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Effects of alternating magnetic field aged on microstructure and mechanical properties of AA2219 aluminum alloy
Journal of Alloys and Compounds 647 (2015) 644e647
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Effects of alternating magnetic field aged on microstructure and mechanical properties of AA2219 aluminum alloy Y.Z. Liu a, b, *, L.H. Zhan a, b, Q.Q. Ma c, Z.Y. Ma a, b, M.H. Huang a, b a
State Key Laboratory of High Performance Complex Manufacturing, Central South University, Changsha 410083, China School of Mechanical and Electrical Engineering, Central South University, Changsha 410083, China c Light Alloy Research Institute, Central South University, Changsha 410083, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 May 2015 Received in revised form 27 May 2015 Accepted 28 May 2015 Available online 12 June 2015
In order to improve the mechanical properties, magnetic field heat treatment was developed, which is a process combined temperature field and magnetic field together. By alternating magnetic field aging test, the effect of the magnetic field aging on mechanical properties of AA2219 aluminum alloy was studied by TEM analysis, Vickers hardness measurement, and mechanical tensile test. It is shown that alternating magnetic field can accelerate the aging precipitation process, and significantly improve the hardness and strength; The TEM images show that there is more precipitated phase and is bigger dispersion degree in the samples than those with the conventional. The microstructure and mechanical properties of the aluminum alloy were improved significantly by alternating magnetic field aging treatment. © 2015 Elsevier B.V. All rights reserved.
Keywords: Alternating magnetic field Aging heat treatment Mechanical properties Microstructure AA2219 aluminum alloy
1. Introduction AleCu alloy is a typical kind of the alloys which can be hardened by aging treatment. It is widely recognized that the precipitation of the alloys is as follows: supersaturated solid solution /G.P. (Ⅰ) zone /G.P. (Ⅱ) zone (q00 phase) /q0 phase /q phase. The early researches on the influence factors of the precipitation of the aging process of the alloys mainly concentrated on the addition of the alloying elements [1,2]. Different alloying elements play a promoting or restraining role in different degrees because of the difference in their influence mechanism. In recent years, the application of the constant magnetic field on the metallic phase transformation has drawn the attention of more and more researchers. There have been studies showing that strong magnetic field poses an influence on the temperature and the pressure of the diffusive [3e5], and the non-diffusive transformation [6,7]. Most present researches on the diffusive transformation focus on the iron-base alloys, while researches on the nonferrous alloys target on nickel-base high temperature alloys, Ale4%Cu alloys and TieNi alloys, etc. For AleCu alloys and nickel-
* Corresponding author. School of Mechanical and Electrical Engineering, Central South University, Changsha 410083, China. E-mail address: [email protected] (Y.Z. Liu). http://dx.doi.org/10.1016/j.jallcom.2015.05.183 0925-8388/© 2015 Elsevier B.V. All rights reserved.
base high temperature alloys, the magnetic field promotes the precipitation [8e10]. For TieNi alloys, strong magnetic field decreases the aging transformation coefficient [7]. The direction and size of constant strong magnetic field keep no change; for the aging precipitation behavior of AleCu alloy, what happen if it is applied alternating magnetic field in the aging stage? There haven't been any researches on the influence of the alternating magnetic field on the precipitation of AleCu alloys during aging treatment. AA2219 aluminum alloy as the research object, this article reveals the influence of the alternating magnetic field on the microstructure of the precipitation and the mechanical properties of the alloy. AA2219 aluminum alloy is of the AleCueMn series welding aluminum alloy, with good comprehensive properties such as high strength, high stress corrosion ability and good toughness. It is wildly used in the field of the aerospace, the military and the civilian equipment. At present, the researches to improve the performance of AA2219 aluminum alloy are only concentrated on conventional heat treatment. Therefore, with the alternating magnetic field generator unit developed independently by our group, the aging treatment with the magnetic field of AA2219 aluminum alloy was investigated. By observing the microstructure and testing the hardness, and the mechanical properties, etc., and comparing with the conventional aging treatment result, the
Y.Z. Liu et al. / Journal of Alloys and Compounds 647 (2015) 644e647
645
2. Materials and experimental methods
located on CSS e 44100 electronic universal testing machines for tensile test to measure its mechanical properties. The value of the tensile strength, the yield strength and the break elongation is the average of the three samples.
