Pressure Dependences of Elastic Constants of AMg6 Aluminum–Magnesium Alloy and n-AMg6/С60 Nanocomposite Alloy

The ultrasonic study results for dependence of the elastic wave velocities and second-order elasticity coefficients of the polycrystalline aluminum alloy AMg6 and its nanocomposite n-AMg6/C₆₀ on hydrostatic pressure up to 1.6 GPa have been described. The ultrasonic research has been carried out using a highpressure ultrasonic piezometer based on the piston-cylinder device. The pressure derivatives of the secondorder elastic constants of these materials established in the present study have been compared with the results of the third-order elastic constants measurements of the test alloys using the Thurston–Brugger method. Involving available literature data, we determined the relationships between the pressure derivatives of the second-order elastic constants of the AMg6 alloy and the Mg-content and nanostructuring.

     

Elastic constants of bismuth polycrystal containing tin,lead, antimony and tellurium impurities

Investigations of ultrasonic velocity and internal friction are carried out in bismuth polycrystals containing Sn, Pb, Sb and Te impurities in the concentration range of 0–1 atomic percent using the composite oscillator technique, and elastic constants are estimated from velocity and density data. The variations of elastic constants are interpreted in terms of changes of lattice parameter, valency of the impurity, the electron-atom ratio, atomic size and Fermi energy. It is observed that the elastic constants are less affected in Bi-Te alloys compared with Bi-Sb, Bi-Pb and Bi-Sn alloys in the above concentration range.     

Lattice specific heat of copper based dilute alloys at low temperature

The low-temperature lattice specific heat of copper-based dilute alloys $\text{Cu}$ Sn, $\text{Cu}$ Ga, $\text{Cu}$ Ge, $\text{Cu}$ Zn and $\text{Cu}$ Ni has been studied theoretically on the basis of Green's function theory. A nearest-neighbour impurity model, with certral and non-central force constant changes has been employed. The effect of volume change is taken into account, and is seen to be important. The calculated results are in excellent agreement with experimental measurements, and the derived force constants are seen to be reasonable.     

Electronic structure and ground state parameters of Ru<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript><Emphasis Type="Italic">Me</Emphasis><Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Al refractory alloys

The electronic structure and ground state parameters of B2 RuAl-based refractory alloys have been investigated in the framework of the density functional theory using the exact muffin-tin orbital method in combination with the coherent potential approximation. It has been demonstrated that the number of states at the Fermi level for the Ru_{1 − x} Me _x Al alloys as a function of the alloying metal content has a minimum, which indicates a change in the Fermi surface topology and the presence of specific features in the behavior of elastic constants. It has been concluded that the electronic structure of the alloys can be described in terms of the rigid band model. The nonlinear variations of the lattice parameters of the alloys has been explained.     

Overcoming limitations in semiconductor alloy design

The phenomenon of giant band gap ‘bowing’ recently observed in several III–V dilute nitride alloys is promising for increasing the flexibility in choice of semiconductor band gaps available for specified lattice constants. However, the poor electrical transport properties that these materials exhibit seriously limit their usefulness. It is proposed that these materials behave as heavily nitrogen-doped semiconductors rather than dilute nitride alloys and that the abnormal or irregular alloy behavior is associated with impurity band formation that manifests itself in the giant bowing and poor transport properties. The potential for regularizing the alloy behavior using isoelectronic co-doping is discussed.     

The Hall coefficient and other transport properties of dilute alloys of copper and silver

The low field Hall coefficient of a number of polycrystalline foils of dilute (2%) alloys of copper and silver has been measured in the temperature range 1.5–50°K, and at room temperature. The alloys chosen wereCu-Au andAg-Au (uncharged impurity),Cu Ge andAg-Sn (charged impurity), andCu-Ni andAg-Pd (transition metal impurity). At 20°K and below, the Hall coefficients of the different copper alloys differ widely from each other,Cu-Ge giving the highest (negative) values (up to twice the room temperature value for pure copper), andCu-Au the lowest (down to 0.7 of this value). There are also significant concentration dependences. The silver alloys show corresponding but smaller changes. A relationship, due to Tsuji, gives the Hall coefficient as a function of the Fermi velocityν and the mean curvature 1/ϱ of the Fermi surface, for the case of an isotropic relaxation time. The integrals over the Fermi surface have been numerically estimated using the known Fermi surface and electron velocities. For both Cu and Ag the results agree with the experimental room temperature values, which we take as evidence thatτ(k) for phonon scattering is here close to isotropic. On the other hand, to account for the Hall coefficients of the alloys, it is necessary to assume that the relaxation timeτ varies over the Fermi surface. It is seen that in Cu and Ag the neck regions contribute relatively little toR since both 1/ϱ andν are small there. The main change inR in different alloys arises from the variation in the relative weighting given to the belly regions by different kinds of impurity scattering. A closer analysis shows that the bulges in the Fermi surface of copper in the 〈100〉 directions contribute relatively heavily because of their high positive curvature. The anisotropy ofτ deduced from the Hall coefficient is compared with that deduced from other measurements.     

