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Metal Casting Technologies : March 2008
www.metals.rala.com.au 40 TECHNICAL FEATURE INTRODUCTION his outline of analysis at the microscopic scale follows on from a previous article in the March 2004 MCT issue that considered the selection of tools and techniques which can be used to characterise materials . Metal casters are all familiar with analysis at the macro-scale since for the majority of their products they have to provide test certificates for chemical composition to show that they are meeting customer specifications. They also know in the general sense about the use of electron microscopy to look at fine scale microstructures and other features such as metal fracture surfaces, sand grains and refractory materials. However many are not aware of how determination of chemical composition at the micro-scale (e.g. micro-segregation, analysis of individual phases, small precipitates and inclusions, etc.) is performed, or of how such information can be used to help solve foundry problems and so lead to improved casting alloys and processes. This short article is intended to encourage non-metallurgical readers in foundries to find out if their local university or research centre has microanalysis facilities, and if so, to consider how such facilities may provide them with technical assistance. ELECTRON BEAM -- SPECIMEN INTERACTIONS In both scanning (SEM) and transmission (TEM) electron microscopes and in electron probe microanalysis (EPMA) instruments electrons are generated at the top of a vacuum column via an electron gun, traditionally a tungsten filament heated by an electric current. In the gun the electrons are attracted away from the filament using a high voltage (normally 5-30kV for SEM or 100-200kV for TEM) between the filament and an anode cap and then accelerated down the column through a small hole in the anode to give a beam smaller than 50μm in diameter. A high vacuum is needed in the column and attached specimen chamber to prevent scattering of the electrons by air or any other gas molecules. As the beam travels down the column its diameter is reduced The Basics of Microanalysis T By Dr. John Pearce Fig 1. The shapes of interaction volumes for thin and bulk specimens