Heterogeneous alloys

Alloys are metallic materials prepared by mixing two or more molten metals. They are used for many purposes, such as construction, and are central to the transportation and electronics industries. Some common alloys are listed in Table 5.5. In homogeneous alloys, atoms of the different elements are distributed uniformly. Examples include brass, bronze, and the coinage alloys. Heterogeneous alloys, such as tin-lead solder and the mercury amalgam sometimes used to fill teeth, consist of a mixture of crystalline phases with different compositions. 

Hess s law A reaction enthalpy is the sum of the enthalpies of any sequence of reactions (at the same temperature and pressure) into which the overall reaction can be divided, heterogenous alloy See alloy. heterogeneous catalyst See catalyst. heterogeneous equilibrium An equilibrium in which at least one substance is in a different phase from the others. Example AgCI(s) Ag+(aq) + Cl "(aq). heterogeneous mixture A mixture in which the... 

Homogeneous alloys have a single glass transition temperature which is determined by the ratio of the components. The physical properties of these alloys are averages based on the composition of the alloy. Heterogeneous alloys can be formed when graft or block copolymers are combined with a compatible polymer. Alloys of incompatible polymers can be formed if an interfacial agent can be found. 

Alloys can be divided into four categories substitutional alloys, interstitial alloys, heterogeneous alloys, and intermetallic compounds. Substitutional and interstitial alloys are both homogeneous mixtures in which components are dispersed randomly and uniformly (A FIGURE 12.15). (Section 1.2) Solids that form homogeneous mixtures are called solid solutions. When atoms of the solute in a solid solution occupy positions normally occupied by a solvent atom, we have a substitutional alloy. When the. solute atoms occupy interstitial positions in the holes between solvent atoms, we have an interstitial alloy (Figure 12.15). 

An element crystallizes in a body-centered cubic lattice. The edge of the unit cell is 2.86 A, and the density of the crystal is 7.92 g/cm. Calculate the atomic weight of the element. Define the term alloy. Distinguish among solid solution alloys, heterogeneous alloys, and intermetallic compounds. Distinguish between substitutional and interstitial alloys. What conditions favor formation of substitutional alloys ... 

Different kinds of alloys—solution alloys, heterogeneous alloys, and intermetallic compounds—are described. 

Alloys can be classified as solution alloys, heterogeneous alloys, and inter-metallic compounds. Solution alloys are homogeneous mixtures in which the components are dispersed randomly and uniformly. Atoms of the solute can take positions normally occupied by a solvent atom, thereby forming a substitutional alloy, or they can occupy interstitial positions, positions in the "holes" between the solvent atoms, fliereby forming an interstitial alloy. These types are diagrammed in Figure 23.18 . 

Chemical properties of metals and alloys (heterogeneities may — be submicroscopic microscopic, or macroscopic)... 

Additional experiments performed with SECM include current-distance measurements, whereby the steady-state tip current is measured as the tip is slowly moved from bulk solution toward the substrate in steps (at a fixed x-y position above the substrate), yielding a probe approach curve (RAC). Theory can be used to fit the experimentally observed PACs, permitting the extraction of rate constants for irreversible substrate kinetics (see Chapter 5). We are currently using this mode to determine electron transfer rates at A1 alloy heterogeneities, using COMSOL Multiphysics to model the PAC for a small active spot (a Cu-rich inclusion at which faster electron transfer occurs) surrounded by a less active area (the A1 alloy matrix) at which slower electron transfer occurs [11]. 

Surface heterogeneity may merely be a reflection of different types of chemisorption and chemisorption sites, as in the examples of Figs. XVIII-9 and XVIII-10. The presence of various crystal planes, as in powders, leads to heterogeneous adsorption behavior the effect may vary with particle size, as in the case of O2 on Pd [107]. Heterogeneity may be deliberate many catalysts consist of combinations of active surfaces, such as bimetallic alloys. In this last case, the surface properties may be intermediate between those of the pure metals (but one component may be in surface excess as with any solution) or they may be distinctly different. In this last case, one speaks of various effects ensemble, dilution, ligand, and kinetic (see Ref. 108 for details). 

XJsorption of gases on to transition metal surfaces is important, and transition metals or alloys are often used as heterogeneous catalysts. 

Heterogeneous alloys can be formed when graft or block copolymers are combined with a compatible polymer. Alloys of incompatible polymers can be formed if an interfacial agent can be found. 

An alloy is cooled from a temperature at which it has a single-phase structure (a) to a temperature at which the equilibrium structure is two-phase (a -i- ji). During cooling, small precipitates of the P phase nucleate heterogeneously at a grain boundaries. The nuclei are lens-shaped as shown below. 

The phase rule is a mathematical expression that describes the behavior of chemical systems in equilibrium. A chemical system is any combination of chemical substances. The substances exist as gas, liquid, or solid phases. The phase rule applies only to systems, called heterogeneous systems, in which two or more distinct phases are in equilibrium. A system cannot contain more than one gas phase, but can contain any number of liquid and solid phases. An alloy of copper and nickel, for example, contains two solid phases. The rule makes possible the simple correlation of very large quantities of physical data and limited prediction of the behavior of chemical systems. It is used particularly in alloy preparation, in chemical engineering, and in geology. 

Polymer Alloys A class of polymer blends, heterogeneous in nature with modified, controlled interfacial properties or morphology. 

Conditions necessary for the onset of corrosion are quite often provided by heterogeneities. These heterogeneities may very well exist within the metal or alloy or may be imposed by external factors. These heterogeneities can give rise to variations in potential on a metal surface immersed in an electrolytic fluid. The galvanic cell thus formed gives rise to flow of current that accompanies corrosion [188]. 

Figure 53.3 illustrates a pit in a stainless steel such as type 534 or 316 austenitic alloy. Pitting starts at heterogeneity in the steel surface, such as an outcropping sulfide inclusion, the shielded region beneath a deposit or even a discontinuity in the naturally present oxide film caused by a scratch or embedded particle of abrasive grit. This initiation phase of pitting corrosion may take seconds... 

Heterogeneities associated with a metal have been classified in Table 1.1 as atomic see Fig. 1.1), microscopic (visible under an optical microscope), and macroscopic, and their effects are considered in various sections of the present work. It is relevant to observe, however, that the detailed mechanism of all aspects of corrosion, e.g. the passage of a metallic cation from the lattice to the solution, specific effects of ions and species in solution in accelerating or inhibiting corrosion or causing stress-corrosion cracking, etc. must involve a consideration of the detailed atomic structure of the metal or alloy. 

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