c05 - 2nd REVISE PAGES 1496T_c05_109-130 7:52 Page 109...

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109 Chapter 5 Diffusion P hotograph of a steel gear that has been “case hardened.” The outer surface layer was selectively hardened by a high-temperature heat treatment during which carbon from the sur- rounding atmosphere diffused into the surface. The “case” appears as the dark outer rim of that segment of the gear that has been sectioned. Actual size. (Photograph courtesy of Surface Division Midland-Ross.) Materials of all types are often heat treated to im- prove their properties. The phenomena that occur during a heat treatment almost always involve atomic diffusion. Often an enhancement of diffusion rate is desired; on occasion measures are taken to reduce it. Heat-treating temperatures and times, and/or cooling rates are often predictable using the mathematics of diffusion and appropriate diffusion constants. The steel gear shown on this page has been case hardened (Section 8.10); that is, its hardness and resistance to failure by fatigue have been enhanced by diffusing excess carbon or nitrogen into the outer surface layer. WHY STUDY Diffusion ? 1496T_c05_109-130 12/20/05 7:52 Page 109 2nd REVISE PAGES
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5.1 INTRODUCTION Many reactions and processes that are important in the treatment of materials rely on the transfer of mass either within a specific solid (ordinarily on a microscopic level) or from a liquid, a gas, or another solid phase. This is necessarily accomplished by diffusion, the phenomenon of material transport by atomic motion. This chapter dis- cusses the atomic mechanisms by which diffusion occurs, the mathematics of diffu- sion, and the influence of temperature and diffusing species on the rate of diffusion. The phenomenon of diffusion may be demonstrated with the use of a diffusion couple, which is formed by joining bars of two different metals together so that there is intimate contact between the two faces; this is illustrated for copper and nickel in Figure 5.1, which includes schematic representations of atom positions and composition across the interface. This couple is heated for an extended period at an elevated temperature (but below the melting temperature of both metals), and Learning Objectives After studying this chapter you should be able to do the following: 1. Name and describe the two atomic mechanisms of diffusion. 2. Distinguish between steady-state and nonsteady-state diffusion. 3. (a) Write Fick’s first and second laws in equa- tion form, and define all parameters. (b) Note the kind of diffusion for which each of these equations is normally applied. 4. Write the solution to Fick’s second law for diffusion into a semi-infinite solid when the concentration of diffusing species at the surface is held constant. Define all parameters in this equation. 5. Calculate the diffusion coefficient for some material at a specified temperature, given the appropriate diffusion constants.
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c05 - 2nd REVISE PAGES 1496T_c05_109-130 7:52 Page 109...

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