Electronic structure and phonon thermodynamics of iron alloys

Inelastic neutron scattering (INS) and nuclear-resonant inelastic x-ray scattering (NRIXS) were used to measure phonon spectra of FeV as a B2- ordered compound and as a bcc solid solution. Contrary to the behavior of ordering alloys studied to date, the phonons in the B2-ordered phase are softer than in the solid solution. Ordering increases the vibrational entropy, which stabilizes the ordered phase to higher temperatures. Ab initio calculations show that the number of electronic states at the Fermi level increases upon ordering, enhancing the screening between ions, and reducing the interatomic force constants. The effect of screening is larger at the V atomic sites than at the Fe atomic sites.

The phonon spectra of Au-rich alloys of fcc Au-Fe were also measured. The main effect on the vibrational entropy of alloying comes from a stiffening of the Au partial phonon density of states (DOS) with Fe concentration that increases the miscibility gap temperature. The magnitude of the effect is non- linear and it is reduced at higher Fe concentrations. Force constants were calculated for several compositions and show a local stiffening of Au–Au bonds close to Fe atoms, but Au–Au bonds that are farther away do not show this effect. Phonon DOS curves calculated from the force constants reproduced the experimental trends. The Au–Fe bond is soft and favors ordering, but a charge transfer from the Fe to the Au atoms stiffens the Au–Au bonds enough to favor unmixing. The stiffening is attributed to two main effects comparable in magnitude: an increase in electron density in the free-electron-like states, and stronger sd-hybridization.

INS and NRIXS measurements were performed at elevated temperatures on B2-ordered FeTi and NRIXS measurements were performed at high pressures. The high-pressure behavior is quasi- harmonic. The softening of the phonon DOS curves with temperature is strongly nonharmonic. Calculations of the force constants and Born-von Karman fits to the experimental data show that the bonds between second nearest neighbors (2nn) are much stiffer than those between 1nn, but fits to the high temperature data show that the former softens at a faster rate with temperature. The Fe–Fe bond softens more than the Ti–Ti bond. The unusual stiffness of the 2nn bond is explained by the calculated charge distribution, which is highly aspherical and localized preferentially in the t2g orbitals. Ab initio molecular dynamics (AIMD) simulations show a charge transfer from the t2g orbitals to the eg orbitals at elevated temperatures. The asphericity decreases linearly with temperature and is more severe at the Fe sites.

In-situ synthesis of titanium carbides in iron alloys using plasma transferred arc welding

The synthesis of Fe-TiC metal matrix composite during metal deposition with laser and arc welding techniques is of technical and economic interest for hard surfacing of engineering components. Recent studies linked the resistance to abrasive wear with the size and morphology of TiC precipitates, which are strongly dependent on the deposition conditions and, more importantly, on the alloy chemistry. In this study, the effect of silicon and manganese on the TiC precipitates was explored and different processing conditions were assessed. The characterisation included optical and scanning electron microscopy, X-ray diffraction and microhardness testing. The results indicate that silicon and manganese can have a significant effect on TiC size and morphology. Therefore, the composition of the matrix alloy offers an effective pathway to modify the microstructure of in-situ precipitated Fe-TiC metal matrix composites. © 2013 Elsevier B.V.

Improved of Cr-Ni-Mo iron base alloys by vacuum melting and casting. Final report.

"Bureau of Ships, Department of the Navy. Contract no. NObs-77019, Index Number NS-013-119."

Ab initio calculation of the BCC Fe-Al-Mo (Iron-Aluminum-Molybdenum) phase diagram: Implications for the nature of the tau(2) phase

The metastable phase diagram of the BCC-based ordering equilibria in the Fe-Al-Mo system has been calculated via a truncated cluster expansion, through the combination of Full-Potential-Linear augmented Plane Wave (FP-LAPW) electronic structure calculations and of Cluster Variation Method (CVM) thermodynamic calculations in the irregular tetrahedron approximation. Four isothermal sections at 1750 K, 2000 K, 2250 K and 2500 K are calculated and correlated with recently published experimental data on the system. The results confirm that the critical temperature for the order-disorder equilibrium between Fe(3)Al-D0(3) and FeAl-B2 is increased by Mo additions, while the critical temperature for the FeAl-B2/A2 equilibrium is kept approximately invariant with increasing Mo contents. The stabilization of the Al-rich A2 phase in equilibrium with overstoichiometric B2-(Fe,Mo)Al is also consistent with the attribution of the A2 structure to the tau(2) phase, stable at high temperatures in overstoichiometric B2-FeAl. (C) 2009 Elsevier Ltd. All rights reserved.

