Alloys of two true metals

ALLOYS OF TWO TRUE METALS The copper-gold system 

A typical example of an alloy of this class, and one which has been extensively investigated, is the system copper-gold. These two metals are chemically closely related, have a similar electronic configuration and have the same crystal structure. The atomic radii are not very different, being 1 28 and 1 44 A, respectively. 

When the system is studied at a high temperature, or, more conveniently, in the form of a quenched specimen, it is found that complete solid solution between copper and gold takes place at all compositions. 

If we start, say, with a specimen of pure gold and add to it progressively more and more copper the atoms of this latter element replace those of gold at random at the sites of the cubic face-centred cell, until ultimately the structure of pure copper results. The statistical replacement of gold atoms by the smaller atoms of copper occasions a slight reduction in the cell side, which varies nearly linearly with composition, but otherwise no alteration in the structure occurs. 

Another system which illustrates the transition from the random arrangement of the solid solution to the ordered structure of the superlattice is that of iron and aluminium. Strictly speaking, this system should be considered under our second group of alloys since we have classed aluminium as a B sub-group metal. We have already emphasized, however, that the position of aluminium is somewhat anomalous in that it also behaves in many respects as a true metal, and it is therefore not out of place to consider this system here. 

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On the basis of this division of the metallic elements into two main groups we may recognize binary alloys of the following three classes (1) Alloys of two true metals (2) alloys of a true metal and a B sub-group metal (3) alloys of two sub-group metals. 

The intermediate phases found in alloys of two true metals have a number of different structures, but many of these are common to more than one system. In addition to the phases already considered in the copper-gold and iron-aluminium systems we shall describe two, one because of its common occurrence and one because of its geometrical interest. 

These are obtained by starting with metal X and gradually replacing its atoms with those of Y. Two true metals yield a TT alloy. TB and BB alloys arc also possible. 

Similar structures usually consist of metals or semiconductors, as in the case of QDs. Although the synthesis of these multicomponent nanosized materials is based on techniques similar to those employed for monocomponent materials, important experimental changes should be adopted [34]. For example, in the case of bimetallic nano-objects, if the synthesis starts from metal salts, the order in which the components are reduced constitutes one of the most important synthetic variables. The simplest method consists of the contemporary reduction of two different metal salts. Alternatively, by performing the reduction of the two metals in two subsequent steps, the second reduction generates a coating on the surface of the first metal. Some authors [156,159] suggest that intimate contact between nano-objects of different composition, as in the bicomponent nanosized material system reported in Fig. 6.14d, may induce properties very similar to those of true alloys of the... 

Adiabatic calorimeters have also been used for direct-reaction calorimetry. Kubaschewski and Walter (1939) designed a calorimeter to study intermetallic compoimds up to 700°C. The procedure involved dropping compressed powders of two metals into the calorimeter and maintaining an equal temperature between the main calorimetric block and a surrounding jacket of refractory alloy. Any rise in temperature due to the reaction of the metal powders in the calorimeter was compensated by electrically heating the surrounding jacket so that its temperature remained the same as the calorimeter. The heat of reaction was then directly a function of the electrical energy needed to maintain the jacket at the same temperature as the calorimeter. One of the main problems with this calorimeter was the low thermal conductivity of the refractory alloy which meant that it was very difficult to maintain true adiabatic conditions. 

There were other, less theoretical but no less persuasive objections. Some substances, such as ammonium chloride, dissociate in the vapor phase. That is, a single particle of vapor turns into two or more particles. Two or more particles occupy two or more times the volume that one particle does. That wreaks havoc with measurements of gas volumes and provides empirical evidence that fails to obey Gay-Lussac s law, making apparent nonsense of Avogadro s hypothesis. It was not until the phenomenon of dissociation was understood, and interpreted in terms of reaction kinetics, that this objection could be countered. Similar objections were raised against Dalton s laws of combining proportions, which work only for compounds of fixed composition. Metallic alloys and salt solutions, to take two of the most obvious exceptions, do seem to share some of the characteristics of chemical compounds, but they do not fit Daltons laws. The simplest way to avoid that objection was to say that only those substances that did fit Dalton s laws were true chemical compounds, but that is a circular argument that did not convince critics. 

Silver-palladium powders which are now used in conductive products are of two different types. One type is co-precipitated powders, wherein silver and palladium are both precipitated from solution to form a powder in which the two metals are intimately associated but do not form a true alloy. Another, more expensive filler, involves first forming a true silver-palladium alloy, and then pulverizing this to a fine powder. 

The law (2) represents the behaviour of a system containing only two elementary processes controlled by the activation energy [21]. Its importance, however, is connected with the experimental observation that it satisfactorily describes the kinetics of a very large class of electrochemical systems. This is particularly true of metals and alloys of technological interest that are subject to uniform corrosion in a great number of aggressive environments. 

Binary Ag alloys are used extensively in the manufacture of thick film hybrids and electronic components. The most commonly used alloy is Ag-Pd. As illustrated in Fig. 8.3, Ag and Pd exhibit complete sohd solubility. These two metals can be prealloyed prior to preparation of the formulation or can be added as two distinct metallic components which alloy during the ensuing firing cycle. X-ray diffraction analysis " can be utilized to determine whether one has a true alloy prior to firing. Similarly, this technique can be utilized to determine if complete sohd solutions have been formed during the firing cycle. 

There are, however, two characteristics, ready oxidation at high temperatures and, in the case of molybdenum and tungsten, brittleness at low temperatures, which limit their applications. Of the refractory metals, tantalum has the widest use in the chemical process industries. Most applications involve acid solutions that cannot be handled with iron or nickel-base alloys. Tantalum, however, is not suitable for hot alkalis, sulfur trioxide, or fluorine. Hydrogen will readily be absorbed by tantalum to form a brittle hydride. This is also true of titanium and zirconium. Tantalum is often used as a cladding metal. 

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