Precipitation hardening, also called age hardening, strengthens metal alloys through heat treatment causing the formation of precipitates. The process involves solution heat treating to dissolve precipitates, quenching to form a supersaturated solid solution, and aging to precipitate nanoscale particles that impede dislocation movement. Aluminum-copper alloys are commonly precipitation hardened, with the aging process increasing strength over time until reaching a peak strength and then decreasing during overaging as precipitates coarsen.

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”.
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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.
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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.

7. Modern Aircraft
Mig–29

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.
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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.
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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.
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11. Al – Cu Equilibrium Diagram

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The aluminum-copper phase diagram and the microstructures that may develop
curing cooling of an Al-4% Cu alloy.
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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
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Zones or precipitate
phase (aging ) with
lattice distortion
Equilibrium phase (Overaging)
without distortion

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(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.
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16. Lattice Strain

17. Hardening
Due to Coherent Precipitate

18. Hardening
toOver Ageing

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.
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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.
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23. Steps in Precipitation Hardening

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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.
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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)
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27. Ageing
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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.
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under license.
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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.
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32. Effects of Aging Temperature and Time
The effect of aging
temperature and time
on the yield strength of
an Al-4% Cu alloy.
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herein under license.
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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.
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Learning™ is a trademark used herein under license.
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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.
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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.
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herein under license.
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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.
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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
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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

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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.
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42. THANKS