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retrogression, the surface of the sample has already been an overtreatment. So the
strength, conductivity and SCC index I SSRT of the alloy via retrogression at 240°C all
are not better than that via other retrogression treatments.

Fig. 4. TEM images of the alloy after different retrogression and re-aging treatments via
under pre-aging: a, b – retrogressed at 160°C for 120 min and re-aged, respectively;
c, d – retrogressed at 200°C for 8 min and re-aged, respectively;
e, f – retrogressed at 240°C for 0.5 min and re-aged, respectively.

CONCLUSION
With the under aging at 120°C for 16 h as pre-aging in RRA treatment, the follo-
wing conclusions have been drawn on three retrogression treatments of spray formed
7075 alloy:
– most of the matrix precipitates (MP) are redissolved in matrix and the grain bo-
undary precipitates (GBP) are discrete obviously after retrogression at 200°C for 8 min.
After re-aging, the UTS, elongation and SCC index of the alloy is 791 MPa, 8.5% and
0.155, respectively.
– MPs are only partly redissoloved in the matrix after retrogression at a low
temperature (160°C), the GBPs are discrete and the obvious PFZ are left at the grain

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