Presence Of Short-range Order Research Articles

Correlation effects in gas absorption by binary ordered alloys

The equilibrium concentration of a gas dissolved in a binary ordered alloy with bcc lattice in the presence of short-range order in both the host system and in the system of host and impurity atoms is computed. It is shown that the correlation in filling the lattice sites by atoms of the alloy is manifested in the solubility only because of the volume effect associated with the dependence of the host and impurity atom interaction energy on the lattice parameter. In alloys in which the solubility increases in the disordered state with rise in temperature, a solubility extremum can be observed in the area of the order-disorder transition point because of the change in the degree of long-range order upon the introduction of the third component. If the transition into the ordered state is accompanied by the growth of absorption, a maximum appears in the temperature dependence of the solubility, otherwise a minimum. Singularities in the solubility associated with the gas not being ideal are examined.

Static structure factor of an electron liquid

It is shown that short range correlations brought into the dielectric function of Tripathy and Mandal through their local field correction, following a decoupling procedure which has the merit of conserving the frequency moments to an infinite order, leads to a peak in the static structure factor, S(K), of an electron liquid at k ≈ 2 k F for r s ⩾ 2. This peak in S(k) indicates that the electrons in metals possess a strongly correlated liquid-like behaviour. This corresponds to an incipient Bragg peak due to the presence of short range order in the electron liquid. Appearance of further peaks in our S(k) at low densities indicates the formation of a Wigner solid.

Effect of short-range order on thermo-emf of binary alloys

The thermo-emf of binary disordered alloys is calculated, taking into account correlations in the distribution of atoms of different kinds over the lattice. It is shown that the presence of short-range order in the alloys may lead to a nonlinear temperature dependence of the thermo-emf and cause a change in its sign.

Magnetism and resistivity of some dilute rare earth-zinc compounds

Measurements have been made from 2K to 300K of the magnetic susceptibilities of polycrystalline specimens of six R2Zn17 and of four RZn12 compounds whose resistivities have also been obtained, where R represents a rare earth. Most of the compounds order antiferromagnetically. The resistivities due to electron-magnon scattering go as T1.38 for GdZn12, T1.45 for TbZn12, T5.9 for SmZn12 and T6.5 for ErZn12, as a function of temperature T. The presence of short range order has been detected far above the Neel points of the two gadolinium compounds.

Short range order and the metal-insulator transition in disordered systems

The effect of short range order on the electronic density of states, the conductivity, and localization is calculated for a model binary alloy in the simple cubic configuration using a cluster extension of CPA. In the presence of short range order a metal insulator transition takes place as the concentration passes through a critical range.

Effect of short-range order on the band-edge behavior of the electronic spectrum of disordered alloys

The electronic spectrum of a one-dimentional disordered alloy was calculated both analytically and numerically for energies close to a band edge. The density of states is shown to be strongly dependent on the presence of short-range order in the form of correlations between the occupancies of neighboring sites.

Plastic deformation of Ni-Cr single crystals

Single crystals of nickel containing up to 21.5 at pct Cr have been deformed in tension over the temperature interval 78 to 623°K. The critical resolved shear stress of alloys containing up to 9 at. pct Cr in the region of the athermal plateau can be fitted into aC1/2 andC2/3 relationship whereC is the concentration of chromium. The observed magnitude of hardening, however is larger than that predicted by the theory of Fleischer or that of Labusch. Rapid solution hardening in alloys having 9 to 21.5 pct Cr arises due to the presence of short range order.

Critical analysis of the 'generalized coherent wave approximation'

The formalism developed by Fletcher (1967) to take account of the presence of short range order in the calculation of the electronic energy spectrum of amorphous covalent semiconductors is examined critically and found to have fundamental difficulties.

Energy gaps in amorphous covalent semiconductors

A calculation of approximate density of states for a disordered covalent semiconductor shows that the energy gap is due to the presence of short range order.

Etude radiocristallographique des transformations d'ordre de type αdésordre<->Iordredans les alliages ternaires or cuivre nickel et l'alliage binaire de composition remarquable AuCu3

The study of the scattering outside the Bragg reflexions shows no local order in the ordered phase I of alloys such as Au (50 at. %) Cu (46 at. %) Ni (4 at. %) even for the smallest degree of long range order. The modulation of the scattering appears only with the presence of short range order in the disordered α phase. There is evidence of a two-phase region on the equilibrium diagram at the AuCu3 stoichiometric composition.

