A TEM Study of Microstructural Changes during Retrogression and Reaging in 7075 Aluminum

NGUYEN CONG DANH, KRISHNA RAJAN, and W. WALLACE

Microstructural changes occurring during retrogression, and during retrogression plus reaging in 7075-T6 aluminum alloy have been investigated by means of transmission electron microscopy, and related to mechanical properties. TEM results indicate that the drop in strength during the initial stage of retrogression was due to the partial dissolution of G.P. zones while the growth of the semi-coherent r/' was responsible for the rapid recovery of strength. It is suggested that the retrogression and reaging treatment resulted in the increase in volume fraction of G.P. zones and especially ~ ' precipitates over both the T6 and retrogressed conditions, therefore significantly improving the strength of the alloy.

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, fol- lowed by reaging, led to the favorable combination of T6 strength and T73 SCC resistance.

During a previous investigation, 2 a modified heat treat- ment 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 sacri- ficing strength. This improvement in SCC resistance cor- related well with the increase in size of grain boundary precipitates and was interpreted in terms of hydrogen em- brittlement 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 in- volved the use of electron microscopy.

II. EXPERIMENTAL PROCEDURE

Heat treatments were carried out on unmachined speci- men 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.

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 re- aging 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) were prepared by conventional jet polishing techniques us- ing a solution of 33 pct nitric acid in methanol at -15 ~ The method of indexing the diffraction patterns for the pre- cipitates was similar to that of DeArdo and Simensen. s

III. RESULTS

A. Microstructural Changes during the Retrogression Treatment

The effects of retrogression at 200 ~ and retrogression plus reaging (120 ~ 24 hours) on yield strength are sum- marized in Figure 1 which is reproduced from Reference 2. It is clear that the shape of the retrogression curve is similar to that obtained by Cina (using hardness measurements) in 7075 alloy ~ and to the curve obtained from yield strength measurements by Groma et al. 6 in an A1-Zn-Mg alloy. Dur- ing retrogression, the strength falls very sharply to reach a minimum before increasing to a 'secondary' peak and then decreasing at long times as the materials overaged. The strength at the secondary peak, however, is not equivalent to that of the original T6 starting material suggesting that there exists a different microstructural component contributing to strengthening compared to the T6 temper. The RRA treated materials have yield strengths which are higher than the T6 minimum values and the "as received" T6 value for the sample tested.

The T6 material showed a typical microstructure with a homogeneous distribution of fine precipitates in the matrix (Figure 2). Streaking was observed along [lll]a~ through matrix reflection spots indicating the existence of "O' plate- lets on [lll]a~. 7 It should be added that G. P. zones would also be expected, in the starting condition as reported by Thomas and Nutting. 7

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Fig. 1 - -Changes in yield strength during retrogression at 200 ~ and after retrogression plus reaging treatment.

was made. The curve suggests that three stages may be present. Interestingly, the times corresponding to the transi- tion from one stage to the next correspond fairly well to the times for each stage of the retrogression curve. A similar curve (b) in terms of Guinier radii of G.P. zones was also obtained by Gueffroy and Lrffler 8 by means of small angle X-ray scattering (SAXS). Their study was conducted at the

Micrographs and the corresponding diffraction patterns of T6 material retrogressed for various times at 200 ~ are shown in Figures 3 through 6. The changes in r/' and "0 precipitate size were measured. The growth of precipitates in terms of the change in particle size obtained by TEM during retrogression treatment is shown by a log-log plot (a) in Figure 7. Each point represents the mean of 50 mea- surements of particle diameter. Since observations were carried out in thin areas of foils and since only the average precipitate size was being measured, no overlap correction

Fig. 2 - - ( a ) Bright-field micrograph of the T6 material, (b) diffraction pattern, and (c) indexing of the pattern shown in (b). There are also r/ precipitates indicated by (11.2) r/ and (00.4) 7/ spots.

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Fig. 3--Bright-field micrographs of specimens retrogressed at various times, z --- [112]; the ~ reflections are indicated by the arrows: (a) 5 rain, (b) 30 mm, and (c) 60 min.

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Fig. 4--Dark-f ield micrographs of specimen retrogressed at 200 ~ for various times (z = []11]) using 20.0 "q' and 00.4 r/ diffraction spots. (a) T6, (b) 5 min, (c) 30 min, and (d) 60 min.

METALLURGICAL TRANSACTIONS A VOLUME 14A, SEPTEMBER 1983-- 1845

Fig. 5--Diffract ion patterns of specimens retrogressed at 200 ~ for various times, z = [112]: (a) 5 rain, (b) 30 min, and (c) 60 min.

