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Precipitation Hardening
Dr. H. K. Khaira
Professor in MSME
MANIT, Bhopal
Precipitation Hardening (or Age
Hardening)
• Precipitation hardening is commonly used to
process aluminum alloys and other nonferrous
metals for commercial use. The examples are
aluminum-copper, copper-beryllium, coppertin, magnesium-aluminum, and some ferrous
alloys
Precipitation Hardening
• the strength and hardness of some metal alloys may be
enhanced by the formation of extremely small uniformly
dispersed particles of a second phase within the original phase
matrix.
• this is accomplished by appropriate heat treatments.
• the process is called precipitation hardening because the small
particles of the new phase are termed "precipitates”.

3
Precipitation Hardening
• “age hardening" is also used to designate this procedure because
the strength develops with time, or as the alloy ages at
designated temperatures below the “solvus” temperature.
• alloys that are hardened by precipitation treatments include AlCu, Cu-Be, Cu-Sn, and Mg-Al; and some ferrous alloys.

Solvus
curve

4

Solvus curve
Precipitation Hardening
• Mechanism of Hardening:
• During plastic deformation:
– Zones or precipitates act as obstacles to
dislocation motion
– Stress must be increased to “push” the
dislocation through the distribution of
precipitates.
– Consequently the alloy becomes harder and
stronger.

5
Precipitation Hardening in the First
Aerospace Alluminum Alloy:
The Wright Flyer Crankcase
• Aluminum has had an essential part in
aerospace history from its very inception. An
aluminum copper alloy (with a Cu composition
of 8 wt%) was used in the engine that
powered the historic first flight of the Wright
brothers in 1903.
Modern Aircraft
Mig–29
Age or Precipitation Hardening
 Age hardening - A special dispersion-strengthening heat treatment.
 By solution treatment, quenching, and aging, a coherent precipitate
forms that provides a substantial strengthening effect. Also known as
precipitation hardening, it is a form of dispersion strengthening.

8
Coherent precipitate

 Coherent precipitate - A precipitate whose crystal structure and atomic
arrangement have a continuous relationship with the matrix from which
the precipitate is formed.

9
Precipitation Hardening
• A composition that
can be precipitation
hardened contains
two phases at room
temperature, but
can be heated to a
temperature that
dissolves the
second phase.

10
Al – Cu Equilibrium Diagram
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license.

The aluminum-copper phase diagram and the microstructures that may develop
curing cooling of an Al-4% Cu alloy.

12
Mechanism of Precipitation Hardening
• Formation of very small particles of a second,
or precipitate, phase.
• During precipitation hardening, lattice strains
are established at the precipitate-matrix
interface.
• There is an increased resistance to dislocation
motion by these lattice strains in the vicinity
of the microscopically small precipitate
particles.
Mechanism of Hardening

Supersaturated α solid
solution

14

Zones or precipitate
phase (aging ) with
lattice distortion

Equilibrium phase (Overaging)
without distortion
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.

(a) A noncoherent precipitate has no relationship with the crystal structure of the
surrounding matrix. (b) A coherent precipitate forms so that there is a definite
relationship between the precipitate’s and the matrix’s crystal structure.

15
Lattice Strain
Hardening
Due to Coherent Precipitate
Hardening
toOver Ageing
Precipitation Hardening
• Small inclusions of secondary phases
strengthen material
• Lattice distortions around these secondary
phases impede dislocation motion
• The precipitates form when the solubility limit
is exceeded
• Precipitation hardening is also called age
hardening because it involves the hardening
of the material over a prolonged time.
Microstructural Evolution in Age or
Precipitation Hardening
 Step 1: Solution Treatment
 Step 2: Ageing
Guinier-Preston (GP) zones - Tiny clusters of atoms that precipitate from
the matrix in the early stages of the age-hardening process.

20
Steps in Precipitation Hardening
• Precipitation hardening is accomplished by two
steps
1. Solution heat treatment
• During solution heat treatment all solute atoms are
dissolved to form a single-phase solid solution Quenching or
rapid cooling to room temperature to form a nonequilibrium
supersaturated solid solution (to prevent diffusion and the
accompanying formation of any second phase)

2. Ageing
• The supersaturated solid solution is heated to an
intermediate temperature within the two-phase region. at
this temperature diffusion rates become appreciable. The
precipitates of the second phase form as finely dispersed
particles.
Precipitation Hardening
• The Process:
• Solution treatment, in
which the alloy is heated to
a temperature above the
solvus line into the alpha
phase and held for a period
sufficient to dissolve the
beta phase.
• Quenching to room
temperature to create a
supersaturated solid
solution
• Precipitation Treatment;
alloy is heated to a
temperature below Ts to
cause precipitation of fine
particles of beta phase.
22
Steps in Precipitation Hardening
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license.

