Page 170 - Materials Science and Engineering An Introduction

142 • Chapter 5 / Diffusion 100

Concentration of Ni, Cu Cu Ni

Diffusion of Cu atoms
Cu Cu–Ni alloy Ni
Diffusion of Ni atoms 0
Position
(a) (b) (c)

Figure 5.2 (a) A copper–nickel diffusion couple after a high-temperature heat treatment, showing the alloyed dif-
fusion zone. (b) Schematic representations of Cu (red circles) and Ni (blue circles) atom locations within the couple.
(c) Concentrations of copper and nickel as a function of position across the couple.

nickel at the two extremities of the couple, separated by an alloyed region. Concentrations
of both metals vary with position as shown in Figure 5.2c. This result indicates that copper
atoms have migrated or diffused into the nickel, and that nickel has diffused into copper.
interdiffusion The process by which atoms of one metal diffuse into another is termed interdiffusion, or
impurity diffusion impurity diffusion.
Interdiffusion may be discerned from a macroscopic perspective by changes in con-
centration that occur over time, as in the example for the Cu–Ni diffusion couple. There
is a net drift or transport of atoms from high- to low-concentration regions. Diffusion
also occurs for pure metals, but all atoms exchanging positions are of the same type; this
self-diffusion is termed self-diffusion. Of course, self-diffusion is not normally subject to observation
by noting compositional changes.

5.2 DIFFUSION MECHANISMS
From an atomic perspective, diffusion is just the stepwise migration of atoms from lattice
site to lattice site. In fact, the atoms in solid materials are in constant motion, rapidly chang-
ing positions. For an atom to make such a move, two conditions must be met: (1) there must
be an empty adjacent site, and (2) the atom must have sufficient energy to break bonds with
its neighbor atoms and then cause some lattice distortion during the displacement. This
energy is vibrational in nature (Section 4.8). At a specific temperature, some small fraction
of the total number of atoms is capable of diffusive motion, by virtue of the magnitudes of
their vibrational energies. This fraction increases with rising temperature.
Several different models for this atomic motion have been proposed; of these pos-
sibilities, two dominate for metallic diffusion.

Vacancy Diffusion
One mechanism involves the interchange of an atom from a normal lattice position to an
adjacent vacant lattice site or vacancy, as represented schematically in Figure 5.3a. This
vacancy diffusion mechanism is aptly termed vacancy diffusion. Of course, this process necessitates the
presence of vacancies, and the extent to which vacancy diffusion can occur is a function
of the number of these defects that are present; significant concentrations of vacancies
may exist in metals at elevated temperatures (Section 4.2). Because diffusing atoms and
vacancies exchange positions, the diffusion of atoms in one direction corresponds to the
motion of vacancies in the opposite direction. Both self-diffusion and interdiffusion oc-
cur by this mechanism; for the latter, the impurity atoms must substitute for host atoms.

Interstitial Diffusion
Tutorial Video:
Diffusion The second type of diffusion involves atoms that migrate from an interstitial position to
Diffusion Mechanisms a neighboring one that is empty. This mechanism is found for interdiffusion of impurities