Sample Preparation for Metallography

Abstract

Abstract Metallography, the study of metal and metallic alloy structure, began at least 150 years ago with early investigations of the science behind metalworking. Metallographic sample preparation has evolved from a black art to the highly precise scientific technique it is today. Its principal objective is the preparation of artifact‐free representative samples suitable for microstructural examination. The particular choice of a sample preparation procedure depends on the alloy system and also on the focus of the examination, which could include process optimization, quality assurance, alloy design, deformation studies, failure analysis, and reverse engineering. The details of how to make the most appropriate choice and perform the sample preparation are the subject of this unit. Metallographic sample preparation is divided broadly into two stages. The aim of the first stage is to obtain a planar, specularly reflective surface, where the scale of the artifacts (e.g., scratches, smears, and surface deformation) is smaller than that of the microstructure. This stage commonly comprises three or four steps: sectioning, mounting (optional), mechanical abrasion, and polishing. The aim of the second stage is to make the microstructure more visible by enhancing the difference between various phases and microstructural features. This is generally accomplished by selective chemical dissolution or film formation—etching. The procedures discussed in this article are also suitable (with slight modifications) for the preparation of metal and intermetallic matrix composites as well as for semiconductors. The modifications are primarily dictated by the specific applications, e.g., the use of coupled chemical‐mechanical polishing for semiconductor junctions. The basic steps in metallographic sample preparation are straightforward, although for each step there may be several options in terms of the techniques and materials used. Also, depending on the application, one or more of the steps may be elaborated or eliminated. This article provides guidance on choosing a suitable path for sample preparation, including advice on recognizing and correcting an unsuitable choice. This discussion assumes access to a laboratory equipped with the requisite equipment for metallographic sample preparation. Listings of typical equipment and supplies and World Wide Web addresses for major commercial suppliers are provided for readers wishing to start or upgrade a metallography laboratory.