Papers by Keyword: Nanocrystalline Structure

Abstract: Transmission electron microscopy and X-ray diffraction were used to study structural changes in the Ti-50.5 at. % Ni alloy upon severe plastic deformation by shear under pressure and subsequent heating. An increase in the degree of deformation leads sequentially to a martensite transformation, twinning of martensite crystals, formation of reorientation bands, development of rotational modes of deformation, formation of a nanocrystalline structure, and finally amorphization. A scheme of the formation of amorphous structure of the alloy during deformation is suggested based on the observed structural changes. It has been found that a reverse martensitic transformation might be one of mechanisms of plastic deformation of the alloy. Therefore, as the degree of deformation increases, first forward and subsequently reverse martensitic transformations can occur. The formation of an amorphous structure starts as the degree of deformation reaches 4.2 (one revolution of Bridgman anvils); at a degree of deformation of 6.8 (5 revolutions of the anvils), the process is virtually completed. The crystallization of the amorphous alloy upon heating starts even at 200°C. However, upon heating up to 300°C (for 0.5-h holdings), the kinetics of crystallization is slow. After annealing at 350°C, the complete crystallization with the formation of a nanocrystalline structure with a grain size of 20-70 nm takes place

Abstract: The effect of micro-and nanocrystalline structural state on magnetic properties and magnetic entropy (ΔS_M) of Gd is investigated. The marked influence of severe plastic deformation on ΔS_M is demonstrated.

Abstract: In the present work, the structure and magnetic properties of strontium hexaferrite powder during milling in various media and subsequent annealing were studied. The milling of the powder leads to an abrupt decrease in the powder particle size and the average crystallite sizes as well as an increase in lattice microstrains of the SrFe₁₂O₁₉ phase. During milling in toluene, no changes in the phase composition were observed, whereas, during milling in water, the Fe₂O₃ phase (type H1.1) is formed. In the powder milled in oleic-acid-containing toluene a small quantity of α-Fe was found. After milling, the saturation magnetization and remanence decrease; at the same time, the coercive force of the powder milled in toluene is unchanged with increasing milling time, but slightly increases after milling in water and in oleic-acid-containing toluene. The annealing allowed us to increase the magnetic properties of the powder. It is conditioned by formation of nanocrystalline state. Moreover, the magnetic properties values depend on the medium and time of milling. The annealed powders are characterized by the following magnetic properties: μ₀Н_ci = 0.42 ÷ 0.49 T, B_r = 0.23 ÷ 0.24 T, (BH)_max = 8-9.6 kJ/m³.

Abstract: In order to obtain a nanocrystalline and amorphous structure in the Mg2Ni-type alloy, the Ni in Mg2Ni alloy has been partially substituted by M (M=Co, Cu), and the melt spinning has been used to fabricate the Mg20Ni10-xMx (M=Co, Cu; x=0-4) hydrogen storage alloys. The microstructures of the alloys were characterized by XRD, SEM and HRTEM. The effects of substituting Ni with M (M=Co, Cu) on the gaseous and electrochemical hydrogen storage kinetics of the as-spun alloys were investigated. The results indicate that the as-spun (M=Co) alloys display a nanocrystalline and amorphous structure, while the as-spun (M=Cu) alloys hold an entire nanocrystalline structure, suggesting that the substitution of Co for Ni facilitates the glass formation in the Mg2Ni-type alloy. The substitution of M (M=Co, Cu) for Ni exerts a trifling impact on the hydriding kinetics of the alloys, but it renders a marked enhancement of dehydriding capacity and kinetics. Furthermore, the measurements of the high rate discharge ability (HRD) and the hydrogen diffusion coefficient (D) as well as the electrochemical impedance spectra (EIS) of the alloys exhibit that the electrochemical kinetics of the as-spun (30 m/s) alloys is significantly ameliorated by substituting Ni with M (M=Co, Cu).

