- Email: [email protected]
Quantitative analysis of GP zones formed at room temperature in a 7150 Al-based alloy
.......~:~,:::~!~'.,:~i~i~i~i~i~::ii!~iiii::!i~i~:~.'..... ~:~:.~:..~
applied surface science ELSEVIER
Applied Surface Science 87/88 (1995) 228-233
Quantitative analysis of GP zones formed at room temperature in a 7150 Al-based alloy C. Schmuck, P. Auger *, F. Danoix, D. Blavette Laboratoire de Microscopie Ionique, URA CNRS 808, Universit~ de Rouen, F-76821 Mont Saint Aignan, France
Received 10 July 1994; accepted for publication 26 July 1994
Abstract The first steps of the decomposition of Al-rich supersaturated solid solutions of A l - Z n - M g - C u alloys, generally lead to a distribution of extremely fine and complex particles of different phases. Although crystallography and morphology of these features have been extensively studied, little information on their chemistry is available. An atom probe study of these phenomena has been undertaken and the first results are reported in this paper. After long term ageing at room temperature, GP zones (GPZ) are formed with a high number density. Although a large scatter exists in the measurements, the mean GPZ composition has been determined. Zinc and magnesium are found up to a total solute level of about 30-35 at% and the Z n / M g atomic ratio is close to 1.3. The copper level in GPZ appears to be similar to that of the solid solution. A preliminary tomographic atom probe study was undertaken. Three-dimensional reconstruction shows that the sphere model used to describe GP zones in this type of alloy and for such ageing conditions, is a rough simplification of a much more complex reality.
I. Introduction W h e n maintained at temperatures lower than 250°C, after a solution treatment performed above 450°C, some aluminium alloys undergo age hardening. This phenomenon, first observed by W i l m [1] in 1906, initiated a large industrial development of these materials. A m o n g the different series of A1 alloys, the A 1 Z n - M g - b a s e d ones are the most important commercial alloys for structural use. This w o r k is focused on
* Corresponding author. Fax: +33 35146652; E-mail: [email protected].
one of these alloys, referred to as 7150, containing additions of copper and zirconium. The sequence of decomposition of the supersaturated solid solution is known to take place via the formation of several phases which may precipitate simultaneously. Depending on thermal treatments, the unmixing processes can be described as follows: supersaturated solid solution GP zones ~ r/' ~ r/ T'--* T.
r/' and ~7 are sometimes known as M ' and M respectively. GP zones are generally described as small soluteenriched spheres. The metastable phase i f , fully coherent with the matrix, precipitates as platelets
0169-4332/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0169-4332(94)00501-X
Refs. [ 2 - 1 5 ] .
C. Schmuck et al. /Applied Surface Science 8 7 / 8 8 (1995) 228-233
with {111} habit planes. The fine dispersion of a high density of particles of this phase is thought to be the main strengthening agent. The structure of r/is similar to that of the equilibrium phase MgZn 2. Particles of this phase appear with different shapes such as platelets, rods, spheres, etc., and different orientations with respect to the matrix. The occurrence and growth of r/is generally associated with a hardness decrease of the material. T', sometimes referred to as X, is a metastable form of the equilibrium phase T [ M g 3 2 (A1, Zn)49]. It appears during low temperature ageings of alloys with Mg contents exceeding 2 wt%. It has been also observed during ageings at higher temperatures ( > 180°C) following a rapid quench. The addition of copper to A1-Zn-Mg-based alloys has a beneficial influence on the major properties of these materials. Copper is thought to stabilize metastable phases into which it may enter [16,17]. These general conclusions were obtained after an intensive work of characterization of both the morphology and crystallography of these phases. Transmission electron microscopy (TEM), X-ray diffraction, small angle X-ray scattering (SAXS) and small angle neutron scattering (SANS) have been the main investigation techniques. Nevertheless, there is still controversy in the literature concerning the range of conditions over which GP zones, 7/' and 77 occur [18]. Determination of the composition of these features by conventional techniques is very difficult because all these metastable phases are found as a distribution of extremely fine particles. This is especially the case for the GP zones with which we are concerned in this preliminary study. Work on various heat treatments leading to ~1', ~/ and T' phases is currently in progress. Because of the possibility of quantitative analysis at t.he sub-nanometer scale, atom probe field ion microscopy (APFIM) is, in principle, ideally suited
229
to study the early stages of these complex unmixing processes [19-22].
