Metallography: Principles and Practices - ASM International

Metallography: Principles and Practice (#06785G)
Author(s): George F. Vander Voort
32 METALLOGRAPHY
While the grains in coarse-grained aluminum castings and wrought products
can be revealed by many of the macroetchants listed in App. A, a number of
investigators have employed color illumination to improve the grain contrast.
Beck has used two etchants, listed in App. A, for revealing grains in aluminum
[21,22]. Illumination was provided by three universal microscope lamps angled to
provide oblique light from three directions. Each lamp was fitted with a different
color filter to increase the contrast of the reflections from adjacent grains. The
sample could be rotated while it was examined at low (10 to 20X) magnification.
The sample was kept rotating while the projected image was traced on a plastic
sheet so that all the grain boundaries could be sketched.
Ryvola has also shown the value of color filters for improving grain contrast in
macroetched aluminum samples [23]. Two illuminators, one with a red filter and
the other with a green filter, were placed on opposite sides of the sample to cast
oblique light. A blue filter was inserted between the sample and the objective of a
stereomicroscope. Rotation of the sample was also used here to reveal all the
grain boundaries. In most studies Ryvola employed Tucker's or Poulton's reagent
(see App. A for composition) as the macroetch.
1-3.8 Alloy Segregation
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Because most engineering alloys freeze over a range of temperatures and liquid
compositions, the various elements in the alloy segregate during the solidification
of ingots and castings. Segregation occurs over short distances, causing microsegregation,
and over long distances, producing macrosegregation. Microsegregation
is a natural result of dendritic solidification because the dendrites are purer
in composition than the interdendritic matter. Macrosegregation manifests itself
in a variety of forms-centerline segregation, negative cone of segregation, A- and
V-type segregates, and banding. These phenomena are the result of the flow of
solute-enriched interdendritic liquid in the mushy zone during solidification; this
flow is a result of solidification shrinkage and gravitational forces.
Macrosegregation can be detected by bulk chemical analysis. Tests on large
ingots generally reveal low concentrations of carbon and alloying elements at the
bottom and sides and enrichment at the top and along the centerline. Macrosegregation
can be detected on fractures and on macroetched discs. In addition to the
use of traditional macroetching and microetching, microsegregation has also been
studied by autoradiography, microradiography, electron-probe microanalysis,
and x-ray fluorescence. The study of segregation has become a relatively simple
matter since the development of the electron microprobe. This instrument is
capable of providing accurate, rapid determinations of compositional differences.
Figure 1-21 illustrates the use of macroetching to reveal segregation and
shows a sample of carbon-manganese-chromium steel which cracked during
extrusion (note the central burst). A transverse disc reveals a spot of segregation
which is more readily observed on the longitudinal section. The streaks are
martensitic with a hardness of 46 to 58 HRC (Rockwell hardness on the C scale),
while the bulk hardness is below 20 HRC. The streak is enriched in C, Mn, and Cr.