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PRE-PRECIPITATE CLUSTERS AND PRECIPITATION PROCESSES IN Al-Mg-Si ALLOYS


Published in Acta mater. Vol. 47, No. 5, pp. 1537-1548, 1999

PRE-PRECIPITATE CLUSTERS AND PRECIPITATION PROCESSES IN Al-Mg-Si ALLOYS
M. Murayama and K. Hono*
National Research Institute for Metals, 1-2-1 Sengen, Tsukuba 305-0047, Japan
Abstract–––The solute clusters and the metastable precipitates in aged Al-Mg-Si alloys have been characterized by a three-dimensional atom probe (3DAP) and transmission electron microscopy (TEM). After long-term natural aging, Mg-Si co-clusters have been detected in addition to separate Si and Mg atom clusters. The particle density of β″ after 10 h artificial aging at 175°C varies depending on pre-aging conditions, i.e., pre-aging at 70°C increases the number density of the β″ precipitates, whereas natural aging reduces it. This suggests that the spherical GP zones formed at 70°C serves as nucleation sites for the β″ in the subsequent artificial aging, whereas co-clusters formed at room temperature do not. Atom probe analysis results have revealed that the Mg:Si ratios of the GP zones and the β″ precipitates in the alloy with excess amount of Si are 1:1, whereas those in the Al-Mg2Si quasi-binary alloy are 2:1. Based on these results, the characteristic two-step age-hardening behavior in Al-Mg-Si alloys is discussed.

1. INTRODUCTION In the continuing drive for automobile weight reduction, the 6000 series Al-Mg-Si alloys are considered the most promising candidates for agehardenable bodysheet materials. Several studies [15] reported that the alloys containing an excess amount of Si above the Al-Mg2Si quasi-binary composition show pronounced hardening by aging at 175°C. However, such hardening response is suppressed when the alloys receive room temperature aging after a solution heat-treatment. Since natural aging can not be avoided in the automobile manufacturing process, understanding the adverse age hardening effect by two-step aging is of great industrial importance. The typical paint-bake cycle in the automobile manufacturing process is 30 min heating at 175°C. Al-Mg-Si alloys reach a peak hardness condition after 10 – 20 h at 175°C, thus the paint-baked body sheet aluminum alloys must be used in an underaged condition. According to Dutta and Allen [6], the precipitation sequence in Al-Mg-Si alloys is solute clusters → GP zones (spherical) → β" (needle) → β' (rod) → β. The formation of solute clusters was first proposed by Pashley et al. [1, 2] to explain the twostep aging behavior, followed by Dutta and Allen [6] based on DSC measurement results. More recently, Edwards et al. [7-9] found direct evidence for the *To whom all correspondence should be addressed. 1

presence of separate Si- and Mg- clusters and Mg-Si co-clusters during aging at 70°C using the atom probe field ion microscope (APFIM) technique. They also reported that the Mg:Si ratios in the Mg-Si co-clusters, GP zones, and β″ precipitates are all close to 1. However, solute clustering during natural aging was not investigated in their study. Since the kinetics of artificial aging are significantly influenced by natural aging, it is necessary to study solute clustering in the naturally aged condition. Using APFIM, we recently reported [10] that clusters of Mg atoms are present in the as-quenched alloys, and separate clusters of Mg and Si atoms and their co-clusters evolve after long-term natural aging. In these previous studies, however, the effect of the solute clusters on the kinetics of the precipitation process has not been discussed in much detail. The purpose of this study is to obtain a better understanding of the mechanism of adverse age hardening effect due to natural aging and the role of excess amount of Si on changing the age hardening response. For this purpose, we attempt to clarify the chemical characteristics of various precipitation products which form after natural aging, pre-aging at 70°C and artificial aging at 175°C in Al-Mg2Si quasibinary and Si-excess alloys by employing a conventional atom probe (AP), a three dimensional atom probe (3DAP) and a transmission electron microscope (TEM). The 3DAP technique is particularly effective in characterizing small clusters and precipitates, because it is capable of mapping individual atoms in the real space with a near-atomic resolution [11, 12], thus it provides accurate information on particle density, composition and morphology of clusters and small precipitates.

