High Quasi-hydrostatic Pressure Research Articles

Magnetic states of iron borate at high quasi-hydrostatic pressure

Magnetic states of iron borate, FeBO3 single crystal subjected to a superposition of high hydrostatic and uniaxial pressures have been analysed in the framework of a thermodynamic approach. It is shown that the uniaxial pressure component in the basal plane can deflect the magnetization from the basal plane of the crystal. The results obtained are in good agreement with available experimental data on magnetic states of FeBO3 at high quasi-hydrostatic pressure.

Suppressing the Equilibrium γ Phase of Fe50Ni25Co25 Ferromagnetic Alloy during Torsion under High Quasi-Hydrostatic Pressure in a Bridgman Chamber

Suppressing the Equilibrium γ Phase of Fe50Ni25Co25 Ferromagnetic Alloy during Torsion under High Quasi-Hydrostatic Pressure in a Bridgman Chamber

Directional amorphization of covalently-bonded solids: A generalized deformation mechanism in extreme loading

Shock compression subjects materials to a unique regime of high quasi-hydrostatic pressure and coupled shear stresses for durations on the order of 1–10 nanoseconds for laser-driven loading of samples. There is, additionally, an attendant temperature increase due to the shock and the mechanisms of plastic deformation in metals whereby dislocations, twins, and phase transitions nucleate and propagate at velocities near the sound speed. Covalently bonded materials have, by virtue of the directionality of their bonds, great difficulty in responding by conventional plastic deformation to this extreme regime of shock compression. We propose that the shear from shock compression induces amorphization, as observed in Si, Ge, B4C, SiC, and olivine ((Mg, Fe)2SO4) and that this is a general deformation mechanism in a broad class of covalently bonded materials. The crystalline structure transforms to amorphous along regions of maximum shear stress, forming nanoscale bands, and thereby relaxing the shear component of the imposed shock stress. This process is usually preceded by the emission and propagation of a critical concentration of dislocations.

Amorphous-Nanocrystalline Composites Prepared by High-Pressure Torsion

This article presents systematic studies of the preparation method and the specific features of the changes in the structure and properties of amorphous-nanocrystalline composites formed from melt-quenched ribbons of iron- and cobalt-based amorphous alloys and the Cu-Nb crystalline nanolaminates by severe plastic deformation by torsion in the Bridgeman chamber at high quasi-hydrostatic pressure.

Peculiarities of structure and properties of titanium during fragmentation and reversal in a torsion process under high pressure

Influence of a reverse rotation of the moving striker of a Bridgeman chamber (torsion under high quasi-hydrostatic pressure at room temperature) on the structure and mechanical properties of technically pure titanium (VT 1-0) was studied. It was established that a change in the rotation direction of the moving striker during torsion had a significant effect on the conditions for separating the ω-phase, as well as on the structural characteristics of deformation fragments and dynamically recrystallized grains formed during large plastic deformations. Influence of the deformation method on the internal distortions of structure and on the mechanical properties was analyzed.

Микроструктура и фазовые соотношения нанокристаллов кобальта, полученного интенсивной пластической деформацией

Nanostructured cobalt samples were obtained by method of severe plastic deformation by torsion. The high quasi-hydrostatic pressure in the working region (up to 8 GPa ) was created at installation of Bridgman anvil type. The anvils were made of tungsten carbide. he method allows to obtain the samples of high purity without pores and impurities. To carry out of X-ray studies, a DRON-7 diffractometer with cobalt radiation was used. X-ray analysis was performed according to the Bragg-Brentano method. For phase analysis the Kα lines were used. The sizes of nanocrystalline cobalt reached about 25 ¸ 50 nm. It is established that during low-temperature annealing a reset occurs in the structure of cobalt. Annealing above 3000C leads to recrystallization of its structure. Derived by severe plastic deformation by torsion nanocrystalline cobalt after heating above the phase transition temperature and cooling below this temperature retains the high-temperature fcc structure. It is shown that the nature of the hcp-fcc delayed transition can be connected with a sizes changes and a strained state of crystallites forming nanocrystalline cobalt.

