Lecture 24: Strengthening and Recrystallization

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Lecture 24 Strengthening and Recrystallization

• PHYS 430/603 material
• Laszlo Takacs
• UMBC Department of Physics

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How can we make a material strong- have large
yield strength?

• We need to make the motion of dislocations
  difficult. That can be achieved by
• Decreasing the particle size of polycrystalline
  materials. Dislocation pile-up in one grain
  creates large stress in the neighboring grain,
  but the number of dislocations in a pile-up
  decreases with decreasing grain size.
• Work hardening. A dislocation in one slip plane
  acts as an obstacle for dislocations in another
  locks form.
• Solution hardening. Size (parelastic) and shear
  modulus (dielastic) effect, vcs for small
  solute concentration. Changing the separation
  between partials.
• Dispersion hardening. Finely dispersed particles
  are obstacles to dislocation motion.
• Precipitation hardening. Similar, but particle
  may be (semi) coherent.
• The above changes in a material influence many
  other properties. In particular, the motion of
  magnetic domain walls is similar to the motion of
  dislocations. What makes a material harder
  mechanically, usually also makes it harder
  magnetically.

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Dispersion hardening

• Finely dispersed particles are obstacles for
  dislocation motion.
• Orowan mechanism If the dislocation meets a pair
  of particles, it can only proceed by bowing out,
  similar to the principle of the Frank-Reed
  source. The stress needed to move the dislocation
  across this barrier is
• ? Gb/(l-2r)

Introducing finely dispersed particles into a
metal is often a difficult task, as the particles
must be wetted by the matrix but not dissolved by
it. Searching for a way to disperse aluminum
oxide particles in Ni-based superalloys lead to
the development of mechanical alloying in the
late 1960s.
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Precipitation hardeninga form of dispersion
hardening

• Precipitates are often coherent or semi-coherent.
  A dislocation can pass through a coherent
  precipitate, but it requires extra stress as a
  stacking fault results and creating it requires
  energy. This mode dominates, if the material
  contains many small particles. If the same total
  volume is in fewer but large particles, the
  Orowan mechanism is preferred. The largest
  hardness is achieved when the two mechanisms
  require the same stress this happens at around
  20 nm.

Precipitates form when a solid solution becomes
supersaturated during cooling and a second phase
crystallizes in the matrix in the form of small
crystallites. They are often coherent (like Ni3Al
in Ni.)
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• In forming operations, sufficient external stress
  is applied to force plastic deformation in spite
  of the obstacles to dislocation motion.
• Plastic deformation results in substantial
  changes in the microstructure
• Shear bands
• Dislocation networks
• Small/large angle grain boundaries
• Grain refinement, rotation, texture
• Annealing can change the microstructure -
  recovery and recrystallization
• Primary recrystallization - newly nucleated and
  growing, practically dislocation-free grains
  replace highly defected grains.
• Grain growth - larger grains grow at the expense
  of smaller grains, decreasing the grain boundary
  energy.
• Read Chapters 7.1-3

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The following images are from the site of
Professor John Humphreys at the Manchester,
Materials Science Centre, UK. There are also a
few very illuminating in situ movies at
http//www.recrystallization.info/

• Dislocation tangle in Al Dislocation cell
  structure in Cu

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Progressive misorientation of subgrains in a
large grain of sodium nitrate, deformed in
dextral simple shear. Note the migration of
subgrain boundaries and the clear changes in
morphology after the appearance of new high angle
grain boundaries. Shear strain at the last photo
is 1. Long edge of each photo is 0.5 mm.M. R.
Drury and J. L. Urai
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Recrystallization of an Al-Zr alloy

• Notice the fine and oriented deformed structure
  and the growing, virtually defect-free
  recrystallized grains.

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Strain rate dependence

• The time / strain rate dependence of the
  stress-strain curve is intuitively anticipated
  and clearly observed, but it is very difficult to
  explain quantitatively. Typically it is
  characterized by the strain rate sensitivity, m,
  defined as

Ball milled and consolidated Cu, average particle
size 32 nm. Babak Farrokh, UMBC
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Temperature dependence

• The strain rate sensitivity is low at low
  temperature (T lt 0.5 Tm) but increases at higher
  temperature. This is understandable, as atomic
  motion is more vigorous at higher temperature,
  diffusion is faster.
• High strain rate sensitivity is usually
  associated with larger strain to failure.
  Consider a tensile experiment. If a random cross
  section decreases in diameter, the strain rate at
  that cross section increases, with enough strain
  rate sensitivity the section becomes harder and
  no further reduction leading to failure occurs.
• In fine grained (lt10 µm) materials close to Tm
  very large strain rate sensitivity and strain to
  failure (up to 100-fold elongation) can be
  observed. This is called superplasticity. It
  depends on grain boundary sliding, rather than
  dislocation mechanisms.
• Nanocrystalline materials contain many grain
  boundaries, superplasticity should be more easily
  achieved.

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Superplasticity of electrodeposited nc Ni andnc
Al-1420 alloy and Ni3Al by severe plastic
deformation

• Notice that superplasticity was achieved at a
  temperature much below typical for conventional
  materials 350C for Ni corresponds to 0.36 Tm!
• McFadden et al. (UC Davis, Ufa, Russia)
• Nature 398 (1999) 684-686

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High RT ductility of a hcp Mg-5Al-5Nd alloy

• Ball milling results in repeated fracturing and
  agglomeration of grains, resulting in a nanometer
  scale microstructure (mean grain size probably 25
  nm). Grain rotation and sliding results in high
  ductility even at room temperature and
• 3x10-4 s-1 stress rate. (Recall that a hcp
  material is normally brittle.)

L. Lu and M.O. Lai, Singapore
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• Ceramic nanocomposite of 40 vol. ZrO2, 30
  Al2MgO4, and 30 Al2O3 shows superplastic
  behavior at 1650C.
• Kim et al. (Tsukuba, Japan)

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Creep
mass flux

• Nabarro-Herring creep Coble creep
• mediated by
• volume diffusion grain boundary diffusion

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Anelasticity and viscoelasticity

• Small time dependent effects can be observed also
  for elastic deformation - e.g. related to
  reversible diffusion of C in a steel under
  stress.
• If a sample is vibrated close to resonance, the
  deviation from perfect elasticity, i.e. the
  existence of dissipative processes, results in a
  change of the resonance curve.
• While technologically unimportant, this is the
  way one can gain information about diffusion and
  other time dependent phenomena at low
  temperature, where their rate is very low and the
  macroscopic effects are not detectable.
• Anelastic under constant stress
• Viscoelastic