The dependence of the mechanism of age hardening in Al-Mg-Si alloys on their Si content, the pre-aging conditions and addition of Cu has been investigated.
THE EFFECT OF AGING ON THE CLUSTERING AND PRECIPITATION PROCESS IN Al-Mg-Si ALLOYS M. Murayama and K. Hono National Research Institute for Metals, Tsukuba 305-0047, Japan Key Words: aluminum alloys, precipitation, solute clustering,, two-step aging, natural aging Abstract The dependence of the mechanism of age hardening in Al-Mg-Si alloys on their Si content, the pre-aging conditions and addition of Cu has been investigated. For this purpose, 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). Atom probe analysis results have revealed that the Mg:Si atomic ratio in the co-clusters, GP zones and β″ is close to that of the alloy composition. This finding suggests that excess Si causes a higher density of clusters and/or precipitates and leads to pronounced age hardening response. On the other hand, the density of GP zones and β″ after artificial aging at 175°C depends on pre-aging conditions, for example, pre-aging at 70°C increase the density of GP zones and β″, whereas natural aging reduces it. Based on these results, the characteristic two-step age-hardening response and the precipitation process of Al-Mg-Si(-Cu) alloys are discussed. Introduction Numerous investigations have been carried out on the precipitation process [1-3], the mechanism of two-step aging [4] and the effect of alloy composition on the age-hardening response [5,6] of Al-Mg-Si(-Cu) alloys. Since Al-Mg-Si(-Cu) alloys are considered to be the most promising candidates for heat-treatable bodysheet materials, study of the precipitation process in these alloys has been received considerable attention by the automobile industry. In the automobile manufacturing process, substantial age-hardening must occur during artificial aging in less than 30 min at about 170°C, because the alloys for body sheet applications must be age-hardened during the paint-baking process. It was reported that the Al-Mg-Si alloys containing an excess amount of Si than the Al-Mg2Si quasibinary composition exhibit more rapid age hardening response [5]. On the other hand, the hardening response is significantly suppressed when the alloy receives a room temperature aging for a prolonged period of time. Since natural aging can not be avoided in the automobile manufacturing process, understanding of the mechanism of this adverse age hardening effect is strongly desired. The first direct observation of the formation of solute clusters was reported by Edwards et al. [7-9] Using the atom probe field ion microscopy (APFIM), the formation of not only the separate Si- clusters but also separate Mg- clusters and Mg-Si co-clusters during 70°C aging was reported. However, the solute clustering during natural aging was not investigated in their study. Furthermore, the influence of the alloy composition is not clearly understood yet. Characterizing features of possible solute clusters during natural aging is important in order to understand 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. There are several reports on the effect of quaternary element additions on age-hardening of Al-Mg-Si alloys [1,10-12]. Cu addition, in general increases the kinetics of precipitation during artificial aging. It also gives beneficial effect in reducing the deterioration of the age-hardening response arising from natural aging of Al-Mg-Si alloy. Recently, Laughlin et al. [12] reported that the Cu level has a large effect on the hardening kinetics especially in the underaged regime and a smaller but noticeable effect on the maximum hardness. Since the typical paint-bake cycle in the automobile manufacturing process involves 30 min heating at around 175°C, the body sheet aluminum alloys must be used in an underaged condition. Therefore, it is important to investigate the effect of Cu in the early stage of artificial aging. The present study aimed at understanding the dependence of the mechanism of age hardening of Al-Mg-Si alloys is on their Si content and the pre-aging conditions. The role of Cu addition to an Al-Mg-Si alloy in enhancing the age hardening response was also investgated. For
