Ultrafine-grained WC-Co bulk materials were prepared by a new method that contains pretreatment of the milled powder mixture and subsequent spark plasma sintering （SPS）. Ball milling parameters and the pretreatment temperature have significant effects on the microstructure and properties of WC-Co cermets. The prepared cermets have a mean grain size of less than 0.5 μm even with a pretreatment temperature as high as 1300℃. The WC-10wt.%Co cermet bulk prepared by the optimized milling, pretreatment, and SPS processing achieves excellent mechanical properties with a Vickers hardness of HV 1643, a fracture toughness of 13.1 MPa.m^1/2 and a transverse rapture strength of 3100 MPa.
The thermal features of the nanograin boundary were described by a developed thermodynamic model. Using the nanocrystalline Cu as an example, the pressure, the bulk modulus, and the volume thermal expansion coef- ficient were calculated to characterize the thermodynamic properties of the grain boundaries on the nanoscale. Based on the parabola-type relationship between the excess free energy and the excess volume of the nanograin boundary, the thermal stability, as well as its evolution characteristics, was analyzed. The experimental re- sults of the temperature-varying grain growth in the nanocrystalline Cu, which exhibited the discontinuous nanograin growth behavior, verified the thermodynamic predictions. In addition, the quantitative relationships correlating the excess volume and the lattice expansion with the nanograin size were discussed.
