Measurement of residual thermal stress in WC–Co by neutron diffraction D. Mari a, * , B. Clausen b , M.A.M. Bourke b , K. Buss a a Ecole Polytechnique Fédérale de Lausanne (EPFL), Institut de Physique de la Matière Complexe, CH-1015 Lausanne, Switzerland b Los Alamos National Laboratory, New Mexico 87545, USA article info Article history: Received 12 September 2008 Accepted 27 November 2008 Keywords: Cemented carbides Cobalt WC Residual stresses Thermal expansion abstract The temperature dependence of residual stresses in a WC–17.8vol.%Co cemented carbide was measured by neutron diffraction. The comparison of the WC lattice parameter within the WC–Co and within stress- free WC reference provides a measurement of lattice elastic strains and, using Hooke’s law, stresses. WC is found to be under hydrostatic compressive stresses of about 400 MPa at room temperature, which decrease monotonically with temperature to a near-zero value at 800 °C. Residual stresses in cobalt also decrease with increasing temperature, but show an apparent increase above 800 °C, which is attributed to an increase in lattice parameter due to W dissolution in the Co phase. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction WC–Co cemented carbides are materials widely used for the manufacturing of cutting tools. They are known for their excep- tional combination of toughness and hardness. However, severe cutting conditions implying high temperature and stress produce plastic deformation of the cutting tool tips. Therefore, a better knowledge of the material behaviour at high temperature is needed to improve the quality of the tools. The stress field influ- encing the microstructure not only derives from the applied load but also from internal stress. As thermal expansion coefficients be- tween WC and Co differ by a factor of three, cooling from the sin- tering temperature of about 1400 °C to lower temperatures can produce extremely high residual stresses. Measurement of residual strains (stresses) in composites is obtained from the comparison of in situ measured lattice parameters for each phase with a stress- free reference. Diffraction measurements of residual thermal stres- ses have been performed by different authors [1–6] both by X-rays and neutrons. The high tungsten content considerably limits the penetration depth of X-rays and measurements are sensitive to polishing and annealing. Hence, neutrons are a better choice to measure bulk properties of WC–Co. On the other hand, in WC– Co, due to the high elastic modulus of WC, a very good precision in measurements is required to obtain elastic strains with ade- quate accuracy. In order to reduce experimental errors in neutron diffraction measurements, differences in positioning and tempera- ture between reference sample and the composite material must be minimum. At room temperature, compressive residual stresses around 500 MPa in WC and tensile stresses around 2000 MPa for cobalt are expected in a WC-18vol.%Co [4]. As temperature is in- creased, thermal stresses are expected to relax. A curious behav- iour was found by [4] where thermal stresses started to increase again by heating above 800 °C. A thermal hysteresis between heat- ing and cooling cycle was also observed. Since strains of the WC lattice are extremely small, it is not clear whether such behaviour should have been attributed to some experimental error. The pur- pose of this work is to measure the residual stress in both phases of a WC-17.8vol.%Co with high accuracy. We use a particular setup and time-of-flight neutron diffraction in order to achieve maxi- mum precision and temperature reproducibility in the measure- ments of the composite materials and of the WC and Co references. The changes of residual stresses in the WC and Co phases are measured up to 1156 °C. 2. Materials and experimental methods WC–Co samples with 17.8vol.% cobalt were prepared by AB Sandvik Coromant (Stockholm, Sweden). This high cobalt grade al- lows the measurement of both WC and Co diffraction peaks with good intensity. Bars of 89  19  4.5 mm 3 were produced by a usual sintering process. The SEM micrograph in Fig. 1 shows the general grain morphology. The average WC grain size is 1.8 lm. Neutron diffraction measurements were performed in ‘‘time-of- flight” mode using the SMARTS diffraction instrument [7] at the pulsed neutron source at the Lujan Center of the Los Alamos Na- tional Laboratory (NM, USA). Detector banks positioned at ±90° an- gle with respect to the incident beam collect a complete diffraction 0263-4368/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2008.11.015 * Corresponding author. Tel.: +41 21 693 4473; fax: +41 21 693 4470. E-mail address: daniele.mari@epfl.ch (D. Mari). Int. Journal of Refractory Metals & Hard Materials 27 (2009) 282–287 Contents lists available at ScienceDirect Int. Journal of Refractory Metals & Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM