Metals solid-solution alloys

Some comments are in order regarding nomenclature First, for metallic alloys, solid solutions are commonly designated by lowercase Greek letters (a, p, y, etc.). With regard to phase boundaries, the line separating the L and a + L phase fields is termed the liq-uidus line, as indicated in Figure 9.3a the hquid phase is present at all temperatures and compositions above this line. The solidus line is located between the a and a + L regions, below which only the solid a phase exists. 

Now, suppose that we have a solid solution of two (2) elemental solids. Would the point defects be the same, or not An easy way to visualize such point defects is shown in the following diagram, given as 3.1.3. on the next page. It is well to note here that homogeneous lattices usually involve metals or solid solutions of metals (alloys) in contrast to heterogeneous lattices which involve compounds such as ZnS. 

High-carbon austenitic structures can be preserved at ambient temperatures if the iron is alloyed with sufficient nickel or manganese, since these metals form solid solutions with 7-Fe but not with a-Fe. If over 11% chromium is also present, we have a typical austenitic stainless steel. Such steels are corrosion resistant, nonmagnetic, and of satisfactory hardness, but, because the a-Fe 7-Fe transition is no longer possible, they cannot be hardened further by heat treatment. Figure 5.9 summarizes these observations. 

Nature of alloys. In view of the fact that alloys are mixtures, it follows that they may be made up of any desired number of metals and may have any desired composition. For simplicity, this discussion is limited largely to the consideration of alloys consisting of two metals (i.e., binary alloys). As a result of extensive study, it has been found that when two metals are melted together and the melt is allowed to cool and solidify, the resulting solid consists of one (or any combination) of the following (1) mixtures of pure crystals of the two component metals, (2) solid solutions, or (3) intermetallic compounds. In any event, the character of the solid alloy is dependent on the specific properties of the metals concerned and on the proportions in which they are mixed. 

The distinction between the two classes of materials considered in this Section per tains to the presence or absence of mixing at the molecular level. Thus in alloys, solid solutions of two or more semiconductors are formed where the lattice sites are interspersed with the alloy components. Semiconductor alloys, unlike their metallic counterparts, have a much more recent history and their development driver has been mainly optoelectronic (e.g., solid state laser) applications. In mixed semiconductor composites, on the other hand, the semiconductor particles are in electronic contact but the composite components do not undergo mixing at the molecular level. 

Silver and gold do not alloy with molybdenum platinum can take at least 16 per cent, of the metal into solid solution near the melting-point, but on cooling the molybdenum separates out. ... 

Steels are homogeneous alloys (solid solutions) made from pig iron. First, pig iron is purified to lower the carbon content and to remove other impurities. Then, appropriate metals are added to form the desired steel. 

The effect of rhenium on the properties of refractory metals has been a research topic of discontinuous relevance since its discovery in the 1950 s [1, 2], Several works have been devoted to the mechanical property enhancement seen in VIb group transition metals when solid solution alloyed with rhenium [3-12], In the 1970 s the works of... 

Enhanced diffusion has been found in Si. It is featured in alloys. Atomic motion in most metals and solid-solution alloys usually occurs by the interchange between atoms and neighboring vacant sites. For such a diffusion process, the atomic diffusion coefficient is given by... 

Iron, nickel, cobalt, and their alloys are the most studied metals for the catalytic growth of CNFs or CNTs. The readiness of these metals to produce metal-carbon solid solutions and to form metastable carbides in the appropriate reaction temperature range should be an important factor to take into account for the comprehension of their reactivity. The different carbon species formed depending on the temperature range employed in the steam reforming of hydrocarbons on nickel catalysts have been discussed [29] and consist of ... 

Here M is a metal, a solid solution alloy or an intermetallic compound, MHj is the hydride and s the molar ratio of hydrogen to metal, H/M. The hydrides frequently show large deviations from stoichiometry. The reaction is in most cases exothermic and reversible, i.e. dihydrogen is recovered by application of heat. The elements which form solid binary metal hydrides are shown in Figure 1. 

In the weld metal, the solid solution of the two alloys contained 2.5 to 2.7% Cu and the eutectics contained 22 to 27% Cu. The distribution of Mn and Fe and Si additions in the solid solution was the same as the HAZ. In the eutectic interlayers of the weld (arrows 10 and 20), the Fe and Si concentration was one order higher than its average content in the alloy. In the melting and subsequent solidification of the weld metal, the Fe and Si additions were distributed more uniformly than in the HAZ, due to their larger quantity and to the dispersion of the secondary phases. Nevertheless, near the fusion line and in the HAZ, isolated eutectic clusters occurred (arrows 9 and 18), but contained less Fe and Si than the interlayers in the central part of the weld. 

