V. A. Volkov

Structural state and magnetic properties of cementite alloyed with manganese

Using X-ray diffraction analysis, Mössbauer spectroscopy, and magnetic measurements, the structure, parameters of hyperfine interactions, localization of Mn atoms in the lattice, coercive force, and specific saturation magnetization have been investigated in the mechanically alloyed and annealed cementite (alloyed with manganese) of compositions (Fe1 − xMnx)3C (x = 0–0.12). It has been shown that strongly deformed cementite resides in the low-coercivity state and, after annealing in the vicinity of 500°C, in the high-coercivity state. Alloying with manganese reduces the coercive force, the specific saturation magnetization, and the Curie temperature of cementite. Inhomogeneities of the distribution of manganese atoms indicate the temperature dependence of the coercive force of mechanically alloyed and annealed cementite samples.

Effect of silicon on the phase formation in mechanically activated systems based on Fe75C25: Mechanosynthesis of composite states

The methods of X-ray diffraction analysis, Mögsbauer spectroscopy, and measurement of dynamic magnetic susceptibility have been used to study stationary phase states that develop at the later stages of mechanical alloying in a planetary ball mill. In the Fe(100 − x)C(x), Fe(75)C(25 − x)Si(x), and Fe(75 − x)C(25)Si(x) (x ≤ 25) systems, the processes of phase formation are determined by the dynamic equilibrium between the crystalline and amorphous phases. Depending on the composition of the alloys, the conditions of this equilibrium are changed, which is reflected in the sets of the crystalline phases that are formed.

Texture transformations upon annealing of aluminum foils: I. Strong texture components

Pole figures have been constructed and textures have been compared for aluminum foils from various manufacturers in the as-delivered state and after additional annealings. Difference in the orientations of the basic texture components have been revealed in annealed foils. The main type of the transformation of a texture upon annealing is the development of a texture component {001}〈100〉 whose orientation differs fundamentally from the basic texture components observed in the foil prior to annealing. In another type of texture transformations, new texture components close to the basic orientations of the deformation texture are developed along with the {001}〈100〉 orientation. In the case of a third type of texture transformations, along with the orientation {001}〈100〉 which is formed upon recrystallization, the major part of the basic deformation-texture components are retained. The orientations of the basic texture components that are formed upon annealing belong to fields of allowed orientations of the basic texture components that were present in the foil before annealing. Apart from regions with a high pole density, regions with a moderate pole density have been found in the pole figures of annealed foils.

Structure and magnetic properties of mechanically synthesized (Fe 1– x Ni x ) 75 C 25 nanocomposites

X-ray diffraction, Mössbauer spectroscopy, and magnetic measurements have been used to study the phase formation in (Fe1–xNix) (with х = 0–0.20) nanocomposites upon severe plastic deformation in a planetary ball mill and subsequent annealings. It has been shown that the mechanical synthesis results in the formation of mainly a nickel-alloyed nanocrystalline (Fe, Ni)3C cementite with a distorted crystal lattice and an amorphous Fe–Ni–C phase. During heating above 300°С, the amorphous phase crystallizes with the formation of cementite, which is characterized by a higher Ni content compared to that in mechanically synthesized cementite. The mechanically synthesized samples exhibit low coercive force (equal to several tens ampere per centimeter). In the course of annealing at temperatures of up to 500–550°С, crystal lattice distortions are removed; this results in reliving the magnetic anisotropy constant and high-coercivity state of cementite. At the same time, Ni-rich cementite areas decompose with the formation of γ-(Fe, Ni, C) phase (austenite); as a result, the average nickel content in the cementite substantially decreases. Annealings at higher temperatures cause the complete decomposition of cementite and lead to an abrupt decrease in the coercive force (Нс) of samples. Alloying with nickel leads to an increase in the Curie temperature of cementite and a decrease in its specific saturation magnetization, coercive force, and thermal stability.

Formation of carbide phases upon the mechanosynthesis of the (Fe0.93Cr0.07)75C25 alloy compared with other carbide-forming processes

Methods of X-ray diffraction, differential thermal analysis, and measurements of the dynamic magnetic susceptibility have been used to investigate the sequence of phase transformations upon the mechanical alloying of a mixture of powders of the initial components of the composition (Fe0.93Cr.07)75C25. It has been shown that, at later stages of mechanical alloying, the phase composition is determined by the conditions of the dynamic equilibrium between the crystalline and amorphous phases. A change in the conditions of mechanical alloying leads to a shift in this equilibrium and to a change in the phase composition of the alloy. A comparison of carbide formation in the Fe–C system upon the mechanosynthesis, tempering of martensite, the saturation of iron with carbon from the gaseous medium, the quenching of the melt, and the sputtering deposition of films has been performed. Some general regularities have been established, from which it follows that an important role in phase formation upon the mechanosynthesis, just as in other abovementioned processes, is played by the thermally activated phenomena.

