- Email: [email protected]
Precipitation kinetics in Al–Zn–Mg commercial alloys
Journal of Materials Processing Technology 141 (2003) 35–40
Precipitation kinetics in Al–Zn–Mg commercial alloys R. Ferraguta, A. Somozaa,*, A. Tolleyb, I. Torrianic a
IFIMAT, Universidad Nacional del Centro de la Provincia de Buenos Aires and CICPBA, Pinto 399, B7000GHG, Tandil, Argentina b Centro Ato´mico Bariloche, Comisio´n Nacional de Energı´a Ato´mica and CONICET, Avda. Bustillo Km. 9.6, R8400AGQ, San Carlos de Bariloche, Argentina c Laborato´rio Nacional de Luz Sı´ncrotron, Universidade Estadual de Campinas, IFGW, CEP 13083-970, 6192 Campinas, SP, Brazil Received 9 August 2002; received in revised form 9 August 2002; accepted 29 October 2002
Abstract The precipitation kinetics of two commercial Al–Zn–Mg alloys (7005 and 7012) was studied by using different experimental techniques. The precipitation sequence was previously investigated by means of positron annihilation lifetime spectroscopy (PALS) and Vickers microhardness. The results were correlated with quantitative information obtained by high resolution and conventional transmission electron microscopy (HREM and CTEM) and small-angle X-ray scattering (SAXS). Special attention was kept on the study of the microstructural evolution during the early stages of artificial aging. The results are discussed in terms of the formation, growth and partial dissolution of solute clusters or Guinier–Preston zones, and nucleation and growth of Z0 precipitates. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Age-hardenable alloys; Al-based alloys; Precipitation
1. Introduction The phenomenon of age-hardening in aluminum alloys was discovered by Alfred Wilm in the first decade of the century and led to the development of the well-known duralumin. The aluminum alloys have been highly used in aerospace industry for more than 80 years [1], and recent advances in aluminum alloy development maintain these alloys as the materials of choice for future commercial aircraft structures. Particularly, the 2xxx and 7xxx series high strength alloys have been developed primarily for aircraft construction. The precipitation sequence usually proposed for alloys with similar Zn and Mg contents to those of the 7012 and 7005 alloys is [2]: a-supersaturated solid solution ða-SSSÞ ! spherical Guinier–Preston (GP) zones ! Z0 precipitates ! Z precipitates. a-SSS is retained after quenching to room temperature (RT). The first stage of decomposition occurs by aging at RT for a few days (natural aging), which results in the formation of GP zones (solutevacancy enriched structures coherent with the matrix). Thermal treatments at temperatures above RT (artificial
aging, between 100 and 200 8C) may lead to the complete decomposition. However, the decomposition stages are more or less developed and overlapped, depending on the details of the thermal treatments. The composition of the alloy is an important factor. In particular, the Zn:Mg ratio has a pronounced effect on the mechanical properties that is ascribed to its influence on the precipitation kinetics. Minority elements and trace additions also influence the precipitation kinetics, but the underlying mechanism is under discussion. The aim of this paper is to show how complementary techniques can be used in order to understand the precipitation in two commercial alloys subjected to a two-step aging treatment, specially in the early stages of artificial aging. The use of a combination of positron annihilation lifetime spectroscopy (PALS), high resolution electron microscopy (HREM), conventional transmission electron microscopy (CTEM) and small-angle X-ray scattering (SAXS) enabled us to follow the microstructure and relate it to the mechanical properties studied by microhardness.
2. Experimental *
Corresponding author. Tel.: þ54-2293-442821; fax: þ54-2293-444190. E-mail address: [email protected] (A. Somoza).
