Chapter 5 - Atom and Ion Movements in Materials.pdf
- 1. 1 The Science and Engineering of Materials, 4th ed Donald R. Askeland Pradeep P. Phul辿 Chapter 5 Atom and Ion Movements in Materials
- 2. 2 Chapter 5 Outline 5.1 Applications of Diffusion 5.2 Stability of Atoms and Ions 5.3 Mechanisms for Diffusion 5.4 Activation Energy for Diffusion 5.5 Factors Affecting Diffusion 5.6 Diffusion and Materials Processing
- 3. 3 Section 5.1 Applications of Diffusion Carburization for Surface Hardening of Steels - If we want a surface, such as the teeth of a gear, to be hard; however, we do not want the entire gear to be hard.
- 4. 4 Creation of Plastic Beverage Bottles - The occurrence of diffusion may not always be beneficial. Using polyethylene terephthalate (PET) to make bottles ensures that the carbonated beverages they contain will not lose their fizz for a reasonable period of time! Oxidation of Aluminum - Aluminum oxidizes (rusts) more easily than iron; however, the aluminum oxide forms a very protective but thin coating on the aluminums surface preventing any further diffusion of oxygen and hindering further oxidation of the underlying aluminum. The oxide coating does not have a color and is thin and, hence, invisible. Conductive Ceramics - Diffusion of ions plays an important role in the electrical conductivity of many conductive ceramics.
- 5. 5 Optical Fibers and Microelectronic Components - Optical fibers made from silica are coated with polymeric materials to prevent diffusion of water molecules. This, in turn, improves the optical and mechanical properties of the fibers.
- 6. 6 Example 5.1 Diffusion of Ar/He and Cu/Ni Consider a box containing an impermeable partition that divides the box into equal volumes. On one side, we have pure argon (Ar) gas; on the other side, we have pure helium (He) gas. Explain what will happen when the partition is opened? What will happen if we replace the Ar side with a Cu single crystal and the He side with a Ni single crystal? Illustration for Diffusion of Ar/He and Cu/Ni .
- 7. 7 Example 5.1 SOLUTION Before the partition is opened, one compartment has no argon and the other has no helium (i.e., there is a concentration gradient of Ar and He). When the partition is opened, Ar atoms will diffuse toward the He side, and vice versa. This diffusion will continue until the entire box has a uniform concentration of both gases. If we open the hypothetical partition between the Ni and Cu single crystals at room temperature, we would find that, similar to the Ar/He situation, the concentration gradients exist but the temperature is too low to see any significant diffusion of Cu atoms into Ni single crystal and vice-versa.
- 8. 8 Section 5.2 Stability of Atoms and Ions Arrhenius equation - The rate of atom or ion movement: c0: a constant R: the gas constant T: the absolute temperature (K) Q: the activation energy (cal/mol) required to cause Avogadros number of atoms or ions to move. Activation energy - The energy required to cause a particular reaction to occur.
- 9. 9 Section 5.3 Mechanisms for Diffusion Self-diffusion - The random movement of atoms within an essentially pure material. Interdiffusion - Diffusion of different atoms in opposite directions. Interdiffusion may eventually produce an equilibrium concentration of atoms within the material.
- 10. 10 Diffusion of copper atoms into nickel. Eventually, the copper atoms are randomly distributed throughout the nickel.
- 11. 11 Vacancy (substitutional) diffusion - Diffusion of atoms when an atom leaves a regular lattice position to fill a vacancy in the crystal. Interstitial diffusion - Diffusion of small atoms from one interstitial position to another in the crystal structure. Mechanisms for Diffusion
- 12. 12 Section 5.4 Activation Energy for Diffusion Diffusion couple - A combination of elements involved in diffusion studies. The activation energy Q is required to squeeze atoms past one another during diffusion. Generally more energy is required for a substitutional atom than for an interstitial atom
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- 14. Temperature Types of Diffusion: Volume diffusion - Diffusion of atoms through the interior of grains. Grain boundary diffusion - Diffusion of atoms along grain boundaries. This is faster than volume diffusion, because the atoms are less closely packed in grain boundaries. Surface diffusion - Diffusion of atoms along surfaces, such as cracks. 14 Section 5.5 Factors Affecting Diffusion
- 15. 15 Time Dependence on Bonding and Crystal Structure Dependence on Concentration of Diffusing Species and Composition of Matrix Factors Affecting Diffusion
- 16. 16 Section 5.6 Diffusion and Materials Processing Sintering - A high-temperature treatment used to join small particles. Diffusion of atoms to points of contact causes bridges to form between the particles. Further diffusion eventually fills in any remaining voids. Powder metallurgy - A method for producing monolithic metallic parts; metal powders are compacted into a desired shape, which is then heated to allow diffusion and sintering to join the powders into a solid mass. Diffusion processes during sintering and powder metallurgy. Atoms diffuse to points of contact, creating bridges and reducing the pore size.
- 17. 17 Diffusion and Materials Processing Melting and casting Grain growth - Movement of grain boundaries by diffusion in order to reduce the amount of grain boundary area. As a result, small grains shrink and disappear and other grains become larger, similar to how some bubbles in soap froth become larger at the expense of smaller bubbles. For grain growth in materials, diffusion of atoms across the grain boundary is required, and, consequently, the growth of the grains is related to the activation energy needed for an atom to jump across the boundary. The driving force for grain growth is reduction in grain boundary area. Grain boundaries are defects and their presence causes the free energy of the material to increase. Thus, the thermodynamic tendency of polycrystalline materials is to transform into materials that have a larger average grain size. High temperatures or low-activation energies increase the size of the grains. Many heat treatments of metals, which include holding the metal at high temperatures, must be carefully controlled to avoid excessive grain growth. This is because, as the average grain size grows, the grain-boundary area decreases, and there is consequently less resistance to motion of dislocations. As a result, the strength of a metallic material will decrease with increasing grain size.
- 18. 18 Grain growth occurs as atoms diffuse across the grain boundary from one grain to another. Grain growth in alumina ceramics can be seen from the SEM micrographs of alumina ceramics. The left micrograph shows the microstructure of an alumina ceramic sintered at 1350o C for 15 hours. The right micrograph shows a sample sintered at 1350o C for 30 hours.