2. Experimental
2.1, Alloy composition and heat treatment Specimens from a 7150-type alloy were prepared from 37 mm thick plates provided by Pechiney CRV. The thick plate exhibits a mixed recrystallized/pancake grain structure with 3-10 /zm diameter subgrains. The composition of this alloy is given in Table 1. Zirconium is added in order to inhibit recrystallization. In these specimens Zr exists only in the form of a low number density of spherical A13Zr particles. Before ageing 1.5 year at room temperature, samples were solution treated at 478 _+ 2°C and then water-quenched.
2.2. Experimental details APFIM experiments were carried out with the energy-compensated atom probe at Universit6 de Rouen [23]. The very good mass resolution ( A M / M = 1/500, full width of peaks at 10% of the maximum height) allows easy separation of intertwined Cu and Zn isotopes. The general experimental conditions were the following: - tip temperature 30 K, - vacuum ~ 10 -8 Pa, - pulse to standing voltage ratio 19%, -the evaporation flow rate was controlled ( ~ 0.01 i o n / p u l s e ) i n order to reduce pile-up effects of aluminium. Under these experimental conditions, atom probe analyses are shown to be quantitative (Table 1). Copper evaporates singly charged, mainly in the
Table 1 Compositions (wt% and at%) of the 7150 studied alloy supplied by Peehiney CRV (here referred to as Pech.) and as determined by atom probe measurements (at% _+ 2 standard deviations)
Peeb. (wt%) Pech. (at%) Atom probe (at%)
Zn
Mg
Cu
Zr a
A1
6 2.7 2.6 + 0.1
2.3 2.6 2.4 + 0.1
2.1 0.9 0.78 + 0,06
0.1 0.03 not determ,
bal. bal. bal.
a Some impurities such as Fe or Si are present with less than 0.03 at%.
230
C. Schmuck et aL /Applied Surface Science 8 7 / 8 8 (1995) 228-233 Table 2 Contingency table (local associations) testing M g / Z n co-segregation (atom probe analysis)
,_12"50I
Mg/Zn < 2 at% > 2 at%
'I
i 12,50
o.oo
12.50, O.OC
25
50
75
100
Analysed depth (nm)
125
150
Fig. 1. Mg, Zn and Cu concentration-depth profiles obtained by a conventional atom probe analysis of the 7150-type alloy aged 1.5 year at room temperature.
form Cu ~-, but CuH~- hydrides were also detected. Zinc evaporates in both the singly and doubly charged states as metal ions. These observations are in agreement with those of Brenner et al. [20] and also with predicted charge states given by Kingham's theory [241. Because GP zones image darkly in the Al-rich a-phase, they are not easy to locate in the FIM image and therefore, selected area analysis is difficult. Nevertheless, the high number density of particles permits random area analysis to be performed,
3. Results and discussions
3.1. Conventional atom probe experiments Zn, Mg and Cu concentration profiles are exhibited in Fig. 1. As expected, it is somewhat difficult to clearly discriminate GP-zone solute enrichments from the statistical fluctuations in the solid solution. Nevertheless, with the help of statistical tests like contingency tables [25], it is possible to check the M g / Z n , M g / C u and Z n / C u preferential associations. As shown in Table 2, it is obvious that zinc and magnesium are strongly associated. The same calculation applied to Z n / C u and M g / C u gives uncertain results. No Strong trend for these cosegregations exists. On the basis of this statistical analysis, it is possible to go back and to isolate Zn + Mg local
< 2 at% 8.6 - 6.2
> 2 at% - 6.5 4.7
Horizontally: Zn concentrations ranging below and above 2 at%, calculated in 50 atom blocks. Vertically: Mg concentrations ranging below and above 2 at%, calculated in the same 50 atom blocks. A high correlation exists between Zn and Mg as demonstrated by the high positive values (8.6 and 4.7 >> 2) associated to low Z n / l o w Mg and high Z n / h i g h Mg levels and reciprocally by the high negative values ( - 6 . 2 and - 6 . 5 << - 2 ) of low Z n / h i g h Mg and high Z n / l o w Mg levels.
concentration enhancements from the integrated concentration-depth profile (ladder plot) as shown in Fig. 2. By doing this, a high number density of these features, supposed to be GP zones, are encountered. Following the literature by assuming a spherical shape for GP zones [3-5,8,12], the derivation of relevant parameters (particle composition, mean size and number density) from profiles is possible. We used here the geometrical model developed by Blavette et al. [26]. This model takes into account, for such small particles, the unavoidable particle/ matrix mixed analysis. Experimental results, as deduced with such a method, were found to be prone to a rather large scatter. Nevertheless, average values could be estimated. The GP-zone mean diameter was found to be about 3 nm and the number density on the order of 3 × 1024 m - 3 . The pre-precipitate volume fraction was evaluated to be 4%.
