HRTEM image contrast of short range order in Ni4Mo

The high resolution transmission electron microscope (HRTEM) imaging of short range order (SRO) in Ni4Mo was investigated by means of multi-slice image simulations. The HRTEM images of Ni4Mo exhibit locally bright dot patterns corresponding to the [001] projections of the N2M2-type (chalcopyrite-like) structure. The multi-slice simulations revealed that the N2M2 patterns are rationalized as the projection patterns of the SRO structure which consists of subunit cell clusters of D1a, D022 and Pt2Mo structures. The N2M2-type image contrast appears when both the fundamental fcc lattice reflections and the 1 1/2 0 diffuse scattering of SRO contribute enough to imaging. This suggests that a good coincidence in intensity distribution between the Fourier power spectra of HRTEM images and the electron diffraction patterns is one of the conditions for the image contrast of SRO to be interpreted in terms of the projection contrast.     

Phase transformations and vacancy formation energies of transition metals by positron annihilation

The technique of the coincidence count rate at the peak of the angular correlation curve (CCR) in positron annihilation has been applied to the investigation of vacancy formation energies in thermal equilibrium in nickel, cobalt, and iron. The monovacancy formation energyE ^1v/F has been determined to (1.55±0.05) eV and (1.34±0.07) eV for nickel and cobalt, and (1.60±0.10) eV for α-iron, and (1.40±0.15) eV for γ-iron, respectively. The structural phase transformations in cobalt (693 K) and iron (1183 K, 1663 K) are exhibited by discontinuities of the CCR. In the case of cobalt the CCR follows exactly the change of the thermal expansion at the transition temperature. The temperature dependence of the CCR in the prevacancy region is found to be proportional to the thermal expansion for all metals investigated.     

Oxidation state and short range order at the surfaces of amorphous Fe40Ni40P14B6 ribbons

Conversion electron Mössbauer spectroscopy (CEMS) was used to study the oxidation state at the surfaces of amorphous Fe₄₀Ni₄₀P₁₄B₆ ribbons obtained by melt-spinning in air and vacuum heated at 583 K. Different concentrations of ferric and ferrous ions depend on the different behaviour of phosphorous diffusion towards the two surfaces during low annealing temperature.     

Calculation of short range magnetic order in iron

A spin fluctuation theory for itinerant electrons that includes short-range magnetic order (SRMO) is used to calculate the Curie-temperature (T_c and the temperature dependence of the magnetization and the susceptibility of bulk Fe. When spin correlations are included the Curie-temperature is reduced by 9% to T_c = 2000 K. The calculated temperature-dependence of the magnetization and the magnetic susceptibility are in excellent agreement with experimental results.     

Short range order in a solid solution Ni-8 at. % Nb

Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 28–30, December, 1990.     

Raman scattering and short range order in amorphous germanium

Variations in short range order of amorphous Ge films are determined from a combination of depolarized Raman scattering, optical absorption, and previous radial distribution function studies. An estimated minimum value of the bond angle width of Δθ ? 9°, obtained for evaporated, anneal stable a-Ge differs significantly from a recent EXAFS estimate. The maximum disorder observed in dc sputtered films deposited at ～90K indicates that an ～20% variation in bond angle width is possible in a-Ge. Substantial modification of the Raman spectrum of the low temperature film annealed at 300K also demonstrates that lower temperature structural relaxation processes involve changes in short range order.     

Rapidity gap correlations in short range order models

We show that short range order models relate the multiplicity distribution produced in a rapidity interval R of the central region, to the distribution of such intervals which all contain a fixed multiplicity N. This is an asymptotic result which furthermore implies that the asymptotic behavior of the Mueller moments be related to certain rapidity gap correlation moments. The behavior of these gap correlations in specific models depends in a direct way on the assumptions made about clustering and about the range of the interaction.     

Temperature dependence of the dislocation starting stress in Ni+17 at.% Cr

The K state can [1–7] have a marked effect on the mechanical parameters of homogeneous solid solutions containing transition elements. The K state is seen as a rise in resistance in low-temperature annealing after quenching from high temperatures and has been ascribed to the formation of short-range order [4, 5, 8–10] or of imperfect long-range order [6, 7]. The mechanical properties are also substantially affected. We have examined the effects on the dislocation starting stress as a function of temperature, as measured by mechanical hysteresis [18–20], in the K state range for Ni+17% Cr.     

Evaluation of the formation entropy of a single vacancy by means of positron annihilation

Analyzing positron annihilation data it was found that the formation energy of a single vacancy is proportional to the characteristic temperature T_c in f.c.c. metals. Assuming that the equilibrium fractional concentrations of single vacancies at T_c are constant, one finds that the formation entropy of a single vacancy is (1.6 ± 0.1)k for all f.c.c. metals. For zinc and cadmium, the formation entropies are 2.5k and 3.3k respectively.     