2.1. Materials
3. Test results and discussion
The material for this experiment is the AleCueMn AA2219 aluminum alloy which can be strengthened with heat treatment. The samples are from cold rolled sheet. The sample size is shown in Fig. 1. The main chemical composition is listed in Table 1.
3.1. Microstructure observation and analysis
effects of the alternating magnetic field aging on the microstructure and the properties of materials are revealed.
Samples of the conventional aging and magnetic field assisted aging were both observed by transmission electron microscopy, as shown in Fig. 3. From Fig. 3a, b, it can be seen that there are suspected rod T phase dispersion points in the alloy substrate with both conventional aging and magnetic field aging, as pointed to with the arrow A. Suspected T phase is the residual phase from the solid solution treatment [11] [12], and is thermally stable. In addition, it can be seen in Fig. 3a that there is precipitation in the conventional aging samples, whose morphology is acicular, as pointed to with the arrow B. However, with the same magnification times, there is no precipitation in Fig. 3b. Enlarge magnification times, 5 times for the conventional aging (as shown in Fig. 3c), 10 for the magnetic aging (as shown in Fig. 3d). It can be seen in Fig. 3c that perpendicular acicular precipitation phase is not evenly distributed; the statistical calculation of the average size of the precipitation phase is about 135 nm. It can be seen in Fig. 3d that perpendicular acicular precipitation phase is fine and evenly distributed; the average size of the precipitation phase with magnetic field aging is about 50 nm. Fig. 3c and d show the typical microstructure characteristics of the semi coherent phase of qʹ. These rich copper qʹ precipitates along the aluminum substrate (100), (010) surface in the perpendicular needle form [13], [14]. The main strengthening phase of AA2219 aluminum alloy is the transition phase (qʹ, qʹʹ) and equilibrium phase q [11]. It can be seen in Fig. 3 that the magnetic aging promoted the precipitation of the qʹ phase which makes the qʹ become finer and dispersed.
2.2. Experimental methods In order to express the effects of the aging with magnetic field on the microstructure and the properties of the material, a conventional aging test was set as a reference. With the same solution and aging parameters, the difference is that the alternating magnetic field is only applied in the aging stage of the magnetic aging. Specific steps are as follows. The solid solution treatment were carried out first, under 535 C for 35 min. The samples were quenched immediately after taking out from the furnace. Then they were preserved under 175 C, with the alternating magnetic field (magnetic aging treatment), the frequency of 50 Hz and the intensity of 0.5 T (root mean square), respectively. The schematic diagram is shown in Fig. 2. 2.2.1. Microstructure observation Precipitation phase morphology of the aging sample of AA2219 aluminum alloy was observed with a TecnaiG220 tem. Sample is treated with the following treatments in sequence: coarse grinding, fine grinding, mechanical thinning to 80 microns, punching into F 3 mm wafer, the double spray processing with mixture of the 25% HNO3 þ 75% CHOH (volume fraction). The double spray instrument adopts MIT2II electrolytic device, liquid nitrogen cooling at the temperature of 35 ~ 25 C, voltage about 15 V. The sample was cleaned with alcohol for 2 ~ 3 min after perforation. The samples for TEM observation were thus made. The microstructure of aluminum alloy was observed under the transmission electron microscope.
3.2. Hardness test The hardness of the samples with the conventional aging and the magnetic aging are shown in Fig. 4. It can be seen from Fig. 4 that the trend of the change in the hardness of the alloy is similar: no matter whether the sample was treated with the magnetic field or not, the micro hardness of
2.2.2. Performance test HXD e 1000 tm/LCD digital optical Vickers hardness tester was adopted in this experiment. The specimen was grinded with metallographic sandpaper. Five points were selected randomly on the surface of a sample, the average was considered as the hardness of the sample. For the mechanical properties test, the samples were
Fig. 2. Test scenarios schematic 1. Test specimen, 2. Magnetic field generator, 3. Aging furnace, 4. Voltage regulator, 5. AC power.
Fig. 1. The size of standard tensile test specimen.