Studies of a wear resistant cast iron

 The objective of this investigation was to set down (on the base of the results obtained by the examination of white cast iron alloys with different content of the alloying elements) a correlation between chemical composition and microstructure, on one hand, and the properties relevant for this group of materials, i.e., abrasion wear resistance and fracture toughness, on the other. Experimental results indicate that the volume fracture of the carbide phase, carbide size and distribution, as well as the morphology of eutectic colonies, had an important influence on the wear resistance of white cast iron alloys under low-stress abrasion conditions, whereas fracture toughness was determined largely by the matrix microstructure.

Electrodeposition of amorphous Fe-Cr-P-Ni-C alloys. Morphological and structural characteristics

The effect of bath composition and electroplating conditions on structure, morphology and composition of amorphous Fe-Cr-P-Ni-C deposits on Cu substrate was investigated. The deposition efficiency of Fe-Ni-P-C alloy increased significantly with the addition of formic acid, but decreased with the addition of Cr to the plating bath. The increase of charge density activates the inclusion of Cr in the deposit. However, above a specific value of charge density, which depends on deposition current density, the Cr content in the deposit decreases. SEM analysis showed that the increase of Ni, Cr or charge deposition promotes susceptibility to microcracking.

Use of conventional electrochemical techniques to produce crystalline FeRh alloys induced by Ag seed layer

By combining galvanic displacement and electrodeposition techniques, an ordered Fe20Rh80 structure deposited onto brass was investigated by X-ray diffractometry, Mössbauer spectroscopy and magnetization measurements. Mössbauer and X-ray diffraction analyses suggest that the Fe-Rh alloy directly electrodeposited onto brass displays a nanocrystalline state while a similar alloy deposited onto Ag/brass shows a faced centered cubic-like structure, with dendrites-like features. These results directly indicate that the presence of Ag seed layer is responsible for the Fe-Rh alloy crystallization process. In addition, room temperature Mössbauer data indicate firstly paramagnetic states for two Fe-species. In the dominant Fe-species (major fraction of the Mössbauer spectra), Fe atoms are situated at a cubic environment and it can be attributed to the γ-Fe20Rh80 alloy based on their hyperfine parameters. In the second species, Fe atoms are placed in a non-local symmetry, which can be related to Fe atoms at the grain boundaries or/and Fe small clusters. These Fe-clusters are in superparamagnetic state at room temperature, but they may be ordered below 45 K, as suggested by magnetization data. © 2013 Elsevier B.V. All rights reserved.

New physical insights into the electromagnetic shielding efficiency in PVDF nanocomposites containing multiwall carbon nanotubes and magnetic nanoparticles

