Composition of alloys

Heterostructures and Superlattices. Although useful devices can be made from binary compound semiconductors, such as GaAs, InP, or InSb, the explosive interest in techniques such as MOCVD and MBE came about from their growth of ternary or quaternary alloy heterostmctures and supedattices. Eor the successful growth of alloys and heterostmctures the composition and interfaces must be accurately controlled. The composition of alloys can be predicted from thermodynamics if the flow in the reactor is optimised. Otherwise, composition and growth rate variations are observed... 

For research of alloys in the liquid state the chemical analysis can be applied only then, when high-temperature samples sharply to chill, that is to temper. But it can cause changes in the state of the system. That is why it is the best to determine composition of alloys in the liquid state. 

The production of emitting sources requires further development in material technology, as the emitting wavelength is controlled by the compositions of alloy semiconductors to form heterostructures such as InGaAs/InAs" " or III-V alloy systems. 

Fig. 7.48 Effect of composition of alloy and deposit on corrosion, shown as weight loss (gcm ) against plotted points. (Shaded area shows most attack, i.e. >0-5gcm )" ...

Table 7.34 Nominal composition of alloys (weight percent)...

The generally accepted theory of electric superconductivity of metals is based upon an assumed interaction between the conduction electrons and phonons in the crystal.1-3 The resonating-valence-bond theory, which is a theoiy of the electronic structure of metals developed about 20 years ago,4-6 provides the basis for a detailed description of the electron-phonon interaction, in relation to the atomic numbers of elements and the composition of alloys, and leads, as described below, to the conclusion that there are two classes of superconductors, crest superconductors and trough superconductors. 

Discuss the role of the surface free-energy in phenomena such as alloy segregation, surface reconstruction, faceting and sintering of small particles. How does a gas environment (air, vacuum) affect the surface composition of alloys ... 

Figure 4. Selectivity in isomerization, dehydrocycllzation and hydrogenolysis (cracking) of Pt/Cu alloys (on Si02). Bulk composition of alloys (% Pt) indicated. (Reproduced with permission from Ref.30. Chem.Soc.London)...

Sachtler WMH. 1973. Surface composition of alloys in equilibrium, Le Vide 164 67. 

Figure 32 is a graph showing the composition of alloys deposited onto copper substrates as a function of Ti2+ concentration and current density in 66.7 m/o AICI3-NaCl [177], Alloys were deposited under a range of current densities for several Ti2+ concentrations. At low Ti2+ concentrations, the alloy composition is dependent upon the applied current density. An alloy having a titanium concentration of 25 a/o is deposited only at low current densities. As the current density is increased, the Ti partial current density becomes limited by the diffusion of Ti2+, and the Ti content of the alloy drops. At a Ti2+ concentration of 150 mmol L 1. the current density... 

Atom probe microscopy is a variation of field ion microscopy in which either the field ionized atoms or evaporated atoms from the tip are detected with a mass spectrometer placed behind an aperture in the imaging screen. This allows one to identify the desorbing ions. If the tip is mounted on a manipulator, one can zoom in on a desired surface plane. The technique has, for example, been used to study the composition of alloy surfaces we refer to Tsong [32,33] for reviews. 

Asami, K. Hashimoto, K. Shimodaira, S. (1978) XPS determination of composition of alloy surfaces and surface oxides on mechanically polished iron chromium alloys. Corrosion Sci. 18 713-723... 

The Cu-Zn system (see Figure 2.7) displays a number of intermediate solid solutions that arise due to limited solubility between the two elements. For example, at low wt% Zn, which incidently is the composition of alloys known as brass, the relatively pure copper a phase is able to accommodate small amounts of Zn as an impurity in the crystal structure. This is known as a terminal solid phase, and the solubility limit where intermediate solid solutions (such as a + /S) begin to occur is called the solvus line. Some of the three-phase transformations that are found in this diagram include a peritectic (5 - - L -> e) and a eutectoid (5 -> y - - e). Remember that these three-phase transformations are defined for equilibrium coohng processes, not heating or nonequihbrium conditions. 

For the following pairs of alloys that are coupled in seawater, predict the possibility of corrosion and if corrosion is possible, note which alloy will corrode (a) Al/Mg (b) Zn/low carbon steel (c) brass/Monel (d) titanium/304 stainless steel and (e) cast iron/315 stainless steel. Clearly state any assumptions you make about compositions of alloys. 

This methodology has been applied for determining the relative composition of alloys [225], amalgams [226], and mixed crystals [227], among others [74-78], based on peak current measurements. The essential requisite is that both electroactive components behave independently—i.e., that the components of a mechanical mixture do not influence each other with respect to their thermodynamic activities in electrochemical reactions [77]. In Fig. 4.4, theoretical calibration plots for the absolute peak current, when the amount of mixture is constant for each measurement (left) and for the percentage peak current (right), are shown [77, 228]. 

Considerable progress achieved in the quantum theory of alloys and in the theory predicting the surface composition of alloys (see, e.g., 5 for a review). 

Development in the field of measurements on metals without carrier can now be considered as satisfying and there is still some progress going on. However, the problem of a reliable determination of the surface composition of alloys on carriers is still too far from being solved. In particular, problems like the detection of small amounts of unalloyed active metals on carrier and the question of homogeneity in distribution of the active metals in an inactive matrix have not been solved yet, and just such problems are most likely responsible for some controversies in the results on alloys. More work has to be done in the future in this field. 

Typical of the many thousands of items that are standardized in the field of engineering are limitations on the sizes and wall thicknesses of piping, specifications of the compositions of alloys, stipulation of the safety factors applied to strengths of construction materials, testing procedures for many kinds of materials, and so on. 

The ascertaining by classical and modern methods that the surface composition of alloys can strongly deviate from the composition of the bulk. These methods have opened up the possibility of correlating catalytic phenomena with catalyst surface composition, thus removing a major drawback of older studies with alloy catalysts. 

Other workers (165) used X-ray photoelectron spectroscopy (XPS) to examine the influence of ammonia oxidation on the surface composition of alloy gauzes. After several months on stream, the surface was covered by the same types of highly faceted structures noted by others. As illustrated in Fig. 14, XPS analysis provides evidence that the top microns, and in particular the top 100 A of the surface, were enriched in rhodium. This enrichment was attributed to the preferential volatilization of platinum oxide. The rhodium in the surface layers was present in the oxide form. Other probes confirm the enrichment of the surface in rhodium after ammonia oxidation (166). Rhodium enrichment has been noted by others (164, 167), and it has been postulated that in some cases it leads to catalyst deactivation (168). 

---