Phase diagrams, bulk alloys

P. Braun. Surf. Sci. 126,714,1983. VEELS study of bulk and surface plasmon energies across Al—Mg alloy phase diagram. 

If neither the AC nor the BC component exhibits in any part of its (zero pressure) (x, T) phase diagram the structure a, which though exists in their solid solution, then the latter is of Type III . In this case, the alloy environment stabilizes a structure which is fundamentally new to at least one of its components. Such alloy-stabilized phases with no counterpart in the phase diagram of the constituent components can be formed in bulk equilibrium growth and may be distinguished from the unusual alloy phases that are known to form in extreme non-equilibrium growth methods and in epitaxial forms. 

In order to examine the possible relationship between the bulk thermodynamics of binary transition metal-aluminum alloys and their tendency to form at underpotentials, the room-temperature free energies of several such alloys were calculated as a function of composition using the CALPHAD (CALculation of PHAse Diagrams) method [85]. The Gibbs energy of a particular phase, G, was calculated by using Eq. (14),... 

We will only briefly discuss the surface enrichment model proposed for the Ni-Cu alloy, since it has been reviewed several times elsewhere (38, 39). The phase diagram of this alloy has a miscibility gap. Figures 1 and 2 show the results of two experiments, which demonstrate composition differences between bulk and surface in Ni-Cu alloys (4a, 4b). The alloys are films deposited under ultrahigh vacuum. After sintering the binary systems all have nearly the same work function, despite the fact that the overall Cu Ni ratio in the copper-rich system is about four times that of the metal-rich system. 

The divergence between the elemental composition at the topmost layer and that in the bulk is appreciated best when the atom-percent composition of Pt (or Co) at the alloy surface is plotted as a function of the monolayer-percent composition of Pt (or Co) in the bulk. Such a plot, which represents the phase diagram of the outermost-layer Pt-Co alloy, is shown in Fig. 7 the open circles are data for when Ptwas deposited initially, whereas the... 

A very good initial guess at the structure of a surface alloy may actually be obtained from the bulk phase diagram for the deposited element-substrate system. This is so, simply because, if there are no specific surface effects, the observed structures would have to be those found in the bulk phase diagram. Since the concentration of the deposited element should be considered small (it is actually "almost" zero, but in the case of local equilibria only the substrate atoms close to the surface may participate in the alloy formation, and thus the "effective" concentration of the deposited element could be quite high), the surface alloy will usually have the structure of the first ordered phase in the... 

Let us consider the deposition of A1 on a (110) surface of Ni. According to the bulk phase diagram, the addition of A1 to Ni in the limit T=0K must lead to the formation of NisAl in pure Ni. Therefore, the surface alloy formed during such a deposition may have a structure which corresponds to NiaAKllO). NisAl has the LI2 structure, and therefore two different truncations are possible for the (110) surface as shown in Fig. 10 The ordered phase can be truncated either by a layer of pure Ni or by an ordered p(2xl)-NiAl layer, which alternate in the [110] direction of ordered NisAl. 

A system which exhibits a behavior somewhat different from Al-Ni is Pd-Cu. The first ordered phase in the Cu-rich region of the Cu-Pd bulk phase diagram [47] is Lh-CusPd, and therefore it is not a surprise that the growth of Pd on Cu(l 10) leads to the formation of surface alloy with the corresponding bulk ordered structure [21]. However, in contrast to the growth of A1 on Ni(l 10), Pd does not segregate to the (110) surface of Cu. This can be seen in Fig. 12 where the first-principles results for different surface alloys of are presented [21]. 

From a theoretical standpoint, the traditional approach for the determination of an alloy structure implies, in principle, a search through any possible configuration until the most energetically favorable is found. While current first-principles methods, coupled with a substantial increase in computational power, have made this approach a standard practice for the calculation of phase diagrams of (mostly binary) bulk alloys, the complexity of surfaces makes quantum approximate methods a necessary tool to supplement the existing techniques and the growing body of experimental data. 

Studies of small particles by Sinfelt [52] have shown that when the particles size becomes very small and when virtually every atom is at the surface, alloy systems display phase diagrams very different from those that characterise the bulk systems. 

The authors have described some fundamental procedures to evaluate the solid-liquid phase relations of nano-sized particles in binary alloys from the calculation of the surface properties as well as the phase equilibira on the basis of thermodynamic databases, which are usually used for the calculation of phase diagrams of the bulk materials. In order to obtain quantitatively precise values of the melting points and liquidus temperatures in alloys, we should carry out further investigation as follows ... 

Having identified Cu as a potential additive to Ag for ethylene epoxidation catalysis, it is useful to examine the properties of Cu in Ag-Cu alloys more closely. The phase diagram of bulk Ag-Cu alloys shows that at almost all compositions, mixtures of Ag and Cu will phase separate into an alloy that is very rich in Ag and an alloy that is very rich in Cu.73 At 200°C, the Ag-rich phase is 99 at.% Ag and the Cu-rich phase contains <1 at.% Ag. This... 

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