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 tempera tures between 1773 and 2573 K using a secondary ion mass spectroscopy technique to determine the diffusion contribution to the coarsening pr ocess. The diffusion process at high temperature is controlled by latt ice diffusion with an activation energy of 335 kJ mol(-1) whereas that at low temperature is governed by grain-boundary diffusion with an ac tivation energy of 170 kJ mol(-1). It appears that the coarsening proc ess is controlled by two energy barriers: one dictated by the diffusiv ity of hafnium and the other by the solubility limit as a function of temperature. The strain energy required to dissociate the carbide part icles into individual species was also considered. The effects of the coarsening of HfC particles in a dispersion-strengthened W-0.4 mol % H fC alloy on recrystallization a nd creep deformation were illustrated using a concerted experimental modelling analysis. Results show that t he strengthening effect of the HfC particles is significantly reduced at temperatures above 1800 K, due to particle coarsening.