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cation of the conductivity is obvious. The conductivities can be improved to 40% IACS
and more.

Fig. 1. Tensile strength and conductivity
of the alloy during retrogression at 160°C (a),
200°C (b) and 240°C (c) and re-aging:
, – re-aged and retrogressed strength,
respectively; , – re-aged and
retrogressed conductivity, respectively.

Properties of the alloy after different retrogressions and re-aging treatment

Pre- Re- UTS, Elongation,
Retrogression, Conductivity,
aging, °C×min aging, % IACS MPa % I SSRT
°C×h °C×h air NaCl air NaCl

120×16 160×120 120×24 37.2 772 710 8.8 6.6 0.287
120×16 200×8 120×24 39.5 791 756 8.5 7.4 0.155
120×16 240×0.5 120×24 38.8 773 737 8.6 7.7 0.136

Figure 2 shows the stress-strain curves of SSRT in dry air and 3.5 wt.% NaCl
solution after three kinds of RRA treatments. From Fig. 2, it can be found that the dec-
rements of UTSs in NaCl solution are similar. However, the decrements of elongations
in NaCl solution among three kinds of RRA treatments are very different. With the
retrogression at 160°C for 120 min, the elongation falls down from 8.8% (in dry air) to
6.6% (in 3.5 wt.% NaCl solution). This decrement of elongation is 25%, which is far
more than those for other RRA treatments with retrogression at 200°C and 240°C.
To judge the SCC resistance, SCC index I SSRT was defined by processing various
mechanical properties of SSRT.

σ NaCl ´ 1 ( + δ NaCl )
I SSRT = 1- ,
σ air ´ 1 ( + δ air )

where s NaCl, s air – the UTS in 3.5 wt.% NaCl solution and in dry air (MPa), respecti-
vely; d NaCl, d air – elongation in 3.5 wt.% NaCl solution and in dry air (%), respectively.
With the I SSRT close to 0, the SCC resistance increases. SSRT properties of spray
formed 7075 alloy after various RRA treatments are also listed in the Table.

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