Topic 6: Case Studies

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Topic 6 Case Studies

• Feb. 3, Feb. 8 Metal casting lab tour (Room 15,
  IMS building basement)
• Feb. 10 Quiz 1
• Reading assignment Ch. 1-13
• Strengthening strategy eliminate or stop
  dislocations
• Work hardening
• Precipitation hardening
• Grain refinement
• Case study Al-Li alloys (precipitation
  hardening)
• Case study High strength low alloy (HSLA) steels
  (grain refining)

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Overview of engineering methods

• Engineering use the learning from science in a
  practical context
• Identify and define problems need
• Define constraints and success criteria
• Conduct analysis/investigation
• Set up specifications or present
  solutions/alternatives
• Verify results

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Case Study I Al-Li alloys for aerospace
applications

• Before World War I (1914-1918), aircraft frames
  were made of wood and wings were covered with
  fabric. The problem with wooden aircraft was
  short durability. Thus, during WWI, all metal
  aircraft were developed (made of steels)
• In 1930s, the availability of strong, corrosion
  resistant Al alloys made aircraft lighter by
  replacing steels with Al alloys. Since then the
  quest for lighter and stronger materials
  continues
• Today, there is severe competition between the
  aluminum industry and composite manufacturers.
  Carbon fiber / epoxy composites are less dense
  than Al (composites 1.8 gm/cc, aluminum 2.8
  gm/cc, steel 7.8 gm/cc)
• So Al manufacturers have been hard at work to
  come up with Al alloys that are lighter and
  stronger than pure Al

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Identify define problem/need

• The less the weight of the plane, the more cargo
  weight that can be carried. To be more precise
  and rigorous, look at the following pie chart
  showing the weight distribution for a typical
  passenger aircraft about to take off on a 1000
  mile journey
• The empty plane accounts for 46 of the total
  weight and the passengers and freight for only
  14.5 . If we can reduce the weight of the empty
  plane by 10 and keep the take off weight the
  same, we could transfer 4.6 of the total weight
  (46 x 10) to passengers freight. This means
  the weight carried for profit has gone up by
  30
• Thus, the key here is to reduce the weight of the
  empty airplane while maintaining or improving its
  strength, durability and performance, i.e., find
  or develop new light weight, high strength
  materials

28 Fuel for normal use
4 Fuel reserves
14.5 Passengers plus freight
46 Manufacturers Empty weight
7.5 Operators items
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Define constraints and criteria

• Materials developed have to be lighter than
  existing Al alloys (density 2.8 g/cc)
• As strong as or stronger than existing Al alloys
  (UTS 77,000 psi, yield strength 62,000 psi)
  1 psi 6890 Pa
• As stiff as or stiffer than existing Al alloys
  (Youngs modulus 11,000,000 psi)
• Should be corrosion resistant
• Should be stable under operating conditions Be
  able to withstand high temperatures due to the
  heat generated via frictional resistance of air.
  For flight speed at or below Mach 2 (twice the
  speed of sound, or 1,200 miles per hour at
  medium altitude), the temperature rise is such
  that Al alloys perform well. When flight speed is
  close to Mach 3 or above, materials with greater
  heat resistance must be used. Stainless steels,
  Ti alloys and Mo alloys are all qualified in this
  regard. These alloys are expensive and difficult
  to manufacture, but their use becomes mandatory
  for aircraft to fly at Mach 3 or above.
• Needs to be relatively inexpensive

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Conduct Analysis/ investigation Existing
materials
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Specific strength Strength/density
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Conclusion Of the existing materials, only Ti
alloys provide advantages over Al alloys in
strength, stiffness, specific strength and
temperature capability. However, Ti alloys are
more expensive and difficult to manufacture than
Al alloys

• Alternatives
• Develop polymer matrix composites reinforced with
  graphite fibers (e.g., epoxy resin/graphite
  fiber) density 1.7 gm/cc, Youngs modulus
  150-300 GPa, Tensile strength 780-1850 MPa),
  specific strength 460-1090, all of which better
  than conventional Al alloys. BUT, not suitable
  for aircraft with speed of Mach 2 or higher.
• Carbon/carbon composites have properties better
  than above and is temperature resistant in
  non-oxidizing atmospheres (up to 2000 Celsius),
  but very expensive
• New Al alloys that are lighter and stronger than
  existing Al alloys.

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Development of Al-Li alloys

• Lithium (Li) is the lightest metal element
  (density a little more than half that of water!).
  If we could use Li to replace some Al atoms
  substitutionally, the new Al alloy will become
  lighter note remember substitutional versus
  interstitial impurity? do not want interstitial
  as this will increase the weight
• The atomic size difference between host atom
  (solvent) and solute is an important factor in
  determining whether the solid solution is
  substitutional or interstitial
• For interstitial solid solution to form, the
  atomic diameter of the interstitial atom must be
  substantially smaller than that of host atoms.
  For example, Carbon (1.42 Å in diameter) forms
  interstitial solid solution in iron (2.48 Å). The
  atomic size factor, d Ddi/d (2.48-1.42)/2.48
  43 . For substititional solid solutions to
  form with appreciable solute concentrations, it
  requires that d lt 15
• Li atom diameter 3 Å, Al atom diameter 2.8 Å
  ? d (3-2.8)/2.8 7 . Thus, addition of Li to
  Al will create a substitutional solid solution,
  which means we can reduce the density of Al
  alloys by adding Li element.
• Second major advantage of adding Li Al-Li alloys
  are stronger than conventional Al alloys due to
  precipitation hardening
• Lithium in Aluminum is just like salt in water
• We find that there is solubility limit of Li in
  Al, which changes with temperature, similar to
  sugar or salt in water
• Once Li concentration exceeds solubility limit,
  precipitates with a composition of Al3Li will
  form. Thus we have opportunities to strengthen
  Al-Li alloys via precipitation hardening

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Point Defects (0-dimensional)

• Intrinsic (vacancies)
• Extrinsic (interstitial and substitutional
  impurity atoms)
• Alter the mechanical properties (by affecting
  slip and dislocation motion), electronic
  properties (doping in semiconductors), etc.