Table 1 The chemical composition of aluminum alloy AA2219 (mass fraction, %). Cu
Si
Fe
Mn
Mg
V
Zr
Zn
Ti
Al
5.6 ~ 6.8
0.2
0.3
0.2 ~ 0.4
0.02
0.05 ~ 0.15
0.1 ~ 0.25
0.1
0.02 ~ 0.1
Bal
646
Y.Z. Liu et al. / Journal of Alloys and Compounds 647 (2015) 644e647
Fig. 3. TEM microstructure photos of aging treatment at 175 C, 6 h (a), (c) The conventional aging treatment (b), (d) The magnetic field aging treatment.
samples increases with time first and then decreases. At the beginning of the aging treatment, the hardness of the sample with the magnetic field increases slightly faster; after a period of time, the difference of the hardness between the two samples narrows down; however, the difference of the hardness becomes even bigger as the time goes on. The hardness of the sample with the conventional aging reaches its peak at about 18 h, while that with the magnetic aging at about 8 h. The magnetic field promotes the diffusion and the aging process of the alloy; with the increase of the aging time, the magnetic field promotes the transformation from G.P. (I) to qʹʹ phase and from qʹʹ phase to qʹ phase, resulting in the improvement of the hardness. The drop of the micro hardness after 12 h of the magnetic aging may be a result of the transformation from qʹ phase to Al2Cu phase. Because of the loss of the coherent
relationship between q phase and the substrate, coherent distortion enhancement reduces and the hardness decreases. 3.3. Mechanical performance test The tensile test of the samples is performed in the universal mechanical testing machine. The mechanical properties are shown in Table 2 and Table 3. It can be seen from Table 2 that, with the increase of the aging time, the yield strength and the tensile strength of the sample increases first and then decreases and the inflection point occurs at about 12 h, and the elongation of the specimen decreases all the time; It can be seen from Table 3 that the inflection point of the strength of the conventional aging occurs at about 18 h, the elongation decreases all the time as well. Comparing Table 2 and Table 3, it can be concluded that the magnetic field promotes the aging process of the alloy and brings forward the peak of the intensity and the trend of the change in the intensity is similar to that in the hardness; in addition, within the same aging time, the elongation of the specimen with the magnetic aging is slightly lower than that with the conventional aging. 4. Analysis and discussion The influence of the magnetic aging on the nucleation, the growth and the distribution of the precipitated phase qʹ, and the material mechanical properties. Table 2 Magnetic aging effects on tensile mechanical properties.
Fig. 4. Effects of conventional aging and magnetic aging on materials micro hardness.
Aging time/h
sb/MPa
ss/MPa
d/%
1 3 6 12 18
353.1 387.7 415.3 436.2 422.3
211.3 251.7 264.3 283.5 280.6
24.3 20.9 18.5 17.3 16.2
Y.Z. Liu et al. / Journal of Alloys and Compounds 647 (2015) 644e647 Table 3 Conventional aging effects on tensile mechanical properties. Aging time/h
sb/MPa
ss/MPa
d/%
1 3 6 12 18 24
347.8 368.0 408.2 418.3 434.9 415.1
210.1 246.7 261.5 275.1 297.5 267.3
27.4 21.3 20.5 20.1 18.0 16.7
The aging process of aluminum alloy is the exsolution of the second phase particles from supersaturated solid solution precipitation, which contains the nucleation and the growth of precipitation particles. The nucleation rate of the homogeneous nucleation is expressed as [15]. :
N ¼ Nv expðDGk =KT ÞexpðDGA =KT Þ
647
under the alternating magnetic field, which enlarge the distance between the fields of stress. When the fields are distanced enough to make the dislocation line bypass the field, the dislocations become “flexible”. When the flexible dislocations flip, every single part can go through the field of stress without the help of other parts. The resistance becomes bigger resulting in the increase in the hardness. In AA2219 aluminum alloy, the formation of G.P. (Ⅰ) zone, G.P. (Ⅱ) zone (q00 phase) and q0 phase is the main cause of the increase in the alloy's hardness. Combined with the TEM observation, it can be concluded that with the same aging treatment, the magnetic field increases the alloy's hardness and enhances the age hardening of the alloy. Meanwhile, after the magnetic aging treatment, qʹ (Al2Cu) phase is finer and more dispersed with lager volume fraction, which improves the strength of the alloy. 5. Conclusions
(1)