The compositions of the two studied alloys are (in wt.%): (i) alloy 7005: Al–4.6Zn–1.4Mg–0.5Mn, also containing 0.1
0924-0136/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 2 ) 0 1 0 4 4 - 0
36
R. Ferragut et al. / Journal of Materials Processing Technology 141 (2003) 35–40
Cr, 0.1 Zr, 0.03 Ti, Fe < 0:4, Si < 0:35 and Cu < 0:1; (ii) alloy 7012: Al–6.0Zn–2.0Mg–1.0Cu, also containing 0.12 Zr, 0.10 Mn, 0.06 Ti, Fe < 0:25, Si < 0:15 and Cr < 0:04. The samples were thermally treated by: (i) solution treatment for 2 h at 475 8C in an electric air circulating furnace; (ii) quenching into water at 20 8C; (iii) pre-aging at RT; (iv) isothermal aging at 150 8C for various times in a glycerin bath and (v) quenching into alcohol at 20 8C. The lifetime spectrometer was a fast–fast timing coincidence system with a time resolution (FWHM) of 255 ps. The total number of counts in each spectrum was about 106, accumulated in approximately 14 000 s. A 20 mCi source of 22 NaCl deposited on a thin Kapton foil was sandwiched between two identical alloy specimens so that the source contributes to the spectra with only one component (tS ¼ 382 ps, IS ¼ 10:6%). Further experimental details are given elsewhere [3]. Specimens for TEM with 3 mm diameter were cut from the heat-treated 10 mm discs and the thickness reduced to 0.2 mm. They were thinned by double-jet electropolishing with a 30% HNO3 solution in methanol at 35 8C and 10 V. The specimens were observed with a Philips CM200UT transmission electron microscope, operating at 200 kV. Bright field CTEM and HREM images were obtained along the h0 1 1i and h0 0 1i zone axes. Due to the relatively limited tilt allowed by the UT objective lens, grains with the appropriate orientation had to be carefully localized. SAXS measurements were performed at RT on the SAS beam-line at the National Synchrotron Light Laborator, LNLS, Brazil. Dimensions of the samples were 5 mm 8 mm 0:15 mm. The selected X-ray wavelength ˚ . The information was analyzed in the range of l was 1.608 A the modulus of the scattering vector q (q ¼ 4p sinðe=2Þ=l, e ˚ 1. being the scattering angle) from 1:7 102 to 0.43 A The distribution of the scattering intensity I(q) was typically obtained in approximately 60 min, except for the asquenched specimen for which it was obtained in 4 min. Spectra were corrected taking into account background, absorption effects and incoherent diffusion in the sample. Particle dimensions were calculated in the low q region by using the Guinier approximation and Porod limit in the zone of large-angle scattering.
3. Results and discussion Fig. 1 shows the variation of the SAXS intensities in the 7012 alloy as a function of the scattering vector obtained immediately after solution treatment and quenching, and after 5 days of pre-aging. The increase in the distribution of scattering intensity in the range of q between 0.06 and ˚ 1 is attributed to the formation of a uniform and 0.4 A fine dispersion of solute clusters or GP zones (a similar behavior is observed in the 7005 alloy). The main operating microstructural mechanism for the formation of pre-precipitates was identified as the diffusion of Zn2-vacancy pairs in
Fig. 1. Small-angle scattering spectra in an as-quenched sample after solution treatment and in another pre-aged sample for 5 days at RT. The spectrum of the as-quenched specimen was smoothed due to its low statistic.
the 7005 alloy [3] and the migration of Mg-vacancy pairs in the 7012 alloy [3,4]. In Fig. 2(a) and (b) two BF images of the 7012 and 7005 alloys, obtained along h0 1 1i and h0 0 1i zone axes corresponding to samples pre-aged for 5 days at RT, respectively, are shown. Small dark spherical particles ˚ in the 7012 alloy and 20 A ˚ in with diameters of about 10 A the 7005 alloy were identified as solute clusters or GP zones. In Fig. 2(c), a HREM image of a sample of the 7012 alloy pre-aged 5.5 months at RT shows spherical regions with ˚ , which are assumed to correspond diameters less than 20 A to the particles observed in Fig. 2(a). No distortion of the matrix lattice planes is observed across these zones, indicating a high degree of coherence. By means of CTEM [5] the density of these particles in the 7012 alloy was estimated to be 1 1018 to 2 1018 cl/cm3 with an average distance ˚ . On the other hand, between pre-precipitates of 100 A from the data of Fig. 1 a nearest neighbor distance of about ˚ for the pre-aged sample was obtained using the 40 A approximation of Synecˇ ek [6]. The region beyond the maximum is out of range to obtain a trustworthy Guinier radius ðRG q 1:3Þ. During pre-aging at RT an important hardness increase correlated with a slight positron lifetime decrease, was observed in both alloys (reported in [3,7]). The positron lifetime t as well as the microhardness HV reach an apparent saturation value after 5 days at RT indicating that the microstructure has arrived at a metastable state. For both alloys this is the initial state corresponding to the artificial aging labeled 1 in Fig. 3(a) and (b). This figure provides a summary of the positron lifetime t during the artificial aging at 150 8C as a function of t1/3 (see [3]). For comparison the microhardness HV behavior is also shown in the inserts in the top-left corner of Fig. 3(a) and (b). In both alloys, the evolution of t and HV shows similar features. Initially, this heat treatment produces a strong decrease of t and HV (not simultaneously), followed by an increase of both parameters which eventually brings them to a new maximum above the initial value. After that, for long aging times a softening as a consequence of overaging is observed.
