A new insight into the ∑=2 grain boundary characteristics in WC powder and in WC-Co sintered materials

In order to gain a better understanding of the WC-Co sintering process, the change in ∑ = 2 grain boundaries was studied in two series of alloys with different carbon contents and increasing Co content in the 10–50 vol% range. The samples were sintered at 1410 °C for 1 h and 5 h. The frequency of ∑ = 2 grain boundaries and the rotation angle distribution were determined using electron backscatter diffraction (EBSD) in order to make a statistical analysis of the effect of composition and sintering time on the special boundary characteristics. Complementary TEM observations were conducted for a more thorough determination of the rotation aspects (angle/axis). The rotation angle is scattered in the WC powder with a maximum peak close to 90°, as expected for ∑ = 2 grain boundaries. However, the average rotation angle tends towards 88.7° in sintered alloys. This deviation is explained by a dislocation array compensating for the misfit and angular deviation in the grain boundary. Its characteristics are calculated using the 0-lattice approach and the associated elastic energy is evaluated from these data. The observed angular deviation is consistent with a minimum elastic energy of the boundary. Particle rearrangement, which can occur during liquid phase sintering, might explain the slight rotation of adjacent crystals in the sintered materials, in order to find a lower energy configuration. A mechanism involving stacking faults in successive prismatic planes is also proposed to explain the formation of ∑ = 2 boundaries and their high frequency in the powder.