100
0.5 nm
100
200
Total number of atoms Fig. 2. Z n + M g integrated concentration-depth profile (ladder plot) obtained by a conventional atom probe analysis of the 7150-type alloy aged 1.5 year at room temperature. The increase of the slope corresponds to the analysis of a GP zone.
C. Schmuck et aL /Applied Surface Science 8 7 / 8 8 (1995) 228-233 Table 3 Compositions (at% ± 2 standard deviations) of GP zones and matrix derived from atom probe data thanks to the geometrical model proposed by Blavette et al. [26]
Zn Mg Cu
GPzones
M~
68±8 17±6 13±6 2±2
94.9±0.3 2.1±0.2 2.2±0.2 0.8±0.1
Composition measurements are presented in Table 3. Under these ageing conditions (i.e. 1.5 year at room temperature), the solute content in GP zones is rather low (30-35 at%). The ratio of Zn to Mg is close to 1.3. The large uncertainties in the copper content prevent any conclusion to be drawn. These results are in good agreement with those of H0no et al. [21] who studied a similar alloy (but with only 0.5 at% Cu) aged at higher temperatures (120°C-130°C). These authors using FIM image and AP selected area analysis found spherical GP zones approximately 9 nm in diameter. As GP zones are metastable, their composition, size and structure (ordering) may be specially sensitive to solute content and heat treatment. GP zones as characterized by Hono et al. [21] are larger than ours (9 nm compared to 3 nm) and contain slightly more Zn and Mg (35-45 instead of 30-35 at%). Also, they are Cu-euriched ( 1 - 3 at%) whereas we cannot decide about the copper content enhancement of our particles, if any. The small differences between Hono's results, as compared with ours (i.e. increase in GPZ size and solute concentrations), may be explained taking into account the treatment temperature (120°C as compared to 20°C in our case). Other work by Ortner et al. [19] describes ordered plate-like GP zones with thicknesses of one or some few {111} atomic planes. The discrepancy between their results and both Hono's observations and ours may be attributed to the higher ageing temperature (150°C) and also, maybe, to the slightly higher zinc content and null copper level in their alloy. These plate-like GP zones can be regarded as a possible transition state between spherical GP zones and 7/' or more likely as a first stage of */' formation.
231
3.2. Tomographic atom probe preliminary results A preliminary tomographic atom probe (TAP) analysis [23] of a sample aged at room temperature was also undertaken. In order to decrease the brittleness of the tip under the applied field stress, the temperature was raised up to 50 K. Under these
13nm x 13nm x 35nm Fig. 3. Three-dimensional reconstructions of an analysed volume (13 × 13 × 35 nm 3) provided by the tomographic atom probe. Each dot represents (a) a Zn atom, (b) a Mg atom and (c) a Cu atom. 7150-type A1 alloy aged 1.5 year at room temperature.
232
C. Schmuck et aL /Applied Surface Science 8 7 / 8 8 (1995) 228-233
conditions, solutes may be prone to preferential evaporation and the composition measurements are likely to be less quantitative. A three-dimensional (3D) reconstruction of the analysed volume (13 × 13 × 35 nm 3) of the 7150type alloy aged 1.5 year at room temperature is provided in Fig. 3. Each black dot represents a zinc atom in Fig. 3a, a magnesium atom in Fig. 3b and a copper atom in Fig. 3c. Because of the projection of the 3D pattern on the 2D sheet, the observation of the microstructure is more difficult in this static representation than in the dynamic images as provided by the graphics work station. Nevertheless, the morphology of the GP-zone distribution is accessible. Rather than isolated spheres homogeneously dispersed in the solute depleted matrix, images exhibit local diffuse concentration enhancements of Zn and Mg. In addition, copper seems to be distributed homogeneously throughout the material with no preferential partition. This is also in agreement with a previous SAXS investigation of a very similar model alloy aged at room temperature [17]. Furthermore, Zn and Mg local concentration enhancements appear to be linked to their neighbours by diffuse filaments which are also Zn + Mg enriched. The general morphology could therefore be described as an isotropic 3D network whose nodes would be the GP zones. It is tempting to correlate this morphology to that observed in TAP investigation of a spinodally decomposed Fe-Cr system [28]. Such an is0tropic spinodal decomposition was already suggested to occur in the A1-Zn system by Delafond et al. [29]. This hypothesis was also proposed by Blaschko et al. [30] to account for SANS data on a very similar model alloy aged at room temperature. Further investigations, especially a kinetics study, are needed to assess this hypothesis.