Theory of the Relaxation of the Uniaxial Anisotropy Induced by Ionic Migration in Iron-Cobalt Ferrites

The relaxation character of the induced uniaxial anisotropy in Co x Fe 3-x O 4 has been analyzed theoretically, employing several models and taking into account the presence of short range ordering and the detailed micro processes of vacancy migration mechanism. It is shown on mathematical grounds that the relaxation times of the processes are inversely proportional to the density of the cation vacancies, ρ, in the crystal. The accurate solution of the relaxation time can be derived specifically for the range of x =10 -(1.5∼2) . It is \begin{aligned} 1/\tau{=}4.65{\cdot}\rho{\cdot}\bar{f}\exp(-\bar{Q}/kT). \end{aligned} The experimental results by Inoue are well described by this equation after putting \bar f =7.5×10 12 sec -1 , and \bar Q =1.0 eV at x =0.063. A theory is presented which takes into account the symmetry of the crystal lattice rigorously. The theory is in excellent agreement with the single crystal results by Palmer. It is pointed out that the cobalt ion neighboring a vacancy is in a trivalent state, and that electron transfer is important in lowering the activation energy of the ionic migration.

Stress-corrosion susceptibility and the dislocation arrangements of austenitic stainless steels

The dislocation arrangements in 304 stainless steel and Incoloy 800 have been determined as a function of temperature and strain. Dislocation pile-ups were observed in both alloys in spite of the fact that the stacking fault energies were considerably different, viz. < 16 erg/cm 2 for the 304 stainless steel and > 30 erg/cm 2 for the Incoloy 800. The stress-corrosion behaviour of the two alloys in magnesium chloride boiling at 142°C was determined. The Incoloy 800 was found to be considerably more resistant than the 304 stainless steel. After critically reviewing the current theories of stress corrosion and structural and stress-corrosion data for a series of austenitic alloys, it was concluded that the data could best be correlated with the theory proposed by Swann. According to this theory, the presence of short range order is expected to markedly influence dislocation arrangement and stress-corrosion behaviour. A review of the literature revealed that there is some indirect evidence to indicate the presence of short range order in certain of the austenitic alloys, but the evidence is not conclusive.

The effect of heating rate and neutron irradiation on the specific heat behavior of a Cu-16.8 at. % A1 and A Cu-17.2 AT. % Ga solid solution

The effect of heating rate on the specific heat behavior of binary solid solutions of copper containing 16.8 at.% Al and 17.2 at.% Ga in the range 40–700°C has been investigated. Also the effect of prior neutron irradiation on the spécifie heat behavior of the Cu-Al alloy was studied. The specific heats were measured with a dynamic adiabatic calorimeter. An anomalous behavior in the specific heat is observed in both solid solutions which is explained in a consistent manner based on the presence of short-range order. The initial slow cooling at approximately 1 2 °C/ min from above about 330°C leaves the alloys prior to reheating with short-range order present. Upon reheating the kinetics of disordering are such that disordering does not begin until about 200°C. The specific heat then shows an anomalous rise of about 7 per cent, then decreases slightly around 300°C. From about 300°C to 700°C the heating rates employed ( 1 2−7 °C/ min ) are low enough to allow instantaneous equilibrium to be established. The specific heats remain anomalously high at least to 700°C. The specific heat curves in the region from 200 to 300°C shift to higher temperatures with increased heating rate. Neutron irradiation at 100°C after slow cooling increases the amount of short-range order present and also introduces excess vacancies. The additional short-range order causes an increase in the specific heat curve as compared to just slow cooling, since more order is destroyed. Due mainly to the excess vacancies the anomalous behavior is observed at lower temperatures than for just slow cooling.

The Change in Colour of a Silver-Gold Alloy on Plastic Deformation

The colour change when deformed samples of the alloy AgAu are annealed has been confInned and an attempt has been made to measure it in terms of the movement of the spectral absorption edge. It is found that on annealing the absorption edge of a heavily deformed sample moves about 5�4 mp, toward shorter wavelengths. No such effect was observed in the pure metals, silver and gold. It is suggested that the effect may be connected with the presence of short-range order and/or with a migration of silver atoms to the surface in the annealing process.