Estimation of the recoil-free fraction of a57Fe atom present as a dilute impurity in copper,gold and palladium

The temperature variation of the recoil-free fraction of the Mössbauer⁵⁷Fe atom present as a dilute impurity in metallic hosts of various masses has been estimated by employing the double-time Green function technique at the high temperature limit. The changes in the force constants due to the⁵⁷Fe atom present as a substitutional impurity in Cu, Au and Pd have been evaluated from the available calculated values of hostfrequency moments and effective host-impurity force constant ratios using neutron dispersion data to estimate theoretically the recoil-free fraction for the resonant nucleus.     

Atomic displacements in bcc dilute alloys

We present here a systematic investigation of the atomic displacements in bcc transition metal (TM) dilute alloys. We have calculated the atomic displacements in bcc (V, Cr, Fe, Nb, Mo, Ta and W) transition metals (TMs) due to 3d, 4d and 5d TMs at the substitutional site using the Kanzaki lattice static method. Wills and Harrison interatomic potential is used to calculate the atomic force constants, the dynamical matrix and the impurity-induced forces. We have thoroughly investigated the atomic displacements using impurities from 3d, 4d and 5d series in the same host metal and the same impurity in different hosts. We have observed a systematic pattern in the atomic displacements for Cr-, Fe-, Nb-, Mo-, Ta-and W-based dilute alloys. The atomic displacements are found to increase with increase in the number of d electrons for all alloys considered except for V dilute alloys. The 3d impurities are found to be more easily dissolved in the 3d host metals than 4d or 5d TMs whereas 4d and 5d impurities show more solubility in 4d and 5d TMs. In general, the relaxation energy calculation suggests that impurities may be easily solvable in 5d TM hosts when compared to 3d or 4d TMs.      

Ab initio structural parameters and transition pressures of GaN x As1− x dilute alloys

The results are presented of an ab initio total energy study of the high-pressure phases of GaN _x As_{1? x} dilute alloys calculated within the full-potential linearized augmented plane wave method in the generalized gradient approximation. Three candidate structures, namely zinc-blende, rocksalt and CsCl, have been considered. In each case, the structural parameters and transition pressures of dilute alloys of interest are calculated. In agreement with earlier calculations of Stenuit and Fahy [Phys. Rev. B 76 (2007) 035201], the deviation of the lattice constants from Vegard's law for zinc-blende GaN _x As_{1? x} alloys is found to be negligible.     

Negative resistivity anomaly of unstable 4f impurities in metals

The minimum of the resistivity of dilute alloys with unstable Ce and Pr impurities is usually attributed to an abnormal increase of the spin scattering cross section of the inner 4f electron at low temperatures (4f Kondo effect). By comparison with the resistivity increments of stable rare earth impurities, we show that nearT=0 the anomalous increment is in fact consistent with simple potential scattering from the outer 5d6s valence electrons of fractional valent impurities. The observed decrease of the increment at elevated temperatures is too large to be due to 4f spin scattering in the unitarity limit or to a valence change. This decrease therefore implies some kind of shunting of the impurity potential scattering by an unknown mechanism connected with the valence instability of the impurity.     

Martensitic transformation of TiNiPd high-temperature shape memory alloys: A first-principles study

Heat of formation, elastic property and electronic structure of TiNiPd high-temperature shape memory alloys have been investigated by first-principles calculations using the pseudopotentials plane-wave method. The results show that the heat of formation difference between austenite and martensite plays an important role in the martensitic transformation. The effect of Pd content on the martensitic transformation temperature and transformation type is clarified based on the elastic constants of the B2 phases. High martensitic transformation temperature can be attributed to a low shear resistance C′. Furthermore, the mechanism of the effect of Pd addition on elastic constants is explained on the basis of the electronic structure.     

Kondo effect and heavy fermions in Yb compounds

The Kondo properties of Yb dilute alloys and intermetallics have been investigated using Mössbauer spectroscopy on¹⁷⁰Yb. In the dilute alloysAuYb andLaBe Yb, the Kondo logarithmic anomaly of the impurity relaxation rate has been detected, and in the concentrated Yb compounds YbBe₁₃, YbP and YbAs, and YbCuAl, the manifestations of the interplay between the Kondo effect and the magnetic ordering due to the RKKY interaction have been characterized.     

Concentration dependences of the elastic constants of the two-dimensional graphene-silicene system

The concentration dependences of the elastic constants of the two-dimensional Si _x C_{1 − x} system have been investigated with the use of the Harrison bonding-orbital method and the Keating model. The central and non-central force constants and the Grüneisen parameter have been considered by means of the bonding-orbital method. All quantities under consideration have been shown to exhibit a nonlinear behavior during the transition from graphene to silicene. A nontrivial role of the short-range repulsion has been discussed. The second-order and third-order elastic constants, the pressure dependences of the second-order elastic constants, as well as the Poisson’s ratio and Young’s modulus have been investigated in the Keating model. It has been found that the elastic constants and Young’s modulus change almost linearly upon the transition from graphene to silicene, whereas the other quantities under consideration exhibit nonlinearity.     

