MONASH UNIVERSITY
Department of Materials Engineering
PRECIPITATION HARDENING OF Al-4Cu
EXPERIMENT T9
AIM
To demonstrate the change in microstructure and in tensile properties associated with
the ageing of a precipitation hardening alloy - in particular, of Al-4 wt.% Cu.
REFERENCES
J.M. Silcock, T.J. Heal, H.K. Hardy, J. Inst. Metals, 82, 1953, p. 239.
R.E. Smallman, "Modern Physical Metallurgy", Butterworths.
J.W. Martin, “Precipitation Hardening”, 2nd Edn., Butterworth-Heinemann, 1998.
D.A. Porter and K.E. Easterling, "Phase Transformations in Metals and Alloys", 2nd
Edn., Chapman & Hall, 1993, pp 291-308.
INTRODUCTION
Structural features
The sequence of precipitation in Al-Cu alloys depends on the degree of
supersaturation of copper and the ageing temperature. When the copper content is
high and ageing temperature is low, the sequence is:
(i)
G.P. Zones - discs of nearly pure copper on (100)Al, one atom plane thick and
of the order of 10 nm diameter.
(ii)
" - coherent precipitates of the order of a few nm thick and 150 nm diameter.
The structure is tetragonal and the composition is already CuA12.
(iii)
' - coherent precipitates which only have weak long range strain fields since
each precipitate is associated with a dislocation loop.
(iv)
 - this is the equilibrium CuAl2 precipitate. Its structure is yet another
tetragonal one and it is commonly regarded as incoherent.
At sufficiently high temperatures or low copper content the sequence is only '
The limiting temperatures at which various intermediate phases can be found are
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conveniently indicated on a "metastable phase diagram" as, for example, a "G.P. zone
solvus".
The rate of precipitation is governed by the rates of diffusion. These in turn, are
sensitive to the vacancy super-saturation, lattice strain and impurities. The last two of
these are a function of the solution treatment temperature and the quench rate.
Strengthening mechanisms
In general, precipitates may be deformable or non-deformable. In the first case, the
strength of the alloy depends on the structure and properties of the precipitate,
whereas in the second case, the strength is controlled by the inter-particle spacing.
Features of the precipitate which may be important to strength are:
(a)
coherency strains
(b)
order-disorder energies
(c)
differences in modulus between particle and matrix, and
(d)
chemical hardening due to the energy of newly created particle-matrix
interface.
The deformation mechanism has a marked effect on the work-hardening rate. If
particles are cut by dislocations, then slip is planar and work-hardening slight:
conversely, bypassing of particles causes turbulent slip and strong work-hardening.
In the Al-Cu system, GP zones and " are generally deformable. The important
feature contributing to strength is probably chemical hardening ((d) above). Small '
precipitates may also deform, but thicker ' are not cut by dislocations.  usually
precipitates in such a dispersion that it, too, is not cut by dislocations.
The full precipitation sequence (i.e. at low ageing temperatures) gives rise to a threestage hardening sequence: hardening due to GP zones, additional hardening due to "
followed by softening as ' and  form. At higher temperatures, only ' and  occur:
there is then initial hardening due to the fine initial dispersion of ' followed by
softening as the ' coarsens:  also appears at or after peak hardness.
EXPERIMENTAL PROCEDURE
Examine the four transmission electron micrographs of Al-4%Cu, quenched from
525°C and aged at 160°C and 190°C for various times. Describe and comment on the
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microstructures shown in the micrographs provided.
You are provided with 5 tensile samples of the same alloy, quenched from 525°C and
aged for various times at 200°C as indicated. Prepare the as-quenched sample and a
sample aged at 200°C for a time between 5-20 min., as determined by the group.
(a)
Measure the hardness of these samples using the Vickers hardness tester.
Make five indents in each of the samples, average the readings and determine
the hardness. Plot Vickers hardness number (VHN) vs (log) ageing time.
(b)
Electropolish the tensile samples in Lenoir’s solution

Protect your eyes by wearing safety glasses.

Wash your hands immediately with cold water if the solution comes into
contact with your skin.

(c)
Wear gloves.
Test the samples in the Instron machine to a strain of 10% using a load cell of
5 kN and speed of 5mm/min. Record the 0.2% proof stress and the initial rate
of work hardening. Examine the surfaces of the deformed specimens with a
microscope and make sketches of the main features you see. Using the tensile
stress-strain curves obtained, plot (i)0.2% proof stress and (ii) initial work
hardening rate vs (log) ageing time.

Wear safety glasses to protect your eyes.

Keep your hands away from the moving parts of the machine.
(d)
Continue the tests to failure and record the tensile strength and elongation to
fracture. Plot tensile strength and elongation to failure vs (log) ageing time.
REPORT
Write a report of approximately 2000-3000 words with emphasis on the link between
the precipitate microstructures, strengthening mechanisms and the measured
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mechanical properties. It worth 6% of total mark for the unit.
ASSESSMENT
Reports will be assessed both for technical content and for technical communication
skills.
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