Precipitation in AlCu System

1
Precipitation in Al-Cu System

• To use Al-Cu system to illustrate the process of
  precipitation which leads to the most important
  strengthening mechamism in non-ferrous metals
  such as Al alloys
• Purpose to bring about a uniform distribution of
  very fine particles, precipitates, in the matrix
  phase
• These precipitates can strengthen the matrix and
  this is called precipitation hardening or age
  hardening.

• Precipitation treatment
• Principle

In the system A-B, an appreciable amount of B may
be dissolved in A to form a solid solution (a) at
an elevated temperature
The solubility of B in A, however, decreases with
lowering temperature.
The excess amount of B at a lower temperature
will "precipitate out" to form a new phase, b
which is based on B.
The trick is in controlling the size, shape,
amount and distribution of the precipitates (b)
in the matrix (a).
Reading 11.7-11.9 (5th ed), 11.9 (6th ed)
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Precipitation in Al-Cu System

• The process

1. Solution treatment heating to a T in the
single a region (e.g. T0 for alloy C0) and
holding for sufficient time to obtain a single a
phase.
2. Quenching rapid cooling to a low T (normally
RT, e.g. T1) to keep the single a phase which is
now supersaturated.
3. Ageing raising T to an elevated level (e.g.
T2) to bring about b precipitates in the matrix
of a.
When T2 gt T1 Artificial ageing When T2 T1
RT Natural ageing
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Precipitation in Al-Cu System
a
b
a
Artificial ageing
Natural ageing
4
Precipitation in Al-Cu System
100 nm
Precipitates h' and h (MgZn2) in a 7150-T651 Al
alloy - TEM micrograph
5
Precipitation in Al-Cu System

• Al-Cu system

Al-4Cu
6
Precipitation in Al-Cu System

• Sequence of precipitation in Al-4Cu

GP zones
q"
Supersaturated a
q'
q
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Precipitation in Al-Cu System

• Age hardening
• strength increases initially with ageing
• strength reaches a peak
• strength decreases eventually

a mixture of q" and q' at the peak
Peak ageing
Underageing
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Precipitation in Al-Cu System

• effect of temperature

The practical ageing temperature is 190C and
ageing time 12 hours