Quantitative evaluation of precipitates in an Al–Zn–Mg–Cu alloy after isothermal aging

Materials Characterization 56 (2006) 121 – 128

Quantitative evaluation of precipitates in an Al–Zn–Mg–Cu alloy after isothermal aging Z.W. Du a,⁎, Z.M. Sun a , B.L. Shao a , T.T. Zhou b , C.Q. Chen b b

a General Research Institute for Nonferrous Metals, Beijing 100088, China School of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics, Beijing 100083, China

Received 7 September 2005; accepted 2 October 2005

Abstract The evolution of microstructure parameters (precipitate size and volume fraction) for an Al–8.0 Zn–2.05 Mg–1.76 Cu alloy during isothermal ageing has been studied by synchrotron-radiation small angle X-ray scattering (SAXS) combining transmission electron microscopy (TEM). The results show that the precipitates are only a few nanometers even at higher temperature 160 °C up to 72 h (5.82 nm). The precipitate volume fraction reaches a plateau except ageing at 120 °C and the maximum is about 0.052– 0.054 in the range 140–160 °C. Models describing the evolution of these two parameters with ageing temperature and time have been constructed for our further predicting the precipitate hardening. The coarsening of precipitate is consistent with LSW (Lifshitz–Slyozov–Wagner) model even in the initial stage where volume fraction is still varying. The activation energy of coarsening regime has been determined to be about 1.25 ± 0.02 eV. © 2005 Elsevier Inc. All rights reserved. Keywords: Al–Zn–Mg–Cu Alloys; Precipitate size; Precipitate volume fraction; Small angle X-ray scattering

1. Introduction 7XXX serials (Al–Zn–Mg–Cu) aluminum alloys are used extensively in aerospace industry for their ultrahigh strength. Their properties can be obtained through control of the precipitation hardening process. Therefore, much research has been concentrated on the precipitation and the related hardening of the alloys both from the experimental and theoretical viewpoint [1–4]. The usual precipitation sequence for the 7XXX serials alloys is generally summarized as follows ⁎ Corresponding author. Tel.: +86 10 82317703. E-mail addresses: [email protected], [email protected] (Z.W. Du). 1044-5803/$ - see front matter © 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2005.10.004

[1a,5,6]: Solid solution → G.P. zones → metastable η′ → stable η (MgZn2). The crystallography and composition of precipitate have also been widely investigated [5–11]. Based on the acquired microstructure information, it is very important to obtain the microstructure parameters, the size and volume fraction of precipitate. This quantitative information is beneficial to predicting the material properties [12]. Seidman and Marquis [13] and Kim and Niinomi [14] have obtained the information by analyzing the TEM micrograph. However, for the limitation of TEM field, the results are not very reliable. In recent years, small angle X-ray scattering (SAXS) has played an important role in studying the precipitates of Al–Li, Al–Li–Cu–Mg, Al–Zn–Mg(–Cu) alloys [1a,2,3,15–20].

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In this paper, the nature of precipitate has been investigated by transmission electron microscopy (TEM). The precipitate mean size and volume fraction have been quantitatively measured by small angle X-ray scattering (SAXS) during ageing in an Al–8.0 Zn–2.05 Mg–1.76 Cu (similar to 7055) alloy. Models describing the evolution of these two parameters with ageing temperature and time have been constructed for our further predicting the precipitate hardening. 2. Materials and methods The composition of the alloy in weight percent is: Zn 8.0, Mg 2.05, Cu 1.76, Mn 0.35, Zr 0.10, Cr 0.24. The samples were solution treated at 490 °C for 1 h in salt bath followed by water quenching. Artificial aging was performed at 120, 140 and 160 °C, respectively, for various durations and at 120 °C for 24 h followed by various durations at 160 °C. The nature and spatial distribution of precipitates were investigated by TEM after artificial aging at 120 °C for 24 h and 140 °C for 48 h. The thin foils were prepared by double-jet and observed on a HITACHI H-800 microscope. Small angle X-ray scattering (SAXS) measurements were performed at Beijing Synchrotron Radiation Facility (BSRF) Division, Institute of High Energy Physics. X-rays were generated by a bending magnet, focused and monochromated to a wavelength of 0.154 nm. Modulus of scattering vector h is in the range from 0.1 to 4 nm− 1. The sample thickness for SAXS is 70 μm. 3. Experimental results 3.1. TEM results Fig. 1 shows the selected area electron diffraction (SAED) patterns along [001] and [1¯14] zone axes of the aluminum matrix after aging at 120 °C for 24 h. The precipitate spots can be observed at 1/3 {220} and 2/3{220} positions along both [001] and [1¯14] zone axes. It can be concluded that the precipitates spots are mainly from η′ and a small amount of η by comparing to the diffraction patterns of precipitates in other Al–Zn–Mg(–Cu) alloys [3,5–7,9,10]. Fig. 2 shows the SAED patterns along [001] and [1¯11] zone axes of the aluminum matrix after ageing at 140 °C for 48 h. In Fig. 2(a), the precipitate spots appear at 1/3{220} and 2/3{220} positions. In Fig. 2(b), besides the spots at 1/3{220} and 2/3{220} positions,