Published in Acta mater. Vol. 47, No. 5, pp. 1537-1548, 1999

Alloy Balanced Si-excess

Mg 0.70 0.65

Table 1 Chemical compositions of the alloys (at. %) Si Cu Fe 0.33 0.024 0.70 0.005

Ti 0.006 -

Al Bal. Bal.

2. EXPERIMENTAL The chemical compositions of the alloys used in this study are listed in Table 1. The balance alloy has an Al-Mg2Si quasi-binary composition, and the Siexcess alloy contains almost an equal number of Si and Mg atoms. These alloys were solution treated at 550°C for 30 min and subsequently water quenched. The solution treated specimens were artificially aged

for 30 min and 10 h at 175°C after various pre-aging conditions; i.e., immediately after the solution treatment, after natural aging for 70 days and after pre-aging at 70°C for 16 h. For atom probe analysis, an energy compensated one dimensional atom probe (1DAP) and a three-dimensional atom probe (3DAP) equipped with CAMECA tomographic atom probe (TAP) detection system [12] were used. Atom probe analyses were carried out at about 30K with a pulse fraction (Vp/Vdc) of 20 % in UHV (~1x10-10 Torr)

Si excess (a) (b)

balance

naturally aged

(c)

(d)

70°C for 16h

10nm
Fig. 1. TEM bright field images and the [001] selected area diffraction patterns obtained from (a) Al0.65Mg-0.70Si (Si-excess) alloy after natural pre-aging for 70 days, (b) Al-0.70Mg-0.33Si (balanced) alloy after natural pre-aging for 70 days, (c) the Si-excess alloy after 70°C pre-aging for 16 h and (d) the balanced alloy after 70°C pre-aging for 16 h. 2

Published in Acta mater. Vol. 47, No. 5, pp. 1537-1548, 1999

after obtaining a clean surface by FIM observation. In the present study, 3DAP data were obtained by locating the [001]Al pole facing to the detector. Microstructures of the specimens were examined with a transmission electron microscope (TEM), Philips CM200, operated at 200 kV. High resolution electron microscopy observations were carried out using JEOL JEM-2000EX, operated at 200 kV. 3. RESULTS 3.1 Natural aging and pre-aging Figures 1 (a) and (b) show TEM bright field images and the [001] selected area diffraction patterns (SADP) obtained from (a) Al-0.65Mg-

0.70Si (Si-excess) alloy and (b) Al-0.70Mg-0.33Si (balance) alloy which were aged at room temperature for 70 days (natural aging). In both specimens, there is no indication of precipitate particles in the bright field images and the diffraction patterns. Figures 1 (c) and (d) show TEM bright field images and the [001] selected area diffraction patterns obtained from (c) the Si-excess alloy and (d) the balance alloy which were aged for 16 h at 70°C (70°C pre-aging). Dark contrast arising from extremely fine particles is observed in Fig. 1(c). The shape of the fine precipitates is not well-defined since they are very fine (~ 2 nm), and the SADP shows neither extra reflection nor diffuse scattering, which suggests that the precipitates are fully coherent with the matrix and do not have any distinct structure. Thus, the precipitates are designated as

(a)

2nm (b)

Fig. 2. HREM images taken at the [001]Al zone axis of Al-0.65Mg-0.70Si (Si-excess) alloy after (a) natural aging and (b) 70°C pre-aging. 3

Published in Acta mater. Vol. 47, No. 5, pp. 1537-1548, 1999
(a)

Mg

Si (b)
Number of Detected Mg / Si Atoms

~80nm

160 140 120 100 80 60 40 20 2 4 6 8 10 12 x10
3

Si co-cluster Mg

14

Total Number of Detected Atoms /

Fig. 3. (a) 3DAP elemental mappings of Mg and Si atoms and (b) 1DAP integrated concentration depth profiles obtained from Al-0.65Mg-0.70Si (Siexcess) alloy after natural pre-aging for 70 days. spherical GP zones following Dutta and Allen [6]. The same contrast is observed in the balance alloy (Fig. 1 (d)), but the particle density of the GP zones is much smaller than that in the Si-excess alloy. An HREM image of a naturally aged Si-excess alloy shows a uniform fringe contrast as shown in Fig. 2 (a). No contrast attributed to the precipitate particles is observed. On the other hand, the contrast arising from the precipitate particles is observed in the Siexcess alloy that was pre-aged at 70°C for 16 h. The HREM image indicates that the precipitates are approximately 2 nm and are coherent with the matrix, justifying that the designation as GP zones is appropriate. Figure 3 (a) shows 3DAP elemental maps of Mg and Si atoms obtained from the naturally aged Siexcess alloy. In these maps, each dot corresponds to