where N is the number of atoms in per unit volume of the parent phase, V is the atomic transition frequency, DGk is the nucleation work, DGA is the diffusion activation energy, K is the Boltzmann constant, T is the thermodynamic temperature. Under the constant magnetic field, the activation energy for the diffusion of Cu atoms increases [13], which accelerated the Cu atom's possession of the defects; with the defects disappear, the energy is transferred to the nucleation of the new phase, resulting in lowering the nucleation energy of the new phase. Considering these two factors together, the equation (1) shows that the magnetic field aging improves the nucleation rate of the aluminum alloy. As the concentration of solute atoms in the sample is certain, the quantity oh the precipitation is certain. The increase of the nucleation rate makes the nucleation area wider and more diffuse. On the other hand, because of the vacancy consumption, the concentration of vacancy drops too fast, which thus inhibits the growing up of the second phase. Similarly, observing the precipitated phase qʹ (Al2Cu) in AA2219 aluminum alloy with TEM, it is found that the size of precipitated phase in the alternating magnetic field aging is smaller than that of the conventional aging, whose possible reason is that the alternating magnetic field aging accelerates the vacancy movement. Cu atoms in AleCu alloy is combined with the vacancy after the solid solution, which speeds up the diffusion of the Cu atoms carrying vacancy and the nucleation of the precipitated phase; meanwhile, the magnetic field aging rapidly reduced the vacancy concentration in the aluminum alloy substrate. The driving force of the growing up of the precipitate phase decreases, which restrains the growing up of the precipitated phase and makes precipitated phase small and dispersed. Compared with that in the strong magnetic field aging sample, the size of the qʹ (Al2Cu) in the alternating magnetic field aging sample is significantly smaller; it may be that the alternating magnetic field strength is weak. In AleCu alloys, the radius of aluminum and copper atoms is quite different. In the beginning period of the aging treatment, the field of stress caused by the misfit degree between the solute atoms and the parent phase is highly diffusive. The dislocation density is too small to create a field of stress which is bigger enough to make the dislocation line bypass the field. The dislocation line remains straight with low resistance. The alloy is soft relatively. With further aging treatment, solute atoms gather together quickly
1)The aging treatment assisted with the alternating magnetic field promotes the aging precipitation of AA2219 aluminum alloy, shortened peak hardiness time. 2)The magnetic field aging specimens observed with the transmission electron microscopy, it is found that the precipitated phase is smaller and more dispersed; the microstructure of AA2219 aluminum alloy is improved. 3)The basic composition of the alloy after aging treatment is a þqʹ (Al2Cu)þ suspected T, among which Al2Cu is the major aging strengthening phase, which is greatly influenced by the magnetic field, while suspected T phase has higher thermal stability. Acknowledgments The authors would like to thank the Key Program of National Natural Science Foundation of China (no. 51235010) and the National Key Basic Research Development Plan Funded Project of China (no. 2014cb046602) for their financial support. References [1] J.M. Silcock, H.M. Flower, Scr. Mater. 46 (2002) 389e394. [2] S.P. Ringer, K. Hono, Mater. Charact. 44 (2000) 101e131. [3] X. Zhao, M.L. Gong, Y.D. Zhang, L. Esling, L. Zuo, G. Vincent, Scr. Mater. 54 (2006) 1897e1900. [4] J.K. Choi, H. Ohtsuka, Y. Xu, W.Y. Choo, Scr. Mater. 43 (2000) 221e226. [5] Y.D. Zhang, N. Gey, C.S. He, X. Zhao, L. Zuo, C. Esling, Acta Mater. 52 (2004) 3467e3474. [6] X.P. Liu, Y.N. Wang, M. Qi, D.Z. Yang, Chin. J. Nonferrous Met. 16 (2006) 2005e2009. [7] Y.W. Ma, S. Awaji, K. Watanabe, M. Matsumoto, N. Kobayash, Solid State Commun. 113 (2000) 671e676. [8] W.C. Liu, S.H. Ji, G.Q. Chen, W.L. Zhou, C.H. Huang, J.T. Guo, Foundry 55 (2006) 338e340. [9] L.Y. Cui, X.N. Li, M. Qi, Chin. J. Nonferrous Met. 17 (2007) 1967e1972. [10] X. Ren, J.W. Zhang, W.L. Zhou, G.Q. Chen, J.S. Zhang, Heat. Treat. Met. 37 (2012) 20e23. [11] B.C. Zhang, Nonferrous Metals and Heat Treatment, Northwest Polytechnical Univ. Press, Xi'an, 2010. [12] W.Q. Gui, J.H. Chen, C.L. Wu, S.B. Wang, J. Chin. Electr. Microsc. Soc. 31 (2012) 302e307. [13] C.G. Cordovilia, E. Louis, J. Mater. Sci. 19 (1984) 279e290. [14] L.F. Mondolfo, Aluminum Alloys: Structure and Properties, Butter Worth and Co., London, 1976. [15] Z.Q. Zheng, Fundamentals of Materials Science, Cent. South Univ. Press, Changsha, 2005.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Effects of alternating magnetic field aged on microstructure and mechanical properties of AA2219 aluminum alloy Y.Z. Liu a, b, *, L.H. Zhan a, b, Q.Q. Ma c, Z.Y. Ma a, b, M.H. Huang a, b a
