E. Z. Valiev

Formation of nanocrystalline structure in the amorphous Ti50Ni25Cu25 alloy upon severe thermomechanical treatment and the size effect of the thermoelastic martensitic B2 ↔ B19 transformation

The results of the comparative analysis of the Ti50Ni25Cu25-alloy structures produced in the initial amorphous state by rapid quenching from the melt (RQM), after severe plastic deformation by torsion under high pressure (HPT), and postdeformation heat treatment (PHT) are presented. The study was carried out using neutron and X-ray diffraction, transmission and scanning electron microscopy, and measurements of electrical properties. The initially amorphous alloy has been established to nanocrystallize after torsion under a pressure of 7 GPa to 0.5 revolutions of the anvil. Then, after 1, 5, 10, and 15 rev, the alloy again undergoes the strain-induced amorphization even with the retention, even after 5–15 rev, of a large number of highly dispersed nanocrystals less than 3–4 nm in size with a distorted B2 lattice in the amorphous matrix. Their crucial role as nuclei of crystallization provides the total low-temperature nanocrystallization during subsequent annealing starting from 250–300°C. It is shown that PHT of the alloy amorphized by HPT makes it possible to produce extremely uniform nanocrystalline (NC), submicrocrystalline (SMC), or bimodal (NC + SMC) austenitic B2-type structures in it. A complete diagram of thermoelastic martensitic transformations in the region of B2-austenite states, from nanostructured state to conventional polycrystalline one, has been constructed. The size effect on the stabilization of martensitic transformation in nanocrystalline B2 alloy has been established.

Structure of zirconium phosphate gels produced by the sol - gel method

Several zirconium phosphate (ZrP) gels of various water contents produced by a sol - gel method were investigated. They were found to be the polymer-class gels using the method of small-angle neutron scattering. X-ray diffraction analysis revealed that their solid-phase framework was formed from branched fragments (layers and blocks) of -ZrP crystal structure. These gels are strongly fluctuating systems when they are on the scale of . Their fluctuation character is shown to be described by two parameters: a correlation radius and a fractal dimension D. It was found that and almost did not depend on the water content and that D increased from to when the water content decreased from 50% to 0% as the gels were dried to a temperature of . Simultaneously, substantial shrinkage of granules, a decrease in porosity and ion-exchange capability, and formation of blocks from -phosphate layers occurred with decrease in the water of the hydrogel as it dried. A probable description of the evolution of the ZrP-gel structure with variation in the water content during drying is suggested on the basis of the data obtained.

Phase and structural transformations in the Ti49.5Ni50.5 alloy with a shape-memory effect during torsion under high pressure

Results of investigations of structural and phase transformations that occur in the titanium-nickelide-based alloy Ti49.5Ni50.5 with a shape memory effect during severe plastic deformation by torsion under high pressure (HPT) are reported. The studies were performed using transmission and scanning electron microscopy, neutron and X-ray diffraction, and measurements of temperature dependences of electrical resistivity. The martensitic B2 → B19′ transformation was found to be induced in the alloy when applying a high pressure. After unloading, the martensitic B19′ phase is retained in the alloy. The fine structure of the B19′ martensite and its evolution into nanocrystalline and, subsequently, amorphous state during HPT with 1/4, 1/2, 1, 5, and 10 rev have been studied. It was shown that, after HPT, all nanosized crystallites whose sizes are less than 30–50 nm have a B2-type structure and, therefore, the reverse martensitic B19′ → B2 transformation is realized in the alloy at room temperature after unloading.

Size effect in nanocrystalline manganites La1−x A xMnO3 (A=Ag, Sr)

Nanocrystalline samples of the manganites La0.9Ag0.1MnO3, La0.7Ag0.3MnO3, and La0.7Sr0.3MnO3 were synthesized through pyrolysis and isothermally annealed. The atomic, subatomic, and magnetic structures of these manganites were studied using magnetic, x-ray, and neutron diffraction measurements. Increasing the annealing temperature from 600 to 1200°C coarsens the grains from 30–40 to 600–700 nm in size. All the samples studied have rhombohedral structure and are ferromagnets. The Curie temperature decreases for the samples doped by silver and increases for the samples doped by strontium as the anneal temperature is increased. The magnetization of the Mn ions increases with nanoparticle size in all the three systems, which indicates the presence of a size effect.