The aluminum-rich end of the aluminum-copper phase diagram showing the three
steps in the age-hardening heat treatment and the microstructures that are produced.

24
Steps in Precipitation Hardening
• By quenching and then reheating an Al-Cu
(4.5 wt%) alloy, a fine dispersion of
precipitates form within the Grains.

• These precipitates are effective in hindering
dislocation motion and
consequently, increasing alloy hardness and
strength
Precipitation Hardening
• aging can also occur at room temperature for some alloys
(natural aging).
• Data represented as hardness or tensile strength vs aging time
(log scale) for T-constant.
• Yield Strength increases as zones or precipitates form
• Strength reaches a peak value and then decreases
(overageing)

26
Ageing

27
Effect of Ageing Temperature on
Strength
Effect of Ageing Temperature on
Ductility
Composition of Al-4% Cu Alloy Phases
Compare the composition of the a solid solution in the Al-4% Cu alloy at room
temperature when the alloy cools under equilibrium conditions with that
when the alloy is quenched.
Figure 1 - The
aluminum-copper
phase diagram and
the microstructures
that may develop
during cooling of an
Al-4% Cu alloy.

©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein
under license.

30
SOLUTION
In Figure - 1, a tie line can be drawn at room temperature. The
composition of the α determined from the tie line is about 0.02% Cu.
However, the composition of the α after quenching is still 4% Cu. Since
α contains more than the equilibrium copper content, the α is
supersaturated with copper.

31
Effects of Aging Temperature and Time

The effect of aging
temperature and time
on the yield strength of
an Al-4% Cu alloy.

©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used
herein under license.

32
Overaging in Precipitation Hardening:
• With increasing time, the strength or hardness
increases, reaches a maximum, and finally
diminishes.
• This reduction in strength and hardness that
occurs after long time periods is known as
overaging.
• Diagram shows strength as a function of the
logarithm of aging time at constant temperature
during the precipitation heat treatment.
Effect of Aging Heat Treatment Time on the Strength
of Aluminum Alloys
The operator of a furnace left for his hour lunch break without removing the
Al-4% Cu alloy from the furnace used for the aging treatment. Compare the
effect on the yield strength of the extra hour of aging for the aging
temperatures of 190oC and 260oC.

Fig – 2 The effect of
aging temperature and
time on the yield
strength of an Al-4% Cu
alloy.

©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson
Learning™ is a trademark used herein under license.

34
SOLUTION
From Fig – 2

At 190oC, the peak strength of 400 MPa (60,000 psi) occurs at
2 h (Figure 11.13). After 3 h, the strength is essentially the same.
At 260oC, the peak strength of 340 MPa (50,000 psi) occurs at
0.06 h. However, after 1 h, the strength decreases to 250 MPa (40,000
psi).
Thus, the higher aging temperature gives lower peak strength
and makes the strength more sensitive to aging time.

35
Design of an Age-Hardening Treatment
The magnesium-aluminum phase diagram is shown in Figure. Suppose a Mg-8% Al
alloy is responsive to an age-hardening heat treatment. Design a heat treatment
for the alloy.

Fig – 3 Portion of
the aluminummagnesium phase
diagram.

©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used
herein under license.

36
SOLUTION
Fig – 3

Step 1: Solution-treat at a temperature between the solvus and the
eutectic to avoid hot shortness. Thus, heat between 340oC and
451oC.
Step 2: Quench to room temperature fast enough to prevent the
precipitate phase β from forming.
Step 3: Age at a temperature below the solvus, that is, below 340oC,
to form a fine dispersion of β phase.

37
Requisite Features on Phase
Diagrams for Precipitation Hardening:
 An appreciable maximum solubility of one
component in the other, of the order of several
percent.
 The alloy system must display decreasing solid
solubility with decreasing temperature.
 The matrix should be relatively soft and ductile,
and the precipitate should be hard and brittle.
 The alloy must be quenchable.
 A coherent precipitate must form.
Use of Age-Hardenable Alloys at High
Temperatures

39
Typical Precipitation Hardened Alloys
• Al 2014 Forged Aircraft Fittings, Al Structures
2024 High strength forgings, Rivets 7075
Aircraft Structures, Olympic Bikes Cu Beryllium
Bronze: Surgical Instruments, Non sparking
tools, Gears Mg AM 100A Sand Castings
AZ80A Extruded products Ni Rene' 41 High
Temperature Inconel 700 up to 1800F Fe A286 High Strength Stainless 17-10P
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license.

