Yu. V. Baldokhin

Crystallochemical aspects of solid state reactions in mechanically alloyed Al–Cu–Fe quasicrystalline powders

Abstract A number of elemental mixtures having initial compositions close to Al65Cu23Fe12 were mechanically alloyed at different energy intensities using a planetary mill. Laboratory X-ray diffraction analysis, differential scanning calorimetry and Mossbauer spectroscopy were used for characterization of the phase and structural state of mechanically alloyed powders after different periods of milling and annealing. Several exothermic effects were found, and these were ascribed to specific solid-state reactions. Quantitative phase analysis was applied in order to identify the mechanism of solid-state reactions taking place in the vicinity of the quasicrystalline phase domain in the Al–Cu–Fe system as a result of mechanical and thermal excitation and homogenization. Metastable intermetallics were identified which possess certain structural and topological elements identical to those found in quasicrystals.

On ferro- and antiferromagnetic ordering in ultrafine particles of Fe-rich Fe–Ni and Fe–Mn alloys

Ultrafine Fe–Ni (28%–32%) and Fe–Mn (30%) particles with an average size of 10–15 nm are studied by combined x-ray diffraction and Mossbauer spectroscopy techniques with the latter being applied at a temperature range from 298 to 4 K. They are produced by evaporation of bulk alloys at 3 Torr Ar pressure. From the x-ray data the ultrafine Fe–Ni (28%–32%) particles are a mixture of bcc and fcc phases, and the ultrafine Fe–Mn (30%) particles contain bcc, fcc, and hcp phases. It is shown that in the former the paramagnetic fcc phase transforms to the antiferromagnetic state with decreasing temperature from 77 down 4 K. As for the latter, the fcc phase is observed to be antiferromagnetic even at room temperature whereas the hcp phase keeps a paramagnetic state right down to 4 K. The results corroborate the Weiss hypothesis that the high temperature face-centered-cubic lattice of Fe-rich alloys can exist in two (ferro- and antiferromagnetic) spin states. The oxide contribution in the spectra is also separated.

COMPARATIVE STUDY OF THE STRUCTURE AND LOCAL MAGNETIC ORDER IN BULK AND ULTRAFINE PARTICLES OF FE-MN (32%-35%) AND FE3PT

A study of Fe–Mn (32% and 34.65%) and Fe3Pt Invar alloys during the transition from bulk to ultrafine (3–20 nm) particles is done at room temperature by combined x-ray diffraction and Mossbauer spectroscopy techniques. The particles obtained by evaporation of foil or filings of bulk material in an Ar atmosphere at pressures from 0.16 to 50 Torr were rapidly quenched during their production. Mossbauer measurements confirmed the availability of two spin states in both Fe–Mn and Fe3Pt fcc particles like it has been observed in previous studies of fine Fe and Fe–Ni (30%–35%) particles. Various heat treatments of the Fe3Pt foil with the initial bcc structure made it possible to obtain its fcc modification in either the entirely ordered or the entirely disordered state. It was shown that the disordered fcc structure in a bulk sample could exist in two spin states (ferromagnetic and paramagnetic) whereas the ordered structure was only in a ferromagnetic state.

Mössbauer spectroscopic studies of passivated layers of iron-containing catalysts for ammonia synthesis

Combined conversion electron (CEMS) and transmission γ-ray (MS) Mössbauer spectroscopy was used to study the structure of passivated layers of promoted and unpromoted iron-containing catalysts. In both catalysts the oxide-coated films were found to consist of small “paramagnetic” (at 300 K) clusters of ferric oxide.AbstractСтруктура пассивационных плёнок промотированных и непромотированных катализаторов синтеза аммиака изучалась в мессбауэровской абсорбционной и конверсионной спектроскопии. В обоих типах катализаторов удалось наблюдать пассивационный слой вблизи поверхноти, состоящий из мелких парамагнитных (при 300 К) кластеров окиси железа.

Preparation and reactivity of metal-containing monomers: 33. Investigation of a cluster-type monomer [Fe3O(OCOCH=CH2)6]OH and its polymerization product by IR, XPS, Mössbauer, and mass spectroscopy

A new cluster-type monomer [Fe3O(OCOCH=CH2)6]OH · 3H2O was synthesized by the reaction of acrylic acid with freshly prepared iron hydroxide. The monomer was characterized by various physicochemical methods. It has been shown that the acid residues are bound to the metal atoms through carboxyl groups in a bridge-like or bidentate fashion, and the double bond does not participate in the coordination. The mechanism of thermal fragmentation of the [Fe3O(OCOCH=CH2)6]+ cluster was studied. It was demonstrated that the radical polymerization of this monomer involved bidentate acrylate groups. No reduction of Fe3+ during the polymerization was observed.

