Mechanodynamic penetration of helium atoms into nanocrystalline iron

A relation between the characteristics of plastic deformation and the specific features of mechanodynamic penetration of helium into nanocrystalline iron compressively strained at 4.2 K is investigated. Iron samples with a grain size of about 200 nm are prepared by the multiple equal-channel angular pressing technique. The samples deformed in giant (6–7%) sample-averaged serrations, which amounts to several thousand percent strain in a shear band. The amount of helium in samples strained to various degrees is measured, and curves of helium extraction from these samples are obtained in the temperature range 300–1400 K. At a strain of ～50%, the amount of helium built up in a sample is found to be substantially higher (more than hundredfold) than that in samples subjected to lower strains. It is found that an increase in the strain rate gives rise to a strain within a serration (the strain localization is enhanced) and that the amount of accumulated helium decreases, most probably, because of the shorter deformation time. The helium extraction curves obtained with increasing temperature exhibit several peaks. The temperature positions of some of them are about the same for samples strained to different extents, while the other peaks are characteristic of samples subjected to a specific strain only. The results obtained suggest the existence of helium traps of different types, which depend on the original structure and the magnitude of the strain and differ both in the amount of helium they contain and in the temperatures at which helium is released from these traps.     

Analysis of the strain-rate sensitivity of flow stresses in nanocrystalline FCC and BCC metals

The dependence of the strain-rate sensitivity coefficient of flow stresses S = dlnσ/dln\(\dot \varepsilon \) on temperature, strain rate, and grain size in nanocrystalline (NC) metals is analyzed quantitatively in terms of the dislocation-kinetics approach taking into account the properties of grain boundaries as sources, sinks, and barriers for moving dislocations. The interaction of moving dislocations with a dislocation forest in nanograin boundaries is shown to be responsible for the fact that the values of this coefficient in NC fcc metals (Cu, Ni) are an order of magnitude greater than those in coarse-grained metals and for the strong dependence of the coefficient S on the above factors. This dependence is largely caused by the annihilation of lattice dislocations in grain boundaries controlled by the activation energy of grain boundary diffusion. The values of the coefficient S in NC bcc metals (α-Fe) are an order of magnitude lower than those in coarse-grained samples, because dislocations move in a Peierls relief in nanograins     

Atomic mechanisms of microcrack propagation in pure and hydrogen-containing FCC and BCC metals

A molecular dynamics method was used to simulate crack propagation in pure and hydrogen-containing aluminum and α-iron for loads far exceeding the critical values. Pairwise interaction potentials calculated within the Heine-Abarenkov-Animalu pseudopotential approximation were applied. It was shown that cracks do not propagate in the pure metals. Their tips become blunt, mouths broaden, and internal stresses are released owing to arising dislocations and necking. This means that mechanisms of viscous fracture come into play. In the presence of hydrogen impurity, the situation is quite different. In aluminum, hydrogen desorbs and the material retains its ductility. In α-iron, hydrogen forms Cottrell clouds around dislocations, thus suppressing their movement and generation. In addition, an increase in the hydrogen concentration in iron near the crack mouth makes the material more prone to α → γ phase transition. As a result, crack propagation is observed; i.e., the material embrittles.     

Mechanodynamic penetration of helium atoms into aluminum and its alloys under strain in liquid helium

The specific features of helium penetration into aluminum and its alloys, V95 and D16T, at a temperature of 4.2 K under uniaxial tension, compression, and reversal of the sign of the load are investigated. The role played by serrated strain in the intensity of the effect under consideration and the influence of impurities on the number of helium atoms penetrating into strained samples are elucidated. It is shown that the condition of additivity of the effect observed under successive reversal of the sign of the load depends on the specific features of the tensile and compressive strains.     

The properties of helium atoms and as impurities in metals

When the atoms of simple metals condense from the gas phase to give a solid, the valence electron wavefunctions are strongly modified by their overlap. At first sight this strong overlap might be expected to produce a complex situation, difficult to treat quantitatively. Yet the simple metals are perhaps the best understood of all solids and the reason is that the electron wavefunctions are plane-wave-like to a first approximation and deviations can be treated by perturbation theory.     