Fig. 1. SAED patterns after 24 h at 120 °C. (a) [001]Al zone axis (b) ¯14]Al zone axis. Precipitate spots are mostly from η′ and small [1 amount of η.

sets of spots are seen around 1/3{422}. It can be concluded that the precipitate spots are mostly η′ and η in the orientation of 1, 2 and 4. The bright field images (Fig. 3) show that the precipitates are almost uniform in size and distribution especially for the sample aged at 120 °C for 24 h. The precipitate sizes consist with the results calculated from the data of SAXS. 3.2. SAXS results The precipitate sizes were calculated using the Guinier approximation, which gives the gyration radius for the precipitates [21]. IðhÞ ¼ Ie Nn2 e−RG h 2

2

=3

ð1Þ

where I(h) is the scattered intensity corrected for background, fluorescence, absorption effects and collimation error, Ie the scattered intensity of an electron, n the total electron number of one particle, N the total particle number in the field irradiated by X-ray, RG the gyration radius of particles, h = 4πsin θ / λ the magnitude of scattering vector, λ the wavelength of X-ray and 2θ the scattering angle. By plotting Guinier curves Ln I (h) vs. h2 (Fig. 4), gyration radius can be easily determined. Deschamps et al. [1a] has indicated that the Guinier plot shows a

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Fig. 4. Guinier curves for the alloy aged at 160 °C for 8 and 72 h.

Fig. 2. SAED patterns after 48 h at 140 °C. (a) [001]Al zone axis (b) ¯11]Al zone axis. Precipitate spots are mostly from η′ and η in variant [1 orientations.

straight line in a large range of scattering vectors (typically in the range 0.8 < h * R < 2, where R is the precipitate effective size) for the mono-disperse system. For

Fig. 3. Bright field image (a) after 24 h at 120 °C (b) after 48 h at 140 °C.

the present alloy, according to our former results [19], the precipitates belong to flat ellipsoid of revolution with half axes approximate to 1 : 1 : 0.4. The mean length of the unequal axes can be approximated with the radius of a sphere with equal gyration radius to the ellipsoid. Then, the effective size of the precipitates can be written as: rffiffiffi 5 R¼ RG 3

ð2Þ

In the following, we consider the equivalent radius as precipitate radius. It is shown in Fig. 5 the evolution of the precipitate radius after ageing at 120, 140, 160 °C for various durations and at 120 °C for 24 h followed by various durations at 160 °C. It clearly appears that the precipitate radius increases with the ageing time at a given temperature. The growth rates of precipitate increase with ageing temperature. After ageing at 120 and 140 °C

Fig. 5. Evolution of the precipitate radius with isothermal ageing time at various temperatures.

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for a binary alloy, integrated intensity can be written as: Z

l

Q¼ 0

Ia ðhÞh2 dh ¼

2p2 ðDZÞ2 ðCp −Cm Þ2 fv ð1−fv Þ Vat2 ð3Þ

Fig. 6. Integrated intensity for the samples aged at 140 °C for various durations.

for 72 h, the precipitate radii are fine about 2.06 and 3.36 nm, nevertheless, they are up to 5.82 nm for the same durations aged at 160 °C with and without pre-ageing. The volume fraction of precipitates can be derived from the integrated intensity measurements. According to Guyot and Cottignies [2], in a two-phase model and

where Ia(h) is the absolute scattered intensity, Vat the atomic volume (approximate to 16.5 Å3), Cp, Cm the concentrations of the solute in the precipitates and in the matrix, fv the volume fraction of precipitates and ΔZ the difference of the atomic number between the solute and the matrix. To obtain fv, parameters Cp, Cm and ΔZ must be determined. To determine Cp, we assume that the composition of the precipitates is equal to that of the η phase proposed by Dechamps and Bigot [20] with atom-probe field ion microscopy (APFIM). The matrix composition Cm is approximated by the matrix composition with η precipitate phase. The difference of atomic number ΔZ is approximated by Zn/Cu and Al/Mg. Fig. 6 shows the evolution of the integrated intensity for the samples aged at 140 °C. The integrated intensity

Fig. 7. (a)–(d) Evolution of the precipitate volume fraction with isothermal ageing time at various temperatures.