R. Ferragut et al. / Journal of Materials Processing Technology 141 (2003) 35–40
37
Fig. 2. Bright-field micrographs for pre-aged samples at RT after solubilization and quenching (labeled 1, Fig. 3): (a) 5 days pre-aging, 7012 alloy, h0 1 1i zone axis; (b) 5 days pre-aging, 7005 alloy, h0 0 1i zone axis. (c) HREM micrograph corresponding to 5.5 months pre-aging, 7012 alloy, h0 1 1i zone axis.
Fig. 4 shows the Guinier plots (ln(I(q)) against q2) for specimens aged 0 min (Fig. 4(a) and (a0 )) and 8 min (Fig. 4(b) and (b0 )) at 150 8C in the 7012 and 7005 alloys. For low q values, a strong first component can be distin˚ in the 7012 alloy and 110 A ˚ in the guished (RG1 90 A
7005 alloy). For deformed samples of an Al–Zn–Mg alloy it was suggested that RG1 represents the typical radius of precipitates formed on dislocations [8] (this component, with a lower intensity, was also observed for non-deformed samples in [8]). In the present study, CTEM results have not
38
R. Ferragut et al. / Journal of Materials Processing Technology 141 (2003) 35–40
Fig. 3. Evolution of the positron lifetime t during isothermal aging at 150 8C, after 5 days of natural aging following solubilization and quenching: (a) 7012 alloy, (b) 7005 alloy. The open symbols labeled 1 and 2 represent those microstructure stages studied by SAXS, TEM and HREM. For the same heat treatment, Vickers microhardness, HV, data are presented in the insert for both alloys.
revealed the presence of particles with similar sizes to RG1, neither in the matrix nor on dislocations. In the 7005 alloy, ˚ , one order of magniparticles with a mean size of 800 A tude higher than RG1, were observed, presumably Mn dispersoids [9]. A second component (RG2) is observed in the alloy 7012. In the alloy 7005 it is not so clear, since there is some curvature in the region following the first component, that could indicate a wide size distribution of particles. For a pre-aged specimen of 7012 alloy (labeled 1 in Fig. 3(a)) the ˚ which corresponds well with Guinier radius RG2 is 25 A the size of the higher pre-precipitates observed by CTEM and HREM. The values of Guinier radius practically do not vary with 8 min of aging. The initial t and HV decrease in the first minutes of aging, observed in Fig. 3(a) and (b) in the studied alloys, is attributed to the partial dissolution of the smallest pre-precipitates [3,5]. PALS results show evidence of reduced trapping in GP zones. It was noted that this phenomenon is not permanent and can be detected only if PALS measurements
Fig. 4. Guinier plots corresponding to samples of the 7012 ((a) and (b)) and 7005 ((a0 ) and (b0 )) alloys aged at 150 8C: 0 min ((a) and (a0 )); 8 min ((b) and (b0 )). The doted lines are visual guides.