4. Conclusions After a long term ageing of this 7150-type alloy at room temperature, GP zones are formed. They appear to be local solute concentration reinforcements of the A1 solid solution (mainly Zn and Mg enrichments up to a total content of 30-35 at% and likely no significant Cu enhancement).
The associated general microstructural morphology can be described as a rather diffuse isotropic network of solutes which spreads throughout the matrix and whose nodes would constitute GP zones. This is consistent with a description in terms of isotropic concentration waves whose amplitude and wavelength increase with ageing time (i.e. spinoda! decomposition). Nevertheless, this has to be confirmed. A more extensive study using TAP could be decisive. Further work on this 7150-type alloy, but dealing with higher ageing temperatures (120°C-160°C) in order to precipitate ~/', ~7 and T' phases, is currently in progress.
Acknowledgements The authors are grateful to Dr. P. Gomiero for fruitful discussions and to Pechiney CR Voreppe France for providing samples and financial support.
References [1] A. Wilm, MrtaUurgie 8 (1911) 225. [2] L.F. Mondolfo, N.A. Gjostein and D.W. Levinson, Trans. AIME 206 (1956) 1378. [3] R. Graf, Comptes Reudus de l'Acadrmie des Sciences 244 (1957) 337. [4] H. Schmalzried and V. Gerold, Z. Metallkd. 6 (1958) 291. [5] J.D. Embury and R.B. Nicholsou, Acta Met. 13 (1965) 403. [6] J.H. Auld and B.E. Williams, Acta Cryst. 21 (1966) 830. [7] J. Gjonnes and C.J. Simensen, Acta Met. 18 (1970) 881. [8] J.M. Raynal, Doctorat d'rtat Thesis, University of Rouen, France, 1971. [9] J.H. Auld and S. Mck. Cousland, Scr. Met. 5 (1971) 765. [10] P. Auger, J.M. Raynal, M. Bernole and R. Graf, Mrm. Sci. Rev. Mrtall. 9 (1974) 557. [11] K.H. Dunkeloh, G. Kralik and V. Gerold, Z. Metallkd. 65 (1974) 291. [12] C.E. Lyman and J.B. Vandersande, Met. Trans. 7A (1976) 1211. [13] T. Ungar, J. Lendvai, I. Kovacs, G. Groma and E. KovacsCsetenyi, Z. Metallkd. 67 (1976) 683. [14] V. Gerold, J.E. Epperson and G. Kostorz, J. Appl. Cryst. 10 (1977) 28. [15] B. Dubost and P. Sainfort, Trait6 de M&aUurgie, Techniques de l'Ingrnieur, Paris, M 240 (1-24), M 241 (1-9). [16] A. Sanlnier and G. Cabane, Rev. M&all. 46 (1949) 13. [17] P. Auger, Doctorat de 3~me cycle Thesis, University of Rouen, France, 1973.
C. Schmuck et al. /Applied Surface Science 8 7 / 8 8 (1995) 228-233 [18] J.K. Park and A.J. Ardell, Scr. Met. 22 (1988) 1115. [19] S.R. Ortner, C.R.M. Grovenor and B.A. Shollock, Scr. Met. 22 (1988) 839. [20] S.S. Brenner, J. Kowalik and Hua Ming-Jian, Surf. Sci. 246 (1991) 210. [21] K. Hono, N. Sano and T. Sakurai, Surf. Sci. 266 (1992) 350. [22] P.J. Warren, C.R.M. G-rovenor and J.S. Crompton, Surf. Sci. 266 (1992) 342. [23] B. Deconihout, A. Menand, M. Bouet and J.M. Sarran, Surf. Sci. 266 (1992) 523. [24] D.R. Kingham, Surf. Sci. 116 (1982) 273. [25] M.K. Miller and G.D.W. Smith, Atom Probe Microanalysis (Mater. Res. Soc., Pittsburgh, PA, 1989).