Figure 11.14
Microstructural
changes that occur in
age-hardened alloys
during fusion welding:
(a) microstructure in
the weld at the peak
temperature, and (b)
microstructure in the
weld after slowly
cooling to room
temperature.

41
THANKS

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precipitation hardening

  • 1. Precipitation Hardening Dr. H. K. Khaira Professor in MSME MANIT, Bhopal
  • 2. Precipitation Hardening (or Age Hardening) • Precipitation hardening is commonly used to process aluminum alloys and other nonferrous metals for commercial use. The examples are aluminum-copper, copper-beryllium, coppertin, magnesium-aluminum, and some ferrous alloys
  • 3. Precipitation Hardening • the strength and hardness of some metal alloys may be enhanced by the formation of extremely small uniformly dispersed particles of a second phase within the original phase matrix. • this is accomplished by appropriate heat treatments. • the process is called precipitation hardening because the small particles of the new phase are termed "precipitates”. 3
  • 4. Precipitation Hardening • “age hardening" is also used to designate this procedure because the strength develops with time, or as the alloy ages at designated temperatures below the “solvus” temperature. • alloys that are hardened by precipitation treatments include AlCu, Cu-Be, Cu-Sn, and Mg-Al; and some ferrous alloys. Solvus curve 4 Solvus curve
  • 5. Precipitation Hardening • Mechanism of Hardening: • During plastic deformation: – Zones or precipitates act as obstacles to dislocation motion – Stress must be increased to “push” the dislocation through the distribution of precipitates. – Consequently the alloy becomes harder and stronger. 5
  • 6. Precipitation Hardening in the First Aerospace Alluminum Alloy: The Wright Flyer Crankcase • Aluminum has had an essential part in aerospace history from its very inception. An aluminum copper alloy (with a Cu composition of 8 wt%) was used in the engine that powered the historic first flight of the Wright brothers in 1903.
  • 8. Age or Precipitation Hardening  Age hardening - A special dispersion-strengthening heat treatment.  By solution treatment, quenching, and aging, a coherent precipitate forms that provides a substantial strengthening effect. Also known as precipitation hardening, it is a form of dispersion strengthening. 8
  • 9. Coherent precipitate  Coherent precipitate - A precipitate whose crystal structure and atomic arrangement have a continuous relationship with the matrix from which the precipitate is formed. 9
  • 10. Precipitation Hardening • A composition that can be precipitation hardened contains two phases at room temperature, but can be heated to a temperature that dissolves the second phase. 10
  • 11. Al – Cu Equilibrium Diagram
  • 12. ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license. The aluminum-copper phase diagram and the microstructures that may develop curing cooling of an Al-4% Cu alloy. 12
  • 13. Mechanism of Precipitation Hardening • Formation of very small particles of a second, or precipitate, phase. • During precipitation hardening, lattice strains are established at the precipitate-matrix interface. • There is an increased resistance to dislocation motion by these lattice strains in the vicinity of the microscopically small precipitate particles.
  • 14. Mechanism of Hardening Supersaturated α solid solution 14 Zones or precipitate phase (aging ) with lattice distortion Equilibrium phase (Overaging) without distortion
  • 15. ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license. (a) A noncoherent precipitate has no relationship with the crystal structure of the surrounding matrix. (b) A coherent precipitate forms so that there is a definite relationship between the precipitate’s and the matrix’s crystal structure. 15
  • 19. Precipitation Hardening • Small inclusions of secondary phases strengthen material • Lattice distortions around these secondary phases impede dislocation motion • The precipitates form when the solubility limit is exceeded • Precipitation hardening is also called age hardening because it involves the hardening of the material over a prolonged time.
  • 20. Microstructural Evolution in Age or Precipitation Hardening  Step 1: Solution Treatment  Step 2: Ageing Guinier-Preston (GP) zones - Tiny clusters of atoms that precipitate from the matrix in the early stages of the age-hardening process. 20
  • 21. Steps in Precipitation Hardening • Precipitation hardening is accomplished by two steps 1. Solution heat treatment • During solution heat treatment all solute atoms are dissolved to form a single-phase solid solution Quenching or rapid cooling to room temperature to form a nonequilibrium supersaturated solid solution (to prevent diffusion and the accompanying formation of any second phase) 2. Ageing • The supersaturated solid solution is heated to an intermediate temperature within the two-phase region. at this temperature diffusion rates become appreciable. The precipitates of the second phase form as finely dispersed particles.
