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Influence of a low-frequency electromagnetic field on precipitation behavior of a high strength aluminum alloy
Materials Science and Engineering A 402 (2005) 1–4
Influence of a low-frequency electromagnetic field on precipitation behavior of a high strength aluminum alloy Xiaotao Liu ∗ , Jianzhong Cui, Engang Wang, Jicheng He The Key Laboratory of Electromagnetic Processing of Materials, Ministry of Education, Northeastern University, Shenyang 110004, China Received 16 August 2004; received in revised form 5 January 2005; accepted 11 January 2005
Abstract The effect of an electromagnetic field on microstructure and hardness of a novel super-strength aluminum alloy was studied. The electromagnetic field accelerates the diffusion process and so the active dissolution of excess phases, which results in high supersaturation of hardening alloy elements in the aluminum matrix upon quenching. As a result, the growth of grains was promoted due to the decreasing of the pinning effect. With application of electromagnetic field the distribution of the hardening alloy elements became homogenous and the grain boundary segregation decrease. The concentration of particles within the matrix is enhanced and a higher value of hardness was obtained after aging due to higher solute concentration within the matrix and more finer and spaced grain boundary precipitates are obtained. A wider precipitate free zone (PFZ) is obtained by the depletion of solutes adjacent to the grain boundary due to formation of grain boundary precipitates. © 2005 Elsevier B.V. All rights reserved. Keywords: Super high-strength aluminum alloy; Electromagnetic field; Matrix precipitates; Grain boundary precipitates; Precipitate free zone
1. Introduction The Al–Zn–Mg–Cu alloys are gaining a commercial importance in aerospace industries where high specific strength is required due to their high response to age-hardening, low density and high strength. The Al–Zn–Mg–Cu alloys are heat treatable alloys commonly used in the artificially aged conditions. The precipitation reactions accompanying heat treatment in this alloy system has been the subject of many investigations because they offer the highest potential for strengthening [1,2]. It was generally recognized that the precipitation takes place by the sequence described below: supersaturated solid solution (SSS) → Guinier–Preston (G.P.) zones → (MgZn2 ) precipitates → (MgZn2 ) precipitates, which results in strengthening of the alloy [3]. The supersaturated solid solution is obtained by a solution treatment followed by quenching to a lower temperature. The decomposition process is highly dependent on the excess vacancy concentration and is thereby sensitive to ∗
Corresponding author. Tel.: +86 024 83687734 E-mail address: [email protected] (X. Liu).
0921-5093/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2005.01.010
the quenching condition. The study of the precipitation in Al–Zn–Mg–Cu type aluminum alloys has focused on three principal microstructural features: (1) the precipitate free zone (PFZ) which forms adjacent to high angle grain boundaries; (2) matrix precipitate structure; and (3) grain boundary precipitate structure. Recently, a novel semi-continuous casting process known as low-frequency electromagnetic casting (LFEC) has been employed with high strength Al–Zn–Mg–Cu alloys to improve the quality of direct chilling (DC) ingots [4]. For example, the microstructures were greatly refined, the solution of alloying elements in grains was effectively promoted and both surface quality and macrosegregation were improved with the application of a low-frequency electromagnetic field during solidification [5,6]. In addition, the investigation of Ban showed that the liquidus and solidus temperatures, of 7075 aluminum alloy, were both increased with the action of a low-frequency electromagnetic field during solidification [7]. It was proposed that low-frequency electromagnetic field promotes the diffusion process accompanying solidification. The purpose of this study is to investigate the effect of solution heat treatment under a low-
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X. Liu et al. / Materials Science and Engineering A 402 (2005) 1–4
Table 1 Chemical composition of the alloy (wt.%) Zn Mg Cu Zr Fe Si Al
9.3 2.8 2.5 0.15 <0.15 <0.1 Balance
frequency electromagnetic field on the precipitation behavior of a novel Al–Zn–Mg–Cu alloy produced by LFEC process.
