Crystal Growth and Size

Abstract

To examine the microstructure of a metal visually, it must be suitably sectioned and prepared in a manner that will show up the grain structure. After the surface to be examined has been exposed, either by removing any unwanted’ surface layer or by sectioning, and flattening with an emery wheel or file, it is polished with successively finer grades of abrasive material. Emery paper may be used for this purpose, or alternatively, diamond dust on a cloth-covered rotating disc. The final lapping is by rouge or magnesia or alumina powder on a cloth-covered disc, after which the surface should be free from scratches that would be visible under a microscope. It is then attacked by a suitable etchant, which may have one of two effects depending upon the metal and the etchant used: either, atoms will be removed at the grain boundaries, or, the surface of each grain will be attacked to expose facets of certain crystallographic planes as in fig. 5.1.