Indium-defect interactions in FCC and BCC metals studied using the modified embedded atom method

With the aim of developing a transferable potential set capable of predicting defect formation, defect association, and diffusion properties in a wide range of intermetallic compounds, the present study was undertaken to test parameterization strategies for determining empirical pair-wise interaction parameters in the modified embedded atom method (MEAM) developed by Baskes and coworkers. This report focuses on indium-solute and indium-vacancy interactions in FCC and BCC metals, for which a large set of experimental data obtained from perturbed angular correlation measurements is available for comparison. Simulation results were found to be in good agreement with experimental values after model parameters had been adjusted to reproduce as best as possible the following two sets of quantities: (1) lattice parameters, formation enthalpies, and bulk moduli of hypothetical equiatomic compounds with the NaCl crystal structure determined using density functional theory and (2) dilute solution enthalpies in metals as predicted by Miedema’s semi-empirical model.     

Influence of the deformation type and medium on the mechanodynamic penetration of nitrogen molecules into surface layers of armco iron

The extraction of nitrogen molecules from deformed samples of armco iron with different initial structures (annealed and subjected to equal-channel angular pressing) and different deformation prehistories (deformation in liquid nitrogen at 77 K, rolling in air at room temperature, and their combination) has been studied. It has been shown that the preliminary deformation in liquid nitrogen increases its concentration in the surface layer of the material and shifts the principal peak of its release toward low temperatures during heating. The results are associated with the existence of different types of nitrogen traps in annealed and nanostructured armco iron and with their changes during subsequent deformation.     

Chemi-ionization and spin exchange upon collisions of metastable helium atoms with potassium atoms in the ground state

The spin exchange and chemi-ionization cross sections have been calculated for the metastable helium atom-potassium atom system in the ground state. The data on the spin exchange cross section in the system He(2³ S ₁)-K(4² S _1/2) have been obtained for the first time. The cross sections were calculated for the collision energies ranging from 2 × 10^?4 to 60 × 10^?4 au The chemi-ionization cross sections obtained have been compared with the data in the literature.     

Influence of hydrogen and helium implantation on the properties of structural materials

The influence of hydrogen and helium impurities on physical and mechanical properties of EP-450 ferrite steel has been studied. The techniques of elastic recoil detection analysis, Rutherford backscattering, acoustic emission, acoustic energy dissipation rate measurements, magnetic and eddy-current methods were used. Microhardness of steel samples containing hydrogen and helium has been studied by nanoindentation. Microscopic investigation of the steel surface after hydrogenation and after exposing to monoenergetic 40-keV helium ions has been performed.     

State to state rotational transfer in collisions between IF molecules and helium atoms

We report measurements of the rate for transfer of population between rotational levels of the B state of iodine monofluoride in collisions with helium atoms. The measured rates are much larger than those observed for transfer of population between vibrational levels, indicating that the rotational population distribution in the collisionally pumped IF laser should be rapidly thermalized. The rates show a dependence on the quantum number of the initial level, in contrast to the results of theoretical calculations.     

Generalized Ginzburg-Landau functionals for studies of phase transformations between FCC,BCC, and HCP structures in metals and alloys

Generalized Ginzburg-Landau functionals describing transitions between bcc, fcc, and hcp structures are studied using four weakly nonuniform transformation parameters: three tensile strains along the principal crystallographic axes and a “phonon” parameter describing relative slip of close-packed planes. Several versions of transformation paths indicated for each of the three transformations considered, which appear to be the most realistic; explicit expressions for atomic displacements are given for each of these paths. It is shown that the gradient terms in these functionals can be explicitly expressed in terms of the dynamic matrix of the crystal on a transformation path; these dynamic matrices can be determined using interpolations of experimental data on phonon spectra between the initial and final phases. The results are used for estimating the characteristics of interphase boundaries between ferrite and cementite in steels. It is found that the width of such interfaces considerably exceeds the interatomic distance, while the estimated values of the gradient terms are in reasonable agreement with the available experimental data.     