233
[26] D. Blavette and S. Chambreland, J. Phys. (Paris) 47 (1987) C'/-503. [27] D. Blavette, B. Deconihout, A. Bostel, J.M. Sarrau, M. Bouet and A. Menand, Rev. Sci. Instrum. 64 (1993) 2911. [28] F. Danoix, P. Auger, S. Chambedand and D. Blavette, Microsc. Microanal. Microstruct. 5 (1994) 1. [29] J. Delafond, A. Junqua, J. Mimault and P. Riviere, Acta Met. 23 (1975) 405. [30] O. Blaschko, G. Ernst, P. Fratzl, M. Bernole and P. Auger, Acta Met. 30 (1982) 547.
applied surface science ELSEVIER
Applied Surface Science 87/88 (1995) 228-233
Quantitative analysis of GP zones formed at room temperature in a 7150 Al-based alloy C. Schmuck, P. Auger *, F. Danoix, D. Blavette Laboratoire de Microscopie Ionique, URA CNRS 808, Universit~ de Rouen, F-76821 Mont Saint Aignan, France
Received 10 July 1994; accepted for publication 26 July 1994
Abstract The first steps of the decomposition of Al-rich supersaturated solid solutions of A l - Z n - M g - C u alloys, generally lead to a distribution of extremely fine and complex particles of different phases. Although crystallography and morphology of these features have been extensively studied, little information on their chemistry is available. An atom probe study of these phenomena has been undertaken and the first results are reported in this paper. After long term ageing at room temperature, GP zones (GPZ) are formed with a high number density. Although a large scatter exists in the measurements, the mean GPZ composition has been determined. Zinc and magnesium are found up to a total solute level of about 30-35 at% and the Z n / M g atomic ratio is close to 1.3. The copper level in GPZ appears to be similar to that of the solid solution. A preliminary tomographic atom probe study was undertaken. Three-dimensional reconstruction shows that the sphere model used to describe GP zones in this type of alloy and for such ageing conditions, is a rough simplification of a much more complex reality.
I. Introduction W h e n maintained at temperatures lower than 250°C, after a solution treatment performed above 450°C, some aluminium alloys undergo age hardening. This phenomenon, first observed by W i l m [1] in 1906, initiated a large industrial development of these materials. A m o n g the different series of A1 alloys, the A 1 Z n - M g - b a s e d ones are the most important commercial alloys for structural use. This w o r k is focused on
* Corresponding author. Fax: +33 35146652; E-mail: [email protected].
one of these alloys, referred to as 7150, containing additions of copper and zirconium. The sequence of decomposition of the supersaturated solid solution is known to take place via the formation of several phases which may precipitate simultaneously. Depending on thermal treatments, the unmixing processes can be described as follows: supersaturated solid solution GP zones ~ r/' ~ r/ T'--* T.
r/' and ~7 are sometimes known as M ' and M respectively. GP zones are generally described as small soluteenriched spheres. The metastable phase i f , fully coherent with the matrix, precipitates as platelets
0169-4332/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0169-4332(94)00501-X
Refs. [ 2 - 1 5 ] .
C. Schmuck et al. /Applied Surface Science 8 7 / 8 8 (1995) 228-233
with {111} habit planes. The fine dispersion of a high density of particles of this phase is thought to be the main strengthening agent. The structure of r/is similar to that of the equilibrium phase MgZn 2. Particles of this phase appear with different shapes such as platelets, rods, spheres, etc., and different orientations with respect to the matrix. The occurrence and growth of r/is generally associated with a hardness decrease of the material. T', sometimes referred to as X, is a metastable form of the equilibrium phase T [ M g 3 2 (A1, Zn)49]. It appears during low temperature ageings of alloys with Mg contents exceeding 2 wt%. It has been also observed during ageings at higher temperatures ( > 180°C) following a rapid quench. The addition of copper to A1-Zn-Mg-based alloys has a beneficial influence on the major properties of these materials. Copper is thought to stabilize metastable phases into which it may enter [16,17]. These general conclusions were obtained after an intensive work of characterization of both the morphology and crystallography of these phases. Transmission electron microscopy (TEM), X-ray diffraction, small angle X-ray scattering (SAXS) and small angle neutron scattering (SANS) have been the main investigation techniques. Nevertheless, there is still controversy in the literature concerning the range of conditions over which GP zones, 7/' and 77 occur [18]. Determination of the composition of these features by conventional techniques is very difficult because all these metastable phases are found as a distribution of extremely fine particles. This is especially the case for the GP zones with which we are concerned in this preliminary study. Work on various heat treatments leading to ~1', ~/ and T' phases is currently in progress. Because of the possibility of quantitative analysis at t.he sub-nanometer scale, atom probe field ion microscopy (APFIM) is, in principle, ideally suited
229
to study the early stages of these complex unmixing processes [19-22].