State Key Laboratory of High Performance Complex Manufacturing, Central South University, Changsha 410083, China School of Mechanical and Electrical Engineering, Central South University, Changsha 410083, China c Light Alloy Research Institute, Central South University, Changsha 410083, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 May 2015 Received in revised form 27 May 2015 Accepted 28 May 2015 Available online 12 June 2015
In order to improve the mechanical properties, magnetic field heat treatment was developed, which is a process combined temperature field and magnetic field together. By alternating magnetic field aging test, the effect of the magnetic field aging on mechanical properties of AA2219 aluminum alloy was studied by TEM analysis, Vickers hardness measurement, and mechanical tensile test. It is shown that alternating magnetic field can accelerate the aging precipitation process, and significantly improve the hardness and strength; The TEM images show that there is more precipitated phase and is bigger dispersion degree in the samples than those with the conventional. The microstructure and mechanical properties of the aluminum alloy were improved significantly by alternating magnetic field aging treatment. © 2015 Elsevier B.V. All rights reserved.
Keywords: Alternating magnetic field Aging heat treatment Mechanical properties Microstructure AA2219 aluminum alloy
1. Introduction AleCu alloy is a typical kind of the alloys which can be hardened by aging treatment. It is widely recognized that the precipitation of the alloys is as follows: supersaturated solid solution /G.P. (Ⅰ) zone /G.P. (Ⅱ) zone (q00 phase) /q0 phase /q phase. The early researches on the influence factors of the precipitation of the aging process of the alloys mainly concentrated on the addition of the alloying elements [1,2]. Different alloying elements play a promoting or restraining role in different degrees because of the difference in their influence mechanism. In recent years, the application of the constant magnetic field on the metallic phase transformation has drawn the attention of more and more researchers. There have been studies showing that strong magnetic field poses an influence on the temperature and the pressure of the diffusive [3e5], and the non-diffusive transformation [6,7]. Most present researches on the diffusive transformation focus on the iron-base alloys, while researches on the nonferrous alloys target on nickel-base high temperature alloys, Ale4%Cu alloys and TieNi alloys, etc. For AleCu alloys and nickel-
* Corresponding author. School of Mechanical and Electrical Engineering, Central South University, Changsha 410083, China. E-mail address: [email protected] (Y.Z. Liu). http://dx.doi.org/10.1016/j.jallcom.2015.05.183 0925-8388/© 2015 Elsevier B.V. All rights reserved.
base high temperature alloys, the magnetic field promotes the precipitation [8e10]. For TieNi alloys, strong magnetic field decreases the aging transformation coefficient [7]. The direction and size of constant strong magnetic field keep no change; for the aging precipitation behavior of AleCu alloy, what happen if it is applied alternating magnetic field in the aging stage? There haven't been any researches on the influence of the alternating magnetic field on the precipitation of AleCu alloys during aging treatment. AA2219 aluminum alloy as the research object, this article reveals the influence of the alternating magnetic field on the microstructure of the precipitation and the mechanical properties of the alloy. AA2219 aluminum alloy is of the AleCueMn series welding aluminum alloy, with good comprehensive properties such as high strength, high stress corrosion ability and good toughness. It is wildly used in the field of the aerospace, the military and the civilian equipment. At present, the researches to improve the performance of AA2219 aluminum alloy are only concentrated on conventional heat treatment. Therefore, with the alternating magnetic field generator unit developed independently by our group, the aging treatment with the magnetic field of AA2219 aluminum alloy was investigated. By observing the microstructure and testing the hardness, and the mechanical properties, etc., and comparing with the conventional aging treatment result, the
Y.Z. Liu et al. / Journal of Alloys and Compounds 647 (2015) 644e647
645
2. Materials and experimental methods
located on CSS e 44100 electronic universal testing machines for tensile test to measure its mechanical properties. The value of the tensile strength, the yield strength and the break elongation is the average of the three samples.
2.1. Materials
3. Test results and discussion
The material for this experiment is the AleCueMn AA2219 aluminum alloy which can be strengthened with heat treatment. The samples are from cold rolled sheet. The sample size is shown in Fig. 1. The main chemical composition is listed in Table 1.
