Page 255 - Materials Science and Engineering An Introduction

7.6 Plastic Deformation of Polycrystalline Materials • 227

Figure 7.10 Slip lines on the
surface of a polycrystalline specimen
of copper that was polished and
subsequently deformed. 173 .
[Photomicrograph courtesy of C. Brady,
National Bureau of Standards (now the Na-
tional Institute of Standards and Technol-
ogy, Gaithersburg, MD).]

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Polycrystalline metals are stronger than their single-crystal equivalents, which
means that greater stresses are required to initiate slip and the attendant yielding. This
is, to a large degree, also a result of geometric constraints that are imposed on the grains
during deformation. Even though a single grain may be favorably oriented with the
applied stress for slip, it cannot deform until the adjacent and less favorably oriented
grains are capable of slip also; this requires a higher applied stress level.

Figure 7.11 Alteration of the grain
structure of a polycrystalline metal
as a result of plastic deformation. (a)
Before deformation the grains are
equiaxed. (b) The deformation has
produced elongated grains. 170 .
(From W. G. Moffatt, G. W. Pearsall,
and J. Wulff, The Structure and Proper-
ties of Materials, Vol. I, Structure, p. 140.
Copyright © 1964 by John Wiley & Sons,
New York.)

(a) (b)

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