1. Strengthening and recrystallization of plastically deformed metals. 2. Material selection according to the mechanical properties

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1. Strengthening and recrystallization of
plastically deformed metals. 2. Material
selection according to the mechanical properties

• Lecture 6

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Why study strengthening mechanisms?

• We can tailor the mechanical properties of an
  engineering material by choosing between strength
  and toughness

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Plastic Deformation

• Plastic deformation is permanent, and strength
  and hardness are measures of a materials
  resistance to this deformation.
• On a microscopic scale, plastic deformation
  corresponds to the net movement of large numbers
  of atoms in response to an applied stress.
• In crystalline solids, plastic deformation most
  often involves the motion of dislocations
• The ability of a metal to plastically deform
  depends on the ability of dislocations to move

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Dislocation Motion

• Cubic hexagonal metals - plastic deformation by
  plastic shear or slip where one plane of atoms
  slides over adjacent plane by defect motion
  (dislocations).

• If dislocations don't move,
  deformation doesn't occur!

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Dislocation Motion

• The process by which plastic deformation is
  produced by dislocation motion is termed slip
• Dislocation moves along slip plane in slip
  direction perpendicular to dislocation line

Edge dislocation
Screw dislocation
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Strengthening of Metals

• Good industrial alloys -gt high strengths yet some
  ductility and toughness
• Since hardness and strength are related to the
  ease with which plastic deformation can be made
  to occur, by reducing the mobility of
  dislocations, the mechanical strength may be
  enhanced
• In contrast, the more unconstrained the
  dislocation motion, the greater is the facility
  with which a metal may deform, and the softer and
  weaker it becomes
• Restricting or hindering dislocation motion
  renders a material harder and stronger

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Strengthening Methods

• grain size reduction,
• solid-solution alloying,
• precipitation hardening/strengthening
• strain hardening/strengthening

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Strategies for Strengthening 1 Reduce Grain
Size
Grain boundaries are barriers to slip.
Barrier "strength" increases with
Increasing angle of misorientation.
Smaller grain size more barriers to
slip. Hall-Petch Equation
d is the average grain diameter. so and ky are
constants for a particular material
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Dependence of Yield Strength on Grain Size

• The influence of grain size on the yield strength
  of a 70 Cu30 Zn brass alloy

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Strategies for Strengthening 2 Solid
Solution Strengthening
Impurity atoms distort the lattice generate
stress. Stress can produce a barrier to
dislocation motion.
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Stress Concentration at Dislocations
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SSS - Impurity Atoms
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SSS Effects of Impurity Atoms

• The resistance to slip is greater when impurity
  atoms are present because the overall lattice
  strain must increase if a dislocation is moved
  away from them.
• The same lattice strain interactions will exist
  between impurity atoms and dislocations that are
  in motion during plastic deformation.
• Thus, a greater applied stress is necessary to
  first initiate and then continue plastic
  deformation for solid-solution alloys, as opposed
  to pure metals

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SSS Strength and Ductility
Variation with nickel content of (a)
tensile strength, (b) yield strength, and (c)
ductility (EL) for coppernickel alloys, showing
strengthening.
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Strategies for Strengthening 3 Precipitation
Strengthening

• Precipitation strengthening, also called age
  hardening, is a heat treatment technique used to
  increase the yield strength of malleable
  materials.
• It relies on changes in solid solubility with
  temperature to produce fine particles of an
  impurity phase, which impede the movement of
  dislocations, or defects in a crystal's lattice.
• Precipitation in solids can produce many
  different sizes of particles, which have
  different properties.
• Alloys must be kept at elevated temperature for
  hours to allow precipitation to take place. This
  time delay is called aging

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Strategies for Strengthening 3 Precipitation
Strengthening
Hard precipitates are difficult to shear.
Ex Ceramics in metals (SiC in Iron or Aluminum).
Large shear stress needed
to move dislocation toward
precipitate and shear it.
Dislocation
advances but
precipitates act as
pinning sites with
S
.
spacing
Result
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Application Precipitation Strengthening
Internal wing structure on Boeing 767
Aluminum is strengthened with precipitates
formed by alloying.
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Strategies for Strengthening 4
Strain-Hardening
Room temperature deformation. Common
forming operations change the cross
sectional area
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Dislocations During Cold Work
Ti alloy after cold working
Dislocations entangle with one another
during cold work. Dislocation motion
becomes more difficult.
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Result of Cold Work

• Dislocation density
• Carefully grown single crystal
• ? 103 mm-2
• Deforming sample increases density
• ? 109-1010 mm-2
• Heat treatment reduces density
• ? 105-106 mm-2

Yield stress increases as dislocation
density increases
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Impact of Cold Work
As cold work is increased
Yield strength (sy) increases.
Tensile strength (TS) increases.
Ductility (EL or AR) decreases.
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Cold Work Analysis
What is the tensile strength ductility
after cold working?
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s- e Behavior vs. Temperature
Results for polycrystalline iron
sy and TS decrease with increasing test
temperature. EL increases with increasing
test temperature. Why? Vacancies help
dislocations move past obstacles.
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Effect of Heating After CW
1 hour treatment at Tanneal...
decreases TS and increases EL. Effects of
cold work are reversed!
3 Annealing stages to discuss...
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Recovery
Annihilation reduces dislocation density.
Scenario 2

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Recrystallization

• Even after recovery is complete, the grains are
  still in a relatively high strain energy state
• Recrystallization is the formation of a new set
  of strain-free and equiaxed grains (i.e., having
  approximately equal dimensions in all directions)
  that have low dislocation densities and are
  characteristic of the precold-worked condition.
• The new grains form as very small nuclei and grow
  until they completely consume the parent
  material, processes that involve short-range
  diffusion

