A TEM study of microstructural changes during retrogression and reaging in 7075 aluminum
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I.
INTRODUCTION
RECENTLY, a heat treatment known as Retrogression and Reaging (RRA) I has been employed with 7075 aluminum to obtain stress corrosion cracking (SCC) resistance equivalent to the T73 temper together with T6 strength levels. This treatment is applied to material in the T6 condition and involves a short time heat treatment in the range 200 to 280 ~ followed by reaging using conditions similar to those used for the original T6 age. During retrogression, strength falls very rapidly, during the first few minutes, reaching a minimum before increasing again to a secondary peak and then decreasing again at long times when the material apparently over-ages. It was claimed by Cina t that processing to the minimum of the retrogression curve, followed by reaging, led to the favorable combination of T6 strength and T73 SCC resistance. During a previous investigation, 2 a modified heat treatment was developed using lower retrogression temperatures which allowed moderately thicker section components to be treated compared to the thin sheet investigated by Cina. It was also confirmed that the RRA treatment can give SCC resistance very similar to the T73 condition without sacrificing strength. This improvement in SCC resistance correlated well with the increase in size of grain boundary precipitates and was interpreted in terms of hydrogen embrittlement concepts. 3'4 The purpose of the work described in this paper is to examine in more detail microstructural changes occurring in the matrix during retrogression, and during retrogression and reaging treatments in relation to mechanical properties of the T6 tempered 7075 alloy. The investigation has involved the use of electron microscopy. II.
EXPERIMENTAL P R O C E D U R E
Heat treatments were carried out on unmachined specimen blanks from 19 mm diameter bar, in a silicone oil bath. The total heat treatment time included an experimentally determined time for the center of the specimen to reach the required retrogression temperature plus the retrogression time required. The nominal alloy composition was 5.1 to NGUYEN CONG DANH, Research Associate, KRISHNA RAJAN, Research Officer, and W. WALLACE, Section Head, are all with National Aeronautical Establishment, National Research Council Canada, Ottawa, Ontario KIA 0R6, Canada. Manuscript submitted September 20, 1982. METALLURGICALTRANSACTIONS A
6.1 w t p c t Z n , 2.1 to 2 . 9 p c t M g , 1.2 to 2 . 0 p c t C u , 0.5 pet Fe, 0.4 pct Si, 0.3 pct Mn, 0.18 to 0.28 pct Cr, 0.2 pet Ti, balance A1. Specimens for tensile testing were machined after full heat treatment. In the present study, microscopy was performed on the starting material and on specimens retrogressed at 200 ~ for five minutes, 30 minutes, and 60 minutes. Specimens retrogressed for five minutes and 30 minutes were also examined after reaging at 120 ~ for 24 hours. These conditions were chosen since it was clear from previous work that retrogression temperatures around 200 ~ gave good SCC resistance. 2'3 Specimens for transmission electron microscopy (TEM) w
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