Effect Of Chemical Order Research Articles

Fe3Ni-type chemical order inFe65Ni35films grown by evaporation: Implications regarding the Invar problem

By use of Monte Carlo simulations of chemical ordering, magnetic ordering, and magnetovolume thermal effects and by reviewing the known effects of chemical order in the face-centered-cubic Fe-Ni alloy system, we show that the observations of Dumpich et al. [Phys. Rev. B 46, 9258 (1992)], who report unique Invar-composition ${\mathrm{Fe}}_{65}{\mathrm{Ni}}_{35}$ samples that exhibit Invar behavior despite being collinear ferromagnets with no deviation from the Slater-Pauling curve, are consistent with the effects of varying degrees of ${\mathrm{Fe}}_{3}\mathrm{Ni}$-type chemical order, which in turn are consistent with the sample preparation and treatment methods used. This allows us to make certain conclusive statements concerning models for Invar behavior and the nature of ${\mathrm{Fe}}_{3}\mathrm{Ni}.$

Vibrational thermodynamics: coupling of chemical order and sizeeffects

The effects of chemical order on the vibrational entropy have been studiedusing first-principles and semi-empirical potential methods. Pseudopotentialcalculations on the Pd3V system show that the vibrationalentropy decreases by 0.07kB upon disordering in the high-temperaturelimit. The decrease in entropy contradicts what would be expected from simplebonding arguments, but can be explained by the influence of size effects onthe vibrations. In addition, the embedded-atom method is used to study theeffects of local environments on the entropic contributions of individual Niand Al atoms in Ni3Al. It is found that increasing numbers ofAl nearest neighbours decreases the vibrational entropy of an atom whenrelaxations are not included. When the system is relaxed, this effectdisappears, and the local entropy is approximately uniform with increasingnumber of Al neighbours. These results are explained in terms of the largesize mismatch between Ni and Al. In addition, a local cluster expansion isused to show how the relaxations increase the importance of long-range andmultisite interactions.

Atomic Density versus Average Coordination Number in Ge–In–Se Glasses

The variation of the atomic density ? with the average coordination number Z for glasses belonging to the Ge x In 6 Se 94-x and Ge x In 12 Se 88-x families of the Ge-In-Se system is investigated. The ?-Z dependence displays broad maxima at Z values of 2.42 and 2.58 for families with 6 and 12 at% In, respectively. These maxima are ascribed to changes from a floppy to a rigid type network and from a two-dimensional layered structure to a three-dimensional network, respectively. The minima exhibited in the ?-Z dependence at Z = 2.63 for the In 6 family and at Z = 2.71 for the In 12 family, corresponding to the tie-line compositions, are attributed to chemical ordering effects.

Correlation of spin and orbital anisotropies with chemical order inFe0.5Pd0.5alloy films using magnetic circular x-ray dichroism

The effects of chemical ordering in thin film ${\mathrm{Fe}}_{0.5}{\mathrm{Pd}}_{0.5}$ alloys, deposited by molecular-beam epitaxy directly on a MgO (001) substrate, have been studied using magnetic circular x-ray dichroism (MCXD) at the Pd and Fe ${L}_{2,3}$ edges. This system exhibits perpendicular magnetic anisotropy if grown in the ${L1}_{0}$ (001) phase and in-plane anisotropy in the disordered $\ensuremath{\gamma}$ phase. We show that MCXD is a sensitive tool for studying variations in the spin and orbital magnetic moments due to changes in the chemical order. We obtain quantitative results for spin and orbital anisotropies and their variations with chemical order in the ${\mathrm{Fe}}_{0.5}{\mathrm{Pd}}_{0.5}$ alloy system.

Chemical Ordering and Microstructural Affects on Magnetic Properties of Sm2Fe17 and Sm2Fe17Nx

ABSTRACTThe effect of chemical order and defect structure on the magnetization process in Sm2Fe17 and Sm2Fe17Nx has been investigated. In Sm2Fe17, chemical disorder results in the development of random anisotropy and a small degree of magnetic hardness. The anisotropy is reduced as long-range order increases. The motion of domain walls in Sm2Fe17Nx is more dependent on the antiphase domain structure than on the amount of long-range order.

