Objectives by mikesanye

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									              Objectives
1. Identify the three major classes of
   strengthening processes for
   polycrystalline metals and explain
   why they work.
2. Explain recovery, recrystallization
   and grain growth.
3. Explain the third Universal Principle:
   Nucleation and Growth processes.
  Elastic Deformation
Stretching of molecular bonds




Will be almost the same in
tension as in compression.
Plastic Deformation: Slip
Slip Happens on “Slip
      Systems”
Close Packed Planes
Close Packed Directions
            Actual Yield

Most Metals are
Polycrystalline 



Neighbors Interfere!
Sy Polycrystals > Sy
Monocrystals
  Dislocations and Plasticity
1. Deformation of pure crystals is greater
   than expected based on number of
   pre-existing dislocations.
2. Dislocations density increases during
   plastic deformation.
3. Pre-existing dislocations do not move
   – they are pinned by “atmospheres” of
   impurities.
4. Conclusion: Dislocations must be
   created!
  Nucleation of Dislocations
1. Homogeneous
    Uniform conditions
    No “Assistance”
    just shear stress
        Nucleation of Dislocations
2. Heterogeneous
    Non uniform conditions
    Assistance by impurities or defects



                                         
Heterogeneous Nucleation
   Frank-Reed Source
Dislocation Loop: Frank Reed
Slip: The Highway Analogy
 Slip systems are freeways
 for Dislocations.
      Slip: Highway Analogy
Three Things Slow Traffic

1. Blocked Freeways
  (construction)
2. Too Many Cars
3. Pedestrians, Stopped
  Cars
 Traffic                 Slip

1.   Blocked Freeways-
     Construction        Grain Boundaries
     Grain Boundaries
3
Dislocations



8
Dislocations

Which is more
difficult?
Yield Strength vs. Grain Size
Traffic         Slip
Too Many Cars
                Dislocation
                  Density
                Called Work
                  Hardening or
                  Strain
                  Hardening
Yield Strength vs. Dislocation
           Density
 Traffic          Slip
Pedestrians,
                  Impurities
   Stopped Cars
                  Alloying Elements
                  Precipitates
Hardness of Steel vs. Interstitial
      Carbon Content
                                 Ex: Solid Solution
   Tensile strength (MPa)
                              Strengthening in Copper
                                                                                180




                                                         Yield strength (MPa)
                                                                                                           Adapted from Fig.
                        400                                                                                7.16 (a) and (b),
                                                                                                           Callister 7e.
                                                                                120
                        300

                       200                                                       60
                             0 10 20 30 40 50                                         0 10 20 30 40 50
                            wt.% Ni, (Concentration C)                                wt.%Ni, (Concentration C)

Tensile strength & yield strength increase with wt% Ni.
Reheating Plastically Deformed
            Metal
1. Recovery – elimination of
   dislocations
2. Recrystallization – formation of
   equiaxed new grains between old
   grains.
3. Grain Growth – equiaxed grains
   grow, absorbing old distorted
   grains.
                          Effect of Heating After %CW
• Effects of cold work are reversed!

                            annealing temperature (ºC)
                            100 200 300 400 500 600 700
 tensile strength (MPa)




                          600                         60
                                tensile strength




                                                           ductility (%EL)
                                                     50
                          500
                                                     40

                          400                        30

                                      ductility      20
                          300
                                                                             Adapted from Fig. 7.22, Callister 7e. (Fig.
                                                                             7.22 is adapted from G. Sachs and K.R. van Horn, Practical
                                                                             Metallurgy, Applied Metallurgy, and the Industrial Processing
                                                                             of Ferrous and Nonferrous Metals and Alloys, American
                                                                             Society for Metals, 1940, p. 139.)
                     Recovery
• Scenario 1     extra half-plane
  Results from           of atoms           Dislocations
                                            annihilate
  diffusion          atoms
                                            and form
                     diffuse
                                            a perfect
                     to regions
                                            atomic
                     of tension
                 extra half-plane           plane.
                          of atoms

Annihilation reduces dislocation density.
                Recrystallization
• New grains are formed that:
  -- have a small dislocation density
  -- are small
  -- consume cold-worked grains.
            0.6 mm                      0.6 mm




 33% cold                    New crystals
 worked                      nucleate after      Adapted from Fig. 7.21
                                                 (a),(b), Callister 7e.
 brass                       3 sec. at 580C.    (Fig. 7.21 (a),(b) are
                                                 courtesy of J.E. Burke,
                                                 General Electric
                                                 Company.)
       Further Recrystallization
• All cold-worked grains are consumed.

             0.6 mm                      0.6 mm




                                                  Adapted from
                                                  Fig. 7.21 (c),(d),
                                                  Callister 7e.
                                                  (Fig. 7.21 (c),(d)
                                                  are courtesy of
                                                  J.E. Burke,
                                                  General Electric
                                                  Company.)



   After 4                   After 8
   seconds                   seconds
                Grain Growth
• At longer times, larger grains consume smaller ones.
• Why?

       0.6 mm           0.6 mm




  After 8 s,       After 15 min,
  580ºC            580ºC


                                               Adapted from Fig. 7.21
                                               (d),(e), Callister 7e.
                                               (Fig. 7.21 (d),(e) are
                                               courtesy of J.E. Burke,
                                               General Electric
                                               Company.)
Characteristic Strength Values
 Material        Yield Strength

 Steels          50-200 ksi

 Aluminum        10-70 ksi

 Polycarbonate   6.5-10 ksi
Nucleation and Growth
Nucleation Processes
  Homogeneous (no
    assistance, high energy)
  Heterogeneous (assistance,
    low energy)
           Nucleation and Growth


Will follow standard incubation/growth laws:
         Nucleation and Growth
Growth Processes
  Often diffusion or heat flow limited
  Competes with further nucleation

								
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