Activated sintering of nickel-doped tungsten: Approach by grain boundary structural transition

The addition of a small amount of Ni to a W powder has been known to enhance the densification of a compact significantly especially in the temperature range of 1,000 approximately 1,200 C. It is highly probable that the abrupt change in the sintering behavior by the addition of a small amount of the second element might be related to a grain boundary structural transition. In relation to this possibility, Lee et al. studied the temperature dependence of the grain boundary diffusivity. They observed that the diffusivity increased abruptly above approximately 1,100 C for Ni-doped W. This result is similar to the previous report by Schintlmeister and Richter on the discontinuous increase in grain boundary diffusivity above approximately 1,200 C in Ni-doped W. Such an abrupt change in grain boundary diffusivity was attributed to the grain boundary transition. On the other hand, the temperature for such a discontinuous change in grain boundary diffusivity coincides with the temperature for the onset of activated sintering of Ni-doped W. Therefore, there is a possibility that the discontinuous increase in the densification of W by Ni might be the result of a structural transition of the grain boundary. The purpose of this paper is to examine this possibility of an Ni-induced grain boundary transition of W grains based on microstructural observations