this purpose, the chemical compositions of the precipitation products in Al-Mg-Si(-Cu) alloys after artificial aging at 175°C has been identified using a three dimensional atom probe (3DAP) and transmission electron microscopy (TEM). 3DAP is capable of mapping individual atoms in the real space with a near-atomic resolution [13,14], thus it can determine chemical compositions of nanoscale precipitates embedded in a matrix phase without any convolution effect. Experimental Chemical compositions of the alloys used in this study are Al-0.70Mg-0.33Si (balance), Al-0.65Mg-0.70Si (Si-excess) and Al-0.61Mg-1.22Si-0.39Cu (Cu bearing) (all in at. %). These alloys were solution treated at 550°C or 525°C for 30 min and subsequently water quenched. The solution treated specimens were subjected to various heat treatments including natural aging for 70 days, pre-aging for 16 h at 70°C, artificial aging for 10 h at 175°C and artificial aging after the pre-aging (two-step aging). For atom probe analyses, an energy compensated time-of-flight one dimensional atom probe (1DAP) and a three dimensional atom probe (3DAP) equipped with CAMECA's tomographic atom probe (TAP) detection system [14] were used. Atom probe analyses were carried out at about 30 K with a pulse fraction (Vp/Vdc) of 20% in UHV (~1x10-10 Torr). Microstructures of the specimens were examined with a transmission electron microscope (TEM), Philips CM200, operated at 200kV. High resolution electron microscopy observations were carried out using JEOL JEM-2000EX, operated at 200 kV. Results and Discussion An HREM image of a naturally aged Si-excess alloy shows a uniform fringe contrast as shown in Fig. 1 (a). No contrast which can be attributed to the precipitates is observed. On the other hand, the contrast arising from the precipitates is observed in the Si-excess alloy that was pre-aged at 70°C for 16 h (Fig. 1 (b)). 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 2 (a) 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 (a) 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 2nm separate clusters of Mg and Si atoms (indicated by the (b) arrowheads). In addition, a cocluster of Mg and Si atoms is detected as indicated by the broken lines in Fig. 2 (a). 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 2 (b) shows 3DAP elemental maps of Mg and Si atoms obtained from the Siexcess alloy after 70°C pre-aging. Fig. 1. HREM images taken at the [001]Al zone axis of Al-0.65MgNote that the size of the dots 0.70Si (Si-excess) alloy after (a) natural aging and (b) 70°C pre-aging.
(a) Number of Detected Mg / Si Atoms
Si
160 140
co-cluster
Mg
120 100 80 60 40 20 2
4
6
8
10
14
12
Total Number of Detected Atoms /
3
x10
(b) GP zone
~14nm
Mg
Si
~120nm
Fig. 2 (a) 1DAP integrated concentration depth profiles obtained from Al-0.65Mg-0.70Si (Si-excess) alloy after natural pre-aging for 70 days and (b) 3DAP elemental mappings of Mg and Si atoms after 70°C preaging for 16 h.
(a)
β”
Mg GP zone
~14nm
[001]
~100nm
Si [010]
(b) β”
GP zone
Mg
~14nm
does not have any physical meaning; it is arbitrarily adjusted only for obtaining a better visualization effect. 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(b). From these results, it is concluded that the chemical nature of the GP zones and the coclusters are similar. However, GP zones are larger than the clusters and the concentration of the solutes is higher. 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 coclusters formed by natural aging. The microstructures after artificial aging have shown that a high 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 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. Figures 3 (a) and (b) show 3DAP elemental maps of Mg and Si atoms obtained from the Si-excess and balance alloys which were artificially aged at 175°C for 10 h after 70°C pre-aging. In the Si-excess alloy, both needle-shaped precipitates and 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 needle-shaped precipitates and the spherical GP zones in the balance alloy are both approximately 17 % Mg and 10 % Si, and their atomic ratio is close to that of Mg2Si, the equilibrium β phase. From these results, it is concluded that the atomic ratio of Mg to Si atoms in the metastable precipitation products are strongly affected by the solute concentration of supersaturated solid solution. Detailed microstructural observations have shown that the density of precipitates in Siexcess alloy are always higher than that in balance one. Therefore, it can be concluded
Si
~100nm
Fig. 3. 3DAP elemental maps of Mg and Si atoms obtained from the (a) Si-excess and (b) balance alloys which were artificially aged at 175°C for 10 h after 70°C pre-aging.
