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The property
change with ageing time is due to the reoganization of the excess copper
in the a-phase matrix. The diagram shows the effect of this reorganization
on the room temperature yield stress, and the photographs show the microstructure
of the GP zones, q''-zones, and
the q'-zones. These states are all metastable.
The final equilibrium state is the q-precipitate
in the a-matrix,
the state that is produced by quasi-static cooling. The atoms in the GP and
q" zones are coherent with the a-matrix and produce local elastic distortion.
The q'-zone is semi-coherent
and the elastic distortion of the matrix is reduced, the q precipitate is incoherent and causes the least elastic distortion
of the matrix. The magnitude of the coherent distortion is largest
for the q" zones and this causes
this microstructure to have the highest yield stress. |
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From:
Ashby and Jones, "Engineering Materials 2," Pergamon (1986), &
Schafer et al, "The Science and Design of Engineering Materials,"
McGraw Hill (1999) |
