Preparing dispersion-hardened materials for transmission electron microscopy

Abstract

The preparation of thin foils for electron microscopy often causes difficulties for dispersion-hardened materials in which the dispersoid (normally an oxide) is not attacked by the eleetropolishing. The metal/ceramic-dispersoid-particle interface tends to produce premature perforation which is then followed by accelerated attack. In the following we shall describe a method that has proved successful in producing very good thin foils from all the dispersion-hardened materials to which it has been applied, namely aluminum with aluminum oxide, zirconium and zircaloy-2 with yttrium oxide, and high-purity iron and austenitic stainless steel with aluminum oxide. First, discs 3 mm in diameter and 0.5 mm thick are made by spark machining (or, if a slight mechanical distortion during preparation is acceptable, discs may be wet-ground to 0.2 mm thickness on emery paper while held in a special jig). The discs are then eleetropolished in an ethanol/perchloric-acid electrolyte containing 20 v/o perchloric acid. The electrolyte, contained in a double-walled beaker, is cooled with ethanol circulated in the cooling jacket from a Fiskars refrigerator; the electrolyte temperature is kept at --20°C±1°C. The arrangement is shown in Fig. 1. A simple rotary pump produces a jet of electrolyte at the specimen. The pump housing is made of P T F E ; it has an inlet in the top center. In the pump housing a magnetic rod covered with P T F E is rotated by the magnetic stirrer on which the whole arrangement stands. The electrolyte runs through two glass tubes ending on each side of the specimen, thus producing the double jet. The specimen is held in a pair of tweezers laquered with Lacomit (except at the contact points), and the tweezers are held in a retort stand. A platinum plate is used as the cathode. The shape and location of the cathode are not found to be critical ; in Fig. 1 it is placed along the back wall of the beaker almost hidden by the reflections from the surface. The specimen can be observed through a stereo microscope so that the current can be switched off immediately after perforation. Before polishing, the rim of the disc is covered with Lacomit while held in the