In-Situ Response of WC-Ni Composites under Compressive Load
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INTRODUCTION
CEMENTED carbide composites are used in applications requiring high strength, wear resistance, and toughness, such as oil, gas, and mineral exploration and mining; high speed machining; and high-temperature dies.[1] Their deformation, fracture, and fracture toughness, which are unusually high for such hard materials, have been the subject of interest for many years.[2–9] The most widely used and studied system, WC-Co, has the complication of a strain-induced phase change of the (metastable) face-centered cubic Co phase to the stable hexagonal close-packed form during deformation. From the point of view of neutron scattering, it is a poor candidate for study because of the high absorption and weak scattering characteristics of the cobalt. For these reasons, the present study addresses the WC-Ni system. The main goals were to (1) understand the stress-strain response of cemented carbides, (2) observe the phasespecific response of these composites under load, (3) investigate the interaction between applied stress and thermal residual stress (TRS), and (4) gain insight into the origins of toughness in this class of materials. Similar results for WC-Co were also obtained[10] and will be compared in a subsequent article. J.W. PAGGETT, Postdoctoral Fellow, is with the Materials Science and Engineering Department, University of Michigan, Ann Arbor, MI 48109-2136, USA. A.D. KRAWITZ, Professor Emeritus, is with the Mechanical and Aerospace Engineering Department, University of Missouri, Columbia, MO 65211, USA. Contact e-mail: krawitza@ missouri.edu. E.F. DRAKE, Director, Materials Technology, is with the ReedHycalog, Houston, TX 77252, USA. M.A.M. BOURKE, Technical Staff Member, Acting Group Leader, B. CLAUSEN and D.W. BROWN, Technical Staff Members, are with the Los Alamos National Laboratory, NM 87545, USA. Manuscript submitted September 13, 2006. Article published online July 3, 2007. 1638—VOLUME 38A, JULY 2007
Neutron diffraction enables relatively large volumes (about 1 cm3) of WC-Ni to be sampled due to the low absorption of neutrons by even the heavy element W and enables phase-specific data to be collected in situ. Use of a pulsed neutron source enables simultaneous collection of all peaks of diffracting planes with interplanar spacings from about 0.5 to 4.0 A˚ in both the axial and transverse directions. In this study, the elastic strain response was monitored in WC-Ni samples during uniaxial compressive loading and unloading. In addition, both the initial TRS (introduced because of differences in the coefficients of thermal expansion between WC and Ni during production) and the final residual stresses after loading were determined. Previous experimental studies of the TRS in cemented carbides have shown that the mean stresses are compressive in the carbide and tensile in the binder and are considerable in magnitude.[10–14] For example, results for WC-15.6 wt pct Ni were 1215 MPa for Ni and –496 MPa for WC.[11] Also, when measured by neutron diffraction, they are independent of sample orientation due to the fact
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