Interstitial and Substitutional Alloys

4 Key excerpts on "Interstitial and Substitutional Alloys"

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  eBook - PDF

  The Study of Metal Structures and Their Mechanical Properties

  Pergamon Unified Engineering Series

  • W. A. Wood, William F. Hughes, Arthur T. Murphy, William H. Davenport(Authors)
  • 2014(Publication Date)
  • Pergamon

    (Publisher)

  CHAPTER FOUR Alloys and Dispersions 4.1 Solid Solution A solid with crystal structure A may dissolve foreign atoms (ions) B by one or both of the processes depicted by Fig. 4.1(a), (b). In (a), interstitial solid-solution, B atoms lodge between atoms of the host structure A. In (b), substitutional solid solution, the B atoms replace A atoms. In both, the B atoms may locally expand, contract, or distort the A structure but they do not upset it. The result is a crystal structure A with variable composition and lattice spacings. (a) (b) * o ß · · A O B · Figure 4.1. (a) Interstitial solid solution and (b) substitutional solid solution. The A structure may be complex, like a zeolite silicate where whole water molecules can enter interstitially into tunnels between its atoms. It may be covalent, like graphite where interstitial ions can lodge between the carbon layers, affecting the way one layer slips over another and thus their lubricating quality. Or it may be ionic, like the silver halides Ag(I/Br) of photographic emulsions where iodine can replace bromine ions in an NaCl-type structure and form substitutional solutions or mixed crystals with various photographic responses. But the structure which above all lends itself to solid solution is the metallic because of its tolerant non-directional bonds. Solid solution then becomes alloying. 59 The Study of Metal Structures and their Properties In the following survey of basic principles in alloying we can without much loss first take as host structure A a simple metal like copper or iron, while allowing B to be metallic or nonmetallic. 4.2 Interstitial Alloying This must depend primarily on two factors, as follows. (i) Ion Size The size of the B atoms or ions must be of the same order as the gaps they are to occupy in A. Since A is a metal, with ions packed closely like spheres, it can provide small gaps only.

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  eBook - ePub

  An Introduction to Metallurgy, Second Edition

  • Sir Alan Cottrell(Author)
  • 2019(Publication Date)
  • CRC Press

    (Publisher)

  § 4.6 ). This type of bond is largely indifferent both to the precise proportions of the component atoms and also to their precise distribution in the crystalline array of atomic sites. Random solutions over wide ranges of composition are thus possible.

  Solid solutions can be either substitutional or interstitial. As Fig. 6.2(a) shows, in a substitutional solution the atoms share a single common array of atomic sites. In interstitial solutions the atoms of one component are small enough to fit into the interstitial spaces between those of the other, which are themselves arranged in a complete crystalline array (Fig. 14.1 ).

  The equilibrium distribution of atoms in a substitutional solution generally depends on temperature. Various distributions are possible, ranging from the fully random state, which is most nearly obtained at high temperatures, to clustered (like neighbours preferred) and ordered (unlike neighbours preferred) states at lower temperatures. Short-range order, in which there is some tendency for unlike atoms to be neighbours but there is no long-range correlation in the distribution of atoms amongst the array of atomic sites, occurs fairly commonly. In addition, some alloys undergo a transformation below a critical ordering temperature to an ordered state in which a superlattice is formed by the regular alternation of unlike atoms through the entire crystal, or at least through large regions of it. A perfect superlattice is of course possible only at a critical and simple proportion of atoms, e.g. 1 to 1, or 3 to 1. Systems which form superlattices at such ratios also show imperfect superlattipes or long-range partial order at neighbouring compositions. Superlattices are found in both primary and secondary solutions. Some typical ones are shown in Fig. 14.2 .

  Fig. 14.1 An interstitial solid solution

  Fig. 14.2

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  eBook - ePub

  Steels: Microstructure and Properties

  • H.K.D.H. Bhadeshia, R.W.K. Honeycombe, Harry Bhadeshia, Robert Honeycombe(Authors)
  • 2017(Publication Date)
  • Butterworth-Heinemann

    (Publisher)

  Chapter 4

  Solutes that Substitute for Iron

  Abstract

  If the wealth of structures available in the binary Fe-C is impressive, the addition of substitutional solutes creates a breathtaking variety of phases and structures, a seemingly endless palette that makes is feasible to custom design alloys of iron. The substitutional solutes influence the thermodynamics of all transformations, but have the most profound effect when they are required to diffuse. While all this is terribly useful, the complexity of the theory necessary to deal with multicomponent steels also increases, ameliorated by the availability of modern computer programs and databases. The understanding necessary to deal with such complexity is introduced so that the reader can make an intelligent use of the mathematical models that are implemented in the software.

  Keywords

  Substitutional alloying; Austenite and ferrite stabilisers; Carbides and nitrides; Alloy pearlite; Interphase precipitation; Ledge mechanism

  4.1 General principles

  The term alloying elements in the context of steels is often used to denote substitutional solutes, which can and do dramatically influence the structure and properties of steels. Indeed, they are responsible for the incredible versatility and utility of steels. The general effects of substitutional solutes are summarised in Fig. 4.1 and will be discussed in context throughout this Chapter. The solutes affect the relative free energies of the relevant phases, a thermodynamic effect that applies to all of the phase transformations irrespective of the detailed atomic mechanisms of transformation. New phases frequently appear, for example ϵ

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  eBook - PDF

  Modern Physical Metallurgy

  • R. E. Smallman(Author)
  • 2016(Publication Date)
  • Butterworth-Heinemann

    (Publisher)

  Chapter 5 The structure of alloys 5.1 Introduction When a metal Β is alloyed to a metal A several different structures and atomic arrangements may be obtained in the alloy, depending upon the relative amounts of the component metals and upon the temperature of the alloy. Thus, if the two types of atoms behave as if they were similar and become homogeneously dispersed amongst each other, a solid solution of the type shown in Figure 1.8(a) will be formed. However, in only a few alloy systems does the solid solution exist over the entire composition range from pure A to pure B, one example being the copper-nickel system. More usually, the second element enters into solid solution only to a hmited extent and, in this case, a primary solid solution is formed which has the same crystal structure as the parent metal (see for example the copper-zinc system. Figure 3.5(a). Then at higher concentrations of the second element, new phases, generally termed intermediate phases, are formed in which the crystal structure usually differs from that of the parent metals. These intermediate phases are also called secondary solid solutions if they exist over wide ranges of composition, or intermetallic compounds if the range of homogeneity is smah. 5.2 Primary substitutional solid solutions As a result of a comparison of the solubilities of various solute elements in the noble metals, copper, silver and gold, several general rules* governing the extent of the primary solid solutions have been formulated. Extension of these experimental observations to solvents from other groups such as magnesium and iron show that, in general, these rules form a useful basis for predicting alloying behaviour. In brief the rules are as follows: (1) The atomic size factor —If the atomic diameter of the solute atom differs by more than 15 per cent from that of the solvent atom the extent of the primary solid solution is small.

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