The fracture toughness of ultrafine WC-Co Alloys

Master Thesis


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University of Cape Town

This thesis examines the fracture toughness behaviour of a series of cemented carbides having carbide grain sizes between 0.35f.Lm and 4f.Lm, cobalt contents ranging from 6 to 15wt%, and vanadium carbide contents from 0 to 0.8wt%. A series of twenty-four ultrafine WC-Co alloys were sintered from powders produced through three different production routes, namely, the spray conversion process and two variations of the conventional powder production methods. A further twelve WC-Co alloys with three different carbide grain size distributions were produced through conventional powder metallurgy processes. Two different fracture toughness test methods have been used to determine the toughness measurements, namely, the Palmqvist Indentation test and the Short Rod test. A Terra Tek Fractometer machine was used to automatically determine the Short Rod toughness values. The Palmqvist testing was carried out on a Vickers hardness machine using indenting loads of 20, 30 and 50kg. The Palmqvist crack lengths were measured by summing the individual crack lengths emanating from the corners of the hardness indentation. These crack lengths were used in a formula developed by Shetty et al., to calculate the Palmqvist fracture toughness. The influence of annealing at 800°C and 900°C on the Palmqvist crack lengths has also been assessed. Microstructural parameters have been determined using ASTM procedures and microscopy techniques and the influence of these parameters on the fracture toughness has also been assessed.

Bibliography: pages 147-152.