In semiconductors, substitutional impurities are
called dopants, and control the amount of charge
carriers
An avenue for atomic motion within the lattice,
in response to an external mechanical or
electrical load
In stainless steel, carbon, which makes it a
steel, is an interstitial impurity in the iron
lattice (and chromium, which makes it stainless,
is a substitutional impurity)
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Al-Li phase diagram
Soluble region
Precipitation region
Solubility limit
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Al3Li precipitates

• Precipitates have fcc structure with Li atoms at
  corners of cube, and Al atoms at face centers
• Two-phase system, with host having lower
  concentration of Li in substitutional positions
  in Al fcc lattice, and precipitates also of fcc
  type but with a higher Li concentration randomly
  oriented wrt host
• To maximize precipitation strengthening effects,
  we want (1) uniform distribution of
  precipitates, and (2) large number of
  precipitates typically, 20 nm diameter
  precipitates with average spacing of 40 nm. Thus
  a dislocation has little chance of moving more
  than 50 nm before it encounters an obstacle!
  Note 1 nm 10 Å 10-9 m
• How do we achieve this? (1) heat (so that Li
  dissolves in Al), (2) fast cooling (so that
  precipitate nuclei do not have time to coalesce
  and grow i.e., large number of small
  precipitates rather than small number of large
  ones)
• Bottom line Two-phase system, with homogeneous
  host having Li in substitutional positions in Al
  fcc lattice (like salt solution), and
  precipitates randomly oriented wrt host (like
  salt precipitates, but not at the bottom of
  beaker as we have a solid solution)

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Verify results

• Impact on properties Al-Li alloys are 10
  lighter, 9 stiffer, 8 stronger for yield
  strength, and 4 stronger for tensile strength in
  comparison with conventional Al alloys
• Al-Li alloys have been produced by Alcoa and used
  in the vertical stabilizer and tailplanes of
  Boeing 777 and Airbus A330/340. They result in
  650 pounds saving in weight at the additional
  expense of less than 150,000. If 650 pounds
  translate into 3 passengers and assuming the
  average ticket price of 250 and two flights a
  day, in 100 days 150,000 will be paid off!

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Case Study II HSLA steels for car bodies (grain
refinement)

• Steel is the traditional material for car bodies
  although there is an incentive to decrease the
  weight by using aluminum and Al-Li alloys, this
  is not cost effective
• Alternative strengthen steels by small amount
  amounts of impurities intentionally added (low
  alloy)
• HSLA steels small amounts of niobium added to
  steel produces niobium carbide precipitates,
  which result in small grain sizes, and increased
  strength
• In Al-Li alloys, Al3Li precipitates pin
  dislocations in HSLA steels niobium carbide
  precipitates pin grain boundaries, thereby
  preventing the grains from becoming big

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Grain refining

• Grain boundaries are barriers to dislocation
  movement because
• dislocations have to change their directions of
  motion since the neighboring grain has different
  orientation
• the atomic disorder within a grain boundary
  region will result in a discontinuity of slip
  planes from one grain to the other
• A dislocation stops when it reaches a grain
  boundary the next dislocation stops behind the
  first, and so on, creating a pile-up or traffic
  jam of dislocations
• Smaller the grain size, less distance does the
  dislocations move (and smaller number of
  dislocations exerting force on the grain
  boundary), and so stronger is the material

Dislocation pileup
Slip plane
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Crystal structures of iron (or steel)

• At room temperature steel exists in a body
  centered cubic (BCC) arrangement
• When heated above 912 C, BCC ? FCC, and reverses
  when cooled below 912 C

BCC
FCC
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Creation of fine grains

• Heat above 912 C ? FCC crystals grains form
• Hot-roll sample ? pancaked grains (Figure in
  page 122)
• Grain boundaries held in place by niobium carbide
  particles
• Cool below 900 C ? BCC crystallites nucleate at
  grain boundaries (just like ice from water, as in
  page 123)
• BCC crystallites grow till they bump into each
  other, finally resulting in fine grained steel

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Result

• Ultimate tensile strength (UTS), or just
  strength, increases by 85 , yield strength
  increases by 190 , no change in Youngs modulus
  (or stiffness) dislocations can still move
  somewhat (unlike in Al-Li alloys), but we have
  stopped the dislocations from moving the grain
  boundary or the next grain
• Made possible by 2 important properties
• Phase transformation from BCC to FCC at 900 C
• Small NbC precipitates formed at grain boundaries
  prevent boundaries from moving

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Summary

• Strengthening strategy eliminate or stop
  dislocations
• Work hardening
• Precipitation hardening
• Grain refinement
• Overview of engineering methods
• Case study Al-Li alloys (precipitation
  hardening)
• Case study High strength low alloy (HSLA) steels
  (grain refining)
• Feb. 3, Feb. 8 Metal casting lab tour (Room 15,
  IMS basement)
• Feb. 10 Quiz 1
• Reading assignment Ch. 1-13