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srs.ki.seSRS_meeting06EliassonAnders_liquid.ppt

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

Anders Eliasson, Lars Ekbom and Hasse
Fredriksson


Royal Institute of Technology
ITM/Casting of Metals
S-100 44 Stockholm

                                        1
  Background
                                                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
                                                    structure.

                                                •   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)



                                                                                     2
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.
   HIP 950 °C           LPS 14 sec
                                      •   When the matrix
                                          composition is further
                                          from equilibrium,
                                          penetration and particle
                                          separation is more
                                          pronounced.
    HIP 1150 °C          LPS 14 sec
    White: Tungsten. Black: Matrix


                                                                     3
      Experimental design

    Ellipsoidal mirror
                                   Sample
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.
                               Alumina   TC
                               tube
                   Stumatite                  •   An thermocouple (TC)
    Alumina disc                                  measures and regulates
      Sample                                      the processing.
                           Focus
       Quartz glass tube




                                                                                4
Liquid phase sintering

                                                  •   1470 C was reached
                                                      and held in the central
                                                      part of the sample, the
                                                      molten zone.
Heated
zone
                                                  •   Melting (at 1450 C) was
                                                      spreading towards the
                                                      outward region, the
                                         Molten       heated zone.
                                         zone

                                                  •   A thermocouple
                                                      regulates the process.
         Position of thermocouple-tube




                                                                                 5
              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.
               Heated zone. Matrix composition at position (2)           26 % W inside the matrix bay
     28

     26
                                                                         24 % W after a sharp slope
     24
     22
                                                                         20 % W after a further slope
     20
                                                                         14 % W in the bulk matrix
%W




     18
     16                                                                   (2 % W between agglomerates)
     14
     12

     10
          0            1                       2                 3
                                      Micron




                                                                                                             6
   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



                                                                           7
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.




                                                        8
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.



                                                           9
Experimental Kirkendall effect in liquids
                                            Displacement in liquid state at 1470 C
                               14

       Displacement (micron)   12

                               10                                x
                               8

                               6

                               4
                                                                                     HIP 950 C
                               2                                                     HIP 1150 C

                               0
                                    0   2        4          6          8        10   12           14
                                                            Time (sec)


              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.




                                                                                                       10
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.



                                                                11

				
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posted:10/19/2012
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