Page 111 - Materials Science and Engineering An Introduction

3.12 Close-Packed Crystal Structures • 83

A A A A

A A A A B B B B B
B B B C C C C
C C C C
A A A A A B B B B B B
B B B B C C C C C
C C C C C
A A A A A A B B B B B B B

(a) (b)
Figure 3.16 (a) A portion of a close-packed plane of atoms; A, B, and C positions are indicated. (b) The AB
stacking sequence for close-packed atomic planes.
(Adapted from W. G. Moffatt, G. W. Pearsall, and J. Wulff, The Structure and Properties of Materials, Vol. I, Structure, p. 50. Copy-
right © 1964 by John Wiley & Sons, New York.)

A second close-packed plane may be positioned with the centers of its atoms over
either B or C sites; at this point, both are equivalent. Suppose that the B positions are ar-
bitrarily chosen; the stacking sequence is termed AB, which is illustrated in Figure 3.16b.
The real distinction between FCC and HCP lies in where the third close-packed layer is
positioned. For HCP, the centers of this layer are aligned directly above the original A
positions. This stacking sequence, ABABAB . . . , is repeated over and over. Of course,
the ACACAC . . . arrangement would be equivalent. These close-packed planes for HCP
are (0001)-type planes, and the correspondence between this and the unit cell representa-
tion is shown in Figure 3.17.
For the face-centered crystal structure, the centers of the third plane are situated
over the C sites of the first plane (Figure 3.18a). This yields an ABCABCABC . . .
stacking sequence; that is, the atomic alignment repeats every third plane. It is more
difficult to correlate the stacking of close-packed planes to the FCC unit cell. However,
this relationship is demonstrated in Figure 3.18b. These planes are of the (111) type; an
FCC unit cell is outlined on the upper left-hand front face of Figure 3.18b to provide
perspective. The significance of these FCC and HCP close-packed planes will become
apparent in Chapter 7.
The concepts detailed in the previous four sections also relate to crystalline ce-
ramic and polymeric materials, which are discussed in Chapters 12 and 14. We may

Figure 3.17 Close-packed plane stacking
sequence for the hexagonal close-packed
structure.
(Adapted from W. G. Moffatt, G. W. Pearsall, and
A
J. Wulff, The Structure and Properties of Materials,
Vol. I, Structure, p. 51. Copyright © 1964 by John
B
Wiley & Sons, New York.)
A

B
A