Homogenization of compacted blends of nickel and tungsten powders

Abstract

The diffusional homogenization of compacted blends of elemental Ni and W powders was studied using quantitative metallography and X-ray compositional line broadening. Homogenization treatments were carried out in the temperature range from 1156 to 1271°C (in the region of the Ni-W phase diagram where only the terminal solid solutions exist). The mean compositions of the compacts studied ranged from 0.06 to 0.14 atomic fraction tungsten (in the Ni-rich solid solution). Other variables studied included the size and shape of the W particles, compaction pressure, and the effect of applied load during the homogenization treatment. Experimental measurements of the solution of the unstable W-rich phase and the increase in W concentration in the Ni-rich matrix were compared to calculated values obtained from a concentric-sphere mathematical model which assumed volume interdiffusion (concentration-independent diffusion coefficients) and local equilibrium at the heterophase interface. The model provided a fairly good approximation of the homogenization kinetics and the effects of the major processing and powder parameters investigated. The primary sources of discrepancy between model predictions and experimental data appeared to result from modest surface diffusion contributions to homogenization and imperfect powder blending.