2. Experimental
2.1, Alloy composition and heat treatment Specimens from a 7150-type alloy were prepared from 37 mm thick plates provided by Pechiney CRV. The thick plate exhibits a mixed recrystallized/pancake grain structure with 3-10 /zm diameter subgrains. The composition of this alloy is given in Table 1. Zirconium is added in order to inhibit recrystallization. In these specimens Zr exists only in the form of a low number density of spherical A13Zr particles. Before ageing 1.5 year at room temperature, samples were solution treated at 478 _+ 2°C and then water-quenched.
2.2. Experimental details APFIM experiments were carried out with the energy-compensated atom probe at Universit6 de Rouen [23]. The very good mass resolution ( A M / M = 1/500, full width of peaks at 10% of the maximum height) allows easy separation of intertwined Cu and Zn isotopes. The general experimental conditions were the following: - tip temperature 30 K, - vacuum ~ 10 -8 Pa, - pulse to standing voltage ratio 19%, -the evaporation flow rate was controlled ( ~ 0.01 i o n / p u l s e ) i n order to reduce pile-up effects of aluminium. Under these experimental conditions, atom probe analyses are shown to be quantitative (Table 1). Copper evaporates singly charged, mainly in the
Table 1 Compositions (wt% and at%) of the 7150 studied alloy supplied by Peehiney CRV (here referred to as Pech.) and as determined by atom probe measurements (at% _+ 2 standard deviations)
Peeb. (wt%) Pech. (at%) Atom probe (at%)
Zn
Mg
Cu
Zr a
A1
6 2.7 2.6 + 0.1
2.3 2.6 2.4 + 0.1
2.1 0.9 0.78 + 0,06
0.1 0.03 not determ,
bal. bal. bal.
a Some impurities such as Fe or Si are present with less than 0.03 at%.
230
C. Schmuck et aL /Applied Surface Science 8 7 / 8 8 (1995) 228-233 Table 2 Contingency table (local associations) testing M g / Z n co-segregation (atom probe analysis)
,_12"50I
Mg/Zn < 2 at% > 2 at%
'I
i 12,50
o.oo
12.50, O.OC
25
50
75
100
Analysed depth (nm)
125
150
Fig. 1. Mg, Zn and Cu concentration-depth profiles obtained by a conventional atom probe analysis of the 7150-type alloy aged 1.5 year at room temperature.
form Cu ~-, but CuH~- hydrides were also detected. Zinc evaporates in both the singly and doubly charged states as metal ions. These observations are in agreement with those of Brenner et al. [20] and also with predicted charge states given by Kingham's theory [241. Because GP zones image darkly in the Al-rich a-phase, they are not easy to locate in the FIM image and therefore, selected area analysis is difficult. Nevertheless, the high number density of particles permits random area analysis to be performed,
3. Results and discussions
3.1. Conventional atom probe experiments Zn, Mg and Cu concentration profiles are exhibited in Fig. 1. As expected, it is somewhat difficult to clearly discriminate GP-zone solute enrichments from the statistical fluctuations in the solid solution. Nevertheless, with the help of statistical tests like contingency tables [25], it is possible to check the M g / Z n , M g / C u and Z n / C u preferential associations. As shown in Table 2, it is obvious that zinc and magnesium are strongly associated. The same calculation applied to Z n / C u and M g / C u gives uncertain results. No Strong trend for these cosegregations exists. On the basis of this statistical analysis, it is possible to go back and to isolate Zn + Mg local
< 2 at% 8.6 - 6.2
> 2 at% - 6.5 4.7
Horizontally: Zn concentrations ranging below and above 2 at%, calculated in 50 atom blocks. Vertically: Mg concentrations ranging below and above 2 at%, calculated in the same 50 atom blocks. A high correlation exists between Zn and Mg as demonstrated by the high positive values (8.6 and 4.7 >> 2) associated to low Z n / l o w Mg and high Z n / h i g h Mg levels and reciprocally by the high negative values ( - 6 . 2 and - 6 . 5 << - 2 ) of low Z n / h i g h Mg and high Z n / l o w Mg levels.
concentration enhancements from the integrated concentration-depth profile (ladder plot) as shown in Fig. 2. By doing this, a high number density of these features, supposed to be GP zones, are encountered. Following the literature by assuming a spherical shape for GP zones [3-5,8,12], the derivation of relevant parameters (particle composition, mean size and number density) from profiles is possible. We used here the geometrical model developed by Blavette et al. [26]. This model takes into account, for such small particles, the unavoidable particle/ matrix mixed analysis. Experimental results, as deduced with such a method, were found to be prone to a rather large scatter. Nevertheless, average values could be estimated. The GP-zone mean diameter was found to be about 3 nm and the number density on the order of 3 × 1024 m - 3 . The pre-precipitate volume fraction was evaluated to be 4%.
