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Acta Metall Sin  2017, Vol. 53 Issue (8): 897-906    DOI: 10.11900/0412.1961.2016.00559
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Quantitative Characterization on the Precipitation of AA 7055 Aluminum Alloy by SAXS
Junzhou CHEN1,2(), Liangxing LV3, Liang ZHEN3, Shenglong DAI1,2
1 Institute of Aluminum Alloy, Beijing Institute of Aeronautical Materials, Beijing 100095, China
2 Beijing Engineering Research Center of Advanced Aluminum Alloys and Applications, Beijing 100095, China
3 School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China
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Abstract  

AA 7055 aluminum alloy is a newly advanced Al-Zn-Mg-Cu alloy. It has been wide applied in aviation and aerospace field due to its attractive combined properties, such as high strength, high fracture toughness, good resistance to the growth of fatigue cracks and good stress corrosion resistance, and so on. It is generally believed that the optimum ageing precipitates are responsible for these good properties. However, the detailed information, such as size and its distribution, volume fraction, and morphology of precipitate in this alloy is still not clear. Although TEM is used to determine these information, the results are mostly qualitative. Small angle X-ray scattering (SAXS) provides a direct technique to determine the size, morphology and volume fraction of nano-scale particles and the sampling size is much larger than that in TEM. In this work, the evolution of the precipitates during ageing at 120 and 160 ℃ in AA 7055 aluminum alloy were investigated systematically and quantitatively by SAXS technique. The results show that, when ageing at 120 ℃, the average radius of the precipitates increases with increasing the ageing time. After ageing for 5 h and later, the average radius of the precipitates is 3.3 nm, and its distribution almost keeps stably. The volume fraction of the precipitates is also increased with increasing the ageing time. When ageing from 5 h to 60 h, the volume fraction increases from 2.4% to 5.2%. When ageing at 160 ℃, however, the average radius of the precipitates increases from 3.1 nm to 11.7 nm with increasing the ageing time from 0.5 h to 72 h. The volume fraction of the precipitates increases from 1.4% to 5.4% with increasing the ageing time from 0.5 h to 16 h. After ageing for 16 h and later, the volume fraction of the precipitates keeps stably. Both ageing at 120 and 160 ℃, the morphology of the precipitates is similar to a flat ellipsoid with an axis ratio between 0.2 and 0.3. Based on these quantitative results of the precipitates, the strength models during ageing will be built possibility.

Key words:  AA 7055 aluminum alloy      ageing precipitation      SAXS      quantitative characterization     
Received:  13 December 2016     
ZTFLH:  TG146.2  

Cite this article: 

Junzhou CHEN, Liangxing LV, Liang ZHEN, Shenglong DAI. Quantitative Characterization on the Precipitation of AA 7055 Aluminum Alloy by SAXS. Acta Metall Sin, 2017, 53(8): 897-906.

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2016.00559     OR     https://www.ams.org.cn/EN/Y2017/V53/I8/897

Fig.1  SAXS curves of the AA 7055 Al alloy aged at 120 ℃ (a) and 160 ℃ (b) for different ageing times (I(h)—scattering intensity, h—scattering vector)
Fig.2  Guinier curves at low angle regions for the AA 7055 Al alloy aged at 120 ℃ for 5 h (a) and 24 h (b) (lnI(h)—natural logarithm of scattering intensity , h2—square of scattering vector)
Fig.3  Guinier curves at low angle regions for the AA 7055 Al alloy aged at 160 ℃ for 0.5 h (a) and 48 h (b)
Fig.4  Gyration radius of precipitate RG in the AA 7055 Al alloy aged at 120 and 160 ℃ for different times
Fig.5  Theoretical scattered curves with different axis ratios ω (R—half-length for unequal axis)
Fig.6  Comparisons of experimental and theoretical scattered curves of precipitations with different ω for the AA 7055 Al alloy aged at 120 ℃ for 5 h (a) and 48 h (b)
Fig.7  Comparisons of experimental and theoretical scattered curves of precipitations with different ω for the AA 7055 Al alloy aged at 160 ℃ for 0.5 h (a) and 48 h (b)
Fig.8  Evolutions of precipitate radius for the AA 7055Al alloy aged at 120 and 160 ℃ for different times (R—precipitate radius)
Fig.9  Porod curves (a, c) and divided scattered curves (b, d) for the AA 7055 Al alloy aged at 120 ℃ for 5 h (a, b) and 24 h (c, d) (h1—scattering vector from low angle, h2—scattering vector from high angle, Kp—Porod constant)
t / h Kp / 10-5 Qh / 10-5 RP μ σ2
5 2.05 3.32 1.55 0.89 2.57
9 3.29 5.03 1.46 0.84 2.56
12 3.32 5.52 1.59 0.91 2.57
16 3.94 6.29 1.52 0.88 2.56
24 3.93 6.52 1.58 0.91 2.58
40 4.23 6.89 1.56 0.95 2.43
48 4.00 6.65 1.59 0.94 2.50
60 4.43 7.21 1.55 0.89 2.56
Table 1  Parameters associated with precipitate size distribution for the AA 7055 Al alloy aged at 120 ℃ for different times
Fig.10  Logarithm Gaussian distributions (p(R)) of precipitate radius for the AA 7055 Al alloy aged at 120 ℃ for different times
Fig.11  Porod curves (a, c) and divided scattered curves (b, d) for the AA 7055 Al alloy aged at 160 ℃ for 0.5 h (a, b) and 48 h (c, d)
Fig.12  Logarithm Gaussian distributions (p(R)) of precipitate radius for the AA 7055 Al alloy aged at 160 ℃ for different times
t / h Kp / 10-5 Qh / 10-5 RP μ σ2
0.5 1.54 1.94 1.20 0.61 2.83
5 2.05 4.00 1.75 0.97 2.64
7 2.18 5.21 2.28 1.35 2.50
12 2.20 6.12 2.66 1.54 2.56
16 2.56 7.40 2.76 1.56 2.60
24 2.29 7.50 3.13 1.73 2.65
48 1.53 6.88 4.28 2.38 2.64
60 1.48 7.52 4.86 2.73 2.62
Table 2  Parameters associated with precipitate size distribution for the AA 7055 Al alloy aged at 160 ℃
Fig.13  Evolution of the precipitates volume fraction for the AA 7055 Al alloy during the ageing
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