G. A. Dorofeev

Determination of nanoparticle sizes by X-ray diffraction

Different procedures for analysis of particle sizes by the X-ray diffraction method are compared by the example of nanoparticles of nickel and iron(3+) oxide (Fe2O3). A modified Warren-Averbach method is proposed for the analysis of the X-ray diffraction line profile based on the approximation by the Voigt function, which yields stable solutions, and the efficiency of the method is shown. The analysis within the frame-work of the Warren-Averbach method makes it possible to restore the distribution function of nanoparticles (crystallites) over true diameters, which satisfactorily correlates with electron microscopy data. The applicability of the Warren-Averbach method to the estimation of crystallite sizes by the analysis of a single diffraction line is substantiated. The range of the applicability of the Scherrer, Williamson-Hall, Warren-Averbach, and modified Warren-Averbach methods to the substructure analysis by the X-ray diffraction is determined as depending on the method of nanostructure formation.

Structure and magnetic properties of Fe100-xSnx (3.2 < × < 62) alloys obtained by mechanical milling

Abstract The structure and magnetic properties of mechanically ground Fe-Sn alloys have been studied by X-ray diffraction, magnetic measurements, and 57Fe and 119Sn Mossbauer spectroscopy. It has been shown that the alloys are nanocrystalline and have the disordered bcc structure with Sn concentrations x 32.7. The concentration dependences of the magnetic ordering temperature, average magnetic moment per Fe atom, average hyperfine magnetic field and isomer shift, as well as those of the bcc lattice parameter and grain size are nonlinear and reveal a number of peculiarities at 12–15, 25 and 50 at% Sn. The ferromagnetic state is shown to form in alloys with x

Effect of aluminum on mechanical solid-state alloying of iron with nitrogen in ball mill

Mössbauer spectroscopy and X-ray diffraction analysis have been used to study mechanical solidstate alloying with nitrogen and chromium of iron and an Fe-3Al alloy in the process of the mechanical activation with chromium nitrides in a ball mill. It is shown that the deformation-induced dissolution of chromium nitrides in iron and Fe-3Al matrices results in the formation of substitutional chromium and interstitial nitrogen solid solutions. The alloying of iron with aluminum accelerates the process of the deformation-induced dissolution of chromium nitrides, but reduces the nitrogen content in the interstitial solid solution. Post-deformation annealing generally leads to the escape of aluminum from the matrix, the substitution of chromium for aluminum, and the formation of fine nitrides AlN.

Mechanical alloying and severe plastic deformation of nanocrystalline high-nitrogen stainless steels

High-nitrogen, nickel-free nanocrystalline stainless steels with Fe-(18–20)Cr-1N and Fe-(23–25)Cr-(10–11)Mn-1N (wt %) compositions were obtained by mechanical alloying (MA) in a high-energy planetary ball mill in an argon atmosphere. As a source of nitrogen, we used chromium and manganese nitrides that enter the composition of the powder mixture with pure metal components. Comparative studies of the evolution of the structure during MA using X-ray diffraction, Mössbauer spectroscopy, and transmission and scanning electron microscopy have demonstrated that, in an Fe-Cr-N system, a ferrite bcc structure is preserved for up to 120 h of MA. In an Fe-Cr-Mn-N system, where nitrogen was supplied by chromium nitrides, complete austenization occurred after 60 h of the MA. The maximum acceleration of the formation kinetics of a high-nitrogen austenite was obtained in an Fe-Cr-Mn-N system, where nitrogen was obtained from a manganese nitride. The mechanisms of solid-phase reactions in powder mixtures during MA are discussed under both the deformation-induced dissolution of nitrides in a metallic matrix and the stability of a nitrogen-supersaturated ferrite with respect to α → γ transformation.