100
0.5 nm
100
200
Total number of atoms Fig. 2. Z n + M g integrated concentration-depth profile (ladder plot) obtained by a conventional atom probe analysis of the 7150-type alloy aged 1.5 year at room temperature. The increase of the slope corresponds to the analysis of a GP zone.
C. Schmuck et aL /Applied Surface Science 8 7 / 8 8 (1995) 228-233 Table 3 Compositions (at% ± 2 standard deviations) of GP zones and matrix derived from atom probe data thanks to the geometrical model proposed by Blavette et al. [26]
Zn Mg Cu
GPzones
M~
68±8 17±6 13±6 2±2
94.9±0.3 2.1±0.2 2.2±0.2 0.8±0.1
Composition measurements are presented in Table 3. Under these ageing conditions (i.e. 1.5 year at room temperature), the solute content in GP zones is rather low (30-35 at%). The ratio of Zn to Mg is close to 1.3. The large uncertainties in the copper content prevent any conclusion to be drawn. These results are in good agreement with those of H0no et al. [21] who studied a similar alloy (but with only 0.5 at% Cu) aged at higher temperatures (120°C-130°C). These authors using FIM image and AP selected area analysis found spherical GP zones approximately 9 nm in diameter. As GP zones are metastable, their composition, size and structure (ordering) may be specially sensitive to solute content and heat treatment. GP zones as characterized by Hono et al. [21] are larger than ours (9 nm compared to 3 nm) and contain slightly more Zn and Mg (35-45 instead of 30-35 at%). Also, they are Cu-euriched ( 1 - 3 at%) whereas we cannot decide about the copper content enhancement of our particles, if any. The small differences between Hono's results, as compared with ours (i.e. increase in GPZ size and solute concentrations), may be explained taking into account the treatment temperature (120°C as compared to 20°C in our case). Other work by Ortner et al. [19] describes ordered plate-like GP zones with thicknesses of one or some few {111} atomic planes. The discrepancy between their results and both Hono's observations and ours may be attributed to the higher ageing temperature (150°C) and also, maybe, to the slightly higher zinc content and null copper level in their alloy. These plate-like GP zones can be regarded as a possible transition state between spherical GP zones and 7/' or more likely as a first stage of */' formation.
231
3.2. Tomographic atom probe preliminary results A preliminary tomographic atom probe (TAP) analysis [23] of a sample aged at room temperature was also undertaken. In order to decrease the brittleness of the tip under the applied field stress, the temperature was raised up to 50 K. Under these
13nm x 13nm x 35nm Fig. 3. Three-dimensional reconstructions of an analysed volume (13 × 13 × 35 nm 3) provided by the tomographic atom probe. Each dot represents (a) a Zn atom, (b) a Mg atom and (c) a Cu atom. 7150-type A1 alloy aged 1.5 year at room temperature.
232
C. Schmuck et aL /Applied Surface Science 8 7 / 8 8 (1995) 228-233
conditions, solutes may be prone to preferential evaporation and the composition measurements are likely to be less quantitative. A three-dimensional (3D) reconstruction of the analysed volume (13 × 13 × 35 nm 3) of the 7150type alloy aged 1.5 year at room temperature is provided in Fig. 3. Each black dot represents a zinc atom in Fig. 3a, a magnesium atom in Fig. 3b and a copper atom in Fig. 3c. Because of the projection of the 3D pattern on the 2D sheet, the observation of the microstructure is more difficult in this static representation than in the dynamic images as provided by the graphics work station. Nevertheless, the morphology of the GP-zone distribution is accessible. Rather than isolated spheres homogeneously dispersed in the solute depleted matrix, images exhibit local diffuse concentration enhancements of Zn and Mg. In addition, copper seems to be distributed homogeneously throughout the material with no preferential partition. This is also in agreement with a previous SAXS investigation of a very similar model alloy aged at room temperature [17]. Furthermore, Zn and Mg local concentration enhancements appear to be linked to their neighbours by diffuse filaments which are also Zn + Mg enriched. The general morphology could therefore be described as an isotropic 3D network whose nodes would be the GP zones. It is tempting to correlate this morphology to that observed in TAP investigation of a spinodally decomposed Fe-Cr system [28]. Such an is0tropic spinodal decomposition was already suggested to occur in the A1-Zn system by Delafond et al. [29]. This hypothesis was also proposed by Blaschko et al. [30] to account for SANS data on a very similar model alloy aged at room temperature. Further investigations, especially a kinetics study, are needed to assess this hypothesis.