the position of the individual atom. Note that the size of the dots does not have any physical meaning; it is arbitrarily adjusted only for obtaining a better visualization effect. The distribution of Mg and Si atoms appears to be homogeneous at a glance. Since the concentrations of Mg and Si is as much as a few atomic percent, small clusters may not be visualized because they are covered in the background from the supersaturated solute atoms. Thus, a statistical method known as contingency table analysis was employed to test whether or not there is a correlation in the distributions of Mg and Si atoms [13]. Table 2 shows contingency tables of the number of detected Mg and Si atoms based on a block size of 146 atoms for the naturally aged Si-excess specimen. In this table, observed frequencies containing categorized numbers of Mg and Si atoms are tabulated. Experimentally observed frequencies and the frequencies for random distribution of Mg and Si are compared using the χ2 test. The calculated value of χ2 is 34.4 with a 9 degree of freedom, thus the null hypothesis that there is no correlation between Mg and Si atoms is rejected with a significance level of 99.999 % (α < 0.001). The table also suggests that there is a positive correlation between Mg and Si atoms (when Mg atoms are detected, there is a tendency for Si atoms to be also detected in the same atom block). Thus, it can be concluded that Mg and Si atoms form co-clusters in this stage. Although the 3DAP elemental maps do not demonstrate the presence of solute clusters visually, local concentration changes can be detected more sensitively by analyzing the data obtained by conventional 1DAP. Figure 3 (b) shows integral profiles of Si and Mg atoms or ladder plots of the naturally aged Si-excess alloy, where the number of detected solute atoms is plotted as a function of the total number of detected atoms. The slopes of the plots represent the local concentration of the alloy, and the horizontal axis corresponds to the depth. Steep changes in the slope are recognized (indicated by arrowheads) in both Mg and Si ladder diagrams. In these regions, the concentration of Mg or Si is significantly higher than the average concentration in the alloy, suggesting that there are separate clusters of Mg and Si atoms (indicated by the arrowheads). In addition, a co-cluster of Mg and Si

Table. 2 Contingency tables for Mg and Si obtained from Si excess alloy after natural pre-aging. Observed Expected Si Si 0 1 2 ≥3 Total 0 1 2 ≥3 Total Mg 0 175 60 25 19 279 0 143.7 70.9 36.6 27.8 279 1 120 69 36 19 244 1 125.7 62.0 32.0 24.3 244 2 66 40 23 24 153 2 78.8 38.9 20.1 15.2 153 ≥3 32 25 16 14 87 ≥3 44.8 22.1 11.4 8.7 87 Total 393 194 100 76 763 Total 393 194 100 76 763 χ2 = 34.396 with 9 degrees of freedom 4

~10nm

Published in Acta mater. Vol. 47, No. 5, pp. 1537-1548, 1999

GP zone

Mg

Si

~120nm

~27nm

Mg

Fig. 4. 3DAP elemental mappings of Mg and Si atoms obtained from Al-0.65Mg-0.70Si 4 (Si-excess) alloy after 70°C pre-aging for 16 h. atoms is detected as indicated by the broken lines in Fig. 3 (b). The ratio of the number of Mg and Si atoms in this co-cluster is close to 1. From these results, it can be concluded that Mg-Si co-clusters are present in the naturally aged specimen. Figure 4 shows 3DAP elemental maps of Mg and Si atoms obtained from the Si-excess alloy after 70°C pre-aging. The presence of particles enriched with Mg and Si atoms is evident from the elemental maps. The shape of the Mg and Si enriched zone is still not well-defined, but these are believed to correspond to the GP zones as characterized by TEM in Fig. 1(c) and 2(b). The contingency table analysis shown in Table 3 clearly indicates the tendency for strong Mg-Si co-clustering. The calculated value for χ2 is 187 with 15 degrees of freedom, thus there is a strong correlation between Mg and Si atoms. The average chemical composition of these spherical GP zones has been determined to be approximately 10 at. % Mg and 10 at. % Si. Some larger zones have been found to have higher concentrations of Mg and Si, but the atomic ratio of Mg:Si in these zones remains approximately 1:1, which is the same as that of the Mg-Si co-clusters observed in the naturally

