Binary alloys characterization

We have already discussed a number of applications of the SEM to materials characterization topographical (SE) imaging, Energy-Dispersive X-Ray analysis (EDS) and the use of backscattering measurements to determine the composition of binary alloy systems. We now shall briefly discuss applications that are, in part, spe-... 

A particular configuration E of a semi-infinite disordered binary alloy Aj,Bi j, is characterized by a set of occupation indices where 7 = 1 if the site R is... 

Kroger FA (1978) Cathodic deposition and characterization of metallic or semiconducting binary alloys or compounds. J Electrochem Soc 125 2028-2034... 

Figure 5.26. Iron binary alloys. Examples of the effects produced by the addition of different metals on the stability of the yFe (cF4-Cu type) field are shown. In the Fe-Ge and Fe-Cr systems the 7 field forms a closed loop surrounded by the a-j two-phase field and, around it, by the a field. Notice in the Fe-Cr diagram a minimum in the a-7 transformation temperature. The iron-rich region of the Fe-Ru diagram shows a different behaviour the 7 field is bounded by several, mutually intersecting, two (and three) phase equilibria. The Fe-Ir alloys are characterized, in certain temperature ranges, by the formation of a continuous fee solid solution between Ir and yFe. Compare with Fig. 5.27 where an indication is given of the effects produced by the different elements of the Periodic Table on the stability and extension of the yFe field.

Owing to their numerous actual and potential applications, several ternary and complex systems of these metals, especially of aluminium, have been investigated a few examples of the systematics of Al-Me-X alloys are presented in 5.18 and in Fig. 5.41. Recent contributions to this subject have been given with the study of the systems R-Al-Cu (Riani et al. 2005, and references there in). These rare earth alloys, characterized by the formation of several intermediate phases, are interesting also as raw materials for the preparation of amorphous alloys. Regularities in the trends of their properties have been underlined. The experimental and calculated data relevant to the binary systems Al-Fe, Al-Ni and Fe-Ni have been examined and discussed in a paper concerning the assessment of the ternary Al-Fe-Ni system (Eleno et al. 2006). 

For example, consider a binary alloy in which the stress-free molar volume is a function of concentration, V(cs). The linear expansion due to the composition change can be inferred from diffraction experiments under stress-free conditions (Vegard s effect) and is characterized by Vegard s parameter, ac [e.g., in cubic or isotropic crystals e ° = e°y 0 = = ac(c — c0)]. The assumption of coherency... 

A more complicated but solvable problem is a definition of the order parameter for antiferromagnetics, binary alloys, superconductors etc. The dimensionless units T/Tc and 77/770 (Fig. 1.4) allow us to present the behaviour of the order parameter rj = r) T) in a form universal for many quite different systems. Moreover, in some cases even quantitative similarities hold which concerns in particular the value of the exponent (3. (The value of 77 = 0 characterizes always disordered phase.)... 

X-ray diffraction work showed the existence of two phases. The work function data suggest that the copper-rich alloy in the two-phase system is located at the surface and the nickel-rich phase below the surface. To check this, CO was admitted at a pressure of 10 8 Torr. The gas is strongly adsorbed on nickel, but not on copper at such low pressures. The work function of copper was not altered. The binary alloys showed a constant increase in work function between 0.04 and 0.11 eV. Therefore, the adsorbing surface belonged to the copper-rich phase. Chemisorption of H2 on Ni-Cu films (40) leads to essentially the same conclusions. At temperatures below the miscibility gap, several classes of alloy systems characterized by their concentration ranges can be distinguished (4c), as illustrated in Fig. 3 ... 

To demonstrate the viability of the synthesis and characterization approach in the present workflow, two binary Pt-Fe alloy libraries were designed, synthesized and characterized by XRD (Figs. 11.4 to 11.6) [19]. One library (Fig. 11.5) was characterized as synthesized, while the other (Fig. 11.6) was annealed at 400 °C for 12 h in a hydrogen/argon atmosphere. Pt-Fe is a well-known binary alloy system, exhibiting both substitutional solid solution compositional ranges and intermetal-lic compounds. 

Grgur, R.N. Markovic, N.M. Ross, P.N. Electro-oxidation of H2, CO and H2/CO mixtures on a well-characterized Pt-Re bulk alloy electrode and comparison with other Pt binary alloys. Electrochim. Acta 1999, 43 (24), 3631-3635. 

