GRAIN SIZE HARDENING AND SOFTENING IN TUNGSTEN CARBIDE AT LOW HOMOLOGOUS TEMPERATURES

摘要

Data in the literature on the effect of grain size d in the range from cm to nm on the flow stress of WC are evaluated, including nanocrystalline materials prepared by a special pulsed laser ablation method. Three grain size regimes were identified: (a) Regime I, d=10~(-2) to 0.5x 10~(-6) m, (b) Regime II, d=0.5xl0~(-6)-10~(-8) m and (c) Regime III, d<10~(-8) m. Grain size hardening occurred in Regimes I and II and softening in Regime III. Both straight and tangled dislocations were observed in Regime I. The major influence of grain size in Regime I could result from either its effect on the total dislocation density or the pile-up of dislocations. Insufficient data are available for Regime II to draw any positive conclusions regarding the governing mechanism. Analysis of the grain size softening in Regime III according to our grain boundary shear model yielded a reasonable activation volume, but a lower-than-expected activation energy. The lower energy could be due to the presence of the amorphous NiAl film at the grain boundaries, employed to obtain the nanocrystalline grain size.