  • 22. Precipitation Hardening • The Process: • Solution treatment, in which the alloy is heated to a temperature above the solvus line into the alpha phase and held for a period sufficient to dissolve the beta phase. • Quenching to room temperature to create a supersaturated solid solution • Precipitation Treatment; alloy is heated to a temperature below Ts to cause precipitation of fine particles of beta phase. 22
  • 24. ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license. The aluminum-rich end of the aluminum-copper phase diagram showing the three steps in the age-hardening heat treatment and the microstructures that are produced. 24
  • 25. Steps in Precipitation Hardening • By quenching and then reheating an Al-Cu (4.5 wt%) alloy, a fine dispersion of precipitates form within the Grains. • These precipitates are effective in hindering dislocation motion and consequently, increasing alloy hardness and strength
  • 26. Precipitation Hardening • aging can also occur at room temperature for some alloys (natural aging). • Data represented as hardness or tensile strength vs aging time (log scale) for T-constant. • Yield Strength increases as zones or precipitates form • Strength reaches a peak value and then decreases (overageing) 26
  • 28. Effect of Ageing Temperature on Strength
  • 29. Effect of Ageing Temperature on Ductility
  • 30. Composition of Al-4% Cu Alloy Phases Compare the composition of the a solid solution in the Al-4% Cu alloy at room temperature when the alloy cools under equilibrium conditions with that when the alloy is quenched. Figure 1 - The aluminum-copper phase diagram and the microstructures that may develop during cooling of an Al-4% Cu alloy. ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license. 30
  • 31. SOLUTION In Figure - 1, a tie line can be drawn at room temperature. The composition of the α determined from the tie line is about 0.02% Cu. However, the composition of the α after quenching is still 4% Cu. Since α contains more than the equilibrium copper content, the α is supersaturated with copper. 31
  • 32. Effects of Aging Temperature and Time The effect of aging temperature and time on the yield strength of an Al-4% Cu alloy. ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license. 32
  • 33. Overaging in Precipitation Hardening: • With increasing time, the strength or hardness increases, reaches a maximum, and finally diminishes. • This reduction in strength and hardness that occurs after long time periods is known as overaging. • Diagram shows strength as a function of the logarithm of aging time at constant temperature during the precipitation heat treatment.
  • 34. Effect of Aging Heat Treatment Time on the Strength of Aluminum Alloys The operator of a furnace left for his hour lunch break without removing the Al-4% Cu alloy from the furnace used for the aging treatment. Compare the effect on the yield strength of the extra hour of aging for the aging temperatures of 190oC and 260oC. Fig – 2 The effect of aging temperature and time on the yield strength of an Al-4% Cu alloy. ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license. 34
  • 35. SOLUTION From Fig – 2 At 190oC, the peak strength of 400 MPa (60,000 psi) occurs at 2 h (Figure 11.13). After 3 h, the strength is essentially the same. At 260oC, the peak strength of 340 MPa (50,000 psi) occurs at 0.06 h. However, after 1 h, the strength decreases to 250 MPa (40,000 psi). Thus, the higher aging temperature gives lower peak strength and makes the strength more sensitive to aging time. 35
  • 36. Design of an Age-Hardening Treatment The magnesium-aluminum phase diagram is shown in Figure. Suppose a Mg-8% Al alloy is responsive to an age-hardening heat treatment. Design a heat treatment for the alloy. Fig – 3 Portion of the aluminummagnesium phase diagram. ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license. 36
  • 37. SOLUTION Fig – 3 Step 1: Solution-treat at a temperature between the solvus and the eutectic to avoid hot shortness. Thus, heat between 340oC and 451oC. Step 2: Quench to room temperature fast enough to prevent the precipitate phase β from forming. Step 3: Age at a temperature below the solvus, that is, below 340oC, to form a fine dispersion of β phase. 37
  • 38. Requisite Features on Phase Diagrams for Precipitation Hardening:  An appreciable maximum solubility of one component in the other, of the order of several percent.  The alloy system must display decreasing solid solubility with decreasing temperature.  The matrix should be relatively soft and ductile, and the precipitate should be hard and brittle.  The alloy must be quenchable.  A coherent precipitate must form.
  • 39. Use of Age-Hardenable Alloys at High Temperatures 39
  • 40. Typical Precipitation Hardened Alloys • Al 2014 Forged Aircraft Fittings, Al Structures 2024 High strength forgings, Rivets 7075 Aircraft Structures, Olympic Bikes Cu Beryllium Bronze: Surgical Instruments, Non sparking tools, Gears Mg AM 100A Sand Castings AZ80A Extruded products Ni Rene' 41 High Temperature Inconel 700 up to 1800F Fe A286 High Strength Stainless 17-10P
  • 41. ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license. Figure 11.14 Microstructural changes that occur in age-hardened alloys during fusion welding: (a) microstructure in the weld at the peak temperature, and (b) microstructure in the weld after slowly cooling to room temperature. 41