Table. 3 Contingency tables for Mg and Si obtained from Si excess alloy after 70°C pre-aging. Observed Expected Si 0 1 2 0 406 224 87 1 247 191 82 2 109 102 34 3 30 39 31 4 15 10 10 ≥5 6 12 12 Total 812 578 256 χ2 = 187.245 with 15 degrees of freedom Mg ≥3 40 51 41 32 15 23 202 Total 757 571 286 132 50 53 1894 0 1 2 3 4 ≥5 Total 0 332.9 251.1 125.8 58.0 22.0 23.3 813 1 236.6 178.5 89.4 41.3 15.6 16.6 578 Si 2 104.8 79.1 39.6 18.3 6.9 7.3 256 ≥3 82.7 62.4 31.2 14.4 5.5 5.8 202 Total 757 571 286 132 50 53 1894

5

~2nm
Si

~14nm

Published in Acta mater. Vol. 47, No. 5, pp. 1537-1548, 1999
Si excess (a) naturally aged + 175°C for 30 min (b) balance

(c) 175°C for 30 min

(d)

(e) 70°C for 16h + 175°C for 30 min

(f)

10nm

Fig. 5. TEM bright field images and the [001] selected area diffraction patterns obtained from (a) Al0.65Mg-0.70Si (Si-excess) alloy and (b) Al-0.70Mg-0.33Si (balanced) alloy aged at 175°C for 30 min after natural pre-aging, (c) the Si-excess alloy and (d) the balanced alloy aged at 175°C for 30 min immediately after solution treatment and (e) the Si-excess alloy and (f) the balanced alloy aged at 175°C for 30 min after 70°C pre-aging. aged specimen. These atom probe results indicates that Mg and Si atoms tend to aggregate together by natural aging and pre-aging. From a chemical point of view, coclusters and GP zones are essentially the same; the only difference is in the size and the density of solute atoms. In our definition, GP zones are the solute clusters with the size and the solute content high enough to give contrast in TEM image. Clusters are solute aggregates with lower solute concentration with undefined morphology that is not sufficient enough to give contrast in TEM images. 3.2 Aging for 30 min at 175°C Figures 5 (a) - (f) show TEM bright field images and the [001] SADPs obtained from the Si-excess alloy and the balance alloy aged at 175°C for 30 min after natural aging, immediately after solution treatment, and after 70°C pre-aging. The distribution of the precipitate particles in the Si-excess alloy is finer and denser than that in the balance alloy. Since the contrast due to the particles is still spherical rather than needle-shaped, and the SADPs show neither extra reflection nor diffuse scattering, the precipitates are designated as the spherical GP zones. Note that the number density of the GP zones in the artificially aged specimens after 70°C pre-aging is much higher than that in the artificially aged specimens immediately after the solution treatment. On the other hand, the number density of the GP zones in the specimen artificially aged after natural aging is lower than that in the specimen artificially aged immediately after the solution treatment. This suggests that natural aging gives adverse effect on the kinetics of subsequent artificial aging, while preaging at 70°C gives positive effect on the artificial aging.

6

Published in Acta mater. Vol. 47, No. 5, pp. 1537-1548, 1999

(a)

GP zone

Mg

~14nm

~7nm

Mg

Si

Si (b)

~65nm GP zone

Mg

Si

~100nm

~14nm

Mg

Si

Fig. 6. 3DAP elemental mappings of Mg and Si atoms obtained from (a) Al-0.65Mg-0.70Si (Si-excess) alloy and (b) Al-0.70Mg-0.33Si (balanced) alloy aged at 175°C for 30 min after 70°C pre-aging. Figures 6 (a) and (b) show 3DAP elemental maps of Mg and Si atoms obtained from the Siexcess alloy and the balance alloy which were aged at 175°C for 30 min after pre-aging at 70°C. The presence of Mg and Si enriched particles is evident from the elemental maps. The shape of the Mg and Si enriched particles are not well-defined (or spherical), but these are believed to correspond to the GP zones observed in Fig. 5 (e) and (f). The chemical composition of the GP zones in each specimen has been determined by selecting the small regions that cover the entire particle. Figures 7 (a) 7 and (b) are integral profiles of the GP zones constructed from the selected regions in Fig. 6 (a) and (b). The Mg:Si ratio of the GP zone in the Siexcess alloy is almost 1:1 and its composition is approximately 7 at.% Mg and 7 at.% Si. The Mg:Si ratio of the GP zone in the balance alloy is close to 2:1, and its composition is approximately 14 at.% Mg and 7 at.% Si. It should be noted that the atomic ratio of the GP zones in the balance alloy is close to that of Mg2Si, but the atomic ratio of the Si-excess alloy is close to that of MgSi, thus all these atomic ratios are close to that of the alloy composition.

