G. Principi

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.

Polymer-derived microcellular SiOC foams with magnetic functionality

SiOC microcellular ceramic foams possessing soft-ferromagnetic properties were produced from a pre-ceramic polymer, poly-methyl-methacrylate microbeads (PMMA) (used as sacrificial pore formers) and iron silicide micro-powders (as functional filler). The interactions between the matrix and the filler were studied as a function of the amount of powders introduced and the pyrolysis temperature. Magnetic and mechanical properties were also investigated.

X-ray diffraction and Mössbauer analyses of SnO disproportionation products

The disproportionation of SnO in the temperature range 200–750 °C for varying lengths of time from 0.5 to 100 h was investigated. The reaction starts at about 250 °C producing metallic tin and an intermediate oxide whose formula has been confirmed as Sn3O4. In the temperature range 250–425 °C the disproportionation products are metallic tin and the intermediate oxide. In the temperature range between 425 ° and 550 °C, Sn3O4 slowly decomposes to Sn and SnO2, after the complete disappearance of SnO. Over 550 °C the disproportionation products are the thermodynamically stable phases Sn and SnO2.

Structural evolution of Fe-Al multilayer thin films for different annealing temperatures

The phase formation during thermal annealing of Fe/Al multilayer thin films prepared by electron-beam evaporation, with an overall atomic concentration ratio of Fe:Al = 1:1, has been studied by Rutherford backscattering spectrometry (RBS), x-ray diffraction spectroscopy (XRD), and conversion-electron Mossbauer spectroscopy (CEMS). At the annealing temperature of 473 K some degree of atomic mixing between Fe and Al layers is revealed only by CEMS. At 573 K a large degree of atomic mixing is indicated also by RBS, leading to the nucleation and growth of the B2 FeAl intermetallic phase, as detected by means of XRD and CEMS. At 673 K all Fe atoms have reacted and the multilayer film is transformed into a defective B2 phase. Annealing at higher temperature increases the structural order of the B2 phase. We suggest that the observed phase formation occurs in three stages: (1) formation of a thin intermixed layer between Fe and Al in the as-deposited sample; (2) Al migration into the initial intermixed layer; (3) B2 phase growth at the interface between the intermixed layer and the Fe layer.

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.

The influence of carbon on nitrogen substitution in iron ε-phases

Mössbauer spectroscopy of nitrided and carbonitrided iron powders and plates with total interstitial atom contents of between 24 and 33 at% has allowed the variation with composition of the hyperfine magnetic fields of metal atoms with 2 or 3 close interstitial atoms to be determined. Fe atoms with three non-metal nearest neighbours and paramagnetic behaviour have also been detected. Only when the content of non-metal atoms goes beyond 30 at%, do Fe atoms in ε-carbonitrides display higher magnetic fields than in the corresponding ε-nitrides. The lower number of electrons contributed to the iron 3d-orbitals by C atoms than by N atoms is considered the cause of the above phenomenon. The variation of other hyperfine parameters with composition is also discussed.

Structural evolution and the kinetics of Cu clustering in the amorphous phase of Fe-Cu-Nb-Si-B alloy

An attempt has been made to investigate the evolution of the structure of the amorphous phase of Fe73.9 Cu0.9 Nb3.1 Si13.2 B8.9 (finemet) alloy by a combination of wide-angle x-ray scattering, small angle x-ray scattering (SAXS), Mossbauer spectroscopy and X-ray absorption near edge spectroscopy on the supposition that they would provide complementary information. Before the onset of nanocrystallization, the amorphous phase undergoes a structural relaxation resulting in small increase in the hyperfine field and a decrease in the width of the first diffraction maxima. There is an increase in the topological ordering in the system, though chemical inhomogeneity sets-in due to the clustering of Cu atoms in the pure amorphous state of this alloy. Annealing at 400 °C (well below the crystallization temperature) for different time durations results in occurrence of Cu clusters having fcc structure. Kinetics of Cu clustering is studied using SAXS. The incubation time for the clustering at 400 °C is ∼120 min. Wit...

Swift heavy ion induced modification of the Co/Si interface; cobalt silicide formation

Electron beam evaporated 60-nm Co films on Si (100) substrates were subjected to 120 MeV Au beam irradiation and subsequent thermal annealing to induce interdiffusion at the interface. Although low energy (keV/nucleon) ion beam induced mixing is known to produce cobalt silicides, the effect of MeV/nucleon radiation or swift heavy ions (SHIs), on the Co/Si system, has not been reported earlier. The irradiated and ex situ annealed samples were analyzed by Rutherford backscattering spectroscopy measurements. Respective crystalline cobalt silicide phases were identified by grazing incidence X-ray diffraction. Complete intermixing of Co and Si in the interfacial region to form CoSi was achieved following thermal annealing (at 400 °C) of the SHI irradiated Co/Si system. The radiation enhanced diffusion mechanism has been invoked to explain the SHI induced intermixing in the Co/Si system after annealing. Thermally assisted (ion generated) defect migration across the Co/Si interface enhanced defect mediated atomic mobility, which led to the formation of cobalt silicides, in accordance with the thermodynamically favored route. The post SHI irradiation annealing temperature required for the formation of crystalline phases (400 °C), is lower than that reported for low energy ion beam mixing cases where post irradiation annealing temperatures in excess of 700 °C are required for the occurrence of phases. Due to lower processing temperatures, SHI induced mixing may be considered as a promising silicidation technique in solid state technology.

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.

Formation of nanocrystalline phases by crystallization of metallic glasses

Abstract The first phase to crystallize out as a result of thermal annealing of amorphous Fe 73.5 Si 16.5 B 6 Cu 1 Nb 3 is a stoichiometric Fe 3 Si phase. With increasing annealing time the Si concentration in the crystalline phase decreases to about 19%. Nanocrystals are fully grown in size after annealing for 5 min at 540°C. The variation of spin texture in the specimen with annealing is markedly different from that in normal metallic glasses and may be understood in terms of an interplay between increasing internal stresses and decreasing magnetostriction.