Precipitation hardening, also known as age hardening, strengthens metal alloys through heat treatment processes. It involves dissolving alloying elements into a metal's crystal structure through heating, then controlling the precipitation of those elements out of solution through precise aging at lower temperatures. This forms uniformly dispersed nano-scale precipitates that impede dislocation motion, strengthening the metal. Common alloys strengthened this way include aluminum-copper, copper-beryllium, and some magnesium alloys. The Wright Flyer's engine in 1903 was one of the earliest uses of precipitation hardening with an aluminum-copper alloy.

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, copper-
tin, magnesium-aluminum, and some ferrous
alloys

3. 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”.

4. 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 Al-
Cu, Cu-Be, Cu-Sn, and Mg-Al; and some ferrous alloys.
Solvus
curve
Solvus curve

5. 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.

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. 8
 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.
Age or Precipitation Hardening

9. 9
 Coherent precipitate - A precipitate whose crystal structure and atomic
arrangement have a continuous relationship with the matrix from which
the precipitate is formed.
Coherent precipitate

10. 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.

11. Al – Cu Equilibrium Diagram

12. 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.

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. 14
Mechanism of Hardening
Supersaturated α solid
solution
Zones or precipitate
phase (aging ) with
lattice distortion
Equilibrium phase (Overaging)
without distortion

15. 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.

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. 20
 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.
Microstructural Evolution in Age or
Precipitation Hardening

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. 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.

23. Steps in Precipitation Hardening

24. 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.

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. 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)

27. 27
Ageing

28. Effect of Ageing Temperature on
Strength

29. Effect of Ageing Temperature on
Ductility

30. 30
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.
Composition of Al-4% Cu Alloy Phases
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein
under license.
Figure 1 - The
aluminum-copper
phase diagram and
the microstructures
that may develop
during cooling of an
Al-4% Cu alloy.

31. 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.

32. 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.

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. 34
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 190o
C and 260o
C.
Effect of Aging Heat Treatment Time on the Strength
of Aluminum Alloys
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson
Learning™ is a trademark used herein under license.
Fig – 2 The effect of
aging temperature and
time on the yield
strength of an Al-4% Cu
alloy.

35. 35
SOLUTION
From Fig – 2
At 190o
C, 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 260o
C, 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.

36. 36
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.
Design of an Age-Hardening Treatment
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used
herein under license.
Fig – 3 Portion of
the aluminum-
magnesium phase
diagram.

37. 37
SOLUTION
Fig – 3
Step 1: Solution-treat at a temperature between the solvus and the
eutectic to avoid hot shortness. Thus, heat between 340o
C and
451o
C.
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 340o
C,
to form a fine dispersion of β phase.

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. 39
Use of Age-Hardenable Alloys at High
Temperatures

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 A-
286 High Strength Stainless 17-10P

41. 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.

42. THANKS