Residual stress in WC-Co measured by neutron diffraction

Large thermal residual microstresses (TRS) can develop in WC-Co composites owing to the difference of the coefficients of thermal expansion (CTE) of the constituents. The variation with temperature of average stresses in a WC-11 wt.% Co sample were studied between room temperature and 1273 K by measuring the cell parameters of cobalt and WC using neutron diffraction. WC powder was also measured to provide stress free reference standards. At room temperature, a hydrostatic compressive stress of about 500 MPa was measured in the WC. The evolution of TRS shows two temperature domains. The low temperature domain (300 < T < 1000 K) is characterised by a decrease of residual stress magnitude as the temperature is increased. The high temperature domain (T > 1000 K) is characterized by an increase of residual stress in WC, a rapid increase of Co lattice parameter, and a hysteresis between the heating and cooling cycles. A model, based on Eshelby's equivalent inclusion method, predicts the observed behavior in both domains. In the low temperature domain, the CTE mismatch between WC and Co accounts for the decrease of TRS upon heating. In the high temperature domain, the system is modelled by the solution of a layer of WC in the Co, which increases the Co lattice parameter and leads to an increase of compressive stress in WC. The model indicates that there is 2.09 at.% W in solution in the cobalt. The hysteresis is attributed to a difference in the heating and cooling kinetics of solution-precipation of W from WC and WCo3. The results are compared with the mechanical properties of WC-Co.