Settling in solid-liquid systems with specific application to liquid phase sintering

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The evolution of microstructure in liquid phase sintered (LPS) alloys as it occurs in a gravity environment is discussed. If the solid phase volume fraction (Vp) is initially sufficiently low, a dispersion of isolated solid particles within the liquid develops first. These isolated particles settle at a velocity approximately equal to the Stokes' velocity and, in a finite container, the settling leads to the formation of a solid skeleton when Vp reaches a critical value. This value is ca. 0.20 for Fe-Cu LPS alloys and because the transition to the skeleton structure is dictated by morphological considerations, we expect this critical value of Vp is system independent. Once formed, a skeletal structure can continue to settle. The settling mechanism is controlled by "extrication" of solid particles from the skeleton and their subsequent directional flow within the liquid as a result of the differing solid and liquid densities. Since particle extrication is a relatively slow process, the "skeletal" settling rate is much reduced in comparison to free settling rates. Particle extrication times are system dependent; they are much less in W-(Fe-Ni-Cu) alloys than in Fe-Cu alloys. The differences are believed related to the variations in average neck size to average particle size ratio between the two materials. Both settling velocities in isolated and skeletal structures and the parameters defining the transitional behavior are discussed in terms of system physiochemistry and morphology. © 1983.

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Acta Metallurgica