In the early 1970s, I came across some articles and a book,Quantitative Stereology, written by the late Ervin E. Underwood. This was the beginning of a lifelong interest in quantifying microstructures and a long-time friendship with a fascinating gentleman.
Stereology is used
to quantify matrix microstructures, as opposed to standard metrology techniques
for measuring case depth, plating thickness or particle size. Microstructural
measurements are made on a two-dimensional “plane of polish” through a
three-dimensional opaque metal. Stereology converts these 2-D measurements into
3-D estimates of microstructural parameters. Most procedures are very simple to
use, but there are special considerations for their validity.
This subject has
been hampered by confusion due to the use of different mathematical symbols for
the same parameters. To minimize this problem, the International Society for
Stereology has promoted a standard nomenclature that is constantly evolving as
new approaches are developed. The most basic symbols are:
P - Point
L - Line
A - Area
S - Surface
V - Volume
N - Number
These symbols can
be combined in a number of ways to generate different symbols. For example, Pp
represents the point fraction (i.e. the fraction of grid points lying in a
phase of interest. While A and S seem to be the same, A is for a flat surface and S is for a curved surface. Thus, S represents the grain-boundary surface
area per unit volume. NAis the number of particles per unit, while NVis the number per unit volume.
Introduction to Quantitative Metallography (part 2)
George Vander Voort is a consultant for Struers, Latrobe Steel and Scot Forge. He is also president of Vander Voort Consulting. Vander Voort has more than 40 years of industry experience and has authored more than 280 publications. A member of ASM International since 1966, he has won 34 awards for his work in metallography.