Strengthening and Softening

1
Strengthening and Softening

• Phenomenon
• alloys are often stronger than pure metals e. g.
  Al-Cu alloys (sy of up to 400 MPa) are stronger
  than pure Al (lt100 MPa)
• alloys are stronger after certain heat treatment
  e. g. artificial ageing of Al-Zn-Mg-Cu alloys (gt
  500 MPa)
• metals get stronger as deformation goes e. g.
  brass (Cu-Zn alloy) from 100 MPa as annealed to
  gt 400 MPa after cold working
• metals with finer grain sizes are stronger e. g.
  nano-crystalline materials

• metals can become softer after exposure at
  elevated temperatures
• What is the mechanism?
• solid solution strengthening
• precipitation strengthening
• strain hardening
• grain size strengthening
• all related to disln movement
• Quantitative description
• effect of alloy contents
• effect of spacing between precipitates
• effect of plastic deformation
• effect of grain size

Reading 7.8-7.12
2
Strengthening and Softening

• How is a material strengthened?
• by stopping the movement of dislocations on their
  slip planes
• no dislocation can get over
• difficult to achieve
• by increasing the difficulty of such movement
• dislocation can still get over but only with
  increased stress

• How to achieve?
• by changing composition
• by manipulating microstructure

So that this cannot be reached or can only be
reached with more force
Blocking
Barriers
Saddling
3
Strengthening and Softening

• many grains
• grain boundaries
• angle of misalignment
• grain sizes

• Strengthening due to grain size reduction
• polycrystalline materials

grain
grain boundary
grain size
Additional Reading 4.5 (5th ed), 4.6 (6th ed)
lt few degrees
4
Strengthening and Softening

• dislocation is blocked - strength is increased
  since more stress is required to force the
  dislocation to move to a different slip plane or
  to cause new dislocation movement
• large angle grain boundaries are more effective

• grain boundary may be a barrier to dislocation
  movement
• dislocation moving on a slip plane in grain A
  until it meets the grain boundary with grain B
• its gliding may not be able to continue as the
  slip plane in grain B is at a different
  orientation

obstacle
5
Strengthening and Softening

• Hall-Petch relationship
• so, ky - constants
• d - average grain diameter
• when d 8, sy so
• yield strength increases rapidly with decreasing
  grain size
• empirical although Hall and Petch modeled it
  based on dislocation theory
• may not work at very fine and very coarse grain
  sizes

Cu-30 Zn (brass)
4.10 (5th ed), 4.11 (6th ed) for determining d
slope ky
so
5 µm
1 mm
smaller grains more grain boundaries more
barriers
6
Strengthening and Softening

• alloying elements solute atoms
• Cu-10Ni
• Ni solute atoms (substitutional)
• Cu host atoms
• Fe-0.01C
• C solute atoms (interstitial)
• Fe host atoms
• Structure that of the host
• Cu-Ni FCC of Cu
• Fe-C BCC of Fe
• Not all elements form solutions
• Some factors to consider
• sizes of atoms
• structures of elements

• Strengthening due to presence of solute atoms
• Solid solution

Additional Reading 4.3
solute atoms
host atoms
7
Strengthening and Softening

• solute atoms bigger
• segregated on dislocation lines ("below")
• "saddling" dislocations

• Solid solution strengthening
• solute atoms smaller
• segregated on dislocation lines ("above")

IMSE Solid Solution Strengthening
tensile stress around it
compressive stress around it
stress field due to dislocation (7.3)
8
Strengthening and Softening

• substantial increase in strength can be achieved

• Solid solution strengthening
• Dsy - increase in yield strength
• Csolute - concentration of solute atoms

Cu-Ni
9
Strengthening and Softening

• yield strength and ultimate tensile strength are
  increased but elongation reduced with increasing
  strain
• percent cold work

• Strengthening due to plastic deformation
• hardening from cold working, i. e. plastic deform
  material at low temperatures (most often room
  temperature)

A0 area before deformation A area after
deformation
10
Strengthening and Softening

• mechanism
• density of dislocations increase and they
  interact with each other making dislocation
  movement more difficult

• effect of CW on stress-strain curve

yield strength tensile strength elongation
Slip plane
Dislocations on other slip planes
Dislocations on the slip plane
11
Strengthening and Softening

• estimate of maximum strengthening
• d - average interparicle spacing

• Strengthening due to precipitates (particles)
• Fine particles from processes such as
  precipitation (precipitates) act as obstables to
  dislocation movement
• dislocations can pass the particles by looping or
  cutting through
• this requires extra stress
• strengthening thus results

Slip plane
Precipitation will be discussed in 2nd
year. Reading 11.7-11.8 (5th ed), 11.9 (6th ed)
12
Strengthening and Softening

• new grains with low disln density will nucleate
  in a matrix of high strain energy (due to high
  density of dislocations)
• dramatic reduction in number of dislns
• significant decrease in strength and increase in
  ductility

• How is a material softened?
• by reducing the barriers to dislocation movement
• Softening after cold working
• Recovery
• heating a cold worked material to a moderately
  elevated T
• dislns will be activated to rearrange themselves
  resulting in
• reduction in number
• low energy configuration
• strength is lowered moderately
• Recrystallisation (reX)
• heating to above reX T corresponding to a
  critical amount of CW

reX T at which recrystallisation reaches
completion in 1 hour
13
Strengthening and Softening
Cu-Zn brass
a
b
c
33CW
3 s at 580C
4 s at 580C
a - as cold worked b - nucleation c - partial
recrystallisation d - complete recrystallisation e
- grain growth
d
e
8 s at 580C
15 min at 580C
14
Strengthening and Softening

• Grain growth
• grains will grow (become larger with time) at
  elevated temperatures
• this is driven by reduction in grain boundary
  area
• some further strength loss (not as dramatic)
• Example
• Design Example 7.1 (self study)

Cu-Zn brass
ReX T
1 hour