are performed shortly after the interruption of the corresponding heat treatment (then t progressively recovers the initial value [3]). Fig. 5 shows a decrease of the integrated intensity Q0 during the early stages of artificial aging in the 7012 alloys (in a similar way in the 7005 alloy). This behavior of Q0 can be correlated with the mentioned fall of the positron lifetime and the softening shown in Fig. 3. Gueffroy and Lo¨ ffler reported a similar result in an Al–Zn– Mg alloy [10], and they attributed it to the dissolution of unstable GP zones. However, during the early stages of aging the decomposition phenomena are overlapping; on the one hand, the smallest pre-precipitates dissolve and, on the other, the metastable Z0 phase is nucleated. Fig. 6 shows a HREM image of the 7012 alloy aged 8 min at 150 8C (labeled 2 in Fig. 3(a)). The edge-on particles with ð1 1 1Þ ˚ long are Z0 platelets and ð1 1 1Þ habit planes of about 70 A [5]. In addition, spherical solute clusters or GP zones can be distinguished. Fig. 3(a) and (b) show that after the mentioned decrease, t increases its value with the aging time up to 220 ps (as occur with Q0 after 4 min of aging), suggesting a progressive formation and growth of the semicoherent Z0 precipitates that increase the hardness of the alloys. In this interpretation, the misfit regions of the matrix–precipitates interfaces, having a higher characteristic positron lifetime, compete with solute clusters or GP zones as positron trapping centers.
Fig. 5. Time evolution of the integral intensity Q0 during the first minutes of aging at 150 8C in the 7012 alloy. The dashed line is only a visual guide.
R. Ferragut et al. / Journal of Materials Processing Technology 141 (2003) 35–40
39
Fig. 6. HREM micrograph of an artificially aged sample for 8 min at 150 8C (labeled 2, Fig. 3 (a)), 7012 alloy, h0 1 1i zone axis.
The different quaternary elements (Mn in alloy 7005 and Cu in alloy 7012) lead to different transformation rates during the precipitation sequence as can be seen in Fig. 3 (see [3]). On the other hand, the size of the precipitates during aging shows a good correlation with the Porod radius RP for the 7012 alloy (results not included here). RP represents the ratio volume/area of the precipitated regions. From the analysis of the first stage of aging the cube of the average radius of the plate-shape Z0 precipitates [5] as well as R3P grow linearly with the aging time according to the LSW (Lifshitz–Slyozov–Wagner) theory.
Acknowledgements The authors wish to acknowledge the support of the National Synchrotron Light Laboratory—LNLS, Brazil
(Project: SAS 407/98). This work was partially financed by Consejo Nacional de Investigaciones Cientı´ficas y Te´ cnicas, Agencia Nacional de Promocio´ n Cientı´fica y Tecnolo´ gica, Comisio´ n de Investigaciones Cientı´ficas de la Provincia de Buenos Aires and Secretarı´a de Ciencia y Te´ cnica of the Universidad Nacional del Centro de la Provincia de Buenos Aires, Argentina.
References [1] I.J. Polmear, Light Alloys: Metallurgy of Light Metals, 3rd ed., Edward Arnold, London, 1995, p. 140. [2] H. Lo¨ ffler, Y. Kovacs, J. Lendvai, J. Mater. Sci. 18 (1983) 2215– 2240. [3] R. Ferragut, A. Somoza, A. Dupasquier, J. Phys. 10 (1998) 3903– 3918. [4] R. Ferragut, A. Somoza, Mater. Sci. Eng. A 334 (2002) 1–5.
40
R. Ferragut et al. / Journal of Materials Processing Technology 141 (2003) 35–40
[5] R. Ferragut, A. Somoza, A. Tolley, Acta Mater. 47 (1999) 4355– 4364. [6] V. Synecˇ ek, J. de Phys. 23 (1962) 828–829. [7] R. Ferragut, A. Somoza, A. Dupasquier, J. Phys. 8 (1996) 8945– 8952.
[8] A. Deschamps, Y. Bre´ chet, P. Guyot, F. Livet, Z. Metallkd. 88 (1997) 601–606. [9] B.O. Kong, S.W. Nam, Mater. Lett. 28 (1996) 385–391. [10] B. Gueffroy, H. Lo¨ ffler, Phys. Stat. Sol. (a) 66 (1981) 585–590.