Fig. 6--Diffract ion patterns for specimens retrogressed at 200 ~ for various times. ] = []11]: (a) T6, (b) 5 min, (c) 30 min, (d) 60 min.

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same temperature as ours on a pure A1-Zn-Mg alloy contain- ing G.P. zones with an initial size of 2.7 nm grown at 100 ~ We shall discuss the interpretation of these changes in more detail later in the paper.

Another microstructural parameter that was measured was the volume fraction of r/' and "r/ precipitates. Given the limitations of selected area diffraction and the small separation of the "0' and r/ reflections, it was difficult to separate the two phases using centered dark-field illumina- tion (Figure 4).

The volume fraction measurements are tabulated for the various heat treatment conditions in Table I. It can be seen that between the T6 and the minimum of the retrogression curve after five minutes (Figure 2), there is no significant

Table I. Volume Fraction of ~7' and 77 during Retrogression and RRA

Heat Treatment Condition Volume Fraction

T6 Retrogressed-- 5 minutes Retrogressed--5 minutes and reaged Retrogressed-- 30 minutes Retrogressed-- 60 minutes

0.029 0.031 0.047 0.071 0.120

change in either precipitate size (Figure 7) or volume frac- tion. After the minimum, there is a noticeable increase in volume fraction along with a gradual increase in precipitate size (2nd stage in Figure 7). After the secondary peak in the retrogression curve is reached at 30 minutes (Figure 2), there is a corresponding increase in the rate of precipitate growth (3rd stage in Figure 7) and a further increase in volume fraction. While diffraction patterns showed that there existed both 77' and 77, after the yield strength reached the secondary peak, the "0' reflection of the type { 10.2} though visible became weaker in intensity with further aging (Figure 5(c)).

B. Microstructures of the RRA Treated Materials

After a short time of five minutes of retrogression at 200 ~ and then reaging under the same condition used for the T6, temper (120 ~ 24 hours), the volume fraction of ~7' and r/ precipitates increased (see Table I) over both the original T6 condition and the corresponding retrogressed condition (the minimum in the retrogression curve, Fig- ure 2). Long streaks were noticed at all matrix spots along the [111]A~ directions indicative of the very thin "r/' plates on [111]A~ (Figures 8(c) and 9(c)). The reaging treatment caused a slight increase in precipitate size compared to that of retrogressed material (9.0 nm for the retrogressed condition after five minutes to 9.4 nm for RRA). Under certain condi- tions, this treatment would increase the volume fraction of semi-coherent r/' precipitates without significantly coars- ening them, and thus improve strength properties. This point will be discussed later in more detail.

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RETROGRESSION TIME (min) Fig. 7--Changes in precipitate size during retrogression. (a) TEM results. (b) Guinier radii data by small angle X-ray scattering (SAXS) measurements obtained by Gueffroy and L6ffler (Ref. 8) also given for comparison.

METALLURGICAL TRANSACTIONS A VOLUME 14A, SEPTEMBER 1983-- 1847

Fig. 8 - - ( a ) Dark-field mi_crograph of specimen retrogressed for 5 min at 200 ~ and then reaged (120 ~ 24 h; z = [111]) using 20.0 ~7' and 00.4 diffraction spots. (b) z = [111 ] diffraction pattern. (c) z = [110] diffraction pattern. Note long streaks along [111]A~ (shown by the arrows) indicating very thin r/ ' plates on (lll)A~. (d) z = [112] diffraction ; , t tern.

Fig. 9 - - ( a ) Dark-field micrograph of specimen retrogressed for 30 min at 200 ~ and then reaged (120 ~ 24 h ; z = [TI1]), using 20.0 aq' and 00.4 r/ diffraction spots. (b) z = [111] diffraction pattern. (c) z = [110] diffraction pattern. Note streaking along [111] indicating thin 7/' plates on (lll)Aj. (d) ~ = [112] diffraction pattern.

IV. DISCUSSION

The precipitation processes in the A1-Zn-Mg alloy system have been the subject of a number of studies.5-7'9'l~ The aging sequence has been generally accepted as: supersaturated solid solution ~ G.P. zones ~ r/' ---> ~7 (MgZnz). Most of the investigations have concentrated on the early stage of

age hardening, involving studies of G.P. zone formation, the structure of G.P. zones, zone kinetics, and their role during the aging process. Unfortunately, there has not been as much microstructural work performed to examine the characteristics of materials given the T6 temper n'12 com- pared to materials aged for shorter times. In our previous study, 3 TEM investigations were carried out mainly on large

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precipitates at grain boundaries, and it was suggested that the retrogression treatment is a precipitate coarsening treat- ment for grain boundary precipitates. In the present study, the examination of the microstructures suggests that the reaging of the retrogressed material allows for nucleation and growth of the semi-coherent "0' phase in the matrix, and possibly the coherent G.P. zone phase also.