2. Experimental procedure The material employed in this study was an Al–Zn–Mg–Cu aluminum alloy ingot 100 mm in diameter produced by LFEC [4]. Table 1 gives the chemical composition of the alloy. After a two-step homogenization at 400 ◦ C for 12 h and at 460 ◦ C for 32 h, the ingots were hot extruded into bars 11 mm in diameter with a three hole flat-faced die. The specimens were solutionized at different temperatures for different times in a resistance furnace both with (B = 0.5 T, f = 10 Hz) and without a low-frequency electromagnetic field and quenched immediately in cold water. Temperature was controlled by a programming thermometer with the K-type thermal couple attached to the specimen, which allowed temperature to be controlled within ±1 ◦ C of the reported value. After quenching, the specimens were aged immediately at 120 ◦ C for 24 h to obtain peak aged condition (T6). The Vicker hardness of the specimen was measured immediately as a measure of strength of the matrix. Each data point was the average of six measurements. Metallographic samples were cut longitudinally from the extruded rods. After polishing, the samples were etched with Keller’s reagent and the microstructure was characterized using standard optical metallography. Grain size distribution was analyzed by IAS-4 graph apparatus. Fine microstructural features were revealed by transmission electron microscopy (TEM). Thin foils for TEM investigation were prepared from 3 mm diameter discs punched out from 0.5 mm thick slices, cut along the axis of extrusion, ground and thinned by electropolishing using a solution mixture of 30% nitric acid in methanol as the electrolyte, maintained at a temperature of −20 ◦ C and at a potential difference of 15 V. The foils were examined in a Philips EM400 and photographed under bright-field (BF).
3. Results and discussion Fig. 1(a and b) shows the microstructure of Al–Zn–Mg–Cu alloy solutionized at 450 ◦ C for 2 h and at 470 ◦ C for 1 h both with (B = 0.5 T) and without (B = 0) an electromagnetic field. In both cases, the grain structure of the alloy, after solution
Fig. 1. Microstructure of recrystallization at 450 ◦ C × 2 h + 470 ◦ C × 1 h: (a) B = 0 T (b) B = 0.5 T.
treatment and quenching, is fully recrystallized. Constituent particles are present in the form of Al7 Cu2 Fe and AlZnMgCu type compound. These constituent particles are aligned in the extrusion direction in conjunction with the recrystallized areas, which suggests that recrystallization has occurred mainly with the particle stimulated nucleation (PSN) mechanism [8]. A larger grain size in the presence of an electromagnetic field was observed when compared to treatment in the absence of the electromagnetic field. The distribution of grain size supports this conclusion (Fig. 2). With the application of an electromagnetic field during solution heat treatment the diffusion of alloying elements and so the dissolution of constituent particles were accelerated and growth of grains were promoted due to a decreasing in the pinning effect [9]. Figs. 3 and 4 show the relationship between hardness and solution treating temperature and time respectively with and without the application of an electromagnetic field. The results show the hardness of the alloy increase with the application of an electromagnetic field. This implies that the magnetic field promotes diffusion of alloying elements and facilitates the dissolution of excess phases. As a result, a higher value of hardness was obtained after aging treatment. Fig. 5 shows the results of transmission electron microscopy after annealing at 450 ◦ C for 2 h and at 470 ◦ C for 1 h without and with the application of an electromag-
X. Liu et al. / Materials Science and Engineering A 402 (2005) 1–4
3
Fig. 2. Distribution of grain size with and without an electromagnetic field.
Fig. 5. TEM of the Al–Zn–Mg–Cu alloy solutionized without and with the electromagnetic field: (a) B = 0 T (b) B = 0.5 T.
Fig. 3. Hardness of Al–Zn–Mg–Cu alloy against solution temperature.