Magnetic-field-induced spin-reorientational transition in TbMn6Sn6

Magnetic state of the intermetallic TbMn6Sn6 compound was studied by neutron diffraction analysis in external magnetic fields of H=0, 4, and 5 kOe. Magnetic moments and their orientations were determined for the Tb and Mn sublattices. Magnetic fields higher than 4 kOe were shown to induce spin-reorientational transition at room temperature from the c axis to the basal plane.

Entropy and magnetocaloric effect in ferromagnets and antiferromagnets

Analytic expressions for the entropy of a ferromagnet and an antiferromagnet that make it possible to calculate the magnetic-field dependence of the entropy in a wide temperature range have been obtained in the mean-field approximation for the Heisenberg model. It has been shown that for Fe, Ni, and Gd the calculated magnitude of the magnetocaloric effect is in satisfactory agreement with experiment. The results of calculations of the dependence of the entropy of an antiferromagnet on the magnitude and direction of a magnetic field have been discussed.

Small-angle neutron scattering from samples of expanded carbon

The subatomic structure of expanded graphite has been investigated by small-angle neutron scattering. Samples were synthesized during quick thermal decomposition of intercalated compounds based on oxidized graphite. They had a low bulk density (up to 0.1 g/cm3) and were characterized by considerable small-angle scattering. It has been established that majority of the volume of expanded graphite samples is occupied by participles with characteristic sizes in two ranges: from 6 to 8 nm and from 20 to 30 nm. Small particles have properties of a surface fractal with the dimension Ds = 2.4–2.6, whereas the larger particles are mainly smooth and have the dimension Ds = 2.0–2.1. The specific surface of the samples studied was determined from the small-angle scattering data.

Magnetic properties of RCo2 compounds in the exchange-striction model of ferrimagnets

The original version of the exchange-striction model of a ferrimagnet has been employed for calculating a number of magnetic properties of RCo2 ferrimagnets, where R = Er, Ho, Dy, Tb, and Gd are rareearth ions. The following magnetic properties are calculated: pressure dependence of the Curie temperature (ТС), temperature dependences of magnetization in sublattices of cobalt and rare-earth atoms, and isotherms of magnetization of these lattices at Т > ТС. For an ErСо2 sample, the Н–Т phase diagram has been constructed and the magnetization in the magnetic fields Н = 0–70 Т has been calculated. The calculated and experimental results have been compared. Based on the exchange-striction model, the qualitative explanation of the difference in the type of the magnetic phase transformation in the intermetallic compounds with R = Tb and Gd and R = Er, Ho, and Dy is given.

Isotropic magnetoelastic interaction and the pressure-induced first-order magnetic phase transition in CoS2

The change in the order of the magnetic phase transition in ferromagnetic CoS2 under pressure recently revealed by Goto et al. [Phys. Rev. B56, 14019 (1997)] is explained in terms of an exchange-magnetostriction model that takes into account the magnetoelastic interactions (MEI) of the first and second order with respect to volume deformations. Using numerical values of MEI constants determined from experimental data, the equilibrium and nonequilibrium magnetic phase diagrams have been calculated for CoS2 in the space of variables T (temperature), P (pressure), and H (magnetic field). The temperature dependence of magnetization at various pressures and the magnetization isotherms have been calculated. The results of the calculations show satisfactory agreement with experiment. Analytical expressions for the critical magnetic field and temperature of the first-order magnetic phase transition (FOMPT) and for the FOMPT curve in a magnetic field at a constant pressure have been obtained. The effect of an applied magnetic field on the characteristics of the FOMPT are analyzed.

Structural state of the claddings of fuel elements of the BN-600 reactor prepared from cold-deformed 16Cr–15Ni–2Mo–2Mn–Ti–V–B steel: Study using the method of high-resolution neutron diffraction

The applicability of the method of high-resolution neutron diffraction for determining defects (dislocations) in the bulk of a material is shown based on the example of a standard sample of deformed copper. The structural state of several samples of fuel-element claddings made of cold-worked steel 16Cr–15Ni–2Mo–2Mn–Ti–V–B (grade ChS68-ID) used in the BN-600 reactor, which were prepared by two producers, i.e., at the PJSC Mashinostroitel’nyi Zavod (MSZ) and at the PJSC Pervoural’sk Novotrubnyi Zavod (PNTZ), have been investigated. The conclusion has been drawn that the claddings have a texture in which the crystallographic planes of grains are oriented along the axis of the shells. It has been shown that the main defects in these claddings are edge dislocations; their density has been determined.