Initial stage of liquid phase sintering: liquid reaction and particle growth in W–Ni–Fe–(Co)

Abstract

The initial stage of liquid phase sintering, involving liquid reaction and particle growth, has been investigated under microgravity in experiments using tungsten heavy alloys for short periods of time (8–14 seconds). The influence of different factors, such as alloy composition, plastic deformation, and non-equilibrium conditions, have been evaluated. During the liquid phase sintering of tungsten heavy alloys at about 1470°C, the liquid matrix penetrates the tungsten particle agglomerates. A fraction of the tungsten particles goes into solution in the liquid phase and the original tungsten powder size will initially be reduced. At the same time, the agglomerates of tungsten particles are effectively separated. In a second stage, larger particles grow in equilibrium with the matrix while pure tungsten particles are dissolved into the matrix. When equilibrium is reached, the tungsten particles start to grow in the liquid Ni–Fe–W matrix phase in accordance with the Ostwald ripening process. A theoretical treatment of the particle solution and growth during these stages is proposed.The addition of iron and cobalt to the W–Ni system reduces the rate of penetration and growth. Non-equilibrium conditions during the formation of a liquid phase have a marked effect on the tungsten particle separation. Milling of the tungsten powder increases the initial growth of tungsten particles.