Magnetic susceptibility of (La,Ce) Al2 alloys

The low-field magnetic susceptibility of (La, Ce)Al₂ alloys with 1–20 at-% Ce was measured between 0.04 and 4 K. Up to 1.5 at-% Ce the impurity contribution to the susceptibility exhibits features which can be described in terms of a combined influence of the crystalline electric field and the Kondo effect. At very low temperatures the onset of interactions between the Ce impurities is indicated. The impurity coupling interactions determine the dependence on temperature and concentration of the more concentrated alloys. For the dilute alloys the impurity magnetization was determined from measurements of the susceptibility in magnetic fields up to 10 kOe. The magnetization as a function of temperature and field shows a typical anomaly which has been observed also in other Kondo systems.     

Elastic constants of lead-bismuth alloys

A study of ultrasonic velocities and internal friction has been carried out in Pb-Bi alloys in the concentration range of 0 to 49.5 atomic % Bi using the composite oscillator technique. From the velocity and density data a set of elastic constants namely, Young’s modulus, rigidity modulus, bulk modulus and Poisson’s ratio are estimated. The results are interpreted in terms of the phase changes occurring in the alloy system. Internal friction is found to be more sensitive than the elastic constants to the phase changes.     

Model calculations of anisotropic electron lifetimes due to dilute substitutional impurities in molybdenum

The de Haas-van Alphen technique forms a sensitive probe of the electronic structure of dilute alloys because of its dependence on the seattering rate in the forward direction. We report the results of a first principle model calculation of the scattering rate of dilute impurities in molybdenum which focuses on the anisotropy of the host wave function on the Fermi surface. This simplified model treats the impurity potential as an atomic potential screened by a Thomas-Fermi function. Substitutional impurities in bcc molybdenum have been studied using the Greens function (KKR) wave functions. The band structure calculated via the KKR method was fitted to a Fourier series representation in order to accurately determine a sufficiently large number of states on the Fermi surface. The KKR wavefunctions were used to calculate scattering rates for the substitutional impurity since the impurity potential can be best described in an angular momentum representation which is inherent to the Greens function method. Our detailed results, to be presented, suggest additional experiment to be done.     

Lattice constants and anisotropic microstrain at low temperature in 242Pu–Ga alloys

We attempted to characterize by neutron powder diffraction the monoclinic α′ phase that is known to form at low temperatures in dilute Pu–Ga alloys. This attempt was unsuccessful, as we did not detect any transformation to the α′ phase, but instead observed a line-broadening effect in the fcc?δ?phase. This effect is large enough to be visible in the raw diffraction data and is highly anisotropic in crystal space. The onset temperature of the line broadening (150?K) coincides with previous observations of the δ–α′ transformation. Bulk α′ was not observed. We believe that the development of α′ nuclei creates a spatially inhomogeneous stress distribution in the?δ?matrix, which in turn exhibits an anisotropic response, governed by its elastic anisotropy. We have analysed this observation of anisotropic microstrains in terms of the fictive microstresses required to produce them by elastic deformation. During the course of this work, we found a pseudo-isotope effect in the room temperature lattice constants of Pu–Ga alloys. The alloys made from nominal ²⁴²Pu isotope show systematically higher lattice constants than the corresponding ²³⁹Pu alloys, and the size of the effect is proportional to the Ga concentration. We believe that this effect is associated with the higher levels of radiation damage from isotopic impurities in the ²⁴²Pu alloys.     

The variation of elastic constants of nickel-iron single crystal alloys from 78.76 TO 300 K

The temperature dependences of the three independent adiabatic elastic stiffness coefficients c₁₁, C₁₂ and C₄₄ of four Ni-Fe single crystal alloys with compositions from 50 to 90 wt.% of Ni have been studied by the ultrasonic pulse superposition technique from 78.76 to 300 K. The elastic constants of the alloys exhibit normal temperature dependencies in the range of temperature investigated. A good fit of the experimental data to Lakkad's phenomenological model for the temperature dependence of elastic constants was obtained.     

稀磁合金的杂质互作用效应

本文用格林函数方法讨论Tk(Kondo温度)时杂质互作用对Kondo效应的影响。对s-d互作用哈密顿量作自洽场近似时,同时计入导致Kondo效应和产生杂质互作用的切断项,求得了杂质系统的Kondo温度和低温电阻。结果表明:在稀磁合金中,杂质间的互作用效应使Kondo温度下降,并且使T《T_k时的电阻率温度变化曲线由(1-AT²)型变为(1+BT²)型(A,B>0),从而可能在Tk温区产生电阻极大。 关键词：