Fig. 12.4. Schematic illustration of conserved and nonconserved order parameters for characterizing the internal state of a binary alloy (adapted from Chen and Wang (1996)) (a) disordered phase (rj = 0) with uniform composition cq, (b) two-phase mixture consisting of disordered phases (rj = 0) with composition Cq, and c, (c) ordered single phase (ri = 1) of single composition ci with an antiphase domain boundary.

Fig. 12.22. Representation configurations from the phase field model of phase separation in a three-dimensional binary alloy (adapted from Sagui et al. (1998)). The progression from (a) through (g) represents different choices of the parameters characterizing the elastic response of the two media, with (a) corresponding to a treatment in which elastic effects are ignored. In those cases where there is more than one picture associated with a given letter, these represent snapshots at different times in the simulation.

Recent years there have been a considerable interest in studying of binary catalytic systems based on stabilized nanocomposites and amorphous alloys of copper with other metals. The reason is that the catalytic activity of such systems in many cases is sufficiently higher than that of individual metals. The most convenient model for theoretical description of binary systems characterized by the absence of far order is a cluster model. However, quantum-chemical study of binary clusters comprises the significantly more c omplicated problem than that o f individual metals, b ecause a correct theoretical description of metal-another metal cluster systems requires that the used method should be in a position to provide good results of calculations of geometrical, electron stmctures and energetic characteristics of both of individual metals. 

A particular configuration T of a semi-infinite disordered binary alloy Aj,Bi j is characterized by a set of occupation indices vr, where 7 = 1 if the site R is occupied by an atom of the type Q, and Tj = 0 o..ierwise. Configurational averaging of occupation indices ( 7r) = yields the local concentrations c. The energy of such a configuration can be expressed in the form of an effective Hamiltonian of Ising type... 

The subsurface composition change expected in a homogeneous binary alloy due to recoil implantation or knock-in is controlled by the maximum energy transfer possible and is characterized by the parameter... 

The synthetic approach developed for the synthesis of platinum-iron binary alloy nanopartides was subsequently adopted for the preparation of several other binary alloy nanopartides, such as FePd [90] or MnPt [91]. However, these alloys required further structural characterization, as well as further developments of the methods for controlling the partide size, shape, and composition. 

The nearly free electron theory developed by Faber and Ziman (1964) is an obvious starting point for discussing liquid alloys of type I. For those cases in which information is available about the three partial interference functions which characterize the structure of binary alloys, close quantitative agreement between theory and experiment has been obtained. We emphasize that a positive da/dT is entirely consistent with metaUic behaviour in Hquid alloys on account of the temperature dependence of the partial interference functions. For this reason many liquid alloys which have in the past been thought of in terms of a semiconducting framework should more properly be regarded as metallic. (It may, in certain cases, be necessary to introduce the Mott g factor but there is little evidence either way on this important point at the present time). Alloys of the second type will form the subject for section 7.7. 

Valence band spectroscopies (AES and XPS) are practically limited to binary alloys because of the complications encountered with an increase in the number of alloy constituents. In contrast with AES and XPS, which probe the occupied DOS of the sample, SXAPS measures the unoccupied conduction band DOS. An important aspect of SXAPS should be pointed out in the present context. Since the matrix element governing the core-hole creation involves the very short-range wave function of the initial core electron state, the technique is expected to reveal a localized DOS. Since the spectra of different constituents are well separated in energy, the application of SXAPS is by no means limited to binary alloys. The changes in SXAPS spectral features and shifts in BE which accompany alloy formation will better characterize the alloys. 

Let us consider another interpretation interdiffusion in a two-phase zone of a ternary system can be formally defined with the help of ordinary Pick s equation system with a 2 x 2 matrix of diffusion coefficients D,j. But here the coefficients are not independent [37] (it means that there is only one thermodynamic degree of freedom at fixed T and p). Here det Dy=0 or D = 0, that is, the matrix of interdiffusion coefficients is degenerated. It should be mentioned that the condition det D,j=0 (or D = 0 for the binary alloy) is inherent for the spinodal, which is also characterized by the stochastization and the final state indefinabihty. 

Among the most important parameters for materials characterization are the compositions of binary Gej Sij ) alloys, ternary Ai B C (e.g.,... 

By XPS spectra, Endo et al. [96] confirmed that formation of binary structure prevented Pd atoms from oxidation in the AuPd and PtPd bimetallic nanoparticles which exhibited higher catal5hic activity than monometallic ones. Wang et al. [112]. characterized PtCu bimetallic alloy nanoparticles Ijy XPS. XPS revealed that both elements in the nanoparticles are in zero-valence and possess the characteristic metallic binding energy. 

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