Reinforcement volume fraction determination in metal matrix composites

The precise determination of reinforcement volume fraction in metal matrix composites (MMC) is a requirement to control the material quality. Depending on the fabrication technique and materials used, this volume fraction determination poses several difficulties. The metallographic technique, besides being time consuming, is prone to preparation artefacts due to intrinsic phase's hardness that produces polishing relief. The proposed method is based on reinforcement extraction by matrix chemical dissolution. The main requisite is that the chemical solution does not dissolve or attack the reinforcement. A special procedure was established for MMC's where the matrix was an aluminium alloy reinforced with silicon and niobium carbides. The method was standardised by means of special samples prepared by powder metallurgy. The data was checked against metallographic samples evaluated by an image analyser. The method was proven reliable and precise.