Abstract: The structure of Nb3Sn-based, bronze-processed Ti-doped multifilamentary superconducting wires has been studied by TEM and SEM after the first (5750C,100 h) and the second (6500C,100 h) stage of the diffusion annealing. The Nb3Sn layers formation in all the composites proceeds by one and the same mechanism and starts with nucleation of particles and very fine grains of this phase in Nb filaments where Sn diffuses from the bronze matrix. Ti, inserted both in the bronze matrix, or Nb filaments, diffuses into the growing superconducting layer and promotes its more active formation. At the first stage of annealing (5750C, 100 h) Nb3Sn grains have an average size of 40 nm, and at the second stage (6500C, 100 h) they increase by a factor of 1.5 and the grain size distribution gets wider. After the two-stage annealing the amount of the residual niobium is small, and some Nb filaments, especially in doped composites, almost completely transform into Nb3Sn. In the Nb3Sn layers of a zone of columnar grains is adjacent to the residual Nb.

Abstract: In this paper we have investigated the possibility of increasing the complex mechanical properties of steel strength category X90 by refinement the structure of hot and warm deformation. Were carried out tests to determine the critical points of recrystallization and built thermokinetic curves of austenite decomposition. After that, for produce disperse ferrite in samples has been applied to the processing mode, which is exactly the same mode of controlled rolling, then, to obtain nanoscale state of the metal deformed at temperatures of bainite transformation with varying degrees of deformation at different temperatures. It was shown that the dispersion and morphology of the structure and, consequently, the mechanical properties vary depending on temperature and strain, and samples show satisfactory ductility at high strength.

Abstract: Bronze-processed Nb3Sn-based multifilamentary composites with external diameter of 0.8 and 0.5 mm and coupled Nb filaments have been studied by transmission (TEM) and scanning (SEM) electron microscopy. After the two-staged annealing, 575°С, 150 h + 650°С, 200 h, commonly used for ITER conductors, a nanocrystalline layer of superconducting Nb3Sn compound is formed in every Nb filament as a result of solid-state reactive diffusion of Sn from the bronze matrix. It is demonstrated that in the wires of smaller external diameter the Nb filaments transformation into the Nb3Sn compound is more pronounced, that is the amount of the residual Nb is smaller. Besides, the nanocrystalline structure of the Nb3Sn diffusion layers is more perfect in 0.5 mm diameter wires, namely, the Nb3Sn grains are finer (their average size being 60 nm compared to 70 nm in 0.8 mm diameter wires) and are more uniform in sizes (the root mean square deviation of grain size distribution is correspondingly 15 and 17 nm).

Abstract: The present work explores an environmental friendly method for forming conversion coatings on AZ91D magnesium alloy by applying a static magnetic field. An amorphous / nanocrystalline conversion coating was developed on magnesium alloys in a phosphate-permanganate aqueous solution under a 0.5 Tesla magnetic field. The examinations of the microstructure and phase compositions reveal that the conversion coating is composed of an amorphous structure and nanocrystalline structure with mean size less than 10 nm. In a corrosive chloride solution, the amorphous/nanocrystalline conversion coating with an artificial defect were investigated by scanning vibrating electrode technique (SVET)/ scanning ion-selective electrode technique (SIET) system. The micro-electrochemical results reveal that the amorphous / nanocrystalline conversion coating as a good barrier layer can provide effective corrosion protection for AZ91D magnesium alloy.

Abstract: A new approach is proposed to control the processes of billets plastic deformation during metal treatment by the methods of severe plastic deformation (SPD). High strength and plasticity have been attained for the processed copper billets after multiple repetitions of equal channel angular hydroextrusion (ECAH) and direct hydroextrusion (HE) techniques and with ECAH and HE implementation in the fractional mode. The combined SPD treatment including ECAH, HE and drawing (D) provided for fire refined tough pitch copper (Cu-FRTP) the ultimate tensile strength =686 MPa, the elongation to failure =2% and the electrical conductivity (EC) at a level of 86.4% IACS and for oxidant free copper (Cu-OF) =576 MPa, =1.9%, EC=96.7% IACS in the 0.5 mm diameter wire. Such treatment is efficient due to the alternative schemes of deformation, the fractional mode and the optimum degrees of plastic deformation and periodic creation of favorable conditions for relaxation and dynamic recrystallization processes in the material.

Abstract: Evolution of structure of Nb subjected to high-pressure torsion (HPT) with various strain at cryogenic and room temperatures and further annealing in the temperature range of 100-7000C has been studied by transmission electron microscopy (TEM) and microhardness measurements. HPT in liquid nitrogen enables to obtain true nanocrystalline structure with crystallite sizes of about 75 nm and the record-breaking microhardness of 4800 MPa. The thermal stability of the structure obtained is analysed.