Fig. 4. Hardness of Al–Zn–Mg–Cu alloy against solution time.
netic field. Without the application of an electromagnetic field the TEM micrographs reveal a very fine precipitation distributed homogeneously inside the grains with coarser, less spaced grain boundary precipitates and a narrow PFZ adjacent to grain boundaries, as can be seen in Fig. 5(a). With the application of an electromagnetic field, there is a corresponding increase in the rate of precipitates and so concentration of particles inside the grains appears to be higher in the microstructure. The grain boundary precipitates appear to be finer and more spaced and width of the precipitate free zone becomes larger compared to that without the electromagnetic field. It is clear that precipitation and development of the PFZ in the aging treatment is controlled by the diffusion of solute atoms [10]. The characteristics in structure found make it possible to hypothesize that the application of an electromagnetic field in process of isothermal solution annealing is associated with a active dissolution of excess phases both within the matrix and at grain boundaries and a supersaturated solution is formed upon quenching. The distribution of hardening alloy elements became homogenous and grain boundary segregation decreases. Therefore, during aging after solution treatment, the nucleation and growth of precipitates on aging were promoted as a result of the sufficiently high solute concentration in the matrix and the hardness level is increased compared to that without an elec-
4
X. Liu et al. / Materials Science and Engineering A 402 (2005) 1–4
tromagnetic field. Owing to the depletion of solute around the grain boundary region due to the formation and growth of grain boundary precipitates, a wider PFZ adjacent to grain boundary was obtained. In conformance with the transportation theory of plasma, mass transport in metals and alloys by chemical diffusion is an ambipolar diffusion process [11]. It takes into account the fact that the diffusion processes of ions and electrons occur at different speeds in a metal. A weak electric field couples the processes together to preserve charge neutrality. The electric field in turn affects the transport resulting in a deviation from purely diffusive behavior. During the solution treatment with the application of the electromagnetic field, the motion of free electrons includes the gyration under the action of Lorentz force and the drift by the guiding center in the electromagnetic field, as a result of which the diffusion of the transported electrons is accelerated and ambipolar diffusion process of the atoms in metal is promoted. The electromagnetic field applied in the process of isothermal solution annealing accelerates the diffusion process and so the dissolution of constituent particles. As a result, a more supersaturated solution of hardening elements is formed after cooling and the hardness, after quenching and aging, is higher.
Owing to the higher solute concentration within the matrix, the rate of the precipitates and concentration of the particles are enhanced, the number of the grain boundary precipitates are decreased, and the PFZ is widened by diffusion of the solute atom nearby the grain boundary to form the grain boundary precipitates.
Acknowledgements This work was financially supported by the National “863” Foundation of China under grant no. 2001AA332030, the National Key Basic Research Program (973) under grant no. G1999064905 and the National Natural Science Foundation of China under grant no. 50401006.
References [1] [2] [3] [4] [5]
4. Conclusion With the application of the electromagnetic field during solution heat treatment, the diffusion of alloying elements was increased and so growth of grains was promoted due to a decrease in the pinning effect. The electromagnetic field facilitates dissolution of excess phases and solution of the alloying elements. A supersaturated solution is formed after cooling and a higher value of hardness was obtained.
[6] [7] [8] [9] [10] [11]
T.S. Srivatsan, J. Mater. Sci. 27 (17) (1992) 4772. T. James, Staley, Adv. Mater. Process. (10) (1997) 17. J.K. Park, A.J. Ardell, Metall. Trans. A 14 (1983) 1957. J. Dong, J.Z. Cui, C.Y. Ban, Metall. Mater. Trans. A 35A (8) (2004) 2487. B.J. Zhang, J.Z. Cui, G.M. Lu, Mater. Lett. 57 (2003) 1707. B.J. Zhang, J.Z. Cui, G.M. Lu., Mater. Sci. Eng. A A355 (2003) 325. C.Y. Ban, Ph.D. Thesis, Northeastern University, Shenyang, 2002. G.G. Garret, J.F. Knott, Metall. Trans. A 9 (1978) 1187. X.T. Liu, J.Z. Cui, J. Zhang, X.M. Wu, Y.H. Guo, J. Mater. Sci. 39 (14) (2004) 4719. T.S. Srivatsan, S. Sriram, D. Veeraraghavan, V.K. Vasudevan, J. Mater. Sci. 32 (11) (1997) 2883. H.T. Lin, D.H. Rich, A. Larsson, J Appl. Phys. 79 (1996) 7014.
