COARSENING OF HAFNIUM CARBIDE PARTICLES IN TUNGSTEN

The coarsening behaviour of finely dispersed HfC particles in a W-HfC alloy was investigated by monitoring the growth rate of the particles. An activation energy of 480 kJ mol(-1) was obtained for the process. Diffusion experiments of hafnium in tungsten were conducted at temperatures between 1773 and 2573 K using a secondary ion mass spectroscopy technique to determine the diffusion contribution to the coarsening process. The diffusion process at high temperature is controlled by lattice diffusion with an activation energy of 335 kJ mol(-1) whereas that at low temperature is governed by grain-boundary diffusion with an activation energy of 170 kJ mol(-1). It appears that the coarsening process is controlled by two energy barriers: one dictated by the diffusivity of hafnium and the other by the solubility limit as a function of temperature. The strain energy required to dissociate the carbide particles into individual species was also considered. The effects of the coarsening of HfC particles in a dispersion-strengthened W-0.4 mol % HfC alloy on recrystallization a nd creep deformation were illustrated using a concerted experimental modelling analysis. Results show that the strengthening effect of the HfC particles is significantly reduced at temperatures above 1800 K, due to particle coarsening.