Particle Separation at Liquid Phase Sintering - Inclusion behaviour in liquids by the Kirkendall Effect

1. Anders Eliasson, Lars Ekbom and Hasse Fredriksson Royal Institute of Technology ITM/Casting of Metals S-100 44 Stockholm Particle Separation at Liquid Phase Sintering-Inclusion behaviour in liquids by the Kirkendall Effect

2. Background This work is about: The very first part of the liquid phase sintering process (LPS < 10 sec). The initial liquid penetration into a solid agglomerate structure. The separation and spreading of the tungsten particles.

3. Why do we get this separation and spreading of the tungsten particles? The separation rate of the particles is much higher for materials produced at a low HIP (Hot Isostatic Compaction) temperature. When the matrix composition is further from equilibrium, penetration and particle separation is more pronounced.

4. Experimental design The sample is placed at the position of the focal point of ellipsoid mirrors. The sample ? 3x8 mm.Isothermal focal length is around 6 mm. An thermocouple (TC) measures and regulates the processing.

5. Liquid phase sintering 1470 ?C was reached and held in the central part of the sample, the molten zone. Melting (at 1450 ?C) was spreading towards the outward region, the heated zone. A thermocouple regulates the process.

6. Initial melting and penetration Initial melting, penetration and particle separation of a tungsten agglomerate by the molten matrix. Large concentration gradients is found in the matrix. 26 % W inside the matrix bay 24 % W after a sharp slope 20 % W after a further slope 14 % W in the bulk matrix (2 % W between agglomerates)

7. Theories of Melt Penetration A penetration of the solid grain structure occurs at low dihedral angels (?). The penetration rate is linked to the gain in free energy of the wetted surfaces. A diffusion process takes place as the liquid penetrates the grain boundaries. The driving force for diffusion is given by the pressure drop, DP, in the liquid by the gain in surface energy + a chemical driving force. A parabolic penetration law is found,

8. Theories of Particle Separation Radial movement due to the melt penetration - retardation in the liquid matrix is too high. Solid-liquid front passing from high tungsten areas to low tungsten areas - might work. Brownian motion - too slow. Marangoni convection acts only at liquid/liquid interfaces, not on solid/liquid interfaces � not relevant.

9. The Kirkendall effect in liquids A fast liquid diffusion of nickel/iron from low-content tungsten areas to high-content tungsten areas, i.e. towards the agglomerate structures. A slower liquid diffusion of tungsten in the opposite direction. Results in a sort of Kirkendall effect in liquid phase. In which the tungsten particles in the agglomerates will move because of crystal lattice rearrangement. With a splitting up of the tungsten agglomerates because of the unequal mass flow between W and Ni/Fe.

10. Experimental Kirkendall effect in liquids Marker displacement in liquid matrix at heat-treatment at 1470 ?C, by a Kirkendall effect, for two different HIP temperatures. (x) is an experimentally observed displacement distance.

11. Conclusions and Further work Liquid penetration can be explained by a combination of differences in interface energy per unit area and wetting under non-equilibrium conditions. Tungsten particle separation and spreading might be explained by differences in diffusion rate and mass flow between Tungsten and Nickel/Iron. The suggested Kirkendall effect in liquids might be an explanation to some other phenomenon like inclusion behaviour in iron base alloys during teeming and deoxidation etc. Further work by controlled diffusion fields is suggested to validate usage of the theory.