Experimental study of the mechanics of fracture in WC-Co alloys
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I.
INTRODUCTION
THE fracture process
in WC-Co has been of considerable interest over recent years. A more complete understanding of the nature of crack propagation will contribute toward efforts aimed at material improvement as well as toward further development of the fracture theory of cemented carbides which requires an accurate description of the fracture mechanism. Although the fracture behavior of cemented carbides has been the subject of intense research work,l ~3 many aspects concerning crack propagation are still a matter of debate. The present paper examines the fracture process in tungsten-carbide/cobalt alloys with up to 25 vol pct Co. It focuses on the differences between macroscopically fast and slow crack propagation and on the sequence and mechanism of cracking in the individual phases. Finally it aims to give the phenomenological basis for an improved model relating the fracture toughness of cemented carbides to the properties of their microstructural components which will be the subject of a second paper.
II.
EXPERIMENTAL
A series of WC-Co alloys was produced under carefully controlled conditions. The heat treatment and the carbon content of each alloy were adjusted to a level of 95 pct of the maximum magnetic saturation. This ensures that the binder contains constant amounts of carbon and tungsten, that no eta phase or free carbon exist, and that the composition of the binder corresponds to a favorable toughness behavior. 14,~5,~6 The binder phase content was varied between 10 and 25 vol pet and the carbide grain size from 0.6 to 2.2 p,m. Point counting and semiautomatic linear image analysis were applied to SEM micrographs to evaluate the microstructural parameters. Table I shows the experimental results of the volume fraction of the binder phase, V~, the mean linear size of the carbide crystals, D ~, and the mean linear path in the binder phase, L,. For the geometric characterization of fracture surfaces x-y-z coordinates of profiles were evaluated from SEM L.S. SIGL. formerly with Max-Planck-lnstitut fiir Metallforschung, Stuttgart, Germany, is wtth the Umverslty of California, Santa Barbara, CA. H E. EXNER is with Max-Planck-lnstitut fiir Metallforschung, Seestrage 92, D-7000 Stuttgart 1, Federal Republic of Germany. Manuscript submitted August 4, 1986. METALLURGICAL TRANSACTIONS A
stereopair micrographs using a Hilger-Watt mirror stereometer attached to a Kontron image analyzer. Details of the instrumentation and the computing procedure are described elsewhere. ~7 For the fracture surfaces analyzed in this work, the following procedure was used. Stereopair micrographs were taken at 2000x magnification in a JEOL 35C SEM at 0 and 5 deg tilt between the incident electron beam and the direction of principal normal stress. Height profiles were evaluated along 20 parallel equidistant lines with a line spacing of 2 /xm, which is of the order of the average carbide grain size. Due to the extensive operating time, stereometric characterization of the fracture surface was limited to alloys 10M
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