Survey on Metallography of Metals Using Microscopy |
DIC has largelyreplaced the older oblique illumination (OI) technique, which wasavailable on reflected light microscopes prior to about 1975. In OI, thevertical illuminator is offset from perpendicular, producing shading effectsthat reveal height differences. This procedure reduces resolution and yieldsuneven illumination across the field of view. Nevertheless, OI was useful whenpeople needed to know if a second phase particle was standing above or wasrecessed below the plane-of-polish, and is still available on a fewmicroscopes. OI can be created on any microscope by placing a piece of paperunder one corner of the mount so that the plane-of-polish is no longerperpendicular to the optical axis. If a specimen is observed at highermagnification, it can be examined with a scanning electron microscope (SEM), ora transmission electron microscope (TEM). When equipped with an energy dispersive spectrometer (EDS),the chemical composition of the microstructural features can be determined. Theability to detect low-atomic number elements, such as carbon, oxygen, andnitrogen,depends upon the nature of the detector used. But, quantification of theseelements by EDS is difficult and their minimum detectable limits are higherthan when a wavelength-dispersive spectrometer (WDS)is used. But quantification of composition by EDS has improved greatly overtime.