Impurity Atoms Alloying

Most metals are used as alloys, and there are far too many of these for a comprehensive review here. A few examples will be considered, however, starting with one of the most simple of cases Ag-Au (Gilman, 2005). 

Silver and gold form a simple alloy system because they have nearly the same atomic diameters 2.89 and 2.88 Angstroms, respectively. Both have f.c.c. crystal structures, and both come from the same column of the Periodic Table so they are isoelectronic. The two metals are mutually soluble with a heat of mixing, AUm = -48meV/atom. The molecular volume, Vm = 8.5 x 10 24cm3, so the heat of mixing density, AUm/Vm is 90.4 x 108ergs/cm3. 

Measurements of the yield stresses of various alloys in this system were made by Sachs and Weerts (1930). These values can be converted into hardness numbers by multiplying by three, and to shear stresses by dividing by two. The general expression for the Au concentration is c (1 - c), where c is the concentration for each alloy. The stress needed to disrupt a Ag-Au pair is about AUm/Vm, and there is a maximum of these pairs when the concentration, c of Au is Vi At this maximum the hardness, H, becomes a maximum  

The hardnesses of the pure metals are approximately equal, so their average is taken to be H0. Then an expression for the hardness of an alloy may be written  

When normal sites in a crystal structure are replaced by impurity atoms, or vacancies, or interstitial atoms, the local electronic structure is disturbed and local electronic states are introduced. Now when a dislocation kink moves into such a site, its energy changes, not by a minute amount but by some significant amount. The resistance to further motion is best described as an increase in the local viscosity coefficient, remembering that plastic deformation is time dependent. A viscosity coefficient, q relates a rate d8/dt with a stress, x  

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Ion implantation (qv) has a large (10 K/s) effective quench rate (64). This surface treatment technique allows a wide variety of atomic species to be introduced into the surface. Sputtering and evaporation methods are other very slow approaches to making amorphous films, atom by atom. The processes involve deposition of a vapor onto a cold substrate. The buildup rate (20 p.m/h) is also sensitive to deposition conditions, including the presence of impurity atoms which can faciUtate the formation of an amorphous stmcture. An approach used for metal—metalloid amorphous alloys is chemical deposition and electro deposition. 

The hardness and strength of alloys can be explained in terms of bonding. The impurity atoms added may form localized and rigid bonds. These tend to prevent the slippage of atoms past each other, which results in a loss of malleability and an increase in hardness. 

These problems have of course different weights for the different metals. The high reactivity of the elements on the left-side of the Periodic Table is well-known. On this subject, relevant examples based on rare earth metals and their alloys and compounds are given in a paper by Gschneidner (1993) Metals, alloys and compounds high purities do make a difference The influence of impurity atoms, especially the interstitial elements, on some of the properties of pure rare earth metals and the stabilization of non-equilibrium structures of the metals are there discussed. The effects of impurities on intermetallic and non-metallic R compounds are also considered, including the composition and structure of line compounds, the nominal vs. true composition of a sample and/or of an intermediate phase, the stabilization of non-existent binary phases which correspond to real new ternary phases, etc. A few examples taken from the above-mentioned paper and reported here are especially relevant. They may be useful to highlight typical problems met in preparative intermetallic chemistry. 

The simple cubic structme, sometimes called the rock salt structure because it is the structme of rock salt (NaCl), is not a close-packed structure (see Figure 1.20). In fact, it contains about 48% void space and as a result, it is not a very dense structure. The large space in the center of the SC structme is called an interstitial site, which is a vacant position between atoms that can be occupied by a small impurity atom or alloying element. In this case, the interstitial site is surrounded by eight atoms. All eight atoms in SC me equivalent and me located at the intersection of eight adjacent unit cells, so that there me 8 x (1/8) = 1 total atoms in the SC unit cell. Notice that... 

The second type of impurity, substitution of a lattice atom with an impurity atom, allows us to enter the world of alloys and intermetallics. Let us diverge slightly for a moment to discuss how control of substitutional impurities can lead to some useful materials, and then we will conclude our description of point defects. An alloy, by definition, is a metallic solid or liquid formed from an intimate combination of two or more elements. By intimate combination, we mean either a liquid or solid solution. In the instance where the solid is crystalline, some of the impurity atoms, usually defined as the minority constituent, occupy sites in the lattice that would normally be occupied by the majority constituent. Alloys need not be crystalline, however. If a liquid alloy is quenched rapidly enough, an amorphous metal can result. The solid material is still an alloy, since the elements are in intimate combination, but there is no crystalline order and hence no substitutional impurities. To aid in our description of substitutional impurities, we will limit the current description to crystalline alloys, but keep in mind that amorphous alloys exist as well. 