In the following discussion, we will follow the micro- structural changes observed along the retrogression curve and then along the RRA curve (i.e., Figure 1--yield strength vs time).

A. Retrogression Process of the T6 Material

The retrogression curve can be dealt with in three stages: the initial rapid drop in strength, the subsequent increase in strength giving rise to a secondary peak, and finally the last stage where the strength begins to drop again.

Given the premise that the primary source of strength- ening in the T6 condition is the presence of a coherent second phase, 11 it is suggested here that the rapid decrease in yield strength is due to the dissolution of that coherent phase. The loss of strength in aged aluminum alloys after heating the material at temperatures higher than the aging temperatures (e.g., above 100 ~ in AI-Zn 13 and A1-Mg 14 alloys) has been ascribed to the reversion of the coherent phase. Thus, we may interpret the initial stage of the retro- gression process in terms of the well known G.P. zone reversion phenomenon. It should also be pointed out that the time required to reach the minimum in the retrogression curve decreases with increasing temperatures. 2 This would be consistent with the concept of faster dissolution kinetics with increasing temperatures. 2

It must be remembered that the initial drop in strength upon retrogression has a limit and in fact it begins to in- crease at some point (Figure 1), and hence there appears the minimum in the retrogression curve. The work of Inoue et al. 15 suggests that the G.P. zone solvus temperature is approximately 130 ~ and therefore one would expect G.P. zone dissolution to be complete at the retrogression tem- perature. However, Ungar et al. 16 have shown that, even at temperatures as high as 160 ~ G.P. zone dissolution would never be reached in alloys with Zn and Mg contents higher than about 5.5 wt pct and 1 wt pct, respectively. Therefore, in the present study it appears that the solute concentrations (5 to 6 wt pct Zn and 2 to 3 wt pct Mg) are high enough to prevent complete G.P. zone dissolution, and indeed to en- hance the formation of the 77' phase.

During the G.P. zone dissolution process, the growth of the preexisting ~7' and the nucleation of new r/' particles on the remaining G.P. zones could occur simultaneously. 6"15-~8 However, the rapid decrease in strength would suggest that these reactions do not occur at the same rate. The TEM investigation of the first stage of retrogression showed that there was no noticeable change in size and distribution of r/' precipitates from the T6 condition (Figure 2(a)) until the minimum of the retrogression curve was reached (Figure 3(a)). This is in agreement with this suggestion that the G.P. zone reversion and not 77' growth dominates during this stage of the process.

In the context of this discussion on the possibility of concurrent formation of r/' and dissolution of G.P. zones during the initial stage of retrogression, it would be worth- while to refer to the work of Delasi and Adler. 12 They found

METALLURGICAL TRANSACTIONS A

from a combination of differential scanning calorimetric measurements and hot stage TEM that G.P. zone dissolution and the formation and growth of "O' precipitates occurred simultaneously in a temperature range up to 217 ~ They pointed out, however, that the first reaction, i.e., the G.P. zone dissolution, was the predominant one. In addition, other workers have found that the dissolution of r/' particles did not take place at temperatures lower than 250 ~ In the context of our study, this would imply that only the dissolution of G.P. zones is responsible for the initial loss of strength. In fact, as mentioned earlier, there was virtually no change in size or volume fraction of ~' and r/ precipitates during the initial stage of retrogression. Thus, any explana- tion of the initial rapid decrease in strength should be attri- butable to the G.P. zones, i.e., G.P. zone reversion.

Let us now turn our attention to the microstructural changes in relation to the increase in yield strength beyond the minimum of the retrogression curve. Our results indicate an increase in volume fraction of r t' and "0 precipitates from the minimum of the retrogression curve to the maximum. We feel that this recovery in strength may be attributed to this increase in volume fraction. Following the suggestions of other workers, 15'19'2~ "q' would provide the predominant contribution to strength at the retrogression temperature. Thus, it may be suggested that the increase in total volume fraction of precipitates was mostly an increase in volume fraction of "q'. It should be recalled that we observed a concurrent increase in precipitate size during the same period of time (2nd stage in Figure 7). Following the sug- gestion of Kovacs et al., 21 an increase in the size of r/' would contribute to a decrease in strength. We feel therefore that the rate of increase in precipitate size was not great enough to dominate the volume fraction effects. As noted in Fig- ure 7, the growth rate increases (3rd stage) noticeably after the secondary peak in the retrogression curve is reached. At that point, the drop in strength is due to simple overaging. It is felt that the growth of the incoherent "O is predominant at this stage as emphasized by the pronounced reflection spots (Figure 5(c)). r/' and rt particles undergo Ostwald ripening at this stage.