Dynamic equilibria of phases in the processes of the mechanosynthesis of an alloy with composition Fe72.6C24.5O1.1N1.8

X-ray diffraction, Mössbauer spectroscopy, and measurements of the dynamic magnetic susceptibility have been used to investigate phase states of the Fe72.6C24.5O1.1N1.8 alloy at different stages of the mechanosynthesis (MS) in a planetary ball mill. The introduction of impurities of O and N into an Fe75C25-based alloy changes the sequence of the formation of phases during MS: instead of Fe3C, the Fe7C3 carbide is first to be formed. The processes of phase formation in the alloy preliminarily subjected to MS have unidirectional nature upon the continuation of the MS and upon annealings and are determined by the interaction of the alloy components with one another under the effect of the accumulated excess energy. The phase compositions of the MS alloys depend on the conditions of the dynamic equilibrium between the crystalline and amorphous phases.

On the use of temperature dependences of the coercive force for an analysis of structural and phase changes that occur upon tempering of alloy carbon steels

The effect of alloying elements Mn, Cr, and Si on the magnetic hysteresis properties of cementite and model steels with a carbon concentration of 0.6 wt % has been studied. It has been shown that alloying with carbide-forming elements (Mn, Cr) reduces the coercive force and the Curie temperature of cementite. Measurements of the temperature dependences of the coercive force of the model steels with carbon content of 0.6 wt % alloyed with manganese, chromium, or silicon have been performed in the temperature range of −196 to +300°C. It has been established that the local maximum of the coercive force of these steels in this temperature range coincides with the Curie point of the precipitates of the carbide phases. Based on an analysis of the temperature dependences of the coercive force, the content of the alloying element in the precipitates of cementite of steels tempered at different temperatures has been estimated. It has been shown that the character of the dependence of the coercive force of alloy steels on the temperature of tempering in the temperature range of 250–700°C is mainly determined by the coercivity and by the kinetics of the formation of cementite precipitates.

Formation of Phases upon the Mechanosynthesis and Subsequent Annealing of Samples of Cementite Composition Alloyed with Chromium and Nickel

Using X-ray, Mössbauer, and magnetic measurements, the formation of phases has been investigated upon mechanosynthesis in a ball planetary mill and upon the subsequent annealing of samples of the cementite composition (Fe0.95–уСr0.05Niy)75C25, where у = 0–0.20, which contains two alloying elements (chromium and nickel). It has been shown that, in the mechanosynthesis process, cementite alloyed with chromium and a small amount of nickel and an amorphous phase alloyed with chromium and nickel have been formed. Upon heating above 300°С, the amorphous phase is crystallized into nickel-enriched cementite. In the process of annealing at higher temperatures, the most nickel-rich cementite decomposes with the formation of austenite. As a consequence, after annealing at medium temperatures, the composition of the alloys contains cementite alloyed mainly with chromium and some amount of alloyed austenite, which can be found in ferromagnetic or paramagnetic states depending on the Ni content. Annealing at 800°С bring about the complete or partial decomposition of cementite contained in the alloys. The intensity of the decomposition has been determined by the nickel content in the samples.

Role of Chromium in the Formation of Phases in Mechanically Synthesized Alloys of Cementite Composition Alloyed with Chromium and Nickel

Phase transformations and aspects of alloying the phases of alloys of cementite composition, alloyed with chromium and nickel in the state immediately after mechanical synthesis and subsequent annealing, are studied. It is shown that cementite after mechanical synthesis is basically alloyed with chromium, while the amorphous phase is alloyed with chromium and nickel. Cementite produced upon the crystallization of the amorphous phase at Tann = 300°C is enriched with nickel. At medium temperatures of annealing, the cementite regions with the highest nickel content decompose to form para- or ferromagnetic austenite.

Effect of silicon on the phase formation in mechanically activated systems based on Fe(75)C(25): Temperature-induced transformations in mechanosynthesized composites

Transformations realized in mechanosynthesized amorphous-nanocrystalline Fe(75)C(25 − x)Si(x) (0 ≤ x ≤ 10 at %) alloys during heating have been studied using dynamic magnetic susceptibility measurements, X-ray diffraction, and metallography. In contrast to mechanosynthesized alloys consisting of α-Fe, Fe3C, and amorphous phases, the annealed alloys with x > 5 at % were found to exhibit the formation of an additional phase such as Fe5SiC. After heating to 700 and 800°C, the powder particles of alloys contain a large amount of uniformly distributed graphite particles of ∼0.5 μm in size. The formation of particles results from the cementite decomposition, which is accelerated at the expense of partial silicon dissolution in cementite and in the presence of α-Fe nanograins as well.