Many theories of adatom-adatom interactions and other interactions are based on a model Hamiltonian developed by Anderson for dilute alloys. Such a Hamiltonian may also be used to explain adatom-impurity atom interactions. 

Despite a possible wide significance of such a topic, there is only one reported study of adatom-substitutional impurity atom interaction, where the interaction of a W adatom with substitutional Re atoms in a W lattice is studied by using a W-3% Re alloy as the substrate.182 The planes used in FIM studies of adatom behavior are usually quite small containing only a few hundred atoms. Thus a plane of a W-3% Re alloy is likely to contain a few Re substitutional atoms. The perturbation to the overall electronic and elastic properties of the substrate lattice should still be relatively small. Therefore the interaction of a single substitutional impurity atom with a diffusing adatom can be investigated. 

From Eqs. (21) and (22) it follows that the atomic order q decreases the solubility of the interstitial impurity in alloys with the examined structure,... 

The appearance impurity atom C in alloy increases its configurational energy Ec by the value p (chemical potential) and energy ET of thermal fluctuations by the value v (phonon potential). We shall examine the distribution of C atoms in potential field, determined by energy uy - p - v, where Uy is the energy of C atom in interstitial site ij. Numbers i, j indicate the number of A atoms in the sites nearest to interstitial site correspondingly of first and second type. 

An impurity atom in a solid induces a variation in the potential acting on the host conduction electrons, which they screen by oscillations in their density. Friedel introduced such oscillations with wave vector 2kp to calculate the conductivity of dilute metallic alloys [10]. In addition to the pronounced effect on the relaxation time of conduction electrons, Friedel oscillations may also be a source of mutual interactions between impurity atoms through the fact that the binding energy of one such atom in the solid depends on the electron density into which it is embedded, and this quantity oscillates around another impurity atom. Lau and Kohn predicted such interactions to depend on distance as cos(2A pr)/r5 [11]. We note that for isotropic Fermi surfaces there is a single kp-value, whereas in the general case one has to insert the Fermi vector pointing into the direction of the interaction [12,13]. The electronic interactions are oscillatory, and their 1 /r5-decay is steeper than the monotonic 1 /r3-decay of elastic interactions [14]. Therefore elastic interactions between bulk impurities dominate the electronic ones from relatively short distances on. 

For the detection of dislocations by electroetching as well as by chemical etching, it is frequently necessary to "decorate " the dislocations by means of one or more impurities in the base metal. The impurity atoms interact with the dislocations. This idea was first put forward by Wyon and Laeombe (11) in the case of the Al. The same approach has also been extended to Fe (12), Si-Fe alloys (13), Zn (14), Cu (15), and others. 

Substitution of one atom for another is a common phenomenon. These mixtures are also called solid solutions. For example, nickel and copper atoms have similar sizes and electronegativities and the same fee crystal structures. Mixtures of the two are stable in any proportion, with random arrangement of the atoms in the alloys. Other combinations that can work well have a very small atom in a lattice of larger atoms. In this case, the small atom occupies one of the interstices in the larger lattice, with small effects on the rest of the lattice but potentially large effects on behavior of the mixture. If the impurity atoms are larger than the holes, lattice strains result and a new solid phase may be formed. 

On the other hand, if a TM metal atom wi h formal ionic valence of more than 4, e.g., V, Cr, Mn, Fe, Co, Ni or Cu from the first row TM atoms, is an intentionally added impurity or alloy atom, and if this atom is resident on a group IV atom site that is fully-bonded to O, then additional occupied d-states can either be incorporated into the otherwise forbidden band gap between the occupied valence band states, and the empty conduction band states of the group IVB host and give rise to excited bound resonance states within the vacuum continuum. Additionally, if the TM d-states are more than half-occupied for a relevant ionic state, then occupied d-states associated with occupancy beyond five d-states sometimes drop into the valence band and are therefore present as bound state resonances [1]. The same description applies to 4f states in the lanthanide rare earth series. At the beginning of the series, there are occupied 4f states above the valence band edge. Later en the series, beyond Gd, a portion of these states drop into the valence band. By the time the third row of transition atoms begins, for example for Hf, the occupied 4f states are below the valence band. 

The majority of the initial Au work has centred on the gold alloys and gold impurity atoms in metals. Interpretation of the data has proved difficult, and for this reason only the more important features are summarised here. 

An early analysis of the large chemical isomer shifts (Table 16.8) found for Pt impurity atoms in Fe, Co, and Ni confirmed that 8R/R is positive [101]. The shift of Au in nickel alloys at 4-2 K decrease linearly by a total of... 

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