3.1. Microstructure observation and analysis
effects of the alternating magnetic field aging on the microstructure and the properties of materials are revealed.
Samples of the conventional aging and magnetic field assisted aging were both observed by transmission electron microscopy, as shown in Fig. 3. From Fig. 3a, b, it can be seen that there are suspected rod T phase dispersion points in the alloy substrate with both conventional aging and magnetic field aging, as pointed to with the arrow A. Suspected T phase is the residual phase from the solid solution treatment [11] [12], and is thermally stable. In addition, it can be seen in Fig. 3a that there is precipitation in the conventional aging samples, whose morphology is acicular, as pointed to with the arrow B. However, with the same magnification times, there is no precipitation in Fig. 3b. Enlarge magnification times, 5 times for the conventional aging (as shown in Fig. 3c), 10 for the magnetic aging (as shown in Fig. 3d). It can be seen in Fig. 3c that perpendicular acicular precipitation phase is not evenly distributed; the statistical calculation of the average size of the precipitation phase is about 135 nm. It can be seen in Fig. 3d that perpendicular acicular precipitation phase is fine and evenly distributed; the average size of the precipitation phase with magnetic field aging is about 50 nm. Fig. 3c and d show the typical microstructure characteristics of the semi coherent phase of qʹ. These rich copper qʹ precipitates along the aluminum substrate (100), (010) surface in the perpendicular needle form [13], [14]. The main strengthening phase of AA2219 aluminum alloy is the transition phase (qʹ, qʹʹ) and equilibrium phase q [11]. It can be seen in Fig. 3 that the magnetic aging promoted the precipitation of the qʹ phase which makes the qʹ become finer and dispersed.
2.2. Experimental methods In order to express the effects of the aging with magnetic field on the microstructure and the properties of the material, a conventional aging test was set as a reference. With the same solution and aging parameters, the difference is that the alternating magnetic field is only applied in the aging stage of the magnetic aging. Specific steps are as follows. The solid solution treatment were carried out first, under 535 C for 35 min. The samples were quenched immediately after taking out from the furnace. Then they were preserved under 175 C, with the alternating magnetic field (magnetic aging treatment), the frequency of 50 Hz and the intensity of 0.5 T (root mean square), respectively. The schematic diagram is shown in Fig. 2. 2.2.1. Microstructure observation Precipitation phase morphology of the aging sample of AA2219 aluminum alloy was observed with a TecnaiG220 tem. Sample is treated with the following treatments in sequence: coarse grinding, fine grinding, mechanical thinning to 80 microns, punching into F 3 mm wafer, the double spray processing with mixture of the 25% HNO3 þ 75% CHOH (volume fraction). The double spray instrument adopts MIT2II electrolytic device, liquid nitrogen cooling at the temperature of 35 ~ 25 C, voltage about 15 V. The sample was cleaned with alcohol for 2 ~ 3 min after perforation. The samples for TEM observation were thus made. The microstructure of aluminum alloy was observed under the transmission electron microscope.
3.2. Hardness test The hardness of the samples with the conventional aging and the magnetic aging are shown in Fig. 4. It can be seen from Fig. 4 that the trend of the change in the hardness of the alloy is similar: no matter whether the sample was treated with the magnetic field or not, the micro hardness of
2.2.2. Performance test HXD e 1000 tm/LCD digital optical Vickers hardness tester was adopted in this experiment. The specimen was grinded with metallographic sandpaper. Five points were selected randomly on the surface of a sample, the average was considered as the hardness of the sample. For the mechanical properties test, the samples were
Fig. 2. Test scenarios schematic 1. Test specimen, 2. Magnetic field generator, 3. Aging furnace, 4. Voltage regulator, 5. AC power.
Fig. 1. The size of standard tensile test specimen.
