Abstract
M.Tech.
A mixture of HfC and TiC powders and a (Hf,Ti)C powder have been hot pressed
with 4wt% Ni. In the absence of Ni the hot pressed temperature was 2000 °C and in
the presence of Ni 1650 °C. The pressure of 30 MPa was applied in both cases.
The starting powders were substoichiometric, as deduced from XRD spectra
analyses, and the (Hf,Ti)C powders consisted of a range of compositions, as
indicated by the width of the XRD peaks.
In the absence of Ni the powders sintered without the formation of a liquid phase. In
the case of HfC and TiC mixture, high- energy dry milled HfC + TiC +C black powder
sintering occurred with simultaneous formation of (Hf,Ti)C solid solution. On
account of mutual solid solubility of two carbides vacancy interdiffusion controlled
the solution as well as the sintering process, assisted by the high concentration of
vacancies in the starting powders. In the case of (Hf,Ti)C powder diffusion was also
the controlling process because t he solid solution was not h omogeneous a nd the
system t ended to homogenization, as shown by t he narrowing of t he XRD peaks
after sintering. Since the diffusion of HfC into TiC did not occur at the same rate as
the diffusion of TiC into HfC (as expected, on account of the different melting points
of the two materials) diffusional porosity was observed in some of the (Hf, Ti) C
grains. Grain growth was substantial but uniform. In the presence of Ni, sintering occurred with the formation of a liquid phase. The
volume fraction of the liquid phase formed was sufficient to yield a low porosity.
Grain growth was less than in the case of material sintered without Ni, probably just
on account of lower sintering temperature.
In the case of high- energy dry milled the reduction in particle size was observed.