This work attempts to bring critical insights into the electromagnetic shielding efficiency in polymeric nanocomposites with respect to the particle size of magnetic nanoparticles added along with or without a conductive inclusion. To gain insight, various Ni-Fe (NixFe1-x; x = 10, 20, 40; Ni: nickel, Fe: iron) alloys were prepared by a vacuum arc melting process and different particle sizes were then achieved by a controlled grinding process for different time scales. Poly(vinylidene fluoride), PVDF based composites involving different particle sizes of the Ni-Fe alloy were prepared with or without multiwall carbon nanotubes (MWNTs) by a wet grinding approach. The Ni-Fe particles were thoroughly characterized with respect to their microstructure and magnetization; and the electromagnetic (EM) shielding efficiency (SE) of the resulting composites was obtained from the scattering parameters using a vector network analyzer in a broad range of frequencies. The saturation magnetization of Ni-Fe nanoparticles and the bulk electrical conductivity of PVDF/Ni-Fe composites scaled with increasing particle size of NiFe. Interestingly, the PVDF/Ni-Fe/MWNT composites showed a different trend where the bulk electrical conductivity and SE scaled with decreasing particle size of the Ni-Fe alloy. A total SE of similar to 35 dB was achieved with 50 wt% of Ni60Fe40 and 3 wt% MWNTs. More interestingly, the PVDF/Ni-Fe composites shielded the EM waves mostly by reflection whereas, the PVDF/Ni-Fe/MWNT shielded mostly by absorption. A minimum reflection loss of similar to 58 dB was achieved in the PVDF/Ni-Fe/MWNT composites in the X-band (8-12 GHz) for a particular size of Ni-Fe alloy nanoparticles. This study brings new insights into the EM shielding efficiency in PVDF/magnetic nanoparticle based composites in the presence and absence of conducting inclusion.

Dynamic model for metal cleanness evaluation by melting in a cold crucible

Melting of metallic samples in a cold crucible causes inclusions to concentrate on the surface owing to the action of the electromagnetic force in the skin layer. This process is dynamic, involving the melting stage, then quasi-stationary particle separation, and finally the solidification in the cold crucible. The proposed modeling technique is based on the pseudospectral solution method for coupled turbulent fluid flow, thermal and electromagnetic fields within the time varying fluid volume contained by the free surface, and partially the solid crucible wall. The model uses two methods for particle tracking: (1) a direct Lagrangian particle path computation and (2) a drifting concentration model. Lagrangian tracking is implemented for arbitrary unsteady flow. A specific numerical time integration scheme is implemented using implicit advancement that permits relatively large time-steps in the Lagrangian model. The drifting concentration model is based on a local equilibrium drift velocity assumption. Both methods are compared and demonstrated to give qualitatively similar results for stationary flow situations. The particular results presented are obtained for iron alloys. Small size particles of the order of 1 μm are shown to be less prone to separation by electromagnetic field action. In contrast, larger particles, 10 to 100 μm, are easily “trapped” by the electromagnetic field and stay on the sample surface at predetermined locations depending on their size and properties. The model allows optimization for melting power, geometry, and solidification rate.

Carbon-decorated FePt nanoparticles

FePt magnetic nanoparticles are an important candidate material for many future magnetic applications. FePt exists as two main phases, that is, a disordered face-centered cubic (fcc) structure, which is generally prepared by chemical methods at low temperatures, and the high-temperature chemically ordered face-centered tetragonal (fct) structure. The fee FePt, with low coercivity but associated with superparamagnetic properties, may find applications as a magnetic fluid or as a nanoscale carrier for chemical or biochemical species in biomedical areas, while fct FePt is proposed for use in ultrahigh-density magnetic recording applications. However, for both of these applications an enhancement of the intrinsically weak magnetic properties, the avoidance of magnetic interferences from neighbor particles, and the improved stability of the small magnetic body remain key practical issues. We report a simple synthetic method for producing FePt nanoparticles that involves hydrothermal treatment of Fe and Pt precursors in glucose followed by calcination at 900 degrees C. This new method produces thermally stable spheroidal graphite nanoparticles (large and fullerene-like) that encapsulate or decorate FePt particles of ca. 5 nm with no severe macroscopic particle coalescence. Also, a low coercivity of the material is recorded; indicative of small magnetic interference from neighboring carbon-coated particles. Thus, this simple synthetic method involves the use of a more environmentally acceptable glucose/aqueous phase to offer a protective coating for FePt nanoparticles. It is also believed that such a synthetic protocol can be readily extended to the preparation of other graphite-coated magnetic iron alloys of controlled size, stoichiometry, and physical properties.

Well-Alloyed PtFe/C Nanocatalysts of Controlled Composition and Same Particle Size: Oxygen Reduction and Methanol Tolerance

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Influência dos elementos de liga nos parâmetros de processo de fundição de ferros fundidos especiais

Pós-graduação em Engenharia Mecânica - FEG