Influence of a low-frequency electromagnetic field on precipitation behavior of a high strength aluminum alloy Xiaotao Liu ∗ , Jianzhong Cui, Engang Wang, Jicheng He The Key Laboratory of Electromagnetic Processing of Materials, Ministry of Education, Northeastern University, Shenyang 110004, China Received 16 August 2004; received in revised form 5 January 2005; accepted 11 January 2005
Abstract The effect of an electromagnetic field on microstructure and hardness of a novel super-strength aluminum alloy was studied. The electromagnetic field accelerates the diffusion process and so the active dissolution of excess phases, which results in high supersaturation of hardening alloy elements in the aluminum matrix upon quenching. As a result, the growth of grains was promoted due to the decreasing of the pinning effect. With application of electromagnetic field the distribution of the hardening alloy elements became homogenous and the grain boundary segregation decrease. The concentration of particles within the matrix is enhanced and a higher value of hardness was obtained after aging due to higher solute concentration within the matrix and more finer and spaced grain boundary precipitates are obtained. A wider precipitate free zone (PFZ) is obtained by the depletion of solutes adjacent to the grain boundary due to formation of grain boundary precipitates. © 2005 Elsevier B.V. All rights reserved. Keywords: Super high-strength aluminum alloy; Electromagnetic field; Matrix precipitates; Grain boundary precipitates; Precipitate free zone
1. Introduction The Al–Zn–Mg–Cu alloys are gaining a commercial importance in aerospace industries where high specific strength is required due to their high response to age-hardening, low density and high strength. The Al–Zn–Mg–Cu alloys are heat treatable alloys commonly used in the artificially aged conditions. The precipitation reactions accompanying heat treatment in this alloy system has been the subject of many investigations because they offer the highest potential for strengthening [1,2]. It was generally recognized that the precipitation takes place by the sequence described below: supersaturated solid solution (SSS) → Guinier–Preston (G.P.) zones → (MgZn2 ) precipitates → (MgZn2 ) precipitates, which results in strengthening of the alloy [3]. The supersaturated solid solution is obtained by a solution treatment followed by quenching to a lower temperature. The decomposition process is highly dependent on the excess vacancy concentration and is thereby sensitive to ∗
Corresponding author. Tel.: +86 024 83687734 E-mail address: [email protected] (X. Liu).
0921-5093/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2005.01.010
the quenching condition. The study of the precipitation in Al–Zn–Mg–Cu type aluminum alloys has focused on three principal microstructural features: (1) the precipitate free zone (PFZ) which forms adjacent to high angle grain boundaries; (2) matrix precipitate structure; and (3) grain boundary precipitate structure. Recently, a novel semi-continuous casting process known as low-frequency electromagnetic casting (LFEC) has been employed with high strength Al–Zn–Mg–Cu alloys to improve the quality of direct chilling (DC) ingots [4]. For example, the microstructures were greatly refined, the solution of alloying elements in grains was effectively promoted and both surface quality and macrosegregation were improved with the application of a low-frequency electromagnetic field during solidification [5,6]. In addition, the investigation of Ban showed that the liquidus and solidus temperatures, of 7075 aluminum alloy, were both increased with the action of a low-frequency electromagnetic field during solidification [7]. It was proposed that low-frequency electromagnetic field promotes the diffusion process accompanying solidification. The purpose of this study is to investigate the effect of solution heat treatment under a low-
2
X. Liu et al. / Materials Science and Engineering A 402 (2005) 1–4
Table 1 Chemical composition of the alloy (wt.%) Zn Mg Cu Zr Fe Si Al
9.3 2.8 2.5 0.15 <0.15 <0.1 Balance
frequency electromagnetic field on the precipitation behavior of a novel Al–Zn–Mg–Cu alloy produced by LFEC process.