4. Conclusions After a long term ageing of this 7150-type alloy at room temperature, GP zones are formed. They appear to be local solute concentration reinforcements of the A1 solid solution (mainly Zn and Mg enrichments up to a total content of 30-35 at% and likely no significant Cu enhancement).
The associated general microstructural morphology can be described as a rather diffuse isotropic network of solutes which spreads throughout the matrix and whose nodes would constitute GP zones. This is consistent with a description in terms of isotropic concentration waves whose amplitude and wavelength increase with ageing time (i.e. spinoda! decomposition). Nevertheless, this has to be confirmed. A more extensive study using TAP could be decisive. Further work on this 7150-type alloy, but dealing with higher ageing temperatures (120°C-160°C) in order to precipitate ~/', ~7 and T' phases, is currently in progress.
Acknowledgements The authors are grateful to Dr. P. Gomiero for fruitful discussions and to Pechiney CR Voreppe France for providing samples and financial support.
References [1] A. Wilm, MrtaUurgie 8 (1911) 225. [2] L.F. Mondolfo, N.A. Gjostein and D.W. Levinson, Trans. AIME 206 (1956) 1378. [3] R. Graf, Comptes Reudus de l'Acadrmie des Sciences 244 (1957) 337. [4] H. Schmalzried and V. Gerold, Z. Metallkd. 6 (1958) 291. [5] J.D. Embury and R.B. Nicholsou, Acta Met. 13 (1965) 403. [6] J.H. Auld and B.E. Williams, Acta Cryst. 21 (1966) 830. [7] J. Gjonnes and C.J. Simensen, Acta Met. 18 (1970) 881. [8] J.M. Raynal, Doctorat d'rtat Thesis, University of Rouen, France, 1971. [9] J.H. Auld and S. Mck. Cousland, Scr. Met. 5 (1971) 765. [10] P. Auger, J.M. Raynal, M. Bernole and R. Graf, Mrm. Sci. Rev. Mrtall. 9 (1974) 557. [11] K.H. Dunkeloh, G. Kralik and V. Gerold, Z. Metallkd. 65 (1974) 291. [12] C.E. Lyman and J.B. Vandersande, Met. Trans. 7A (1976) 1211. [13] T. Ungar, J. Lendvai, I. Kovacs, G. Groma and E. KovacsCsetenyi, Z. Metallkd. 67 (1976) 683. [14] V. Gerold, J.E. Epperson and G. Kostorz, J. Appl. Cryst. 10 (1977) 28. [15] B. Dubost and P. Sainfort, Trait6 de M&aUurgie, Techniques de l'Ingrnieur, Paris, M 240 (1-24), M 241 (1-9). [16] A. Sanlnier and G. Cabane, Rev. M&all. 46 (1949) 13. [17] P. Auger, Doctorat de 3~me cycle Thesis, University of Rouen, France, 1973.
C. Schmuck et al. /Applied Surface Science 8 7 / 8 8 (1995) 228-233 [18] J.K. Park and A.J. Ardell, Scr. Met. 22 (1988) 1115. [19] S.R. Ortner, C.R.M. Grovenor and B.A. Shollock, Scr. Met. 22 (1988) 839. [20] S.S. Brenner, J. Kowalik and Hua Ming-Jian, Surf. Sci. 246 (1991) 210. [21] K. Hono, N. Sano and T. Sakurai, Surf. Sci. 266 (1992) 350. [22] P.J. Warren, C.R.M. G-rovenor and J.S. Crompton, Surf. Sci. 266 (1992) 342. [23] B. Deconihout, A. Menand, M. Bouet and J.M. Sarran, Surf. Sci. 266 (1992) 523. [24] D.R. Kingham, Surf. Sci. 116 (1982) 273. [25] M.K. Miller and G.D.W. Smith, Atom Probe Microanalysis (Mater. Res. Soc., Pittsburgh, PA, 1989).
233
[26] D. Blavette and S. Chambreland, J. Phys. (Paris) 47 (1987) C'/-503. [27] D. Blavette, B. Deconihout, A. Bostel, J.M. Sarrau, M. Bouet and A. Menand, Rev. Sci. Instrum. 64 (1993) 2911. [28] F. Danoix, P. Auger, S. Chambedand and D. Blavette, Microsc. Microanal. Microstruct. 5 (1994) 1. [29] J. Delafond, A. Junqua, J. Mimault and P. Riviere, Acta Met. 23 (1975) 405. [30] O. Blaschko, G. Ernst, P. Fratzl, M. Bernole and P. Auger, Acta Met. 30 (1982) 547.