~2nm

~14nm

~2nm

Published in Acta mater. Vol. 47, No. 5, pp. 1537-1548, 1999

(a) Si-excess
50 40 30 20 10 0 0 500 1000 1500

Mg Si

2000

Total number of detected atoms

(b) balance
50 40 30 20 10 0 0 200 400 600

Mg Si

Mg/Si

alloy, both the needle-shaped precipitates and the spherical GP zones are observed. The needle-shaped precipitates in the Si-excess alloy are composed of Mg and Si atoms, and their composition is approximately 23% Mg and 21% Si. The chemical composition of the spherical GP zones which are observed in this condition is 12 % Mg and 12 % Si. Thus, Mg and Si concentration of the β" precipitates is almost twice as high as that of the GP zones, but the atomic ratio of Mg to Si atoms remains the same, i.e., 1:1. In contrast, the compositions of the needleshaped precipitates and the spherical GP zones in the balance alloy are both approximately 17 % Mg and 10 % Si, and their atomic ratio is similar to that of Mg2Si, the equilibrium β phase. 4. DISCUSSION

Mg/Si

800

Total number of detected atoms

4.1 Effect of pre-aging Based on the present AP results, it can be concluded that Mg and Si atoms aggregate to form co-clusters in the early stage and these gradually grow to GP zones during pre-aging. The chemical nature of the GP zones and the co-clusters are similar. However, GP zones are larger than the clusters and the concentration of the solutes is higher, thus GP zones give contrast in TEM images. As the thermal stability of coherent precipitates depends on their size due to the capillary effect, the GP zones formed by 70°C pre-aging are expected to be thermally more stable than the co-clusters formed by natural aging. The microstructures after artificial aging have shown that a high number density of spherical GP zones formed by 70°C pre-aging results in an increased number density of the β" precipitates in both the Si-excess and the balance alloys. On the other hand, a decrease in the number density of β" precipitates by artificial aging after natural aging indicates that the co-clusters do not work as nucleation sites for the β" precipitates. Based on the present 3DAP and TEM observation results, the adverse effect of natural aging on the subsequent artificial age-hardening response is given as below, following the account for the two step aging in the Al-Zn-Mg alloys by Lorimer and Nicholson [16]. Figures 9 (a) and (b) show schematic size distribution of co-clusters and GP zones formed after natural aging and 70°C preaging. The shaded distribution diagram represents the size distribution which is expected right after heating the specimens to the artificial aging temperature (T=175°C). The size of the co-clusters that are formed by natural aging is smaller than the critical size which is necessary for nucleation at 175°C, rc(T=175°C). Thus, on heating the specimen to the artificial aging temperature, all co-clusters formed during natural aging may be reverted. The precipitation kinetics after the reversion of co8

Fig. 7. Integrated profiles of Si and Mg atoms obtained from the selected regions that cover a GP zone in Fig. 6 (a) and (b). Since GP zones do not have a distinct structure, it is reasonable that the available number of atoms, i.e. the alloy composition, affects the composition of the GP zones. From these observations, it can be concluded that only GP zones are present under the paint-bake condition. 3.3 Aging for 10 h at 175°C Figures 8 (a) – (d) show TEM bright field images and the [001] SADPs obtained from the Siexcess alloys and the balance alloys aged for 10 h at 175 °C after pre-aging. In all of the bright field images, the needle-shaped strain field contrast is observed. In addition, streaks along the [001] directions are observed in the SADPs shown in Fig. 7(a), (c) and (d). These streaks are from the needleshaped precipitates aligned in the <001> directions, which are attributed to the β" precipitates [14, 15]. The number density of the β" precipitates in the Siexcess alloy is higher than that of the balance alloy. The number density of the β" precipitates in the specimen artificially aged after natural aging is much lower than that of the specimen artificially aged after 70°C pre-aging. Thus, it is concluded that pre-aging the Si-excess alloy somehow causes a higher density of nucleation sites for the β" precipitates during artificial aging, whereas the nucleation of the β" precipitates during artificial aging is significantly suppressed after natural aging. Figures 9 (a) and (b) show 3DAP elemental maps of Mg and Si atoms obtained from the balance and Si-excess alloys which were artificially aged at 175°C for 10 h after 70°C pre-aging. In the Si-excess