Table 1 The chemical composition of aluminum alloy AA2219 (mass fraction, %). Cu
Si
Fe
Mn
Mg
V
Zr
Zn
Ti
Al
5.6 ~ 6.8
0.2
0.3
0.2 ~ 0.4
0.02
0.05 ~ 0.15
0.1 ~ 0.25
0.1
0.02 ~ 0.1
Bal
646
Y.Z. Liu et al. / Journal of Alloys and Compounds 647 (2015) 644e647
Fig. 3. TEM microstructure photos of aging treatment at 175 C, 6 h (a), (c) The conventional aging treatment (b), (d) The magnetic field aging treatment.
samples increases with time first and then decreases. At the beginning of the aging treatment, the hardness of the sample with the magnetic field increases slightly faster; after a period of time, the difference of the hardness between the two samples narrows down; however, the difference of the hardness becomes even bigger as the time goes on. The hardness of the sample with the conventional aging reaches its peak at about 18 h, while that with the magnetic aging at about 8 h. The magnetic field promotes the diffusion and the aging process of the alloy; with the increase of the aging time, the magnetic field promotes the transformation from G.P. (I) to qʹʹ phase and from qʹʹ phase to qʹ phase, resulting in the improvement of the hardness. The drop of the micro hardness after 12 h of the magnetic aging may be a result of the transformation from qʹ phase to Al2Cu phase. Because of the loss of the coherent
relationship between q phase and the substrate, coherent distortion enhancement reduces and the hardness decreases. 3.3. Mechanical performance test The tensile test of the samples is performed in the universal mechanical testing machine. The mechanical properties are shown in Table 2 and Table 3. It can be seen from Table 2 that, with the increase of the aging time, the yield strength and the tensile strength of the sample increases first and then decreases and the inflection point occurs at about 12 h, and the elongation of the specimen decreases all the time; It can be seen from Table 3 that the inflection point of the strength of the conventional aging occurs at about 18 h, the elongation decreases all the time as well. Comparing Table 2 and Table 3, it can be concluded that the magnetic field promotes the aging process of the alloy and brings forward the peak of the intensity and the trend of the change in the intensity is similar to that in the hardness; in addition, within the same aging time, the elongation of the specimen with the magnetic aging is slightly lower than that with the conventional aging. 4. Analysis and discussion The influence of the magnetic aging on the nucleation, the growth and the distribution of the precipitated phase qʹ, and the material mechanical properties. Table 2 Magnetic aging effects on tensile mechanical properties.
Fig. 4. Effects of conventional aging and magnetic aging on materials micro hardness.
Aging time/h
sb/MPa
ss/MPa
d/%
1 3 6 12 18
353.1 387.7 415.3 436.2 422.3
211.3 251.7 264.3 283.5 280.6
24.3 20.9 18.5 17.3 16.2
Y.Z. Liu et al. / Journal of Alloys and Compounds 647 (2015) 644e647 Table 3 Conventional aging effects on tensile mechanical properties. Aging time/h
sb/MPa
ss/MPa
d/%
1 3 6 12 18 24
347.8 368.0 408.2 418.3 434.9 415.1
210.1 246.7 261.5 275.1 297.5 267.3
27.4 21.3 20.5 20.1 18.0 16.7
The aging process of aluminum alloy is the exsolution of the second phase particles from supersaturated solid solution precipitation, which contains the nucleation and the growth of precipitation particles. The nucleation rate of the homogeneous nucleation is expressed as [15]. :
N ¼ Nv expðDGk =KT ÞexpðDGA =KT Þ
647
under the alternating magnetic field, which enlarge the distance between the fields of stress. When the fields are distanced enough to make the dislocation line bypass the field, the dislocations become “flexible”. When the flexible dislocations flip, every single part can go through the field of stress without the help of other parts. The resistance becomes bigger resulting in the increase in the hardness. In AA2219 aluminum alloy, the formation of G.P. (Ⅰ) zone, G.P. (Ⅱ) zone (q00 phase) and q0 phase is the main cause of the increase in the alloy's hardness. Combined with the TEM observation, it can be concluded that with the same aging treatment, the magnetic field increases the alloy's hardness and enhances the age hardening of the alloy. Meanwhile, after the magnetic aging treatment, qʹ (Al2Cu) phase is finer and more dispersed with lager volume fraction, which improves the strength of the alloy. 5. Conclusions
(1)
