Investigation of Secondary Phases in Alloys by Electron Diffraction and the Electron Microscope

Abstract

The application of electron diffraction reflection methods and the electron microscope to investigations of metallurgical structures is reported. It is shown that these instruments can yield valuable information concerning very small amounts of secondary phases and impurities in metals. Thus, for example, when martensite decomposes at 200°C, the slow etching component is highly dispersed Fe3N. The decomposition at 400°C yields the usual Fe3C. Further, the Fe3N formed at 200°C transforms to Fe3C when annealed at 350°C. Other examples are presented. The successful application of these instruments is dependent upon the production of clean, etched surfaces. Surface preparation is discussed and a satisfactory technique for rinsing etched surfaces is described. In some alloy systems, the etching reaction may result in the redeposition of a component onto the etched surface. In the case of a β-brass containing 0.0005–0.001 percent silver, the silver is deposited in the form of spherical particles about 1000A in diameter. Aluminum alloys appear to offer a serious problem in discovering satisfactory etchants due to the position of aluminum in the electromotive series relative to copper, silver, etc.