Precipitation kinetics in Al–Zn–Mg commercial alloys R. Ferraguta, A. Somozaa,*, A. Tolleyb, I. Torrianic a
IFIMAT, Universidad Nacional del Centro de la Provincia de Buenos Aires and CICPBA, Pinto 399, B7000GHG, Tandil, Argentina b Centro Ato´mico Bariloche, Comisio´n Nacional de Energı´a Ato´mica and CONICET, Avda. Bustillo Km. 9.6, R8400AGQ, San Carlos de Bariloche, Argentina c Laborato´rio Nacional de Luz Sı´ncrotron, Universidade Estadual de Campinas, IFGW, CEP 13083-970, 6192 Campinas, SP, Brazil Received 9 August 2002; received in revised form 9 August 2002; accepted 29 October 2002
Abstract The precipitation kinetics of two commercial Al–Zn–Mg alloys (7005 and 7012) was studied by using different experimental techniques. The precipitation sequence was previously investigated by means of positron annihilation lifetime spectroscopy (PALS) and Vickers microhardness. The results were correlated with quantitative information obtained by high resolution and conventional transmission electron microscopy (HREM and CTEM) and small-angle X-ray scattering (SAXS). Special attention was kept on the study of the microstructural evolution during the early stages of artificial aging. The results are discussed in terms of the formation, growth and partial dissolution of solute clusters or Guinier–Preston zones, and nucleation and growth of Z0 precipitates. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Age-hardenable alloys; Al-based alloys; Precipitation
1. Introduction The phenomenon of age-hardening in aluminum alloys was discovered by Alfred Wilm in the first decade of the century and led to the development of the well-known duralumin. The aluminum alloys have been highly used in aerospace industry for more than 80 years [1], and recent advances in aluminum alloy development maintain these alloys as the materials of choice for future commercial aircraft structures. Particularly, the 2xxx and 7xxx series high strength alloys have been developed primarily for aircraft construction. The precipitation sequence usually proposed for alloys with similar Zn and Mg contents to those of the 7012 and 7005 alloys is [2]: a-supersaturated solid solution ða-SSSÞ ! spherical Guinier–Preston (GP) zones ! Z0 precipitates ! Z precipitates. a-SSS is retained after quenching to room temperature (RT). The first stage of decomposition occurs by aging at RT for a few days (natural aging), which results in the formation of GP zones (solutevacancy enriched structures coherent with the matrix). Thermal treatments at temperatures above RT (artificial
aging, between 100 and 200 8C) may lead to the complete decomposition. However, the decomposition stages are more or less developed and overlapped, depending on the details of the thermal treatments. The composition of the alloy is an important factor. In particular, the Zn:Mg ratio has a pronounced effect on the mechanical properties that is ascribed to its influence on the precipitation kinetics. Minority elements and trace additions also influence the precipitation kinetics, but the underlying mechanism is under discussion. The aim of this paper is to show how complementary techniques can be used in order to understand the precipitation in two commercial alloys subjected to a two-step aging treatment, specially in the early stages of artificial aging. The use of a combination of positron annihilation lifetime spectroscopy (PALS), high resolution electron microscopy (HREM), conventional transmission electron microscopy (CTEM) and small-angle X-ray scattering (SAXS) enabled us to follow the microstructure and relate it to the mechanical properties studied by microhardness.
2. Experimental *
Corresponding author. Tel.: þ54-2293-442821; fax: þ54-2293-444190. E-mail address: [email protected] (A. Somoza).