B. Effects o f RRA Treatment on Mechanical Properties

The reaging treatment carried out on retrogressed mate- rials allowed a maximum increase in yield strength of about 5 pct with respect to that of the T6 condition (Figure 1). It should be noted that even after long times of retrogression where coarsening of precipitates took place (Figure 3(c)), the reaging treatment could still improve yield strength by up to 4 pct compared to retrogressed specimens. The formation of r/' precipitates during reaging is primarily re- sponsible for this improvement (Table I).

The RRA treatment can be simply considered as a two- step aging process; the first step involves retrogression and the second represents the reaging treatment. In the initial step, retrogression causes the partial dissolution of the pre- existing G.P.zones. However this dissolution may not nec- essarily be complete as has been noted by some workers. 6'15 The remaining G.P. zones then can act as nucleation sites for "0' particles. 16.17.18 The dissolution of G.P. zones, further- more, enriches the matrix in Zn and Mg, which in turn promotes the nucleation and growth of the rt' phase. In the second reaging process, both G.P. zones and the "0' precipi- tates can nucleate and grow. However, the nucleation and

VOLUME 14A, SEPTEMBER 1983--1849

growth of-t/' should be dominant over the nucleation and growth of G.P. zones following the suggestions of Ryum 2~ and Inoue et al . 15 as described earlier in the discussion. This is consistent with our observed increase in volume fraction of precipitates over both the original T6 condition and the corresponding retrogressed condition (Table I). Kovacs et al . 21 also found that the 77' contribution to strengthening through the Orowan mechanism was larger than that of the coherent G.P. zones which are sheared by dislocations. The high density and homogeneous distribution of the r / par- ticles observed in Figures 8 and 9 and the high yield strength of the alloy are in good agreement with the suggestion of Kovacs et al .

In the cases of materials retrogressed for long time (60 minutes), the increased strength achieved after the re- aging treatment may be interpreted by the fact that solute concentration in the matrix still remained sufficiently high for the nucleation and growth of "O' on reaging.

Finally, the reader should be reminded that RRA is de- signed to provide both high strength (equivalent to T6) and high stress corrosion resistance (equivalent to T73) in a single temper. This paper has considered only the micro- structural changes affecting the strength of the alloy, and the reader is referred to References 2 and 3 for the correspond- ing information on stress corrosion response.

V. CONCLUSIONS

TEM has been used to investigate microstructural changes occurring during retrogression and during RRA treatments, and the results were examined in relation to mechanical properties of 7075-T6 alloy. There were many processes overlapping each other in the various stages of retrogression. Three main reactions, however, can be listed as follows: (1) partial dissolution of G.P. zones, (2) formation and growth of 7' particles, and (3) coarsening of precipitates. The initial stage I in Figure 10, which represents the schematic variation in yield strength observed during retrogression treatment, should be classified under ( l ) in the above scheme. The formation and growth of 77' (2) corresponds to stage II. The decrease in strength shown by stage III reflects an excessive coarsening of precipitates, as indicated in (3) above.

REFERENCES 1. B.M. Cina: U.S. Patent 3856584 (December 24, 1974). 2. W. Wallace, J. C. Beddoes, and M. C. de Malherbe: Canadian Aero-

retrogressed ~ reaged

T6

I I , p rr -I~ ,nr

I I I I

R E T R O G R E S S I O N T I M E

Fig. 10-- Schematic representation of the changes in yield strength during retrogression and RRA treatments.

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198t, vol. 12A, pp. 1429-34. 16. T. Ungar, J. Lendvai, and I. Kovacs: Aluminium, 1979, vol. 55,

pp. 663-68. 17. G.W. Lorimer and R.B. Nicholson: Acta Metall., 1966, vol. 14,

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vol. 65, pp. 291-96. 19. J. Gjonnes and Chr. J. Simensen: Acta Metall., 1970, vol. 18,

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Metall., 1980, vol. 28, pp. 1621-31.

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