2. Experimental procedure The material employed in this study was an Al–Zn–Mg–Cu aluminum alloy ingot 100 mm in diameter produced by LFEC [4]. Table 1 gives the chemical composition of the alloy. After a two-step homogenization at 400 ◦ C for 12 h and at 460 ◦ C for 32 h, the ingots were hot extruded into bars 11 mm in diameter with a three hole flat-faced die. The specimens were solutionized at different temperatures for different times in a resistance furnace both with (B = 0.5 T, f = 10 Hz) and without a low-frequency electromagnetic field and quenched immediately in cold water. Temperature was controlled by a programming thermometer with the K-type thermal couple attached to the specimen, which allowed temperature to be controlled within ±1 ◦ C of the reported value. After quenching, the specimens were aged immediately at 120 ◦ C for 24 h to obtain peak aged condition (T6). The Vicker hardness of the specimen was measured immediately as a measure of strength of the matrix. Each data point was the average of six measurements. Metallographic samples were cut longitudinally from the extruded rods. After polishing, the samples were etched with Keller’s reagent and the microstructure was characterized using standard optical metallography. Grain size distribution was analyzed by IAS-4 graph apparatus. Fine microstructural features were revealed by transmission electron microscopy (TEM). Thin foils for TEM investigation were prepared from 3 mm diameter discs punched out from 0.5 mm thick slices, cut along the axis of extrusion, ground and thinned by electropolishing using a solution mixture of 30% nitric acid in methanol as the electrolyte, maintained at a temperature of −20 ◦ C and at a potential difference of 15 V. The foils were examined in a Philips EM400 and photographed under bright-field (BF).
3. Results and discussion Fig. 1(a and b) shows the microstructure of Al–Zn–Mg–Cu alloy solutionized at 450 ◦ C for 2 h and at 470 ◦ C for 1 h both with (B = 0.5 T) and without (B = 0) an electromagnetic field. In both cases, the grain structure of the alloy, after solution
Fig. 1. Microstructure of recrystallization at 450 ◦ C × 2 h + 470 ◦ C × 1 h: (a) B = 0 T (b) B = 0.5 T.
treatment and quenching, is fully recrystallized. Constituent particles are present in the form of Al7 Cu2 Fe and AlZnMgCu type compound. These constituent particles are aligned in the extrusion direction in conjunction with the recrystallized areas, which suggests that recrystallization has occurred mainly with the particle stimulated nucleation (PSN) mechanism [8]. A larger grain size in the presence of an electromagnetic field was observed when compared to treatment in the absence of the electromagnetic field. The distribution of grain size supports this conclusion (Fig. 2). With the application of an electromagnetic field during solution heat treatment the diffusion of alloying elements and so the dissolution of constituent particles were accelerated and growth of grains were promoted due to a decreasing in the pinning effect [9]. Figs. 3 and 4 show the relationship between hardness and solution treating temperature and time respectively with and without the application of an electromagnetic field. The results show the hardness of the alloy increase with the application of an electromagnetic field. This implies that the magnetic field promotes diffusion of alloying elements and facilitates the dissolution of excess phases. As a result, a higher value of hardness was obtained after aging treatment. Fig. 5 shows the results of transmission electron microscopy after annealing at 450 ◦ C for 2 h and at 470 ◦ C for 1 h without and with the application of an electromag-
X. Liu et al. / Materials Science and Engineering A 402 (2005) 1–4
3
Fig. 2. Distribution of grain size with and without an electromagnetic field.
Fig. 5. TEM of the Al–Zn–Mg–Cu alloy solutionized without and with the electromagnetic field: (a) B = 0 T (b) B = 0.5 T.
Fig. 3. Hardness of Al–Zn–Mg–Cu alloy against solution temperature.