Concentration / at. %
~9nm
that the densities of the co-clusters, GP zones and β" are all determined by the available number of β” Si atoms, rather than by that of Mg atoms. Thus, (a) higher densities of GP zones and β" precipitates Mg precipitate in the Si-excess alloy. Figure 4 (a) shows 3DAP elemental maps of Mg, Si and Cu atoms and (b) the concentration depth profiles across the needle-shape precipitate GP zone and GP zone obtained from the specimen aged Si for 10 h at 175°C after 70°C pre-aging. The 3DAP elemental maps of Mg and Si show the presence of both needle-shape β″ precipitates and GP zones. The Cu elemental map shows that Cu there is no enrichment of Cu in the GP zone, while Cu is partitioned in the needle-shape precipitates. The chemical composition of the needle-shape precipitates has been found to be ~100nm approximately 14 at.% Mg, 14 at.% Si and 4 at.% ( b) GP zone β” Cu. On the other hand, the spherical GP zones are 16 Mg Mg 10 composed of Mg and Si atoms, and their 12 8 6 8 composition is approximately 7.5 at.% Mg and 4 4 8.5 at.% Si. Thus, we can conclude that the Cu 2 0 0 partitioning occurs when the GP zones are 16 10 evolved to the β″ precipitates. It should be noted Si Si 8 12 6 8 that the atomic ratios of Mg:Si in the GP zones 4 4 2 and β″ precipitates are approximately 1:1. The 0 0 4 microstructures at the peak hardness condition 4 Cu Cu 2 2 show that the size and distribution of the β″ 0 0 2 8 10 0 0 2 4 6 1 precipitates are not affected by the pre-aging treatments. This is in contrast to the case of De pth / ~nm ternary alloy, where naturally aged specimen shows lower hardness at the peak hardness Fig. 4 (a) 3DAP elemental maps of Mg, Si and Cu condition. Comparison with the Cu-free alloy atoms and (b) concentration depth profiles across the with same Mg:Si ratio has shown that Cu is needle-shape precipitate and GP zone obtained from specimen aged for 10 h at 175°C after 70°C preeffective in reducing the deterioration due to the aging. natural aging in the peak hardness condition, but its effectiveness is questionable in the early stage. In agreement with the hardness measurement results, the size and density of β″ phase in the pre-aged specimen are almost the same as that in the directly aged specimen. Therefore, it is concluded the additions of Cu enhances the formation of β″ phase during artificial aging at 175°C. References 1) D.W. Pashley, J.W. Rhodes and A. Sendorek: J. Inst. Metals, 94(1966), 41. 2) G. Thomas: J. Inst. Metalls, 90(1961/62), 57. 3) I. Dutta and S.M. Allen: J. Mater. Sci. Lett., 10(1991), 323. 4) D.W. Pashley, M.H. Jacobs and J.T. Vietz: Phil. Mag., 16(1967), 51. 5) S. Ceresara, E. Dirusso, P. Fiorini and A. Giarda: Mater. Sci. Eng., 5(1969/70), 220. 6) T. Moons, P. Ratchev, P. DeSmet, B. Verlinden and P. Van Houtte: Scripta Mater., 35(1996), 939. 7) G.A. Edwards, K. Stiller, G.L. Dunlop: Appl. Surf. Sci., 76/77(1994), 219. 8) G.A. Edwards, K. Stiller, G.L. Dunlop, M.J. Couper: Mater. Sci. Forum, 217-222(1996), 713. 9) G.A. Edwards, K. Stiller, G.L. Dunlop, M.J. Couper: Acta Mater., 46(1998), 3893. 10) D.L.W. Collins: J. Inst. Metals., 86(1957/58), 325. 11) D.K. Chatterjee, K.M. Entwistle: J. Inst. Metals, 101(1973), 53. 12) D.E. Laughlin, W.F. Miao, L.M. Karabin and D.J. Chakrabarti: Automotive Alloys II, (S.K.
13) 14)
Das ed.) (TMS, 1998) A. Cerezo, T.J. Godfrey, G.D.W. Smith: Rev. Sci. Instrum., 59(1988), 862. D. Blavette, B. Deconihout, A. Bostel, J.M. Sarrau, M. Bouet, A. Menand: Rev. Sci. Instrum., 64(1993), 2911.
