Dr. H. K. Khaira
Professor in MSME
Precipitation Hardening (or Age
• 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
• 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
• this is accomplished by appropriate heat treatments.
• the process is called precipitation hardening because the small
particles of the new phase are termed "precipitates”.
• “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.
• Mechanism of Hardening:
• During plastic deformation:
– Zones or precipitates act as obstacles to
– Stress must be increased to “push” the
dislocation through the distribution of
– Consequently the alloy becomes harder and
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.
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.
Coherent precipitate - A precipitate whose crystal structure and atomic
arrangement have a continuous relationship with the matrix from which
the precipitate is formed.
• A composition that
can be precipitation
two phases at room
can be heated to a
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
• There is an increased resistance to dislocation
motion by these lattice strains in the vicinity
of the microscopically small precipitate
Mechanism of Hardening
Supersaturated α solid
Zones or precipitate
phase (aging ) with
Equilibrium phase (Overaging)
• Small inclusions of secondary phases
• Lattice distortions around these secondary
phases impede dislocation motion
• The precipitates form when the solubility limit
• Precipitation hardening is also called age
hardening because it involves the hardening
of the material over a prolonged time.
Microstructural Evolution in Age or
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.
Steps in Precipitation Hardening
• Precipitation hardening is accomplished by two
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)
• 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
• 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
• Quenching to room
temperature to create a
• Precipitation Treatment;
alloy is heated to a
temperature below Ts to
cause precipitation of fine
particles of beta phase.
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
• aging can also occur at room temperature for some alloys
• 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
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.
Overaging in Precipitation Hardening:
• With increasing time, the strength or hardness
increases, reaches a maximum, and finally
• This reduction in strength and hardness that
occurs after long time periods is known as
• Diagram shows strength as a function of the
logarithm of aging time at constant temperature
during the precipitation heat treatment.
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
Thus, the higher aging temperature gives lower peak strength
and makes the strength more sensitive to aging time.
Fig – 3
Step 1: Solution-treat at a temperature between the solvus and the
eutectic to avoid hot shortness. Thus, heat between 340oC and
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.
Requisite Features on Phase
Diagrams for Precipitation Hardening:
An appreciable maximum solubility of one
component in the other, of the order of several
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
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