Many studies carried out with bimetallic materials show that Pt-Ru is today one of the best options to oxidize methanol. Thus, a simple way to prepare active Pt-Ru catalysts involves the deposition of metallic nanoparticles from a suspention onto the carbon microparticles by the method known as formic acid method [24]. Considering that the crystal structures of Pt and Ru are different, Pt being fee and Ru hep, the final crystal structure of the alloy depends on the composition. For Ru atomic fractions up to 0.6-0.7, the two metals form solid solutions in which Ru atoms replace Pt lattice points in the fee structure. The opposite situation, Pt atoms replacing Ru atoms in the hep structure is found for Ru atomic fractions higher than about 0.7. However, the crystal structure seems to depend also on the physical state of the material. When nanoparticles are prepared by reduction of ionic metal species, there is at least one report [25] claiming that the fee stmeture prevails up to 80 at.%Ru. On the other hand in sputtered films the hep structure is predominant even at low Ru fractions [26]. 

Fig. 14. Solution formation by the triva-lent rare earth metals. Extensive solid solutions means that 5 at.% or more of R in M or M in R is found in most of the R-M (or R-R ) systems when M (or R ) is shaded, where R is a second rare earth element in an intra rare earth binary alloy system. For M = Ga there is some evidenee that gallium dissolves extensively in the high-temperature bee form of the rare earth metals.

Concerning miscibility between the metal constituents of an alloy, all types of alloys could be obtained by electrodeposition eutectic-type alloys, solid solution-type alloys, alloys with intermediate phases, and/or intermetallic compounds [1]. According to Krastev and Dobrovolska [40], self-organization phenomena during the electrodeposition of alloys, resulting in pattern and spatiotemporal structure... 

Dental-filling alloy Solid Solution of a liquid (Hg) in a sohd (Ag plus other metals)... 

Figure 19.3 Electroplating. An electrolytic ceU is used to add a coating of a relatively expensive metal to a less expensive base metal. The base metal for the Oscar statues shown here is britta-nium, an alloy (solid solution) of tin, copper, and antimony. They are then sequentially electroplated with copper, nickel, silver, and finally, 24-karat gold. 

The constant A contains many terms including b, the frequency factor, the entropy term, etc. Experimentally, the exponent on stress is observed for pure metals and solid solution alloys to be in the range 3-8. There is not a general explanation for the variation in n to the author s knowledge, and discussion of this subject is beyond the scope of the present chapter. The creep equation is generalized allowing the exponent on stress to be a variable giving the well-known Dom equation... 

Metals A and B form an alloy or solid solution. To take a hypothetical case, suppose that the structure is simple cubic, so that each interior atom has six nearest neighbors and each surface atom has five. A particular alloy has a bulk mole fraction XA = 0.50, the side of the unit cell is 4.0 A, and the energies of vaporization Ea and Eb are 30 and 35 kcal/mol for the respective pure metals. The A—A bond energy is aa and the B—B bond energy is bb assume that ab = j( aa + bb)- Calculate the surface energy as a function of surface composition. What should the surface composition be at 0 K In what direction should it change on heaf)pg, and why ... 

Another method by which metals can be protected from corrosion is called alloying. An alloy is a multicomponent solid solution whose physical and chemical properties can be tailored by varying the alloy composition. 

The tables in this section contain values of the enthalpy and Gibbs energy of formation, entropy, and heat capacity at 298.15 K (25°C). No values are given in these tables for metal alloys or other solid solutions, for fused salts, or for substances of undefined chemical composition. 

In the examples given below, the physical effects are described of an order-disorder transformation which does not change the overall composition, the separation of an inter-metallic compound from a solid solution the range of which decreases as the temperature decreases, and die separation of an alloy into two phases by spinodal decomposition. 

As you can see from the tables in Chapter 1, few metals are used in their pure state -they nearly always have other elements added to them which turn them into alloys and give them better mechanical properties. The alloying elements will always dissolve in the basic metal to form solid solutions, although the solubility can vary between <0.01% and 100% depending on the combinations of elements we choose. As examples, the iron in a carbon steel can only dissolve 0.007% carbon at room temperature the copper in brass can dissolve more than 30% zinc and the copper-nickel system - the basis of the monels and the cupronickels - has complete solid solubility. 

A phase is a region of material that has uniform physical and chemical properties. Phases are often given Greek symbols, like a or fi. But when a phase consists of a solid solution of an alloying element in a host metal, a clearer symbol can be used. As an example, the phases in the lead-tin system may be symbolised as (Pb) - for the solution of tin in lead, and (Sn) - for the solution of lead in tin. 

Fig. 17.1. (a) Dislocation motion is intrinsically easy in pure metals - though alloying to give solid solutions or precipitates con moke it more difficult. (b) Dislocation motion in covalent solids is intrinsically difficult because the interatomic bonds must be broken and reformed. ( ) Dislocation motion in ionic crystals is easy on some planes, but hard on others. The hard systems usually dominate. 

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