Fig. 4. Hardness of Al–Zn–Mg–Cu alloy against solution time.
netic field. Without the application of an electromagnetic field the TEM micrographs reveal a very fine precipitation distributed homogeneously inside the grains with coarser, less spaced grain boundary precipitates and a narrow PFZ adjacent to grain boundaries, as can be seen in Fig. 5(a). With the application of an electromagnetic field, there is a corresponding increase in the rate of precipitates and so concentration of particles inside the grains appears to be higher in the microstructure. The grain boundary precipitates appear to be finer and more spaced and width of the precipitate free zone becomes larger compared to that without the electromagnetic field. It is clear that precipitation and development of the PFZ in the aging treatment is controlled by the diffusion of solute atoms [10]. The characteristics in structure found make it possible to hypothesize that the application of an electromagnetic field in process of isothermal solution annealing is associated with a active dissolution of excess phases both within the matrix and at grain boundaries and a supersaturated solution is formed upon quenching. The distribution of hardening alloy elements became homogenous and grain boundary segregation decreases. Therefore, during aging after solution treatment, the nucleation and growth of precipitates on aging were promoted as a result of the sufficiently high solute concentration in the matrix and the hardness level is increased compared to that without an elec-
4
X. Liu et al. / Materials Science and Engineering A 402 (2005) 1–4
tromagnetic field. Owing to the depletion of solute around the grain boundary region due to the formation and growth of grain boundary precipitates, a wider PFZ adjacent to grain boundary was obtained. In conformance with the transportation theory of plasma, mass transport in metals and alloys by chemical diffusion is an ambipolar diffusion process [11]. It takes into account the fact that the diffusion processes of ions and electrons occur at different speeds in a metal. A weak electric field couples the processes together to preserve charge neutrality. The electric field in turn affects the transport resulting in a deviation from purely diffusive behavior. During the solution treatment with the application of the electromagnetic field, the motion of free electrons includes the gyration under the action of Lorentz force and the drift by the guiding center in the electromagnetic field, as a result of which the diffusion of the transported electrons is accelerated and ambipolar diffusion process of the atoms in metal is promoted. The electromagnetic field applied in the process of isothermal solution annealing accelerates the diffusion process and so the dissolution of constituent particles. As a result, a more supersaturated solution of hardening elements is formed after cooling and the hardness, after quenching and aging, is higher.
Owing to the higher solute concentration within the matrix, the rate of the precipitates and concentration of the particles are enhanced, the number of the grain boundary precipitates are decreased, and the PFZ is widened by diffusion of the solute atom nearby the grain boundary to form the grain boundary precipitates.
Acknowledgements This work was financially supported by the National “863” Foundation of China under grant no. 2001AA332030, the National Key Basic Research Program (973) under grant no. G1999064905 and the National Natural Science Foundation of China under grant no. 50401006.
References [1] [2] [3] [4] [5]
4. Conclusion With the application of the electromagnetic field during solution heat treatment, the diffusion of alloying elements was increased and so growth of grains was promoted due to a decrease in the pinning effect. The electromagnetic field facilitates dissolution of excess phases and solution of the alloying elements. A supersaturated solution is formed after cooling and a higher value of hardness was obtained.
[6] [7] [8] [9] [10] [11]
T.S. Srivatsan, J. Mater. Sci. 27 (17) (1992) 4772. T. James, Staley, Adv. Mater. Process. (10) (1997) 17. J.K. Park, A.J. Ardell, Metall. Trans. A 14 (1983) 1957. J. Dong, J.Z. Cui, C.Y. Ban, Metall. Mater. Trans. A 35A (8) (2004) 2487. B.J. Zhang, J.Z. Cui, G.M. Lu, Mater. Lett. 57 (2003) 1707. B.J. Zhang, J.Z. Cui, G.M. Lu., Mater. Sci. Eng. A A355 (2003) 325. C.Y. Ban, Ph.D. Thesis, Northeastern University, Shenyang, 2002. G.G. Garret, J.F. Knott, Metall. Trans. A 9 (1978) 1187. X.T. Liu, J.Z. Cui, J. Zhang, X.M. Wu, Y.H. Guo, J. Mater. Sci. 39 (14) (2004) 4719. T.S. Srivatsan, S. Sriram, D. Veeraraghavan, V.K. Vasudevan, J. Mater. Sci. 32 (11) (1997) 2883. H.T. Lin, D.H. Rich, A. Larsson, J Appl. Phys. 79 (1996) 7014.