Page 152 - Materials Science and Engineering An Introduction

124 • Chapter 4 / Imperfections in Solids

4.10 MICROSCOPIC TECHNIQUES
Optical Microscopy
With optical microscopy, the light microscope is used to study the microstructure; opti-
cal and illumination systems are its basic elements. For materials that are opaque to
visible light (all metals and many ceramics and polymers), only the surface is subject to
observation, and the light microscope must be used in a reflecting mode. Contrasts in
the image produced result from differences in reflectivity of the various regions of the
microstructure. Investigations of this type are often termed metallographic because met-
als were first examined using this technique.
Normally, careful and meticulous surface preparations are necessary to reveal the
important details of the microstructure. The specimen surface must first be ground and
polished to a smooth and mirror-like finish. This is accomplished by using successively
finer abrasive papers and powders. The microstructure is revealed by a surface treat-
ment using an appropriate chemical reagent in a procedure termed etching. The chemi-
cal reactivity of the grains of some single-phase materials depends on crystallographic
orientation. Consequently, in a polycrystalline specimen, etching characteristics vary
from grain to grain. Figure 4.14b shows how normally incident light is reflected by three
etched surface grains, each having a different orientation. Figure 4.14a depicts the sur-
face structure as it might appear when viewed with the microscope; the luster or texture
of each grain depends on its reflectance properties. A photomicrograph of a polycrystal-
line specimen exhibiting these characteristics is shown in Figure 4.14c.

(a)

Microscope

Polished and Photomicrograph courtesy of J. E. Burke, General Electric Co.
etched surface

(b) (c)
Figure 4.14 (a) Polished and etched grains as they might appear when viewed with an optical microscope. (b) Section
taken through these grains showing how the etching characteristics and resulting surface texture vary from grain to grain
because of differences in crystallographic orientation. (c) Photomicrograph of a polycrystalline brass specimen, 60 .