Microscopic examination

А number of methods and tools have been developed in order to reveal the constitution and structure of metals and alloys. In the early 1800's, scientists first started looking at metals under the microscope. These early investigators discovered the relationship between grain size and hardness, the presence of defoliation lines during work hardening, the hardening effect of alloying elements, and finally, the relationship between, microstructure and the properties of quenched and tempered steel. The results of these discoveries were later applied to process control - specifying, for example, the best structure for machi­ning, or the best grain size for a deep drawing operation.

In recent years metallurgists have been able to capitalize on the information gained and develop new alloys with the desired micro structure and properties. A number of methods of metallur­gical examination have been developed including the development of new tools such as the electron microscope and x-ray diffrac­tion apparatus which have greatlyу helped in changing metallurgy from an art to a science.

The primary purpose of microscopic examination is to reveal details of metal structures which are too small to be seen with the unaided eye. Metallurgical microscopes can be used to deter­mine grain size, inclusions, previous heat treatments, possible causes for failures, deformation. In short, microscopic exami­nation can reveal a great deal about the past history of a metal specimen and how it will act in service.

Maximum magnification of optical microscopes is limited to about 2000 diameters. Higher magnification does not reveal greater detail and is called “empty magnification”.

Contrary to popular opinion, high magnifying power does not necessarily reveal fine detail. It is the numerical aperture (N.A.) or light-gathering ability to resolve fine detail.

It has been found by experience that the maximum total mag­nification necessary to observe details by a particular objecti­ve should not exceed 1000 times the numerical aperture of the lens. For example, a lens whose numerical aperture is 0.65 should not normally be used at greater than 650 x magnification if the photomicrograph is to appear reasonably sharp and distinct when viewed from a distance of about 10 inches.

One of the most important contributions to microscopy has been the development of electron microscopes. The electron microscopes represents the latest achievement in the search for systems capable of greater resolving power than is obtainable with optical microscopes. The electron microscope is capable of direct magnifications of 10.000 to 30.000 diameters, and with auxiliary optical equipment, may be extended to as high as 500.000 diameters.

The resolution attainable with the electron microscope of the order of .002 microns (0.000000078") as compared to 0.2 microns (0.0000078") for the optical microscope, is due to the very short wave length of the electron beam used to "illuminate" the specimen. The electron microscope is capable of great depth of focus, approximately 30 times that of the optical microscope.

Unfortunately, the standard electron microscope cannot be used directly with metallographic specimens. The usual procedure is to make a thin, transparent replica of the specimen which carefully defines the contour of the etched surface o£ a metal­lographic specimen. The newly developed scanning type electron microscope is limited to about 50.000X but it has an exceptional depth of focus. Polishing of the specimen is unnecessary making it possible to examine the surfaces of fractured specimens, coatings, and other three-dimensional surfaces.