Microstructural characterization of cemented carbides using SEM based automatic image analysis

Abstract

Methods for SEM based automatic image analysis of cemented carbide microstructure were developed. Two different categories of hard metal were investigated: WC-Co grades and WC-γ-Co grades, i.e. cemented carbides containing cubic carbides (γ-phase) as well as tungsten carbide WC. The materials represented a wide range in grain size, chemical composition and structure complexity. Automatic object detection and evaluation of carbide grain size, carbide contiguity and volume fractions were possible through the combination of optimized specimen preparation, high resolution microscopy and image processing in a software oriented image analyzer. The importance of careful specimen preparation in order to obtain maximum contrast and structure definition was emphasized. High spatial resolution in SEM was achieved by working at low accelerating voltages. Two different back scattered electron contrast modes were utilized: atomic number (Z) contrast and channelling contrast. Mean values of WC and cubic carbide (γ-phase) grain size in the range 0·–2 μm were measured; values which were considerably lower than values obtained according to conventional methods. The difference was explained by the low detection limit in the SEM based analysis, enabling detection of fine carbide grain populations. Binder mean free path increased and carbide contiguity decreased with increasing binder volume and with increasing carbide grain size in the WC-Co alloys. The results of the automatic measurements were of fully sufficient accuracy for a wide range of hard metals. In cases of very weak contrast, limited manual editing was sufficient to obtain results close to actual values. It was concluded that automatic methods can be successfully applied to the characterization of complex cemented carbide microstructures.