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increases initially, later decreases, and then increases again up to a plateau, finally decreases slightly for long ageing time. Similar behavior can be observed in the evolution of precipitate volume fraction (Fig. 7(b)– (d)). The initial increase is attributed to the nucleation and growth of large number of G.P. zones and metastable phase η′. The decrease after initial increase maybe results in the partial reversion of G.P. zones and η′. According to Deschamps et al. [1a], the decrease for long aging time is caused by the loss of the scattered intensity corresponding to the largest precipitates in the beam-stop. However, the evolution of volume fraction after ageing at 120 °C (Fig. 7(a)) seems different from other conditions. The initial decrease appears later than other conditions, furthermore, no plateau can be observed for long ageing time up to 96 h, which suggests that the solute atoms are still beyond saturation and volume fraction will increase continuously with isothermal time. Therefore, the evolution process is in fact similar to other conditions and only the low diffusion speed of solutes at the lower temperature retard the precipitation. In addition, it can be observed that the maximum of precipitate volume fraction for the present alloy in the range 140–160 °C is about 0.052–0.054. Such large amount of precipitate will play an important role in the precipitate hardening. If we neglect the deviation caused by SAXS at long time aging, it can be seen from Fig. 7 that the evolution of volume fraction is consistent with the classical twostep precipitation process, i.e. nucleation and growth of η′ followed by Ostwald ripening stage. At the first stage, both integrated intensity and volume fraction increase steadily and at the second stage they keep constant. 4. Results analysis and discussion To describe the evolution of precipitate size and volume fraction by model, some assumptions are made. The alloy is considered to be pseudo-binary. The dynamic variation of the precipitate composition is neglected. No phase transformation from the η′ to η has been taken into account. The partial dissolution of metastable phase during isothermal ageing is not considered. 4.1. Evolution of precipitate volume fraction According to Shercliff and Ashby [22], the precipitate volume fraction formed during ageing is directly proportional to solute loss. Therefore, the volume frac-

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tion of precipitate at given ageing condition can be derived from the mean solute concentration c (t) in the matrix. cðtÞ ¼ c0 þ ðcs −c0 Þexpð−t=s1 Þ

ð4Þ

where cs is the initial solute concentration, c0 the equilibrium concentration at an ageing temperature and τ1 a temperature-dependent time constant. According to Johnson–Mehl–Avrami equation, it can be rewritten by cðtÞ ¼ c0 þ ðcs −c0 Þexpð−K1 t n Þ

ð5Þ

where n is Avrami factor and K1 a temperature-dependent constant. The volume fraction, f (t), of precipitate is directly proportional to solute loss cs–c (t), tending to a final equilibrium value, f0, Thus: f ðtÞ cs −cðtÞ ¼ ¼ 1−expð−K1 t n Þ f0 cs −c0

ð6Þ

where f0 depends on how far the ageing temperature T lies below solid solvus temperature, Ts, and is expressed as [22]   cs −c0 f0 ¼ fmax ð7Þ cs For binary or pseudo-binary alloys, the equilibrium concentration of solute at a fixed temperature T < Ts is given by [22,23]:   Qs 1 1 − c0 ¼ cs exp− ð8Þ R T Ts where cs is the solvus boundary (the initial concentration of the solid solution), Qs the free energy of solution, Ts the solvus temperature and R the gas constant. Then f0 is described by    Qs 1 1 − f0 ¼ fmax 1−exp− ð9Þ R T Ts where fmax is the maximum value of f0, i.e. when all alloying elements precipitate. Due to the parameters for the present alloy in Eq. (9) is unknown. According to Starink and Wang [23], we take reference alloy Al–6.1 Zn–2.35 Mg–0.1 Zr to determine f0.    cs −c0 cs Qs 1 1 − f0 ¼ fmax;R ¼ fmax;R 1−exp− cs;R cs;R R T Ts;R ð10Þ

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Deschamps has studied this alloy combining SAXS and APFIM techniques and found that the volume fraction for this alloy aged at 160 °C for 50 h is about 0.044 (approximated as f0 at 160 °C) [20]. According to Refs. [23,24], the solution enthalpy of η′ is 26.6 kJ/mol and Qs can be gV derived from it ðQs ¼ 13DHsol ¼ 8:9kJ =mol Þ. Ts is about 285 °C [23]. Therefore, fmax,R can be derived from Eq. (9). cs and cs,R (initial concentration for the present and reference alloy) are approximated as a linear combination of the concentrations of the main alloying elements (in at.%). Combining Eqs. (6) and (10), the variation of volume fraction with Fig. 9. Diagram of R3 vs. t for the samples aged at various temperatures.