Austenitic structure formation in an Fe-32% Ni alloy during slow heating in the critical temperature range

Electron diffraction is used to show (for the first time) that the reverse α → γ transformation in an Fe-32% Ni during slow heating develops via the formation of an intermediate paramagnetic 9R phase. Coarse extended lamellae form according to a shear mechanism in the central part of the temperature range of the reverse transformation, which is called the critical range (here, the physical properties of the alloy change anomalously). The extended lamellae consist of 9R-phase lamellae with γ-phase interlayers. A high density of periodic stacking faults in the structure of the 9R phase and a high density of chaotic stacking faults in the complex 9R + γ phase determine the nature of phase transformation-induced hardening.     

Evidence for fcc-like short range order in Fe-Ni-B metallic glasses

Soft X-ray appearance potential spectra have been measured for iron and nickel in (Fe_0.5Ni_0.5)_100?xB_x metallic glasses with x = 16, 18, 20, 22, 24, 26. For x = 16 both iron and nickel spectra exhibit structural features characteristic for the electronic structure of fcc transition metals. It is concluded that in the amorphous state Ni and Fe are arranged in fcc-like geometry beyond nearest neighbours.     

Molecular dynamics characterization of icosahedral short range order in undercooled copper

The stability of undercooled simple metals is still an intriguing problem for materials science and technology. There is not consensus on the role played by the icosahedral short range order during undercooling. The scenario is even less clear for undercooled metals under external pressure. Extensive molecular dynamics simulations, based on an empirical tight-binding interatomic potential, are performed to explain experimental results recently obtained on liquid and undercooled liquid copper. A common neighbour analysis is used to fully characterize the icosahedral short range order in both undercooled and liquid systems. Moreover, the effect of pressure on icosahedral short range order, is addressed and rationalized. External pressure increases the probability to find atomic bonds with icosahedral symmetry both in the liquid and in the undercooled copper.     

NMR study of chemical short range order in Fe-B metallic glasses

¹¹B NMR study in glassy Fe_100?xB_x (12 ? x ? 24) suggests that B atoms will be fitted into some larger holes of Bernal polyhedrals only for x < 18, as the Polk model described. But for x ? 18, B atoms will occupy some positions of vertex sites of some polyhedras besides filling more larger holes until all of the larger holes are filled, like the Gaskell model pointed. Using this pattern, the big changes of the concentration dependence of some properties such as magnetic moments and Curie temperature at x = 18 can be interpreted.     

The deuteron wave function at short range and the triton

It is shown that if one assumes something between zero and the prediction of the scaling model with dipole fit for the neutron electric form factor, then a variety of short-range behaviour for the deuteron wave function is consistent with existing experimental data on the deuteron electric form factor. This still relatively wide latitude for the inner deuteron wave function, consistent with existing experimental electromagnetic data, gives rise to an off-shell variation of approximately 1.2 MeV in the triton binding energy with a fixed ¹S₀ interaction and a P_D varying from 4.5 to 6.5 %. Interactions with greater densities of matter at short range bind the triton more strongly and closer to the experimental value. An off-shell variation of 0.7 MeV is obtained with a fixed p_d and singlet interaction. However, a single measurement of the deuteron tensor polarization at about q² = 20 fm^?2 would severely restrict this variation.     

Work hardening of alloys with short range order of atoms

The study presents analysis of the experimental flow curves for polycrystals of fcc concentrated solid solutions Ni-Cr and Ni-Mn. It was demonstrated that the flow curves of these alloys can be divided into a number of stages of work hardening, and stage II bisects into two substages: with parabolic, and linear hardening. It was demonstrated that the shape of the curves of work hardening is determined by the type of dependence L = f(σ), where L is the average path length of the motion of the dislocation;σ is flow stress. Some possible critical stresses of transition from stage to stage are discussed.     

On the short range magnetic order of iron aboveT c

A derivation of the ordering part of the free energy functional based on an expansion about the free energy of a random CPA tight-binding paramagnetic effective medium is presented. Considering the dominant term of this functional, which is of Heisenberg form, the static magnetic correlation function is calculated within a spherical model approximation. For some parameters a behaviour characteristics for systems with tendency to form helicoidal ordered structure belowT _c is observed.     

Influence of spatial short range order on impure ising paramagnets

Magnetic susceptibility is calculated consistently to β² for an Ising paramagnet with arbitary concentrations of non-magnetic impurities. Both magnetic and non-magnetic interactions are explicitly considered, as are spatial and spin correlations. By fitting to a Curie-Weiss form, expressions for the Curie temperature and the Curie-Weiss constant are obtained. For small magnetic concentrations the Curie-Weiss constant can change sign due to the lattice interactions.     

Effect of vacancy charge state on positron annihilation in silicon

The charge-state-dependent lattice relaxation of mono-vacancy in silicon is studied using the first-principles pseu- dopotential plane-wave method. We observe that the structural relaxation for the first-neighbor atoms of the mono-vacancy is strongly dependent on its charge state. The difference in total electron density between with and without charge states in mono-vacancy and its relevant change due to the localized positron are also examined by means of first-principles simu- lation, demonstrating the strong interplay between positron and electron. Our calculations reveal that the positron lifetime decreases with absolute charge value increasing.