The effect of pre-ageing on the microstructure and properties of 7050 alloy
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In the present work, the effect of pre-ageing temperature and time variations on the mechanical properties and electrical conductivity of the Retrogression and re-aging (RRA) treated 7050 has been investigated. The results reveal that the electronic conductivity and hardness of RRA-treated samples are sensitive to the pre-ageing tempers. The RRA-treated samples with 120 °C/2 h pre-ageing 1180 °C/2 h retrogression 1120 °C/24 h re-ageing temper can be tailored toward a good combination of strength and elongation, while the electrical conductivity of re-ageing samples is also higher than that of 120 °C/24 h pre-ageing RRA-treated samples. With an intermediate pre-ageing temperature of 80 °C/24 h RRA-treated samples possess a higher re-aged electronic conductivity, while no significant differences can be found between hardness of 120 °C/2 h and 120 °C/24 h pre-ageing RRA-treated samples. The variation of hardness and electronic conductivity during retrogression depends on the pre-ageing tempers. For under-aged sample, the retrogression hardness appears a stage of hardness increasing followed by a further decrease in hardness results, owing to disappearance of dissolving stage of fine GP zone and g9 phase during pre-ageing.
I. INTRODUCTION
7xxx series alloys have been extensively used in automobile and aerospace structures along with many other high strength structural components due to their excellent mechanical properties, high thermal and electrical conductivity, and superior resistance to corrosion.1–7 A lot of research activities has been paid for obtaining the combination of high strength and stress corrosion cracking (SCC) resistance, since the beginning of the use of the high strength 7075 aluminum alloy for aerospace applications.8–11 Single-aging(T6 temper) has been developed to obtain highest strength, while stress corrosion resistance is very low and susceptibility to stress-corrosion cracking and exfoliation corrosion is quite high. To improve stress corrosion resistance, the two-step aging (T76/T74/T73 temper) has been developed. However, the consideration of two-step aging is at the expense of strength. The main differences between the T6 and T7 tempers are related to the precipitates distribution and dislocation density.12–15 For the purpose of achieving a better combination of high strength and stress corrosion resistance, triple-stage aging (T77 temper) has been developed. The triple-stage aging is called RRA. Retrogression and re-aging (RRA), developed by Cina and his colleagues in 1974, is an attempt
Contributing Editor: Yang-T. Cheng a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2015.372 J. Mater. Res., Vol. 30, No. 24, Dec 28, 2015
to improve SCC resistance with high levels of both strength. That means 7xxx aluminum alloys treated through RRA have high corrosion resistance equivalent to the T73 temper, together with the T6 strength levels. This so-called RRA treatment involves taking the 7000 series aluminum alloy in the T6 temper, retrogressing it by hea