The compositions of the two studied alloys are (in wt.%): (i) alloy 7005: Al–4.6Zn–1.4Mg–0.5Mn, also containing 0.1
0924-0136/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 2 ) 0 1 0 4 4 - 0
36
R. Ferragut et al. / Journal of Materials Processing Technology 141 (2003) 35–40
Cr, 0.1 Zr, 0.03 Ti, Fe < 0:4, Si < 0:35 and Cu < 0:1; (ii) alloy 7012: Al–6.0Zn–2.0Mg–1.0Cu, also containing 0.12 Zr, 0.10 Mn, 0.06 Ti, Fe < 0:25, Si < 0:15 and Cr < 0:04. The samples were thermally treated by: (i) solution treatment for 2 h at 475 8C in an electric air circulating furnace; (ii) quenching into water at 20 8C; (iii) pre-aging at RT; (iv) isothermal aging at 150 8C for various times in a glycerin bath and (v) quenching into alcohol at 20 8C. The lifetime spectrometer was a fast–fast timing coincidence system with a time resolution (FWHM) of 255 ps. The total number of counts in each spectrum was about 106, accumulated in approximately 14 000 s. A 20 mCi source of 22 NaCl deposited on a thin Kapton foil was sandwiched between two identical alloy specimens so that the source contributes to the spectra with only one component (tS ¼ 382 ps, IS ¼ 10:6%). Further experimental details are given elsewhere [3]. Specimens for TEM with 3 mm diameter were cut from the heat-treated 10 mm discs and the thickness reduced to 0.2 mm. They were thinned by double-jet electropolishing with a 30% HNO3 solution in methanol at 35 8C and 10 V. The specimens were observed with a Philips CM200UT transmission electron microscope, operating at 200 kV. Bright field CTEM and HREM images were obtained along the h0 1 1i and h0 0 1i zone axes. Due to the relatively limited tilt allowed by the UT objective lens, grains with the appropriate orientation had to be carefully localized. SAXS measurements were performed at RT on the SAS beam-line at the National Synchrotron Light Laborator, LNLS, Brazil. Dimensions of the samples were 5 mm 8 mm 0:15 mm. The selected X-ray wavelength ˚ . The information was analyzed in the range of l was 1.608 A the modulus of the scattering vector q (q ¼ 4p sinðe=2Þ=l, e ˚ 1. being the scattering angle) from 1:7 102 to 0.43 A The distribution of the scattering intensity I(q) was typically obtained in approximately 60 min, except for the asquenched specimen for which it was obtained in 4 min. Spectra were corrected taking into account background, absorption effects and incoherent diffusion in the sample. Particle dimensions were calculated in the low q region by using the Guinier approximation and Porod limit in the zone of large-angle scattering.
3. Results and discussion Fig. 1 shows the variation of the SAXS intensities in the 7012 alloy as a function of the scattering vector obtained immediately after solution treatment and quenching, and after 5 days of pre-aging. The increase in the distribution of scattering intensity in the range of q between 0.06 and ˚ 1 is attributed to the formation of a uniform and 0.4 A fine dispersion of solute clusters or GP zones (a similar behavior is observed in the 7005 alloy). The main operating microstructural mechanism for the formation of pre-precipitates was identified as the diffusion of Zn2-vacancy pairs in
Fig. 1. Small-angle scattering spectra in an as-quenched sample after solution treatment and in another pre-aged sample for 5 days at RT. The spectrum of the as-quenched specimen was smoothed due to its low statistic.
the 7005 alloy [3] and the migration of Mg-vacancy pairs in the 7012 alloy [3,4]. In Fig. 2(a) and (b) two BF images of the 7012 and 7005 alloys, obtained along h0 1 1i and h0 0 1i zone axes corresponding to samples pre-aged for 5 days at RT, respectively, are shown. Small dark spherical particles ˚ in the 7012 alloy and 20 A ˚ in with diameters of about 10 A the 7005 alloy were identified as solute clusters or GP zones. In Fig. 2(c), a HREM image of a sample of the 7012 alloy pre-aged 5.5 months at RT shows spherical regions with ˚ , which are assumed to correspond diameters less than 20 A to the particles observed in Fig. 2(a). No distortion of the matrix lattice planes is observed across these zones, indicating a high degree of coherence. By means of CTEM [5] the density of these particles in the 7012 alloy was estimated to be 1 1018 to 2 1018 cl/cm3 with an average distance ˚ . On the other hand, between pre-precipitates of 100 A from the data of Fig. 1 a nearest neighbor distance of about ˚ for the pre-aged sample was obtained using the 40 A approximation of Synecˇ ek [6]. The region beyond the maximum is out of range to obtain a trustworthy Guinier radius ðRG q 1:3Þ. During pre-aging at RT an important hardness increase correlated with a slight positron lifetime decrease, was observed in both alloys (reported in [3,7]). The positron lifetime t as well as the microhardness HV reach an apparent saturation value after 5 days at RT indicating that the microstructure has arrived at a metastable state. For both alloys this is the initial state corresponding to the artificial aging labeled 1 in Fig. 3(a) and (b). This figure provides a summary of the positron lifetime t during the artificial aging at 150 8C as a function of t1/3 (see [3]). For comparison the microhardness HV behavior is also shown in the inserts in the top-left corner of Fig. 3(a) and (b). In both alloys, the evolution of t and HV shows similar features. Initially, this heat treatment produces a strong decrease of t and HV (not simultaneously), followed by an increase of both parameters which eventually brings them to a new maximum above the initial value. After that, for long aging times a softening as a consequence of overaging is observed.
