The diagrams
show the atomic configuration associated with the various precipitation
stages in the Al-4 wt % Cu system. As the diffusion processes at the heat
treatment temperature proceed the previous precipitate tends to dissolve and
its atoms migrate to the lower energy precipitate that is next formed. The
GP zones have dimensions of 1 nm x 10 nm and are copper disc precipitates that
are coherent with the aluminum matrix. The q"-zones
are 10 nm x 100 nm and copper and aluminum form a regular
lattice structure. All of the faces of the zone are coherent with the aluminum
matrix. The q'-zones have grown
to 1 m diameter and only their large area disc faces are coherent with the aluminum
matrix. The structure is also that of a substitutional alloy of Cu and
Al. The final q-zones (CuAl2)
are completely incoherent with the matrix and the precipitates tend to favor
a spherical shape to minimize their surface energy. This phase is an Intermetallic
compound. |