Stress anomaly in Al-rich Ti-Al single crystals deformed by the motion of 1/2«110] ordinary dislocations

The microstructure and plastic deformation behaviour of Al-rich Ti-Al single crystals containing 54.7 and 58.0 at.% Al have been examined, focusing on the effect of chemical ordering of a Al5Ti3 superstructure on anomalous strengthening. Fine precipitates with the Al5Ti3 superstructure were developed in the L10 matrix of the Ti-58.0 at.% Al alloy. The size and volume fraction of the precipitates varied depending on temperature. An anomalous increase in the yield stress of the two alloys appeared at around 800oC. This strengthening mechanism is discussed on the basis of the difference in antiphase-boundary energies on (111), (110) and (001) planes in the Al5Ti3 phase.

Mössbauer effect in atomic ordered and disordered Fe0.75−xMnxAl0.25 alloys

AbstractA Mössbauer effect study of the effect of chemical order on hyperfine magnetic field distributions in the quasi‐binary alloy system Fe0.75−xMnxAl0.25 (based on substitution of Fe by Mn in Fe3Al) shows that a preferential substitution of Mn at one of the sites leads to a B2 order in the ternary system. The kinetics of the ordering process in Fe0.45Mn0.3Al0.25 shows distinctly different relaxation behaviours at the two possible inequivalent sites in the ordered structure.

On the chemical threshold in chalcogenide glasses

The values of the glass-transition temperature ( T g ) and compactness ( C) of glasses in the Ge-As-Se, Ge-Sb-Se, Ge-Ga-Se and Ge-In-Se systems were examined in order to display the chemical threshold in these systems. The rationalization of data ( T g , C) was made by plotting T g and C values versus a quantity R which expresses the ratio of the bonding possibilities of chalcogen atoms to bonding possibilities of non-chalcogen atoms. In all other systems except for the Ge-As-Se system, the well pronounced chemical threshold was observed in T g ( R) and C ( R) dependences at R= 1, i.e. for the case of the existence of only heteropolar bonds in a given system. It is thus supposed that the chemical ordering effect is mainly responsible for the extreme values of T g and C, respectively, in these systems. On the other hand, no significant evidence of the chemical threshold was found in Ge-As-Se glasses in the T g ( R) or C ( R) plots, while the C(〈 CN 〉) plot indicated well pronounced Tanaka's threshold at the mean coordination number 〈 CN 〉 ≈ 2.7.

Volumetric effect of topology in chalcogenide glass systems

Topological models of glass structure predict features in the property versus the average coordination number, Z, plots at Z ⋍ 2.40 and at Z ⋍ 2.67 . Topological and chemical ordering effects coexist in chalcogenide network glasses and therefore features due to topological effects are not clearly seen in many systems. The volumetric effect, V t, of topological origin has been determined by subtracting from the measured mean atomic volume, V m, the volume, V c, calculated by taking into account the effects of chemical ordering. The V m versus Z and V t versus Z plots have been examined for several binary and ternary systems. While the V m versus Z plots of these systems have different appearances, their V t versus Z plots exhibit similar features, with a minimum in V t at Z ⋍ 2.40 and a local maximum in V t at Z ⋍ 2.67 .

Structural and magnetic studies on binary mixtures of molten noble metal halides

Neutron diffraction and magnetic susceptibility measurements have been carried out on molten cuprous halides. The structure factors of molten CuBr and CuI show a distinct prepeak caused by a chemical ordering effect. The structure of molten CuBr and CuI is interpreted in terms of local configurations similar to those in the solid phase. The magnetic susceptibilities of the molten CuCl x Br 1− x , CuCl x I 1− x and CuBr x I 1− x systems have been measured by using the Curie-Faraday method. The molten systems of CuCl-CuI and CuBr-CuI obey the Wiedemann rule, while the molten CuCl-CuBr system exhibits a large deviation.

