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most sizes are 1…2 nm, but some big size precipitates are more than 10 nm, which are
formed by precipitates growing during aging (Fig. 3b) GBPs are discrete, but close,
like a chain. There are few PFZ at grain boundaries after pre-aging treatments.
Continuous or chain-shaped GBPs are harmful to SCC resistance of the alloy, so the
SCC sensibility is high in 7075 alloy for T6 treatment.

Fig. 3. TEM images of alloy pre-aged at 120°C for 16 h (a) and 24 h (b).
Figure 4 shows TEM images of the alloy after different retrogression and re-aging
treatments. The MP are only partly redissoloved in the matrix after retrogression at a
low temperature (160°C) via pre-aging at 120°C for 16 h, GBPs are discrete and the
obvious PFZ are left at the grain boundaries (Fig. 4a). After RRA with under pre-aging
and low temperature retrogression, thin homogeneous dispersive MP are separated out
again in the matrix, which average sizes are less than 5 nm. PFZ width is 15…20 nm.
After RRA treatment (Fig. 4b) it is because of high strength that the MP are
separated out by alloy elements where lots of the MPs smaller than critical dimension
are dissolved under retrogression treatment. GBPs are thick and discrete. Those GBPs
can constitute an obstacle to forming the galvanic corrosions and improve corrosion
resistance of the alloy. The data shows that the conductivity increases and the SCC
index I SSRT decreases [23].
It can be seen (Fig. 4c) that most of MP have been redissolved in the matrix after
retrogression at 200°C. Parts of GBP have been redissolved, and others grow up along
the grain boundaries. So the GBPs are long and discrete. After re-aging, the MPs are
thin, homogeneous and dispersive and GBPs are rounded and discrete obviously. The
MP size is about 2 nm. The average size and spacing of the GBP is 5…7 nm and more
than 10 nm respectively (Fig. 4d). The SCC resistance of the alloy is improved by
those discrete GBPs.
With the retrogression at 240°C (Fig 4e) the situation of MP mostly redissolved in
matrix is similar to that after retrogression at 200°C. But the morphology at grain
boundary is different. The GBPs are semi-continuous, the sizes and spacing of the
GBPs are small and some GBP are arranged side by side. After re-aging, the MPs are
coarsened and grown, whose sizes increased from 1…2 nm to 3…5 nm. The GBPs are
still semi-continuous and the phenomenon of GBP arranged side by side disappeared.
The PFZ are widened to 5 nm, but they are still less narrow than at others effects of
retrogression treatments, as shown in Fig. 4f. The SCC resistance of the alloy can be
improved to a certain extent by aforementioned grain boundary structures.
The short time of retrogression at 240°C is the main factor of the alloy after RRA
treatment effect on the properties. Because the retrogression time is only dozens of
seconds, there occur the phenomena of nonuniform heat treatment of the samples, even
though thin sheets. When the optimum effect has been found on the surface of the sample,
the inside of the sample is uncompleted. When the inside of the sample is suited by

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