R. Ferragut et al. / Journal of Materials Processing Technology 141 (2003) 35–40
37
Fig. 2. Bright-field micrographs for pre-aged samples at RT after solubilization and quenching (labeled 1, Fig. 3): (a) 5 days pre-aging, 7012 alloy, h0 1 1i zone axis; (b) 5 days pre-aging, 7005 alloy, h0 0 1i zone axis. (c) HREM micrograph corresponding to 5.5 months pre-aging, 7012 alloy, h0 1 1i zone axis.
Fig. 4 shows the Guinier plots (ln(I(q)) against q2) for specimens aged 0 min (Fig. 4(a) and (a0 )) and 8 min (Fig. 4(b) and (b0 )) at 150 8C in the 7012 and 7005 alloys. For low q values, a strong first component can be distin˚ in the 7012 alloy and 110 A ˚ in the guished (RG1 90 A
7005 alloy). For deformed samples of an Al–Zn–Mg alloy it was suggested that RG1 represents the typical radius of precipitates formed on dislocations [8] (this component, with a lower intensity, was also observed for non-deformed samples in [8]). In the present study, CTEM results have not
38
R. Ferragut et al. / Journal of Materials Processing Technology 141 (2003) 35–40
Fig. 3. Evolution of the positron lifetime t during isothermal aging at 150 8C, after 5 days of natural aging following solubilization and quenching: (a) 7012 alloy, (b) 7005 alloy. The open symbols labeled 1 and 2 represent those microstructure stages studied by SAXS, TEM and HREM. For the same heat treatment, Vickers microhardness, HV, data are presented in the insert for both alloys.
revealed the presence of particles with similar sizes to RG1, neither in the matrix nor on dislocations. In the 7005 alloy, ˚ , one order of magniparticles with a mean size of 800 A tude higher than RG1, were observed, presumably Mn dispersoids [9]. A second component (RG2) is observed in the alloy 7012. In the alloy 7005 it is not so clear, since there is some curvature in the region following the first component, that could indicate a wide size distribution of particles. For a pre-aged specimen of 7012 alloy (labeled 1 in Fig. 3(a)) the ˚ which corresponds well with Guinier radius RG2 is 25 A the size of the higher pre-precipitates observed by CTEM and HREM. The values of Guinier radius practically do not vary with 8 min of aging. The initial t and HV decrease in the first minutes of aging, observed in Fig. 3(a) and (b) in the studied alloys, is attributed to the partial dissolution of the smallest pre-precipitates [3,5]. PALS results show evidence of reduced trapping in GP zones. It was noted that this phenomenon is not permanent and can be detected only if PALS measurements
Fig. 4. Guinier plots corresponding to samples of the 7012 ((a) and (b)) and 7005 ((a0 ) and (b0 )) alloys aged at 150 8C: 0 min ((a) and (a0 )); 8 min ((b) and (b0 )). The doted lines are visual guides.