Atomic structure of melt-spun amorphous Ni 56Dy 44 studied by neutron diffraction

The atomic structure of a melt-spun amorphous Ni 56Dy 44 alloy was studied by neutron diffraction using the isotopic substitution technique with nickel isotopes. The three Faber-Ziman partial structure factors as well as the Bhatia-Thornton partial structure factors together with the corresponding correlation functions were evaluated. The short-range order parameter, calculated from the partial coordination numbers, reflects a chemical ordering effect in amorphous Ni 56Dy 44.

Density andT g of glasses of the As-Al-Te system

Results of measurements of the density and the glass transition temperature (Tg) on 25 glass compositions of the As-Al-Te system are reported and discussed. A linear increase of density and a decrease of Tg are seen with increasing As content across the AsAlTe4-As2Te3 tie-line compositions. In the density-composition and mean atomic volume-composition dependences for the AsxAl5Te95-x? AsxAl8Te92-x and AsxAl12Te88-x glasses, a change in slope is seen at the respective tie-line compositions, indicating chemical ordering in these glasses. When he dependence of density on the average coordination number Z are considered, the effects of chemical ordering are obscured; instead, the effects of topological nature emerge, with changes in slope at the topological thresholds of Z ? 2.40 and Z ? 2.67. The results further indicate that the dependence of mean atomic volume v on Z is controlled by the Te content in these glasses. Due to this, the generally observed features in the dependence of v on Z for chalcogenide glasses, namely a minimum in v to Z ? 2.40, a peak in v for Z ? 2.67 and a decrease of v for Z amp;gt; 2.67, are suppressed.

Phase stability of artificial metallic superlattices: Chemical-order effect.

With the advance of atom by atom deposition techniques, such as magnetron sputtering and molecular-beam epitaxy, it has become possible to design and manufacture materials with composition modulations in one dimension on an atomic scale. A theoretical understanding of the thermodynamic phase stability of these layered materials is still lacking. Therefore, as a first step, the role of short-range interactions within the context of the first-principles electronic theory of phase stability of substitutional bulk alloys on the stability of superlattices and interfaces at $T=0$ K is examined.

Floppy to rigid transition and chemical ordering in Ge [sbnd]Sb(As) [sbnd]Se glasses

The dependence of mean atomic volume and the glass transition temperature on the average coordination number, lt;igt;Zlt;/igt;, have been examined for glasses of the Ge-Sb(As)-Se systems, in the light of recent models proposed for network glasses. The results indicate the occurrence of additional features at lt;igt;Zlt;/igt; values other than the topological thresholds of amp;sime;2.40 and amp;sime;2.67. These additional features are attributed to effects of chemical ordering. The occurrence of these features in addition to those due to the two topological effects, namely, the floppy to rigid transition and the structural transition, leads to the conclusion that the effects of the chemical ordering and the effects which have topological origin coexist in these glasses

Structures of Molten CuCl, CuBr and CuI

The structures of molten CuCl, CuBr and CuI have been measured at two temperatures, using the neutron diffraction. The structure factors of molten CuI and CuBr show a distinct prepeak at about 0.9 Å -1 . These peaks may be caused by a kind of chemical ordering effect. The information of partial structures are estimated from the total pair distribution functions and the total radial distribution functions using the Davidon-Fletcher-Powell method. In molten CuCl, the first peak positions of the partial distribution functions agreed with those obtained by the isotope enrichment method.

Sphere and distance models for binary disordered systems

Abstract First-neighbour geometrical models are presented and shown to provide a good approximation for the structural description of binary disordered systems with metallic, covalent or ionic bonding. Sphere mixture models taking into account atomic size and chemical order effects are first studied and analytical expressions for the partial coordination numbers are derived which only involve packing fraction, diameter ratio, concentration and chemical order parameter. Relations between the concentration thresholds separating different chemical order regimes and site percolation theory are established. Close-packed sphere mixtures are finally shown to be a good representation of liquid or glassy alloys. Hole sphere models are then introduced and shown to provide a satisfactory approximation to the structure of loose-packed covalent glasses of silicon and germanium. Finally, first-neighbour distance models are presented and the variations of their partial structure factors with non-additivity parameter are...

