Particle Separation at Liquid
Inclusion behaviour in liquids
by the Kirkendall Effect
Anders Eliasson, Lars Ekbom and Hasse
Royal Institute of Technology
ITM/Casting of Metals
S-100 44 Stockholm
This work is about:
• The very first part of the
liquid phase sintering
process (LPS < 10 sec).
HIP structure LPS ~1 sec
• The initial liquid penetration
into a solid agglomerate
• The separation and
spreading of the tungsten
LPS ~2 sec LPS ~8 sec particles.
Illustration of the initial stage of LPS in a
W-Ni-Fe alloy (25 vol% W-particles)
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
HIP 950 °C LPS 14 sec
• When the matrix
composition is further
penetration and particle
separation is more
HIP 1150 °C LPS 14 sec
White: Tungsten. Black: Matrix
Halogen lamp • 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.
Stumatite • An thermocouple (TC)
Alumina disc measures and regulates
Sample the processing.
Quartz glass tube
Liquid phase sintering
• 1470 C was reached
and held in the central
part of the sample, the
• Melting (at 1450 C) was
spreading towards the
outward region, the
Molten heated zone.
• A thermocouple
regulates the process.
Position of thermocouple-tube
Initial melting and penetration
• Initial melting, penetration and
particle separation of a tungsten
agglomerate by the molten
• Large concentration gradients is
found in the matrix.
Heated zone. Matrix composition at position (2) 26 % W inside the matrix bay
24 % W after a sharp slope
20 % W after a further slope
14 % W in the bulk matrix
16 (2 % W between agglomerates)
0 1 2 3
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,
2 SL cos / 2 SS DP, in the liquid by the gain
in surface energy + a
chemical driving force.
Wetting or penetration is described
by the dihedral angle, . • A parabolic penetration law is
found, l C t
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
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
• With a splitting up of the tungsten
agglomerates because of the unequal
mass flow between W and Ni/Fe.
Experimental Kirkendall effect in liquids
Displacement in liquid state at 1470 C
Displacement (micron) 12
HIP 950 C
2 HIP 1150 C
0 2 4 6 8 10 12 14
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.
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
• Further work by controlled diffusion fields is suggested
to validate usage of the theory.