HEAT TREATMENT OF ALLOYS
• STEELS
• Hardening and toughening by quenching and tempering
was known empirically for thousands of years
• 1906 Wilm realized that Al+Cu could also be hardened by
quenching and tempering (ageing) Al-4% Cu
• 1920 Mercia, Walternberg and Scott discovered that
hardening occurred during ageing due to the
decomposition of a supersaturated solid solution by the
quench
HEAT TREATMENT OF ALLOYS
• The requirement is that the phase diagram
had to have a solid solubility limit that
decreased with temperature
• This fortunately is a common feature of
many phase diagrams
• Potentially there are many possibilities of
age hardening alloy systems
• Also an appreciable maximum solubility of
one element in the other
Hypothetical phase diagram for a precipitation
hardenable alloy of composition Co
PRECIPITATION HARDENING
• This technique is used in systems which have
two phases at room temperature but only one
of the two is stable at elevated temperatures
• For example the Pb-Sn system has the 𝛼 𝑎𝑛𝑑 𝛽
phases stable over the majority of
compositions at room temperature, but at
higher temperatures (> 183°𝐶) this range is
reduced and at less than 19wt%Sn, only the 𝛼
phase is stable
• At less than3wt%Pb only the 𝛽 phase is stable
The Pb-Sn Phase diagram
PRECIPITATION HARDENING
• Consider a 16 wt% Sn alloy
• Under equilibrium cooling to room temperature, the
structure seen consist of 𝛼 phase grains with 𝛽 phase
precipitated out along the grain boundaries
• Heating this alloy to 183oC, the excess Sn in the 𝛽 phase is
taken up in solution and the 𝛽 phase disappears
• Rapid quenching from this temperature freezes the the Sn
in the 𝛼 solid phase and no 𝛽 forms
• This process is known as SOLUTION TREATMENT
PRECIPITATION HARDENING
• By re-heating the metal so slightly to temperatures
higher than room temperature, but well below the
eutectic temperature, the metal can be aged and by
slow, short range diffusion, allow Sn to form fine, evenly
dispersed precipitates within the grains
• This increases the hardness of the material without
affecting the ductility too much
• This is called precipitation hardening
THE HARDENING MECHANISM
• Hardening is brought about by the production of a fine
dispersion of precipitates that act as obstacles to
dislocation glide.
• Note that dislocation glide results in yielding of crystalline
materials
• This prevention of glide results in hardening of materials
• Main factors
• Obstacle spacing
• Obstacle strength
• Strong dislocation-obstacle interaction
THE HARDENING MECHANISM
• Optimum hardness is gained from the
production of a fine dispersion of strongly
bonded, semi-coherent precipitates from
ageing a supersaturated solid solution
• The temperature chosen for ageing depends
on the condition of the alloy
• Too high temperatures may result in overageing and loss of peak strength
AGE HARDENING OF AL ALLOYS
• ADVANTAGES OF AL ALLOYS
• Low density
• Comparatively cheap (but not compared to steel)
• Wide range of properties from work hardening and
ageing, example pure Aluminium has a strength of
60MPa, while heat treated alloys may have strengths as
much as 600 Mpa
• Good corrosion resistance
• Good electrical and thermal conductivities
• Good ductility (easy to fabricate)
ALUMINIUM ALLOYS
• DISADVANTAGES
• Low melting point, as such applications limited to less
than 150oC to 200oC.
• Low elastic modulus e.g 70 GPa compared 200GPa for
steel
HEAT TREATMENT OFoAL-4%CU
ALLOY
o
• Solution heat treat at 530 C – 540 C; microstructure –
Single 𝛼 phase field
• Quench to room temperature- Frozen microstructure of 𝛼
phase grains but now in the 𝛼 + 𝜃 field, so the 𝜃 begin to
precipitate out
• Type of precipitate formed is a function of time spent and
ageing temperature
• 𝑆𝑆𝑆𝑆 → 𝐺𝑢𝑖𝑛𝑒𝑎𝑟 𝑃𝑟𝑒𝑠𝑡𝑜𝑛 𝑍𝑜𝑛𝑒𝑠 → 𝜃 ′′ → 𝜃 ′ → 𝜃
The Al rich side of the Al-Cu phase diagram
GUINIER PRESTON (GP) ZONES
• This is a very fine dispersion of small flat discs of Cu
atoms, 1 atom thick and 3 atoms diameter
• They are fully coherent with the matrix and take the
same FCC crystal structure as the aluminium matrix
"
𝜃 Phase
• This is a fine dispesion of plates 2 nm
thick and 40 nm diameter
• They have a different crystal structure to
the matrix
• They are coherent to the matrix but with
large misfit strains
′
𝜃 Phase
• Forms on a coarser scale than 𝜃 " probably
from 𝜃 "
• They grow at the expense of other
precipitates
• Form at longer times or directly at
temperatures >2000C
• They are partly incoherent with the matrix
and have little elastic strains
𝜃 Phase
• These are CuAl2 intermetallics
• This equilibrium phase form after a very
long ageing times or at temperatures >
3500
• It is a very brittle phase
• It forms as an intergranular precipitate
with all planes fully incoherent with the
aluminium matrix
• It is associated with over-ageing
• Highest strengthening is obtained by the
production of a dispersion of coherent or
"
′
semi-coherent 𝜃 and 𝜃 precipitates in a
reasonable ageing time
• After 2 to 3 days GP zones transform to 𝜃 "
′
and 𝜃 precipitates which give maximum
hardening
′
• Beyond peak hardening, 𝜃 → 𝜃 results
which gives a low strength alloy with poor
toughness due to the brittle nature of the
equilibrium phase AlCu2
Strength and hardness as a function of ageing
time at constant temperature
PLOT OF HARDNESS AGAINST TIME
1300
𝜃^′
𝜃^′′
1200
𝜃
HARDNESS
1100
1000
900
800
700
600
1
10
AGEING TIME IN HOURS
100
1000