Effect of High-Pressure Torsion on the Lattice Parameters of α-Fe and α-Fe-Based Solid Solutions

X-ray diffraction and computer simulation are used to study the effect of deformation by torsion under a high quasi-hydrostatic pressure on the lattice parameter of α-Fe and its solid solutions. These results are discussed in terms of possible mechanisms of interaction of dissolved elements with the vacancies formed upon deformation and the applicability of the Le Chatelier principle

Stages of Mechanical Alloying in Systems with Different Solubility Cu–Zn and Au–Co in the Case of Cold and Low-Temperature Deformation by Torsion Under Pressure

Methods of X-ray structural analysis and durometry, as well as electron microscopy were used to identify the stages of mechanical alloying in the case of torsion under high quasi-hydrostatic pressure on Bridgman anvils in the Cu–Zn and Au–Co systems that have different mutual solubility and enthalpy of mixing. It was established that decrease in temperature of mechanical alloying from room temperature (cold deformation) to the boiling temperature of liquid nitrogen (80 К, low-temperature deformation) has a considerable impact on mechanical alloying at different processing stages and on characteristics of an alloy synthesized by deformation. In the Cu–Zn system, when the ratio of powder components corresponds to the solid solution of α-brass in equilibrium state, as deformation increased, one observed consecutive change of evolution stages of the powder mix to the state of copper-based solid solution. At the same time, when processing temperature decreases, larger deformation is required to achieve analogous structural changes. In the Au–Co system characterized by absence of solubility at room and lower temperatures, one also observes the stages of powder mix evolution with the increase in deformation. However, complete dissolution occurs in the case of processing at 80 К, while larger deformation is required in the case of increase in mechanical alloying temperature. The paper examines possible mechanisms of solid solution formation in the system of components that are mutually insoluble under equilibrium conditions.

Thermal Stability of Microstructure and Microhardness of Heterophase BCC-Alloys After Torsional Deformation on Bridgman Anvils

The results of investigations of thermal stability of microstructure and microhardness of alloys of the V–4Ti–4Cr and Mo–47Re systems, subjected to torsional deformation by high quasi-hydrostatic pressure at room temperature, are reported. It is shown that submicrocrystalline and nanocrystalline states, and the respective high values of microhardness, persist up to the upper bound (~0.4 Тmelt) of the temperature interval of their recovery and polygonization in a single-phase state. The main factors ensuring thermal stability of highlydefective states in heterophase alloys are discussed.

Структурно-фазовые превращения в монокристаллическом цирконии при деформации сдвигом под давлением

The structural-phase transformation in pseudo-single crystal of zirconium during deformation on Bridgman anvils is studied by X-ray spectroscopy with synchrotron radiation. The angles of rotation in the Bridgman anvils turning varied from φ = 0 deg to φ = 45 deg. It was shown that the α-phase lattice is compressed at a true strain value e = 4.6, which is reflected in its parameters: the α-phase parameter "α" decreases by 5.3%, and the parameter "c" by 0.3%. On the contrary, the ω-phase arising in the process of deformation under pressure showed a tendency to stretching, if the parameter "a" of this phase practically does not change with an increase in the degree of deformation, then the parameter "c" is insignificant, by 0.14%, but increases. This behavior can probably be explained by the small atomic density and strong anisotropy in terms of the atomic density of the planes and series of the ω-phase. It is determined that the structure arising in the process of deformation of the baric ω-phase, deformation conditions from e = 0.5 to e = 4.6, does not undergo textural changes. At this step of deformation, only the stage of nucleation of the ω-phase is observed only in area of the α-phase, which have a favorable crystallographic orientation. The formation of groups of planar defects in the ω phase is a mechanism for the compensation of elastic stresses during lattice transformation α → ω under conditions of high quasihydrostatic pressure.

Bimodal Structures of Solids Obtained under Megaplastic Strain

The evolution of the defect structure of aluminum alloys of different compositions under megaplastic strain at a high quasi-hydrostatic pressure has been studied. The theoretical analysis and numerical calculation of the evolution of the defect structure of solids (metals) under this impact have been performed within the three-defect model of nonequilibrium evolution thermodynamics. The calculation of defect kinetics shows that the presence of coarse grains submerged into a matrix of fine grains at specified parameters and coefficients leads to the additional generation of dislocations and, as a consequence, to the greater dislocation strengthening of a material.

MgB2-based superconductors for fault current limiters

A promising solution of the fault current problem in power systems is the application of fast-operating nonlinear superconducting fault current limiters (SFCLs) with the capability of rapidly increasing their impedance, and thus limiting high fault currents. We report the results of experiments with models of inductive (transformer type) SFCLs based on the ring-shaped bulk MgB2 prepared under high quasihydrostatic pressure (2 GPa) and by hot pressing technique (30 MPa). It was shown that the SFCLs meet the main requirements to fault current limiters: they possess low impedance in the nominal regime of the protected circuit and can fast increase their impedance limiting both the transient and the steady-state fault currents. The study of quenching currents of MgB2 rings (SFCL activation current) and AC losses in the rings shows that the quenching current density and critical current density determined from AC losses can be 10-20 times less than the critical current determined from the magnetization experiments.