The Effect of Silicon on the Electronic Structure and Corrosion–Electrochemical Behavior of Austenitic Steels1

The regularities of the effect produced by alloying with silicon (up to 6% Si) on the corrosion–electrochemical behavior of austenitic stainless steels of various structure types (single-phase or with the precipitation of second phases) in a solution of 1 mol/l HClO4 + 0.25 mol/l NaCl at 22°C and in 27 and 65% HNO3 at 70°C and at the boiling point are clarified. Nuclear γ-resonance (Mössbauer) spectroscopic studies enabled us to develop a mechanism of enhancing the passivability of the steels studied at potentials corresponding to their active–passive transition and accelerating the transpassivation process by silicon. The hypothesis proposed is based on the change in the electron density distribution around iron nuclei induced by silicon and accordingly the change in the chemisorption interactions between the surface iron atoms and the polar molecular and ionic components of the solution.

New concept of the mechanism of intergranular corrosion of stainless steels

The electronic structure of chromium-nickel austenite stainless steels of type X20H20 and X10H20, in particular, added with 4 to 6% Si, is studied using nuclear γ-resonant (Mössbauer) spectroscopy. The structure varies significantly depending on the chromium and silicon content. Low-chromium steels are used for the modeling of chromium-depleted boundary zones that neighbor chromium-rich excess phases. Eventually, a mechanism of the intergranular corrosion of stainless steels that takes into account the changes in the electronic structure of the chromium-depleted boundary zones is first suggested. These changes in the chemisorption interaction of the iron carcass ions with corrosive components of the medium, cannot but affect the zones’ stability. It is this interaction that synergistically intensifius the intergranular corrosion of austenite stainless steels containing combinations of silicon dopant and carbon impurity, or the phosphorus and carbon impurities.

Phase Transformations during Deformation of Fe-Ni and Fe-Mn Alloys Produced by Mechanical Alloying

Compositions of Fe(100 − x)Mnx (x = 10 and 12 at. %) and Fe(100 − y)Niy (y = 18 and 20 at. %) were produced by combined mechanical alloying of pure-metal powders and annealed in the austenitic field. After annealing and cooling to room temperature, the alloys had a single-phase austenitic structure. During deformation, the γ phase partially transforms into the α2 phase (and/or ɛ phase in Fe-Mn alloys). The phase composition of the alloys after deformation depends on the amount of alloying elements and the predeformation annealing regime. The amount of martensite in the structure of a bulk alloy obtained by powder compacting grows proportionally to the degree of deformation of the sample.

The Effect of Silicon on Electron Structure and Corrosion-Electrochemical Behavior of a X20H20 Type Phosphorus-Containing Steel

The effect of silicon (0.13–5.40%) on the corrosion–electrochemical behavior of phosphorus-containing (∼0.1% P) austenitic stainless steels of a X20H20 type is studied in a 1.0 mol/l HClO4 + 0.25 mol/l NaCl solution at 22°C, as well as boiling 27 and 65% HNO3 solutions. The steel is tested either in quenched or sensitized states (i.e., with a single-phase or heterogeneous structure with precipitated second phases, respectively). The variation in the electron density on 57Fe nuclei under the action of silicon is studied with the use of Mössbauer spectroscopy by determining the isomeric (chemical) shift δ (mm/s) of singlet and doublet, the quadrupole electric splitting Δeq (mm/s) of doublet, and a fraction of area S (%) of each spectrum component in the total spectrum. The mechanism of the effect of silicon on the passivability of test steels and their susceptibility to intergranular corrosion at potentials of the active–passive transition and the end of the passive range is proposed based on the corrosion–electrochemical data and the calculated parameters of Mössbauer spectra.

Internal Stresses, Corrosion and Electrochemical Behavior in Soils, and Stress Corrosion of Pipe Steels

The results of field tests carried out at main gas pipelines (MP) are shown. The distribution of internal stresses along the MP perimeter is obtained. Polarization currents at differently strained pipeline fragments are determined in soils taken at different distances from the failure epicenter. The mechanoelectrochemical effect is shown to be twice greater in failure soils. Its value can serve as a criterion of the stress corrosion probability.