Lecture 27: Mechanical Alloying

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Lecture 27 Mechanical Alloying
PHYS 430/603 material Laszlo Takacs UMBC
Department of Physics
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Mechanical alloying takes place via repeated
plastic deformation, fracturing, and cold welding
of powder particles in a high-energy ball
mill. It is a method that can produce extremely
small grain size (to below 10 nm), metastable
phases (both crystalline and amorphous), and high
concentration of lattice defects. The figure
below is a very schematic representation of the
process in a mixture of two ductile materials.
Notice the formation of layers that get
randomized later.
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The equipment of mechanical alloyingBall mills
produce a mixture of impact and shearing/friction
between the balls producing the mixing/alloying
needed.

• Planetary mills and attritors produce more
  friction, the dominant form of action in
  vibratory and shaker mills is impact/
  compression.
• Available mills range from small laboratory
  versions to large industrial mills.

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Szegvari attritor model 1-ST Fritsch P-7
planetary mill
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SPEX 8000 Mixer Mill50 mm, 20 Hz, vmax 6
m/sFlat-ended vial

• Home made vibratory mill
• 4 mm, 50 Hz , vmax 1.3 m/s
• Cylindrical vial

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Interior of the milling container of the
vibratory mill.Seeing the powder is difficult
as it is a small fraction of the ball mass,
typically about 1/10. Some powder coats the balls
and the wall of the container, the rest collects
at the bottom, under the balls.

• Internal diameter 3.5 cm
• Volume 36 ml
• Balls 5 pieces of 8-mm and 100 g of 3.5-mm

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• Industrial tumbler mill and the insider of a
  typical clinker mill. The clain is often maid
  that mechanical alloying can be scaled up to
  industrial quantities, as the necessary equipment
  is available.

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History of mechanical alloying

• Developed to produce oxide dispersion
  strengthened nickel alloys, INCO Alloys, Gilman
  and Benjamin, 1968
• Ball milling can produce amorphous alloys, C.C.
  Koch, UNC, 1983 (top photo)
• Mechanically induced self-sustaining reactions,
  MSR,
• P.G. McCormick, 1989
• The first comprehensive book on mechanical
  alloying,
• C Suryanarayana, UCF, 2004 (bottom photo)
• Research on the chemical effects of mechanical
  milling has been ongoing since the 1950s, but the
  two areas did not have contact until the 1990s.

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The microstructure develops according to the
mechanical properties of the components
(Koch,1989). The length scale is an important
characteristic of the degree of activation.

• Ni-Al, a ductile-ductile system
  Cu-W, a ductile brittle system
• The only metal-metal showing MSR
• D.L. Zhang, 2005

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Fe-Zr, a system forming amorphous alloy.This is
an early stages of the process, the lamellae
become too thin for viewing by SEM
quickly.Mixing takes place by direct mechanical
effect and also by enhanced diffusion due to the
lattice defects created by milling.
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• What happens during mechanical alloying?
• Balls and the container wall gets coated with the
  processed powder, part of the powder remains
  free. Collisions can both deposit and remove
  powder from the surface, leading to mixing and
  homogenization.
• Powder gets caught between colliding balls and a
  ball and the container wall. Collisions provide
  impact and shear, they both fracture and cold
  weld particles.
• Grain size decreases, lattice defects accumulate.
• Alloying, solid state reactions take place, both
  by direct mechanical effect and defect-enhanced
  diffusion.
• Uniform powder is formed.
• The product of mechanical alloying is a fine
  powder with particles typically a few time 10µm
  in diameter, consisting of grains that are as
  small as 10 nm.

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Questions related to the mechanism of mechanical
alloying

• The mechanics of the ball mill. Quite complicated
  if non-central collisions, the rotation of the
  balls, partial inelasticity of the collisions due
  to powder coating, etc. are allowed.
• The motion of powder inside the mill, transfer
  from free particles to coating on the surfaces
  and back.
• Plastic deformation in the miniforging volume
  between colliding surfaces.
• Defect formation, diffusion, alloying, nucleation
  of new phases in the particles under mechanical
  action.
• Mechanical alloying is a very complex process
  that requires understanding on multiple length
  and time scales.
• General aspects
• The effect of mechanical action on the
  thermodynamic behavior.
• The energetics of the process.

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Delogu and Cocco, 2006

• lt------
• Ni asperity impacts a Zr surface.
• ------gt
• Impact with shear between a Ni and a Zr asperity.

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The effect of milling on the grain size of some
fcc metals

• The grain size and strain effects were separated
  using the diffraction angle dependence of the
  line width.
• Eckert et al., 1992

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MA for 60 hours in a planetary mill under
hexane.Notice the limited solubility of Mn in Ti
in the equilibrium state.Zhang et al. 2009
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Ni-Fe-Ag mixture milled for 1, 3, and hours in a
SPEX 8000 mill
Notice that Fe and Ni form an fcc alloy and up to
7 Ag dissolves in this matrix, based on shift of
lattice parameter. Bennett et al. 1995
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Reaction between Fe3O4 and Zn
Pardavi, Takacs, 1995
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The Mechanically induced Self-propagating
Reactions can occur, if a strongly exothermic
chemical reaction (with high adiabatic
temperature) is possible in the powder mixture.
MSRs consist of three steps 1. Activation 2.
Ignition 3. Self-sustaining reactionThe usual
indication of an MSR is the jump of the
temperature of the milling container, signaling
ignition.
Ti C (Sassari, Italy) Fe3O4 Zr
0.4 Sn 0.4 Zn S
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Ignition of Ti Si 5 3

• SPEX 8000 mill, quartz vial, one steel ball, 9.5
  mm diameter.
• Tad 2550 K
• Time from first smoldering spot to maximum
  brightness 250 ms

Temperature of the outside surface of the milling
vial.
Temperature inside the milling vial close to
ignition.
Monagheddu, Doppiu, Deidda, Cocco, 2003