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International Journal of Modern Physics B
Vol. 23, Nos. 6 & 7 (2009) 900–905
World Scientific Publishing Company
900
THE EFFECT OF RRA ON THE MICROSTRUCTURE AND PROPERTIES OF
A NOVEL Al-Zn-Mg-Cu-Zr ALLOY
XIWU LI
*
, BAIQING XIONG, YONGAN ZHANG, BAOHONG ZHU, HONGWEI LIU and ZHIHUI LI
State Key Laboratory for Fabrication and Processing of Non-ferrous Metals,
General Research Institute for Non-ferrous Metals,
Beijing, People’s Republic of China
*
xiwulee@yahoo.com.cn
A novel Al-7.5Zn-1.6Mg-1.4Cu-0.12Zr alloy was subjected to the retrogression and re-ageing
(RRA) treatments at a lower range of retrogression temperatures from 170 to 200°C. The effect of
RRA on the mechanical properties, electrical conductivity and the microstructure of the alloy has
been investigated. The results indicate that an increase in the yield strength of the alloy can be
achieved after RRA treatment. With increase of the retrogression temperature, the retrogression time
for keeping the strength levels similar to T6 temper decreases rapidly. When submitted to RRA at
170°C for 90-120 min, the alloy can obtain a good performance on both SCC and mechanical
strength; the electrical conductivity was above 40%IACS, reaching values typical for T76 condition
and the tensile yield strength values were 552-570 MPa which is higher than that of the T6 temper.
The microstructure is a very fine distribution of GP zones and η′ precipitates inside grains, similar to
T6 condition and η precipitates on grain boundaries distributed similarly to T7 temper.
Keywords: Al-Zn-Mg-Cu alloy; RRA; properties; microstructure.
1. Introduction
Al-Zn-Mg-(Cu) (7xxx series) alloys have been widely used as structural materials in the
aerospace industry, due to their desirable specific mechanical properties. The Cu-rich
7xxx series alloys include the highest strength conventional Al alloys currently in use on
commercial aircraft: AA7050, 7150, 7X49 and 7055, which are generally difficult to
satisfy the need for thicker components due to high quench sensitivity
[1]
. However, the
new generation aircraft uses thicker components than ever because of its large size and
today’s complex requirements. To meet these stringent requirements, some novel alloys –
such as AA7040, 7140 and 7085 – have been recently developed for this application.
Particularly, AA7085 has even lower quench sensitivity and a much higher strength-
toughness combination
[1-3]
. To pursue the alloy development and achieve optimized
performance, a novel Al-7.5Zn-1.6Mg-1.4Cu-0.12Zr alloy has been investigated with
modifications in solute content and in particular in Zn/Mg/Cu ratios in the present work.
It is an increasingly important issues for applications that 7xxx Al alloys in T6
condition is very susceptibility to stress-corrosion cracking (SCC). Considerable research
indicated that susceptibility to SCC could be reduced by over-ageing the alloy. However,
this was achieved at the cost of a 10-15% reduction in strength compared to the T6
condition
[4-6]
. In the aerospace industry, such a loss represents a substantial decrease in