Published in Acta mater. Vol. 47, No. 5, pp. 1537-1548, 1999
Si excess (a) (b) balance

naturally aged + 175°C for 10h

(c)

(d)

70°C for 16h + 175°C for 10h

25nm

Fig. 8. TEM bright field images and the [001] selected area diffraction patterns obtained from (a) Al0.65Mg-0.70Si (Si-excess) alloy and (b) Al-0.70Mg-0.33Si (balanced) alloy aged at 175°C for 10 h after natural pre-aging, (c) the Si-excess alloy and (d) the balanced alloy aged at 175°C for 10 h after 70°C preaging. clusters become slower than the as-quenched specimen, because the quenched-in vacancies are annealed out at the temperature for artificial aging, and the vacancy concentration that controls the subsequent precipitation kinetics is reduced to the equilibrium concentration at 175°C, thereby suppressing the precipitation kinetics of subsequent artificial aging. The size distribution of the GP zones that would be formed by pre-aging at 70°C is schematically shown in Fig. 9 (b). Some of the GP zones are larger than rc(T=175°C), thus only smaller GP zones are reverted at the temperature for artificial aging. Only the GP zones larger than rc(T=175°C) would remain after heating to 175°C, and they may evolve to β" or may serve as nuclei for the β" precipitates. Thus, whereas prior natural aging causes adverse effect on an artificial age-hardening 9 response, pre-aging at 70°C shows beneficial effect on the subsequent artificial age-hardening response. Direct observations of co-clusters by 3DAP indicate that co-clusters and GP zones are essentially the same differing only in size and solute density, which would influence the thermal stability of these products. 4.2 Effect of excess Si In the case of the Si-excess alloy, the spherical GP zones and the needle-shape β" precipitates contain ~ 20 at.% Mg and ~ 20 at.% Si. Thus, the atomic ratios of Mg and Si of co-clusters, GP zones and the β" precipitates in the Si-excess alloy are all close to 1:1. In contrast, the GP zones in the balanced alloy contains 13 at.% Mg and 7 at.% Si,

Published in Acta mater. Vol. 47, No. 5, pp. 1537-1548, 1999

(a)

β”

Mg GP zone [001]

Si [010] (b)

~100nm

β”

GP zone

Mg

Si

~100nm

Fig. 9. 3DAP elemental mappings of Mg and Si atoms obtained from (a) Al-0.65Mg-0.70Si (Si-excess) alloy and (b) Al-0.70Mg-0.33Si (balanced) alloy aged at 175°C for 10 h after 70°C pre-aging. and the β" precipitates contains approximately 16% Mg and 11% Si which is closer to the atomic ratio expected from the equilibrium β phase, Mg2Si. Considering the mass balance of the solutes, it is reasonable that the β" precipitates in the Si-excess alloy contains more Si than those in the balanced alloy, otherwise the excess Si has to form its own precipitates or clusters. After pre-aging or artificial aging, no precipitates containing only Si were found, thus GP zones are believed to be the only precipitate product after pre-aging at 70°C. Figures 10 (a) and (b) schematically illustrate atomic ratios of the clusters or the GP zones observed in the balance alloy and in the Si-excess alloy. In the case of the balance alloy, the atomic ratio of Mg to Si in the GP 10 zones and the β" precipitates is 2:1, which is the same as that of the equilibrium Mg2Si. On the other hand, in the Si-excess alloy, only one Mg atom is available for one Si atom, thus the atomic ratio of the GP zones and the β" becomes 1:1. This suggests that the densities of the co-clusters, GP zones and β" are all determined by the available number of Si atoms, rather than by that of Mg atoms. Thus, higher densities of GP zones and β" precipitates precipitate in the Si-excess alloy. However, since the stoichiometric composition of the equilibrium β phase is Mg 2Si, the atomic ratio of Mg to Si in more stable precipitates such as β′ should be closer to 2:1. This means that the excess amount of Si must eventually form Si precipitates.

