Extraction replicas for the analysis of second phases in nonferrous alloys using Analytical Electron Microscopy

Abstract

The technique for extracting second-phase particles from steels onto a carbon support film for analysis in the transmission electron microscope (TEM) has been in continuous use since its development by Bradley, nearly 30 years ago. However, the availability of EDX and EELS accessories, capable of giving chemical information, has provided an incentive for the technique to be used with other alloys.The purpose of this presentation is to review some applications of extraction replication to non-ferrous alloys.Briefly, extraction involves chemically etching only the matrix phase of an alloysothat second-phase particles stand proud and can be extracted onto a carbon film. The most straightforward procedure is to make a single-stage direct-carbon replica, where carbon is evapourated onto the etchedsurface of a mctallographic specimen and subsequently stripped by further etching.An alternative approach is the two-stage technique, where the particles are first stripped from an etched specimen using a plastic film, e.g. cellulose acetate, such as "Bioden". The plastic surface containing theparticles is then coated with a layer of carbon, typically 10˜20nm thick, and the plastic is dissolved, leaving the particles attached to the carbon film. Although the two-stage technique involves anextra operation, it has some advantages. In particular, it is possible to strip films after differentdegrees of etching, without the necessity to re-polish the specimen. Also, with the single-stage technique, an etchant suitable for removing the replica may react so strongly that the thin carbon film breaks up during stripping.