Effect of reversible torsion on the structure and mechanical properties of iron under severe plastic deformations in a Bridgman camera

The effect of the reversible direction of rotation of the movable anvil of the Bridgman camera (torsion under high quasi-hydrostatic pressure at room temperature) on the structure and mechanical properties of commercially pure iron (steel 08kp) is investigated. It is established that a change in the direction of rotation of the movable anvil under torsion substantially effects the structural characteristics of deformation fragments and dynamically recrystallized grains formed under severe plastic deformations. The effect of the method of deformation on the internal distortions of structure and on the mechanical properties is analyzed.

Influence of high-pressure torsion on the structure of an FeCoSiBNb alloy with an aperiodic phase in the initial state

The transformation of the initial structure (cubic quasicrystal) of the Fe60Co15Nb6Si15B4 (at %) alloy during severe torsion deformation under a high quasi-hydrostatic pressure is studied. It is found that, at the first stage, a substantially misoriented and fragmented structure forms without changes in the phase composition; at the final stage, the structure consists of a mixture of an amorphous phase and bcc α-Fe-based nanocrystals. The results are compared to the changes in the alloy structure under action of severe deformation of another type, namely, milling. The role of compressive stresses in structure formation is discussed.

Effect of the fraction and direction of high-pressure torsion deformation in a Bridgman cell on the structure and mechanical properties of commercial-purity iron

The effect of the fraction and the scheme of deformation under high quasi-hydrostatic pressure in a Bridgman cell at room temperature and on the structure and the microhardness of commercial-purity iron is studied. Deformation is performed by one continuous or portioned loading to the same total strain. The number of cycles and the direction of rotation of the lower movable anvil are varied. It is determined that the number of revolutions and the direction of rotation considerably affect the sizes of the deformation-induced fragments and the recrystallized grains formed upon megaplastic deformation. The correlation between the structure parameters and the microhardness is analyzed.

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.

Structure of an AMts aluminum alloy after high-pressure torsion in liquid nitrogen

We have presented the experimental data on the structure formation in commercial AMts aluminum-manganese alloy subjected to severe plastic deformation by torsion under high quasi-hydrostatic pressure in Bridgman anvils. The dependences of structural characteristics and hardness on the degree of deformation at room temperature and the temperature of liquid-nitrogen have been analyzed.

Structural study of helical polyfluorene under high quasihydrostatic pressure

We report on an x-ray diffraction (XRD) study of helical poly[9,9-bis(2-ethylhexyl)fluorene] (PF2/6) under high quasihydrostatic pressure and show an effect of pressure on the torsion angle (dihedral angle) between adjunct repeat units and on the hexagonal unit cell. A model for helical backbone conformation is constructed. The theoretical position for the most prominent 00l x-ray reflection is calculated as a function of torsion angle. The XRD of high molecular weight PF2/6 (M(n)=30 kg/mol) is measured through a diamond anvil cell upon pressure increase from 1 to 10 GPa. The theoretically considered 00l reflection is experimentally identified, and its shift with the increasing pressure is found to be consistent with the decreasing torsion angle between 2 and 6 GPa. This indicates partial backbone planarization towards a more open helical structure. The h00 peak is identified, and its shift together with the broadening of 00l implies impairment of the ambient hexagonal order, which begins at or below 2 GPa. Previously collected high-pressure photoluminescence data are reanalyzed and are found to be qualitatively consistent with the XRD data. This paper provides an example of how the helical π-conjugated backbone structure can be controlled by applying high quasihydrostatic pressure without modifications in its chemical structure. Moreover, it paves the way for wider use of high-pressure x-ray scattering in the research of π-conjugated polymers.

Comparing specific features of the structural formation of aluminum alloys during severe and intense plastic deformation

We compare the deformation behavior and specific features of the structural formation of two aluminum alloys, AMTs and V95, upon megaplastic (MPD) and severe plastic (SPD) deformation. It is established that upon SPD by dynamic channel angular pressing, a submicron crystalline structure is formed with grain sizes of 200 to 600 nm; and that upon MPD by high quasi-hydrostatic pressure shearing, a nanostructure is formed with grain sizes of 55 to 100 nm. The sequence of the phase and structure transition is established for an increase in the rate of deformation and velocity of the materials. Mechanisms of elastic energy relaxation are determined as a function of the extent of dislocation mobility.

Microstructure and mechanical properties of medium-carbon steel subjected to severe plastic deformation

The microstructure and properties of medium-carbon steel (0.45% C) are studied after torsional severe plastic deformation (SPD) at a high quasi-hydrostatic pressure and elevated temperatures of from 300 to 450°C. The initial treatment prior to the SPD is hardening for martensite. Analysis of the results shows that the SPD is effective for raising the characteristics of strength and microhardness at satisfactory ductility.