are performed shortly after the interruption of the corresponding heat treatment (then t progressively recovers the initial value [3]). Fig. 5 shows a decrease of the integrated intensity Q0 during the early stages of artificial aging in the 7012 alloys (in a similar way in the 7005 alloy). This behavior of Q0 can be correlated with the mentioned fall of the positron lifetime and the softening shown in Fig. 3. Gueffroy and Lo¨ ffler reported a similar result in an Al–Zn– Mg alloy [10], and they attributed it to the dissolution of unstable GP zones. However, during the early stages of aging the decomposition phenomena are overlapping; on the one hand, the smallest pre-precipitates dissolve and, on the other, the metastable Z0 phase is nucleated. Fig. 6 shows a HREM image of the 7012 alloy aged 8 min at 150 8C (labeled 2 in Fig. 3(a)). The edge-on particles with ð1 1 1Þ ˚ long are Z0 platelets and ð1 1 1Þ habit planes of about 70 A [5]. In addition, spherical solute clusters or GP zones can be distinguished. Fig. 3(a) and (b) show that after the mentioned decrease, t increases its value with the aging time up to 220 ps (as occur with Q0 after 4 min of aging), suggesting a progressive formation and growth of the semicoherent Z0 precipitates that increase the hardness of the alloys. In this interpretation, the misfit regions of the matrix–precipitates interfaces, having a higher characteristic positron lifetime, compete with solute clusters or GP zones as positron trapping centers.
Fig. 5. Time evolution of the integral intensity Q0 during the first minutes of aging at 150 8C in the 7012 alloy. The dashed line is only a visual guide.
R. Ferragut et al. / Journal of Materials Processing Technology 141 (2003) 35–40
39
Fig. 6. HREM micrograph of an artificially aged sample for 8 min at 150 8C (labeled 2, Fig. 3 (a)), 7012 alloy, h0 1 1i zone axis.
The different quaternary elements (Mn in alloy 7005 and Cu in alloy 7012) lead to different transformation rates during the precipitation sequence as can be seen in Fig. 3 (see [3]). On the other hand, the size of the precipitates during aging shows a good correlation with the Porod radius RP for the 7012 alloy (results not included here). RP represents the ratio volume/area of the precipitated regions. From the analysis of the first stage of aging the cube of the average radius of the plate-shape Z0 precipitates [5] as well as R3P grow linearly with the aging time according to the LSW (Lifshitz–Slyozov–Wagner) theory.
Acknowledgements The authors wish to acknowledge the support of the National Synchrotron Light Laboratory—LNLS, Brazil
(Project: SAS 407/98). This work was partially financed by Consejo Nacional de Investigaciones Cientı´ficas y Te´ cnicas, Agencia Nacional de Promocio´ n Cientı´fica y Tecnolo´ gica, Comisio´ n de Investigaciones Cientı´ficas de la Provincia de Buenos Aires and Secretarı´a de Ciencia y Te´ cnica of the Universidad Nacional del Centro de la Provincia de Buenos Aires, Argentina.
References [1] I.J. Polmear, Light Alloys: Metallurgy of Light Metals, 3rd ed., Edward Arnold, London, 1995, p. 140. [2] H. Lo¨ ffler, Y. Kovacs, J. Lendvai, J. Mater. Sci. 18 (1983) 2215– 2240. [3] R. Ferragut, A. Somoza, A. Dupasquier, J. Phys. 10 (1998) 3903– 3918. [4] R. Ferragut, A. Somoza, Mater. Sci. Eng. A 334 (2002) 1–5.
40
R. Ferragut et al. / Journal of Materials Processing Technology 141 (2003) 35–40
[5] R. Ferragut, A. Somoza, A. Tolley, Acta Mater. 47 (1999) 4355– 4364. [6] V. Synecˇ ek, J. de Phys. 23 (1962) 828–829. [7] R. Ferragut, A. Somoza, A. Dupasquier, J. Phys. 8 (1996) 8945– 8952.
[8] A. Deschamps, Y. Bre´ chet, P. Guyot, F. Livet, Z. Metallkd. 88 (1997) 601–606. [9] B.O. Kong, S.W. Nam, Mater. Lett. 28 (1996) 385–391. [10] B. Gueffroy, H. Lo¨ ffler, Phys. Stat. Sol. (a) 66 (1981) 585–590.