental powders. A structural refinement of X-ray powder data on the mechanically alloyed products according to the Rietveld method has detailed the progressive dissolution of aluminium into the lattice of α-iron as a function of MA time. With respect to pure iron, a volume expansion of ≈ 3% is measured in both compositions mechanically alloyed for 32 h. In the iron and aluminium phases, the Debye-Waller static disorder increases as a function of MA time and the intrinsic shape of the peak profiles becomes predominantly Cauchy. These changes are accompanied by an increase in the average microstrain and by a reduction in the average crystallite size (which includes also the effect of dislocations). The Mossbauer spectra show that in the equiatomic case the initial sharp magnetic sextet of α-iron is progressively reduced and is replaced by a doublet, while for the Fe 75 Al 25 composition a broad magnetic sextet is eventually obtained. Thermal scans at 600°C of the specimens mechanically alloyed for 2, 4 and 8 h precipitate essentially the Al 5 Fe 2 phase. In the case of the Fe 50 Al 50 specimens, annealing of the powders mechanically alloyed for 16 and 32 h precipitates mainly the partially ordered FeAl intermetallic compound, whilst no ordering is obtained in the Fe 75 Al 25 case.

A study of nanocrystalline iron and aluminium metals and Fe3Al intermetallic by mechanical alloying

Pure iron and aluminium powders and a mixture of composition Fe75Al25 were treated mechanically in a high-energy mill for up to 40 h. X-ray diffraction and analytical transmission electron microscopy were coupled to elastic energy dissipation and dynamic Youngs modulus measurements to study the structural transformation of the specimens induced by the mechanical treatment. A quantitative comparison between the structural behaviour of the pure elements and of the mixture was carried out. The role of the parameters such as the composition, the grain size and the activation energy during the process was examined in relation to the competing mechanisms of plastic deformation and recovery.

Liquid phase acetophenone hydrogenation on Ru/Cr/B catalysts supported on silica

The liquid phase hydrogenation of acetophenone was studied over a series of silica supported bimetallic catalysts with various Ru/Cr atomic ratios. The catalysts were prepared by reduction of the metal salts with NaBH4 aqueous solutions and were characterised by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) (ESCA). The hydrogenation reaction was carried out in batch at a hydrogen pressure of 9 bar. The selectivity towards reduction of the carbonyl function increases with the increasing amount of Cr ions added.

A study of Cu50Fe50 produced by mechanical alloying and its thermal treatment

A specimen of Cu50Fe50 equiatomic composition was mechanically alloyed (MA) by ball milling starting from the pure elements, which are immiscible according to the equilibrium phase diagram. Structural analysis by x‐ray and neutron diffraction has shown that the mechanical process initially reduces the crystallite size of both elements as a function of the milling time. The diffraction data show that the bcc iron phase is subsequently consumed, due to progressive incorporation of the iron atoms into the fcc copper matrix. The Mossbauer spectra of a specimen MA for 16 h has a broad magnetic profile typical of a Fe‐Cu extended solid solution, with some evidence of two local environments of the iron atoms and a small admixture of the γ‐Fe. The annealing of these MA treated specimens effects a decomposition of the extended solid solution into FCC copper and both α‐ and γ‐iron allotropes. This decomposition process is discussed in relation to spinodal decomposition and to nucleation‐and‐growth mechanisms.

Raman spectra of water in the translational and librational regions: I Study of the depolarization ratio

Polarized and depolarized Raman spectra of room temperature liquid H20 and D20 have been measured up to the bending region. Both spectral shapes and depolarization ratios are analysed in the translational and librational regions. An isotropic spectral component extending over all the investigated frequency range is derived and interpreted in terms of interaction induced mechanisms involving the polarizabilities of the isolated molecule.

Temperature evolution of single particle correlation functions of liquid water

Molecular dynamics simulations of liquid water at various temperatures is reported. The autocorrelation functions of linear and angular velocities and of the first and second rank rotation matrices are studied and compared with previous data obtained using different potential models. Comparisons with the results of molecular dynamics simulations of heavy water and hydrogen sulfide are also shown.

Induced contributions in the rayleigh spectra of water: A molecular dynamics simulation

Abstract A molecular dynamics simulation of TIP4P water at T =280 K has been performed to calculate the depolarized and isotropic Raman spectra in the translational and librational region. Comparison of the isotropic spectra with experiment confirms the assignment of the low-frequency scattered light to collision-induced phenomena and allows identification of the spectral contributions arising from polarizability modulation due to short-range interactions.

Structural evolution of mechanical alloyed Fe-Al powders after consolidation and thermal ageing

Elastic energy dissipation and dynamic Young’s modulus measurements coupled with x‐ray and energy‐dispersive analyses were employed to follow the structural transformation of bulk samples prepared by mechanical alloying Fe‐Al powders mixed in the atomic ratio Fe/Al=3. The results show that it is possible to synthesize a nanocrystalline bulk Fe3Al intermetallic phase by properly combining mechanical treatments of the powders with suitable temperature thermal aging. Driving mechanisms and transformation paths leading to this stable phase, not otherwise observed in the simply thermally aged Fe‐Al crystalline powders, are examined and discussed.

Mössbauer surface study of a nitrogen-implanted medium-carbon steel

The effect of nitrogen implantation on C40 medium-carbon steel is investigated by means of Mössbauer electron backscattering spectroscopy. Samples were implanted at various doses, after quenching from the austenitizing temperature and after quenching plus tempering, in order to detect the influence of the structure previous to the implantation. A different sequence of surface compound formation is observed in the two considered cases.

Structural Characterization of Ceria–zirconia Powder Catalysts Prepared by High-energy Mechanical Milling: A Neutron Diffraction Study

Neutron diffraction measurements were carried out on samples of CeO 2 –ZrO 2 powder catalysts prepared by high-energy mechanical milling. The formation of solid solution was evidenced across the entire composition range examined. Quantitative phase evaluation by the Rietveld method indicated formation of tetragonal structure for low CeO 2 content, whereas cubic solid solutions were the stable form at high CeO 2 loading. In addition, a pseudocubic or tetragonal t″ cell with axial ratio of unity and with internal deformation of the oxygen sublattice was observed at intermediate composition (50 mol% CeO 2 ). Thermal annealing up to 1000 °C showed expansion of the unit cell parameters; an increase in the degree of tetragonality at the expense of cubic and monoclinic phase was observed for composition Ce x Zr 1− x O 2 ( x