8. What is Diffusion The phenomenon of material transport by atomic motion. Diffusion couple Cu&Ni is heated to high temperature and cooled. Cu atoms migrated or diffused into Ni and Ni atoms into Cu. The process in which atoms of one metal migrate into another - Inter-diffusion or impurity diffusion Diffusion occurring in pure metals –atoms exchanging positions- called self diffusion
9. Mechanisms of diffusion Diffusion is a stepwise migration of atoms from lattice site to lattice site. For an atom to make such a move, There must be an empty adjacent site. The atom must have sufficient energy to break bonds with its neighboring atom. Vacancy diffusion Interchange of an atom with an adjacent vacant lattice site. (self- and inter-diffusion) Interstitial diffusion Migration of atoms from an interstitial position to a neighboring empty one.
11. Diffusion – time function? Steady-state and Non-steady-state diffusion processes are distinguished by the parameter – diffusion flux, J. Flux is defined as number of atoms crossing a unit area perpendicular to a given direction per unit time. Flux has units of atoms/m2.sec or moles/m2.sec. If the flux is independent of time, then the diffusion process is called steady-state diffusion. On the other hand, for non-steady-state diffusion process, flux is dependent on time.
15. Theory of alloys What is an alloy Types of alloys Solid solution Substitutional Interstitial Intermetallic compounds Hume – Rothery Rules Superalloys
16. Alloys An alloy is a phase comprising of one or more components. There are three distinguishable types: Substitutional: solute substitutes the solvent in the crystal lattice without structural changes. Interstitial: solute does not occupy the sites in the lattice of the solvent, but resides in crystallographic pores. Transformational: A completely new lattice is formed. Usually occurs as a result of intermetallic compound formation.
17. Substitutional Solid Solution In substitutional solid solution, the arrangement of the solute atoms may be disordered (random) or ordered. Some alloy systems exhibit complete solid solubility (e.g. Cu-Ni, Cd-Mg), others show only limited solubility at any temperature. Several factors determine the limits of solubility. These are expressed as a series of rules often called Hume-Rothery Rules.
19. Hume-Rothery Rules The Hume-Rothery rules are a set of basic rules describing the conditions under which an element could dissolve in a metal, forming a solid solution. There are two sets of rules, one which refers to substitutional solid solutions, and another which refers to interstitial solid solutions.
20. Hume-Rothery Rules Hume-Rothery Rule 1:Atomic Size Factor Rule. The atomic radii of the solute and solvent atoms must differ by no more than 15%: Extensive substitutional solid solution occurs only if the relative difference between the atomic diameters (radii) of the two species is less than 15%. If the difference >15%, the solubility is limited.
21. Hume-Rothery Rules Hume-Rothery Rule 2:Crystal Structure Rule . The crystal structures of solute and solvent must match. For appreciable solid solubility, the crystal structures of the two elements must be identical. Hume-Rothery Rule 3: Valency Rule . Maximum solubility occurs when the solvent and solute have the same valency. Metals with lower valency will tend to dissolve metals with higher valency. A metal will dissolve a metal of higher valency to a greater extent than one of lower valency.
22. Hume-Rothery Rules Hume-Rothery Rule 4: The Electronegativity Rule The solute and solvent should have similar electronegativity. If the electronegativity difference is too great, the metals will tend to form intermetallic compounds instead of solid solutions.
23. Interstitial solid solution Interstitial solid solutions are formed if a solute is smaller than the pores in the lattice of a solvent a solute has approximately the same electronegativity as a solvent. There are very few elements that create ions, small enough to fit in interstitial positions, therefore, appreciable solubility is rare for interstitial solid solutions. Ions that often may be a solute in solid solutions are: H, Li, Na, B.
24. Hume-Rothery Rules For interstitial solid solutions, the Hume-Rothery rules are: Hume-Rothery Rule 1: Solute atoms must be smaller than the pores in the solvent lattice. Hume-Rothery Rule 2: The solute and solvent should have similar electronegativity.
27. Superalloys Superalloys is a name for a group of alloys that retain high strength at elevated temperatures. The main strengthening mechanism is preventing grain boundaries from sliding via dislocation. Most of the superalloys are Substitutional solutions, where one of the components tends to form covalent bonds (Al in Ni3Al, W in Ta or Nb.)
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