Binder deformation in WC-(Co, Ni) cemented carbide composites
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INTRODUCTION
CEMENTED carbides (cermets) comprise a class of composite materials widely used for their unusual combination of deformation-resistance, wear-resistance, and high toughness. Cermets are produced by a powder metallurgy and liquid phase sintering methodology,1 resulting in a fully dense composite containing a major fraction of a metal carbide and a minor fraction of metallic binder. Fundamental understanding of mechanical behavior in cermets has lagged conventional wisdom due to the complexity and fine scale of their microstructures, the uncertain character and role of the sizable differential thermal residual stresses which develop between carbide and binder during cooling, and lack of knowledge about the nature of deformation in and between the constituents. These factors affect loadsharing between the phases and plastic accommodation of compatibility strains in the deformed composite. The present work addresses the role of the binder phase in the mechanical response of model cermets in which binder phase stability is systematically altered through alloy additions. This report focuses on the binder microstructure and deformation character in the WC-(Co, Ni) system resulting from a range of mechanical treatments. The WC-Co family of cermets has long been recognized for superior performance in ambient-temperature applications, The cobalt binder in these alloys remains predominantly in its high-temperature fcc crystal structure upon cooling to room temperature. The normally sluggish allotropic transformation to the stable hcp form is thought to be further retarded in cermets by the effects of carbide restraint and solutes acquired during sintering. More recently it has become clear that the metastable fcc cobalt binder phase undergoes a strain-induced transformation to hcp under C.H. VASEL, formerly Graduate Research Assistant, University of Missouri, Columbia, MO, is now Research Engineer, Exxon Production Research Company, Houston, TX 77001. A.D. KRAWITZ is Associate Professor of Mechanical and Aerospace Engineering, University of Missouri, Columbia, MO 65211. E. F. DRAKE is Staff Metallurgist, Reed Tool Company, Houston, TX 77252. E. A. KENIK is Research Staff Member, Metals and Ceramics Division, ORNL, Oak Ridge, TN 37830. Manuscript submitted May 29, 1984. METALLURGICAL TRANSACTIONS A
load. 2-5 This martensitic transformation is well known in bulk cobalt and its alloys6-1~and now has been shown to proceed to appreciable levels in WC-Co composites under conditions of monotonic3'5 and cyclic5 loading, and in percussive 4 and rotary5 rock-drilling service. The role of this transformation in the mechanical behavior 9of cermets has been considered by Satin and Johanneson,3 who used TEM to study WC-Co alloys having 6 and 15 wt pct cobalt. They reported transformation of up to 10 vol pct of the binder to hcp after loading to 85 pct of the ultimate compressive strengths of the alloys, which corresponds to 1.0 and 1.3 pct strain, respectively. A deformation and failure model was proposed in which the bi
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