Free surface segregation in boron doped Ni3Al

Surface segregation in boron-doped Ni3Al alloys containing 24–26% Al has been studied for temperatures between 600 and 1000°C. In agreement with earlier studies, boron was not observed to segregate to free surfaces at 1000°C, while sulfur was observed to segregate strongly. Between 600 and 800°C, both boron and nitrogen, as well as sulfur, were observed to segregate to surfaces damaged by sputter ion etching. Boron and nitrogen did not segregate strongly to well annealed surfaces in this temperature range, however, indicating possible effects of chemical order, defect structure, and /or nickel enrichment on the extent of interfacial boron segregation.

Long-range order and short-range order in Pd3V: Breakdown of the mean-field theory.

The electronic structure calculations of pair potential interactions have been widely developed in the case of transition metal alloys [1]. However, few experimental data exist in this case. Among the possible systems we choose the Pd-V one, which is not magnetic [2] and shows little size effects (small disparity between Pd and V atomic size). In this way, we hope to isolate chemical order effects and then, to perform suitable experiments to compare with theoretical calculations. We present results on Pd3V which are quite complete. We performed diffuse scattering experiments of neutrons and X-rays which give rise to intensity maxima at positions (100). This indicates a local order different from that of the low temperature DO22 ordered phase, characterized by (1½0) diffraction spots. After comparing in detail both experiments, we show how this result is in contradiction with the usual mean field theory. However, the cluster variation method (octahedron-tetrahedron approximation) with first and second neighbor interactions can explain this effect. Introducing third and fourth neighbor interactions V3, V4 while keeping the tetrahedron-octahedron entropy expression, we propose a solution for the four first interactions in good overall agreement with experimental data.

Neutron and X-ray diffraction study of the Ti 84Si 16 metallic glass

The total structure factors of amorphous Ti 84Si 16 have been evaluated by X-ray diffraction as well as by neutron diffraction. From the combined X-ray and neutron data the TiTi and TiSi atomic pair correlation functions and the corresponding distances and coordination numbers have been derived. The results show that the short-range order is characterized by chemical bonding between the Ti and the Si atoms. On the basic assumption that in metal-metalloid glasses there is no metalloid-metalloid nearest-neighbour contact, the density-concentration correlation functions are discussed, and it is shown that besides the chemical ordering effect there is a marked size effect in amorphous Ti 84Ti 16. The results of this study are compared with those of other metal-metalloid glasses, and common features of this type of amorphous alloys are shown.

Magnetic polarization clouds, percolation, and the critical concentration for ferromagnetism

The magnetic properties of many alloys are described in terms of magnetic polarization clouds. This is true in particular of dilute alloys of magnetic atoms in strongly exchange-enhanced hosts and of concentrated but magnetically dilute alloys of Ni in nonmagnetic hosts. In such alloys the locally averaged magnetization density is observed by diffuse neutron scattering to fall off as a Yukawa function away from the center of a polarization cloud. We present numerical calculations which confirm this Yukawa form and present an approximate theoretical derivation which gives a formula for the inverse range κ of a polarization cloud in an alloy in terms of the uniform exchange-enhancement factor α and the band susceptibility Γ(q) for the alloy. Assuming ferromagnetism in such alloys to arise from percolation amongst the polarization clouds, we also derive a relationship between the critical concentration c*cr, of polarization clouds for the onset of ferromagnetism and the average magnetization μ*cr and inverse range κcr of those clouds. This relationship is found to be in good agreement with experiment. Approximate calculated values of c*cr, μ*, and κ are presented for two cases. The effects of chemical ordering and clustering are considered briefly.