where N is the number of atoms in per unit volume of the parent phase, V is the atomic transition frequency, DGk is the nucleation work, DGA is the diffusion activation energy, K is the Boltzmann constant, T is the thermodynamic temperature. Under the constant magnetic field, the activation energy for the diffusion of Cu atoms increases [13], which accelerated the Cu atom's possession of the defects; with the defects disappear, the energy is transferred to the nucleation of the new phase, resulting in lowering the nucleation energy of the new phase. Considering these two factors together, the equation (1) shows that the magnetic field aging improves the nucleation rate of the aluminum alloy. As the concentration of solute atoms in the sample is certain, the quantity oh the precipitation is certain. The increase of the nucleation rate makes the nucleation area wider and more diffuse. On the other hand, because of the vacancy consumption, the concentration of vacancy drops too fast, which thus inhibits the growing up of the second phase. Similarly, observing the precipitated phase qʹ (Al2Cu) in AA2219 aluminum alloy with TEM, it is found that the size of precipitated phase in the alternating magnetic field aging is smaller than that of the conventional aging, whose possible reason is that the alternating magnetic field aging accelerates the vacancy movement. Cu atoms in AleCu alloy is combined with the vacancy after the solid solution, which speeds up the diffusion of the Cu atoms carrying vacancy and the nucleation of the precipitated phase; meanwhile, the magnetic field aging rapidly reduced the vacancy concentration in the aluminum alloy substrate. The driving force of the growing up of the precipitate phase decreases, which restrains the growing up of the precipitated phase and makes precipitated phase small and dispersed. Compared with that in the strong magnetic field aging sample, the size of the qʹ (Al2Cu) in the alternating magnetic field aging sample is significantly smaller; it may be that the alternating magnetic field strength is weak. In AleCu alloys, the radius of aluminum and copper atoms is quite different. In the beginning period of the aging treatment, the field of stress caused by the misfit degree between the solute atoms and the parent phase is highly diffusive. The dislocation density is too small to create a field of stress which is bigger enough to make the dislocation line bypass the field. The dislocation line remains straight with low resistance. The alloy is soft relatively. With further aging treatment, solute atoms gather together quickly
1)The aging treatment assisted with the alternating magnetic field promotes the aging precipitation of AA2219 aluminum alloy, shortened peak hardiness time. 2)The magnetic field aging specimens observed with the transmission electron microscopy, it is found that the precipitated phase is smaller and more dispersed; the microstructure of AA2219 aluminum alloy is improved. 3)The basic composition of the alloy after aging treatment is a þqʹ (Al2Cu)þ suspected T, among which Al2Cu is the major aging strengthening phase, which is greatly influenced by the magnetic field, while suspected T phase has higher thermal stability. Acknowledgments The authors would like to thank the Key Program of National Natural Science Foundation of China (no. 51235010) and the National Key Basic Research Development Plan Funded Project of China (no. 2014cb046602) for their financial support. References [1] J.M. Silcock, H.M. Flower, Scr. Mater. 46 (2002) 389e394. [2] S.P. Ringer, K. Hono, Mater. Charact. 44 (2000) 101e131. [3] X. Zhao, M.L. Gong, Y.D. Zhang, L. Esling, L. Zuo, G. Vincent, Scr. Mater. 54 (2006) 1897e1900. [4] J.K. Choi, H. Ohtsuka, Y. Xu, W.Y. Choo, Scr. Mater. 43 (2000) 221e226. [5] Y.D. Zhang, N. Gey, C.S. He, X. Zhao, L. Zuo, C. Esling, Acta Mater. 52 (2004) 3467e3474. [6] X.P. Liu, Y.N. Wang, M. Qi, D.Z. Yang, Chin. J. Nonferrous Met. 16 (2006) 2005e2009. [7] Y.W. Ma, S. Awaji, K. Watanabe, M. Matsumoto, N. Kobayash, Solid State Commun. 113 (2000) 671e676. [8] W.C. Liu, S.H. Ji, G.Q. Chen, W.L. Zhou, C.H. Huang, J.T. Guo, Foundry 55 (2006) 338e340. [9] L.Y. Cui, X.N. Li, M. Qi, Chin. J. Nonferrous Met. 17 (2007) 1967e1972. [10] X. Ren, J.W. Zhang, W.L. Zhou, G.Q. Chen, J.S. Zhang, Heat. Treat. Met. 37 (2012) 20e23. [11] B.C. Zhang, Nonferrous Metals and Heat Treatment, Northwest Polytechnical Univ. Press, Xi'an, 2010. [12] W.Q. Gui, J.H. Chen, C.L. Wu, S.B. Wang, J. Chin. Electr. Microsc. Soc. 31 (2012) 302e307. [13] C.G. Cordovilia, E. Louis, J. Mater. Sci. 19 (1984) 279e290. [14] L.F. Mondolfo, Aluminum Alloys: Structure and Properties, Butter Worth and Co., London, 1976. [15] Z.Q. Zheng, Fundamentals of Materials Science, Cent. South Univ. Press, Changsha, 2005.