Transformation-toughening in cemented carbides: Part I. Binder composition control
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I.
INTRODUCTION
C E M E N T E D carbides of which WC-Co is a classic example, typically consist of hard, refractory grains of a transition-metal carbide bonded with a relatively soft metal from group VIII of the periodic table. These materials are processed using conventional powder metallurgy techniques and densified by liquid phase sintering. The unique combination of elastic modulus, compressive strength, hardness, and fracture toughness of WC-Co cemented carbides has made them indispensable in a variety of scientific and industrial applications. Significant research effort has gone into exploring the relationship between microstructural parameters such as binder content, binder mean free path, carbide grain size and distribution, and the mechanical behavior of WC-Co and other cemented carbides. Economic and strategic considerations have motivated several attempts to find a suitable alternate to WC-Co with equivalent mechanical properties. These attempts have not met with much success. The uniqueness of WC-Co is amply borne out when the fracture toughness/hardness behavior of several cemented carbide systems are compared. 2The excellent wetting of WC by Co, and the specific yield and work hardening behavior of the cobalt binder, are in part responsible for the high fracture toughness/hardness combination exhibited by WC-Co. More importantly, there are several features that are unique to WC and distinguish its behavior from that of other transition metal carbides. WC can undergo plastic deformation at room temperature, 3 The cleavage energy of WC is several times higher than that of other carbides, 4 and its elastic modulus is very high. s These considerations lead one to believe that at least for the short term, WC will be the principal hard phase in new cermet systems under consideration for further development. If the choice of the hard phase is restricted to WC, attention can then be focused on the requirements for a suitable alternate binder material. Since these materials are processed to near-full density by liquid phase sintering, some restrictions on the choice of a binder are apparent, The selected binder should: (a) wet the carbide, (b) have solubility for both W and C, R K. VISWANADHAM, formerly Manager, Research-Development, Reed Tool Company, Houston, TX, is Staff Scientist, Martin Marietta Laboratories, 1450 South Rolling Road, Baltimore, MD 21227-3898. P. G. LINDQUIST ~s a Research Assistant m the Department of Materials Science and Engineering, University of Illinois at Champaign-Urbana, Urbana. IL 61801. Manuscript submitted November 6, 1986 METALLURGICAL TRANSACTIONS A
(c) form a liquid with W and C at not too high a temperature, (d) be a non carbide former, and (e) not form intermetallic compounds. In addition, the composition of the binder has to be adjusted so that neither free carbon nor the mixed eta carbide ( M 6 C ) is formed. Metals from group VIII of the periodic table satisfy these requirements to varying degrees. If Co is excluded, then the choice is limited to Fe, Ni, and their a
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