Posted by Andrew Binz on March 31, 2010 at 12:11:38:
In Reply to: Metallographic Preparation posted by Joelle on January 25, 2010 at 19:32:35:
Selective corrosive attack. Different etchants attack different phases/chemical compositions for various reasons. Etchants are not interchangeable, and only etchants intended for a particular purpose for a particular alloy using a particular procedure should be performed.
Different constituents are revealed because the etchant selectively removes small amounts of material. When under a microscope the light reflects differently off the different planes and angles revealing the constituents. There are also some etchants that leave layers of material on the surface of the specimens. As light travels through these different layers different phases/constituents are revealed.