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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 hightemperature 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 MidlandRoss.)
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.
Heattreating 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
?
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Page 109
2nd REVISE PAGES
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View Full Document5.1 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 steadystate and
nonsteadystate 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 semiinfinite 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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 Spring '09
 LIBERA

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