Heat Treatment Annealing of ColdWorked Metals

1
Heat Treatment (Annealing) of Cold-Worked Metals

• Annealing heat treatment whereby
  ________________________________
• __________________________________________________
  ____________
• i. Cold working increase ?y, TS decrease
  ductility (see next slide)
• ii. Annealing decrease ?y, TS increase
  ductility

• Three Stages of Anealling with
• Increasing Temp.
• -
• -
• -

(NoteAnnealling stages may also occur by holding
T above Trecrys)
2
Recall.
Cold working (strain-hardening, work
hardening)? Plastic deformation ? Increase of
or dislocation density (now, very high
dislocation density) ? __________________________
_ (many atoms out of place) ? Dislocation motion
_______________________________
_____________ ___________________________________
? __________________________________________
3

• 3 Stages of Annealing
• 1. Recovery
• As temp. increases, ______
• _______________ increases
• Some dislocations move to relieve strain
• Slight _________________ in of dislocations
• Minor changes in mechanical properties

Brass alloy
4

• 2. Recrystallization
• _______________________
• ________________________
• ________________________
• New set of ___________ grains
• Have ______ dislocation density
• Gradually consume cold-worked grains
• Occurs when T increased above Trecrys or held at
  T gt Trecrys
• Trecrys min temp required for complete
  recrystallization in 1 h
• Trecrys ½ to 1/3 of Tmelt
• T in Kelvin
• see photomicrographs magnification 75X

Brass alloy
crystals

5
Cold-worked grains
3 s _at_ 580 ?C
Initial recrystallization
Brass Trecrys 450 ?C
4 s _at_ 580 ?C
8 s _at_ 580 ?C
Complete recrystallization
(Annealling stages occur by holding T above
Trecrys)
6
Next Stage grain growth
15 min _at_ 580 ?C
10 min _at_ 700 ?C
New set of strain-free grains w/ low dislocation
density
7

• 3. Grain Growth
• If left at elevated temperature following
  recrystallization
• _________________________ (coarsening)
• some grow while others shrink
• see photomicrographs magnification 75X

Brass alloy

• Cold-worked Grains
• _______ dislocation density
• _______ strain
• ? stronger
• After Annealing, Grains
• _______ dislocation density
• _______ strain
• ? more ductile

8

• Cold-worked Metals
• Grains
• ______ dislocation density
• 109 1010 / mm2 (highly deformed)
• ______ strain state
• ? stronger
• After Annealing, Metals
• Grains
• ______ dislocation density
• 105 106 / mm2
• ______strain state
• ? more ductile

Brass alloy
9
Hot-Working

• metal forming operation performed at T gt
  Trecryst
• Metal remains ductile or (if previously
  cold-worked) becomes more ductile

pp 383-384
10
Mechanical Properties of Ceramics (12.8-12.11,
13.11)
11
Elastic Modulus
Most Ceramics E ___________ GPa Most Metals
E _____________ GPa
Most polymers E lt ____ GPa
12
Density
13

• Most ceramics fail/fracture _____________________
  _____________ (see below)
• Very __________ (not tough, not ductile)
• Exhibits ___________________________________
  (which parallels?)
• ______________________ (ionic) prevents
  dislocation motion (i.e. slip)

Recall Ionic bonds 600-1500 kJ/mol Metallic
bonds lt850 kJ/mol Covalent bonds 346 kJ/mol
(for C-C)
14

• Al2O3 and Soda-lime glass

15
Why Flexural Strength (not Tensile Strength) for
Ceramics?

• Difficult to prepare and test specimens with the
  required geometry
• Difficult to grip brittle materials into tensile
  tester clamps w/o fracture
• Ceramics fail after 0.1 strain

s
Material
(MPa) E(GPa)
fs
Si nitride Si carbide Al2O3 Zirconia (ZrO2) glass
(soda)
250-1000 100-820 275-700 800-1500 69
304 345 380 205 69
16
Measuring Flexural Strength (?fs)
_____________________________ test to measure
strength.
Adapted from Fig. 12.32, Callister 7e.
For rectangular specimen

• Specimen
• - rod with circular or rectangular cross-section
• Apply force
• - top face in compression
• - bottom face in tension
• 3. Fracture occurs on tension face
• Why? For ceramics TS 1/10 of compressive
  strength
• So, flexural test good substitute for tensile test

?fs (3Ff L)/(2bd2)
Ff load at fracture L distance between
support pts
For rectangular specimen
?fs (Ff L)/(?R2)
R radius of specimen
17
Influence of Pores in Ceramics
Figure 13.16
powder
With pressure
Figure 13.14
pore
With sintering
Pores introduced during powder processing
18
Fig. 12.35
Fig. 12.36
?fs versus P
E versus P
?fs ?o exp (-nP)
E Eo(1-1.9P 0.9P2)
?0, n experimental constants Increase P ?
_______________ ?fs 10 porosity ? decrease ?fs
50 (vs non-porous)
Eo elastic modulus of non-porous material P
volume fraction porosity Increase P ?
__________________ E