Color has historically seen limited use in metallography, mainly due to the cost of film and prints and the difficulty and cost of reproducing images in publications. With the growth of digital imaging, however, capturing color images is much simpler and cheaper. Also, printing images in color is inexpensive for in-house reports and can be distributed cheaply on CDs or flash drives, although reproduction in journals is still expensive.
Color does have many advantages over black and white. First, the human eye is sensitive to only about 40 shades of gray from white to black, but it is sensitive to a vast number of colors. Tint etchants reveal features in the microstructure that often cannot be revealed using standard black-and-white etchants. Color etchants are sensitive to crystallographic orientation and can reveal if the grains have a random or a preferred crystallographic texture. They are also very sensitive to variations in composition and residual deformation. Further, color etchants are usually selective to certain phases, and this is valuable in quantitative microscopy.
The use of color in metallography has a long history with color micrographs published over the past 80 years. But to do good work in color does require the specimens to be prepared as damage-free as possible. Since the major source for preparation-induced damage is the sectioning step, it is obvious that special attention be applied to this process. This means that production cutting devices should not be used to produce the plane to be polished. Instead, choose the least damaging technique, such as a precision saw (e.g., Accutom or the Secotom), which allows you to cut somewhat larger specimens. A laboratory abrasive cutter may also be used – always with the correct blade for the material being sectioned. If the edges are critical, mount with a high-quality resin that yields good edge retention, such as DuroFast.
We will continue our discussion of this topic in a series of three or four blogs.