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Recrystallization
New grains are formed that -- have a
small dislocation density -- are small --
consume cold-worked grains.
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Further Recrystallization
All cold-worked grains are consumed.
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Recrystallization

• During recrystallization, the mechanical
  properties that were changed as a result of cold
  working are restored to their precold-worked
  values that is, the metal becomes softer,
  weaker, yet more ductile
• Recrystallization is a process the extent of
  which depends on both time and temperature. The
  degree (or fraction) of recrystallization
  increases with time
• For pure metals, the recrystallization
  temperature is normally 0.3Tm where Tm is the
  absolute melting temperature

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Grain Growth

• After recrystallization is complete, the
  strain-free grains will continue to grow if the
  metal specimen is left at the elevated
  temperature this phenomenon is called grain
  growth
• Grain growth does not need to be preceded by
  recovery and recrystallization it may occur in
  all polycrystalline materials, metals and
  ceramics alike

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Grain Growth
At longer times, larger grains consume smaller
ones. Why? Grain boundary area (and
therefore energy) is reduced.
After 10 min, 700ºC
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º
TR recrystallization temperature
º
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Time and Temperature Dependent Grain Growth
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Recrystallization Temperature, TR

• TR recrystallization temperature point of
  highest rate of property change
• Tm gt TR ? 0.3-0.6 Tm (K)
• Due to diffusion ? annealing time? TR f(t)
  shorter annealing time gt higher TR
• Pure metals lower TR due to dislocation movements
• Easier to move in pure metals gt lower TR

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Coldwork Calculations

• A cylindrical rod of brass originally 0.40in
  (10.2mm) in diameter is to be cold worked by
  drawing. The circular cross section will be
  maintained during deformation. A cold-worked
  tensile strength in excess of 55,000psi (380MPa)
  and a ductility of at least 15EL are desired.
  Furthermore, the final diameter must be 0.30in
  (7.6mm). Explain how this may be accomplished.

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Coldwork Calculations Solution

• If we directly draw to the final diameter what
  happens?

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Coldwork Calc Solution Cont.

• For CW 43.8

• ?y 420 MPa

• TS 540 MPa gt 380 MPa

• EL 6 lt 15

• This doesnt satisfy criteria what can we do?

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Coldwork Calc Solution Cont.
For TS gt 380 MPa
For EL lt 15
? our working range is limited to CW 12-27
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Coldwork Calc Soln Recrystallization

• Cold draw-anneal-cold draw again
• For objective we need a cold work of CW ? 12 -
  27
• Well use CW 20
• Diameter after first cold draw (before 2nd cold
  draw)?
• must be calculated as follows

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Coldwork Calculations Solution

• Summary
• Cold work D01 0.40 in ? Df1 0.335
  in
• Anneal to remove all CW effects D02 Df1
• Cold work D02 0.335 in ? Df 2 0.30 in
• Therefore, meets all requirements

?
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Material Selection According to the Mechanical
Properties
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Material Selection The Basics

• Getting the optimum balance of performance,
  quality, and cost requires a careful combination
  of material and part design
• The ideal product is one that will just meet all
  requirements.
• Anything better will usually incur added cost
  through higher-grade materials, enhanced
  processing, or improved properties that may not
  be necessary.
• Anything worse will likely cause product failure,
  dissatisfied customers, and the possibility of
  unemployment

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Material Selection The Basics

• The interdependence between materials and their
  processing must also be recognized.
• New processes frequently accompany new materials,
  and their implementation can often cut production
  costs and improve product quality.
• A change in material may well require a change in
  the manufacturing process
• Improper processing of a well-chosen material can
  definitely result in a defective product.

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Materials and Manufacturing

• An engineering material may possess different
  properties depending upon its structure.
• Processing of that material can alter the
  structure, which in turn will alter the
  properties.
• Altered properties certainly alter performance.

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PROCEDURES FOR MATERIAL SELECTION

• Every engineered item goes through a sequence of
  activities that includes
• design
• material selection
• process selection
• production
• evaluation
• possible redesign or modification

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Requirements for Material Selection

• (1) shape or geometry considerations,
• (2) property requirements,
• (3) manufacturing concerns

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1. GEOMETRIC CONSIDERATIONS

• What is the relative size of the component?
• How complex is its shape?
• What are the surface-finish requirements? Must
  all surfaces be finished?
• Could a minor change in part geometry increase
  the ease of manufacture or improve the
  performance (fracture resistance, fatigue
  resistance, etc.) of the part?

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2. Mechanical Properties

• How much static strength is required?
• If the part is accidentally overloaded, is it
  permissible to have a sudden brittle fracture, or
  is plastic deformation and distortion a desirable
  precursor to failure?
• How much can the material bend, stretch, twist,
  or compress under load and still function
  properly?
• Are any impact loadings anticipated? If so, of
  what type, magnitude, and velocity?

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2. Mechanical Properties

• Can you envision vibrations or cyclic loadings?
  If so, of what type, magnitude, and frequency?
• Is wear resistance desired? Where? How much? How
  deep?
• Will all of the above requirements be needed over
  the entire range of operating temperature? If
  not, which properties are needed at the lowest
  extreme? At the highest extreme?

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Environmental Considerations

• What are the lowest, highest, and normal
  temperatures the product will see? Will
  temperature changes be cyclic? How fast will
  temperature changes occur?
• What is the most severe environment that is
  anticipated as far as corrosion or deterioration
  of material properties is concerned?
• What is the desired service lifetime for the
  product?

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3. Manufacturing Concerns

• How many of the components are to be produced? At
  what rate?
• What is the desired level of quality compared to
  similar products on the market?
• Has the design addressed the requirements that
  will facilitate ease of manufacture?

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Ashby Material Selection Charts
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