Solid-Phase Mechanical Alloying of BCC Iron Alloys by Nitrogen in Ball Mills

The methods of Mössbauer spectroscopy and X-ray diffraction analysis have been used to study the processes of a solid-phase alloying of the iron alloys with a bcc lattice by nitrogen that occur upon ball-mill mechanical activation in the presence of chromium nitrides. It is shown that a deformation-induced dissolution of chromium nitrides in the matrix of pure iron and in that of the alloys Fe–3Al and Fe–6V results in the formation of the substitutional chromium and interstitial nitrogen bcc solid solutions. An additional alloying of iron with aluminum or vanadium under the deformation dissolution of nitrides leads to the escape of aluminum and vanadium from the matrix and to a decrease in the nitrogen content characteristic of the interstitial solid solution proper due to the strong chemical bonding of alloying elements with nitrogen. The subsequent annealing leads to the decomposition of already formed solid solutions with the formation of aluminum, vanadium, and chromium nitrides of extreme dispersion.

On the problem of the cementite structure

Evolution of the crystalline structure of cementite that was formed in contact with α-Fe upon heat treatments of a mechanically alloyed iron-amorphous Fe-C phase nanocomposite has been studied using X-ray diffraction and Mössbauer spectroscopy.

Continuous detection of the torque during shear deformation as a method for estimating the evolution of structure-phase transformations

The behavior of pure metals and alloys are studied during the shear deformation in Bridgman anvils under a high quasi-hydrostatic pressure. Structural evolution is estimated from the change of the torque detected continuously during deformation. It is shown that a comparison of the dependence of the torque on the number of anvil revolutions obtained for a certain material with the reference curve of a pure metal can be successfully used to detect the structure-phase transformations during severe plastic deformation.

Nanocrystallization of the amorphous Co-B-Si alloys formed by melt spinning and mechanical alloying

X-ray diffraction, differential scanning calorimetry, and transmission electron microscopy were used to investigate the thermal stability and peculiarities of nanocrystallization upon heating of amorphous Co65.5S18B10M6.5 (M = Cr, Fe) alloys obtained by melt-spinning (MS) and mechanical alloying (MA) of mixtures of elemental components. It is shown that, irrespective of the method of manufacture, the amorphous iron-containing alloys, in general, have a lower thermal stability than the chromium-containg alloys. The thermal stability of MA alloys is lower than that of the MS alloys of the same chemical composition. The processes of nanocrystallization during heating of MS and MA amorphous alloys differ. The first crystallize, when heating, through the formation of intermediate phases, and the second, with the formation of stable crystalline phases directly from the amorphous matrix. After the crystallization, the alloys have a nanocrystalline structure with a crystallite size of about 20 nm and the same phase composition of crystallization products for the alloys of the same nominal composition obtained by MA and MS: ɛ-Co, Co2B, and β-Co2Si for Co65.5Si18B10Cr6.5 and α-Co, Co2B, β-Co2Si, and (Fe,Co)3Si for Co65.5Si18B10Fe6.5. The results are discussed in terms of the excess free volume of the amorphous phase and of the value of its specific surface area.

Iron-Cementite Nanocomposites Obtained by Mechanical Alloying and Subsequent Magnetic Pulsed Pressing

X-ray diffraction, atomic-force microscopy, and density, microhardness, and coercive force measurements were used to study bulk iron-cementite nanocomposites obtained by different regimes of magnetic pulsed pressing of mechanically alloyed samples with a total carbon content ranging from 5 to 25 at. %.

Deformation-induced dissolution of the Fe2B boride in nanocrystalline α-Fe

Structural-phase transformations in the α-Fe+Fe2B composite with a total boron content of 15 at. % during mechanical grinding in a planetary ball mill have been studied using X-ray diffraction, Mössbauer spectroscopy, and magnetic measurements. It has been established that dissolution of the Fe2B boride with the formation of an amorphous phase and segregations of boron atoms at grain boundaries occur after the grain size reaches 3 nm. In this work a comparative analysis of the deformation-induced dissolution of Fe2B and Fe3C in the Fe-B and Fe-C systems has been made and the possible mechanisms of the structural-phase transformations have been discussed.