The effect of temperature gradient on liquid coagulation during liquid phase sintering of W-Ni-Fe alloy has been investigated. Powder compacts were prepared from 4.6㎛ Ni, 5 ㎛ Fe and W powders of 1,5 and 10㎛ size. For model experiments spherical Ni particles of 125 $\mu$m diameter were used. Cylindrical compacts have been sintered mostly above the melting point of Ni in the presence of liquid matrix. The heating rate has been about $50^\circ{C}/min$.
The liquid coagulation was most pronounced in W (10㎛)-1Ni-1Fe specimens. After sintering at $1455^\circ{C}$ for "0" time, a region of high matrix content and low porosity has been formed at the specimen center along with an outer ring of low matrix content and high porosity. Upon isothermal sintering the porosity in the inner and outer regions has decreased, while matrix distribution has became uniform. Since the liquid coagulation was most likely to occur from the temperature gradient in the specimen during heating, the melting and spreading behavior of Ni was observed in model specimens of W-6Ni mixtures prepared from 1㎛ W powder and 125㎛ spherical Ni particles. The microstructural changes in these specimens clearly show that liquid coagulation can occur from temperature gradient. As the outer region of the specimen near its surface reaches the melting point of Ni, Ni melts spread into the capillaries between W powders leaving large spherical pores at the sites of Ni particles. Some large pores between W grains are left unfilled as Ni melts continue to flow into the capillaries in the inner region following the advancement of the high temperature front. When Ni particles in the inner region begins to melt, its melt can fill only a few of the remaining pores between W grains. part of the Ni melts still remain at the Ni sites in the inner region with spherical pores at their centers. These are subsequently filled again with liquid, while the spherical pores near the surface remain unfilled. Furthermore, since the surface region remains at high temperature for longer time grain shape accommodation is further advanced in this region than the inner region. Therefore, the matrix content between W grains is also higher in the inner region. Thus a liquid coagulation structure with the melts filling all the spherical pores and occupying large space between W grains results, while the spherical pores in the outer region are unfilled.
Based on observations on this model system the coagulation behavior in the mixtures of fine powders is reexamined. In this specimens prepared from fine mixtures, densification is so rapid that liquid coagulation occurs with low liquid content.