~14nm

~14nm

Published in Acta mater. Vol. 47, No. 5, pp. 1537-1548, 1999

Actually, Suzuki et al. [17] and Matsuda et al. [18] reported the presence of Si precipitate in the overaged Si-excess alloy, while the equilibrium β phase was the only precipitation product that was observed in the overaged balanced alloy. However, such late over-aging reaction is not reverent to the precipitation hardening reaction which occurs during the paint-baking cycle. 5. CONCLUSIONS The solute clusters, the GP zones and the β" precipitates which precipitate in the Al-0.65Mg0.70Si (Si-excess) and the Al-0.7Mg-0.33Si (balance) alloys have been quantitatively characterized by 3DAP. Separate Mg- and Siclusters are present in the as-quenched condition, but Mg and Si atoms aggregate during natural aging to form Mg-Si co-clusters. By aging at 70°C, spherical GP zones are formed, the chemical nature of which is similar to the co-clusters. GP zones give contrast in TEM images but the co-clusters do not, because the former has a higher solute concentration than the later. The precipitation products after artificial aging for 30 min at 175°C are GP zones regardless of the prior aging conditions, but the number density of the GP zones is significantly affected by the pre-aging conditions. Natural aging suppresses the Fig. 11. Schematic atomic ratios of clusters or GP zones observed in (a) the Si-excess alloy and (b) the balanced alloy.
co-cluster

(a)
density

r c(T=175°C)

size

(b)
GP zone

precipitation kinetics of the GP zones in artificial aging, but pre-aging at 70°C increases the number density of the GP zones in the artificially aged alloy. This suggests that the GP zones formed in the preaging condition grow in the subsequent artificial aging process, but the co-clusters formed by natural aging are completely reverted. Thus, it can be concluded that the major contributor for agehardening in the paint-baking cycle is the GP zones. An increase in the number density of β" precipitates by artificial aging after 70°C pre-aging indicates that the spherical GP zones provide heterogeneous nucleation sites for the β" precipitate. On the other hand, a low number density of β" precipitates with natural aging suggests that co-clusters are reverted at the temperature for artificial aging, resulting in reduced precipitation kinetics. This study also revealed that the atomic ratio in the co-clusters, GP zones and β" are close to that of the alloy composition. This finding suggests that excess Si causes a higher number density of precipitates. Acknowledgement-––The authors acknowledge Dr. Saga, Nippon Steel Corporation for provision of the alloys used in the present study. This work was partly supported by the New Energy Development 11

density

r c(T=175°C)

size

Fig. 10. Schematic size distribution of co-clusters and GP zones formed after (a) natural aging and (b) 70°C pre-aging.

Published in Acta mater. Vol. 47, No. 5, pp. 1537-1548, 1999

Organization (NEDO) International Joint Research Grant. REFERENCES

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Edwards, G.A., Stiller, K., Dunlop, G.L., Couper, M.J., Acta Mater., 1998, 46, 3893. Murayama, M., Hono, K., Saga, M., Kikuchi, K., Mater. Sci. Eng. A, in press. Cerezo, A., Godfrey, T.J., Smith, G.D.W., Rev. Sci. Instrum., 1988, 59, 862. Blavette, D., Deconihout, B., Bostel, A., Sarrau, J.M., Bouet, M., Menand, A., Rev. Sci. Instrum., 1993, 64, 2911. Hetherington, M. G., Cerezo, A., Hyde, J., Smith, G. D. W. and worrall, G. M., J. de Phys., 1986, 47, C7-495. Lynch, J.P., Brown, L.M., Jacobs, M.H., Acta Metall., 1982, 301, 389. Thomas, G., J. Inst. Metalls, 1961/62, 90, 57. Lorimer, G.W., and Nicholson, R.B., The Mechanism of Phase Transformations in Crystalline Solids, 1969, The Institute of Metals, London, p. 36. Suzuki, H., Kanno, M., Shiraishi, Y., J. Japan Inst. Light Metals, 1978, 28, 233. Matsuda, K., Tada, S., Ikeno, S., J. Japan Inst. Metals, 1994, 58, 252.

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