ageing time at a given temperature can be expressed by    cs Qs 1 1 f ðt; T Þ ¼ fmax;R − 1−exp− cs;R R T Ts;R d½1−expð−K1 t n Þ


ð11Þ

Fig. 8 shows the evolution of precipitate volume fraction by modeling, which shows good agreement with the results by SAXS measurements. The equilibrium volume fraction calculated by Eq. (10) for 120, 140 and 160 °C is, respectively, 0.0638, 0.054, 0.049. 4.2. Particle coarsening To model the evolution of the average size of the precipitates, we use LSW (Lifshitz–Slyozov–Wagner) model [2]. The LSW model is given by Eq. (12), which

Fig. 8. (a)–(c) Modeled and experimental results of precipitate volume fraction after one-step ageing.

Fig. 10. Diagram of Ln (KT) vs. 1 / T.

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It was called “kinetic strength” by Shercliff and Ashby [22]. The kinetic strength characterizes the advance of coarsening that result from a given aging treatment. By using the kinetic strength Ks, we can describe the coarsening process given by Eq. (12) in a modified LSW model for various time t and temperature T. R3 −R30 ¼ cKs

Fig. 11. Diagram of R3 vs. Ks for the samples aged at various conditions.

predicts the time dependence of the precipitate radius. The growth coefficient K is given by Eq. (13) R3 −R30 ¼ Kt

ð12Þ

  8Vat2 gceq D 8Vat2 gceq D0 Q K¼ ¼ exp − kB T 9kB T 9kB T

ð13Þ

where γ is the precipitate-matrix interface energy, Vat the average atomic volume, ceq the equilibrium solute concentration, D the solute diffusion coefficient expressed by D = D0e− Q/kBT, Q the activation energy and T the aging temperature. In the narrow temperature range 120∼160 °C, it is assumed that the change in the concentration and the interface energy can be neglected. Then the growth coefficient K can be rewritten as   c Q K ¼ exp − ð14Þ T kB T where c is approximate as constant. For each aging temperature, K was derived from the slope of curves R3 vs. t (Fig. 9). Then the activation energy Q for the Ostwald ripening stage was obtained using Arrhenius plot ln (KT) vs. 1 / T (Fig. 10). The activation energy estimated from Eq. (14) is about 1.25 ± 0.02 eV, this value is intermediate between those of Cu (1.4 eV), Mg (1.17–1.34 eV), Zn (1.25–1.34 eV) for diffusion in aluminum, as well as the activation energy for selfdiffusion in Al (1.47 eV) [2]. To gather all experimental results, for various time t and temperature T, on a master plot, we use here an integrated variable as given by Z dt −Q=kB T ðtÞ Ks ¼ e ð15Þ T ðtÞ

ð16Þ

As shown in Fig. 11, the increase of R3 with variable Ks follows a linear behavior. So the coarsening of precipitates conforms to the principle of LSW (R0 = 1.44 nm and c = 1.074E14 nm3 K/s) even in initial stage where volume fraction is still varying. 5. Conclusions The nature of precipitate has been investigated by TEM after ageing at 120 °C for 24 h and 140 °C for 48 h. The precipitates are mostly metastable η′ and equilibrium η with several orientation variants. The precipitate mean size and volume fraction have been quantified by small angle X-ray scattering (SAXS) measurements after ageing at 120, 140, 160 °C for various durations and at 120 °C for 24 h followed by various durations at 160 °C. The precipitates are only a few nanometers even at higher temperature 160 °C up to 72 h (5.82 nm). The precipitate volume fraction reaches the plateau except ageing at 120 °C, the maximum is about 0.052–0.054 in the range 140–160 °C. Models describing the evolution of these two parameters with ageing temperature and time have been constructed for our further predicting the precipitate hardening. The coarsening behavior of the precipitate has been quantified with LSW model. The activation energy of coarsening regime has been determined to be about 1.25 ± 0.02 eV. The model predicting the evolution of precipitate volume fraction is based on the relation between the solute loss in the matrix and the increase of precipitate amount. Acknowledgements This research was supported by National 863 High Technology project of China (No. 2001AA332030) and National Key Fundamental Research Project of China (No. G19990649). References [1] (a) Deschamps A, Livet F, Bréchet Y. Influence of predeformation on ageing in an Al–Zn–Mg alloy: I. Microstructure evolution and mechanical properties. Acta Mater 1999;47:281.

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