Structural properties of BCC and FCC iron: LMTO-ASA-Stoner calculations

The LMTO-ASA (Linear Muffin-Tin Orbitals - Atomic Spheres Approximation) calculations of structural properties of BCC and FCC iron were performed. The ground-state magnetic properties: ferromagnetic contributions to the total energy and magnetic moments, were found by making use of the Stoner theory of itenerant ferromagnetism, rather than spin-polarized calculations. This enabled us to circumvent the difficulty resulting from using the traditional local spin-density approximation which is known to have failed predicting correctly the energetics of iron phases. The Stoner exchange parameter, I, was calculated from the linear response theory as a function of volume. Then a constant enhancement factor, β, was introduced and the new Stoner parameter, ββI, was used in all the calculations. The factor β was found by fitting the equilibrium atomic volume of the ferromagnetic BCC phase to its experimental value. No other adjustments of any quantities were performed. The calculations revealed that the nonmagnetic BCC modification was unstable with respect to the spontaneous magnetization. Ferromagnetism stabilizes the BCC phase. The calculated bulk moduli, magnetic moments and the BCC-FCC energy difference are in good agreement with the available experimental data.     

Analysis of structural factors that control necking during tension of FCC metals and alloys

The relation between the strength and ductility of structural materials is theoretically analyzed using stress-strain curves for a number of fcc metals and alloys. The theoretical analysis is based on the criterion of necking in a tensile specimen and on a stress-strain curve, which reflects the evolution of the dislocation density in a material with increasing strain and the effect of structural factors on this evolution. Theoretical relationships are obtained for the uniform strain and the ultimate tensile strength. The effects of the stacking-fault energy, the solid-solution hardening, and the grain size on these quantities are considered.     

CVM study of FCC and BCC Ising magnets with generalised spin number

Calculations using the cluster variation method (CVM) in the tetrahedron approximation have been applied to multicomponent FCC and BCC Ising systems with spin number up to s=7/2. The magnetic specific heat capacity (C_H) of ferro- and antiferromagnetic FCC and BCC systems has been calculated in the first neighbour approximation and compared with data obtained with other methods. A concavity in the C_H curve at T≈T_C/2 develops with increasing spin number.     

Influence of impurity atoms on the temperature dependence of the electric field gradient in nontransition metals

The temperature dependence of the quadrupole coupling constants of¹¹¹CdIn,¹¹¹CdSn, and¹²⁰SbSn was measured by means of the perturbed angular correlation (distribution) technique. The results are discussed within a generalized model for the electric field gradient in nontransition metals.This work was supported by the Bundesministerium für Forschung und Technologie.     

Magnetic dichroism of potassium atoms on the surface of helium nanodroplets

The population ratio of Zeeman sublevels of atoms on the surface of superfluid helium droplets (T=0.37 K) has been measured. Laser induced fluorescence spectra of K atoms are measured in the presence of a moderately strong magnetic field (2.9 kG). The relative difference between the two states of circular polarization of the exciting laser is used to determine the electron spin polarization of the ensemble. Equal fluorescence levels indicate that the two spin sublevels of the ground-state K atom are equipopulated, within 1%. Thermalization to 0.37 K would give a population ratio of 0.35. We deduce that the rate of spin relaxation induced by the droplet must be <520/s. For the K2 triplet dimer we find instead full thermalization of the spin.     

Atomic simulation of the vacancies in BCC metals with MAEAM

The formation energy of the mono-vacancy and both the formation energy and binding energy of the di-and tri-vacancy in BCC alkali metals and transition metals have been calculated by using the modified analytical embedded-atom method (MAEAM). The formation energy of each type of configuration of the vacancies in the alkali metals is much lower than that in the transition metals. From minimum of the formation energy or maximum of the binding energy, the favorable configuration of the di-vacancy and tri-vacancy respectively is the first-nearest-neighbor (FN) or second-nearest-neighbor (SN) di-vacancy and the [112] tri-vacancy constructed by two first-and one second-nearest-neighbor vacancies. It is indicated that there is a concentration tendency for vacancies in BCC metals.