L05C: Surface defects

1
L05C Surface defects

• Defects arise in all stages of production and
  processing.
• CASTING
• A common phase in production of metals is
  casting.
• A melt is put in a mold and heat is extracted
  through the mold wall.
• Crystals nucleate on the mold wall where the
  temperature is lowest and grow inward. This
  inward growth produces a columnar structure,
  with the lengthwise crystallographic orientation
  of each grain about the same, in the
  preferential growth direction).
• For example, a cast ingot of pure copper

Last revised on April 4, 2014 by W.R. Wilcox,
Clarkson University
2
Simulation of casting of an Al-Si alloy with
nucleation of new grains in the melt.
(http//www.tms.org/pubs/journals/jom/0201/thevoz/
thevoz-0201.html ) View in projection mode to see
the action.

• Microstructure depends on many things
• the alloy composition
• how close the melt is to the freezing point
  when poured it
• how rapidly it's cooled
• whether the cooling's all around or mostly on
  the bottom
• etc.

Grain Refiner - added to make smaller, more
uniform, equiaxed grains.
3
Casting of alloys

• Alloy crystals tend to grow as dendrites?
  https//www.youtube.com/watch?vS07fPo45BvM
• If the melt falls below its melting point while
  being added to the mold, small crystals may have
  already nucleated in the melt and be floating
  around.
• Dendrite arms may detach and float around in the
  melt.
• After solidification is complete, grains formed
  by the floating crystals have random shapes and
  orientation. The region occupied by these in
  the casting is called equiaxed.
• Example Ti47.2 Al1.50

http//www.sciencedirect.com/science/article/pii/S
0966979507000817
4
Polycrystalline Materials

• Grain Boundaries
• regions between crystals
• transition from lattice of one side to that of
  the other
• High-angle grain boundaries
• highly disordered
• low density
• high impurity diffusivity
• high chemical reactivity

• Low-angle grain boundaries
• slightly disordered
• made up of a line of dislocations, which can be
  seen by usual methods of revealing dislocations.

5
Low-angle grain boundaries

• If formed only by edge dislocations its a tilt
  boundary

• If formed only by screw dislocations its a twist
  boundary. Most are mixed.
• Dividing line between high-angle and low-angle
  boundaries is fuzzy, roughly between 10o and 20o
• If individual dislocations can be seen, can be
  considered low angle.
• For example, etch pits on NaCl YAlO3

6
Stacking faults

• Found in closed-packed face-centered cubic and
  hexagonal crystals because only the
  second-nearest neighbors are different at the
  fault
• Reminder Close-packed planes in FCCare in order
  ABCABC 111, while in HCP the order is ABAB
  0001.
• Example Austenitic steel

• FCChexagonal stacking faults also common with
  the diamond structure and the zinc-blende
  structure. (Diamond Lonsdalite, zinc-blende
  wurtzite.)

7
Twin boundarieshttp//en.wikipedia.org/wiki/Cryst
al_twinning

• Twins are two grains whose lattices are at a
  definite, reproducible, orientation with respect
  to one another.
• Crystal lattices in the twins may be mirror
  images of one another, i.e. reflection twins.
  Si, for example? http//www.tf.uni-kiel.de/matwi
  s/amat/def_en/kap_7/backbone/r7_1_1.html
• When the two lattices share all atoms at the
  boundary they are called coherent. Common, but
  not always.
• Twinning can occur during plastic deformation,
  transformation to a different crystal structure,
  or crystal growth.
• The mechanisms for twinning during deformation
  and transformation are generally well understood.
• The mechanisms for twinning during crystal growth
  are generally unknown.
• Twin boundaries are often planar, and appear as
  straight lines in a section.
• But sometimes twin boundaries jog so that they
  appear curved at low magnification.
• Twin boundaries are often parallel to one
  another.Copper, for example ?
  http//www.nature.com/am/journal/2009/200904/full/
  am2009128a.html

8
Examples of stacking faults twins in metals

• The spheres labeled A in the figure to the
  right from VMSE represent metal atoms in a
  close-packed plane. Positions B and C show the
  two possible locations for the next close-packed
  plane on top of this one.

• Planes stacked in the order ABCABC generate a
  FCC crystal.
• In FCC, one type of stacking fault can be
  represented by ABCBCABC.
• In FCC, a reflection twin can be represented by
  ABCBCABC.
• Planes stacked in the order ABABAB generate a
  HCP crystal.
• In HCP one type of basal plane (0001) stacking
  fault can be represented by ABACABA
• Many twin planes observed in HCP and much more
  difficult to illustrate.
• More complex twins in BCC.

9
Interface between two phases

• Another type of surface defect. For example
• Second phase inside the solid
• Thin films (extremely important technologically)
• Small solid particles in a gas or liquid.
• Notice that the interface may have a structure
  quite different from those of the adjacent bulk
  phases (http//en.wikipedia.org/wiki/Surface_recon
  struction)
• If we assume the crystal structure exists up to
  the surface, several types of defects can exist
  at this surface

Some chemical reactions may take place only at
specific surface sites.
10
Solid Catalysts and Surface Defects

• A catalyst increases the rate of a chemical
  reaction without being consumed
• Active sites on catalysts are normally surface
  defects

11
Volume defects

• Second phase in solid. Can be void, gas bubble,
  or another solid.
• When insoluble foreign particles are present in a
  melt, these may be trapped in the solid during
  solidification.
• If the impurity is soluble in the solid at the
  melting point, it may precipitate out as the
  solid is cooled. (Solid solubility normally
  decreases as temperature is decreased.) These
  precipitates may be gas bubbles, impurity itself,
  or compound between impurity and solid.
• Example carbon flakes in gray cast iron

• Other methods of forming composite materials
• Mixing of concrete and then hardening by
  formation of hydrate crytals.
• Mixing of fibers with a monomer and then
  polymerizing.

12
Defects in Polymers

• Defects due in part to chain packing errors and
  impurities such as chain ends and side chains

Adapted from Fig. 5.7, Callister Rethwisch 4e.