- Email: [email protected]
Microstructure and mechanical properties of rare-earth-modified Al−1Fe binary alloys
Materials Science & Engineering A 632 (2015) 62–71
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Microstructure and mechanical properties of rare-earth-modified Al 1Fe binary alloys Z.M. Shi n, K. Gao, Y.T. Shi, Y. Wang School of Materials Science and Engineering, Inner Mongolia University of Technology, 010051 Hohhot, China
art ic l e i nf o
a b s t r a c t
Article history: Received 25 January 2015 Received in revised form 23 February 2015 Accepted 24 February 2015 Available online 5 March 2015
Rare-earth (RE) mischmetal modification, homogenous annealing and rolling techniques were used to improve the microstructure and mechanical properties of an Al 1Fe alloy. Al3Fe, Al6Fe and Al3Ce intermetallic phases were found in the RE-modified alloys in the as-cast state. The RE modification refined the α-Al grains, resulting in a divorced eutectic and the formation of the Al Fe phases in the forms of discontinuous networks, flakes and particles. Elemental La dissolved in the Al Ce phases but did not dissolve in the Al Fe phases. The dissolution of elemental Ce in the Al Fe phase along with homogeneous annealing caused the claw-like Al Fe phases to transform into short flakes and particles, which were uniformly distributed in the matrix by the rolling processes. A 0.3 wt% RE modification had the optimized effect on the microstructure and mechanical properties, while a 0.4 wt% RE addition resulted in the aggregation of the Al Ce particles, which then deteriorated the mechanical properties of the alloys. & 2015 Elsevier B.V. All rights reserved.
Keywords: Mechanical characterization Microanalysis X-ray diffraction Aluminum alloys Casting Grain refinement
1. Introduction Aluminum alloys are increasingly being used in the fields of transportation and architecture due to the conceptualization of lightweight manufacturing. Conversely, a tremendous amount of wasted aluminum products need to be recycled for use. In these products, the increased Fe content is a primary problem. In most cases, the Fe content in wrought aluminum alloys is limited to a smaller amount because the Al Fe phases present are in the form of needles, flakes and lath shapes when the content is greater than approximately 0.1 wt%. Conversely, in 8xxx series wrought aluminum alloys, the Fe content can be as great as 2.0 wt% (8006), and these alloys are only used to prepare some unimportant products, such as bottle caps, heat radiators and foils. Çorlu et al. [1], Karlík et al. [2], Moldovan et al. [3] and Xie [4] studied the effects of heat treatment and rolling on the microstructure and mechanical properties of AA8006 and AA8011 alloys. It was found that the coarse Al Fe Mn, Al Fe Si and Al FeMn Si phases serve as crack sourced and extensively damaged the plastic processing properties. Therefore, many methods have been used to eliminate the negative effects of the Al Fe intermetallic phases by reducing their size and changing their morphology and distribution. Liu et al. [5], Lu [6] and Zhang [7] treated the 8011 alloy melts using Al Ti B, Al Ti C and Al 5Ti 1B grain refiners,
respectively. Although the α-Al grains were refined to some extent, the mechanical properties were not enhanced because limited grain refinement cannot improve the distribution and the morphology of the coarse Al Fe phases. Li et al. [8] found that 0.4 wt% La-rich rare earth mixtures had a good modification effect on the Al3Fe phases in an Al 2Fe alloy. The addition of 0.6 wt% rare earth inhibited the preferred growth of eutectic cells. Li et al. [8] and Xing et al. [9,10] prepared an AA8011 alloy with fine α-Al grains using accumulative rolling at 650 1C. Annealing at 200 1C resulted in further refined α-Al grains and the α-AlFeSi, Al3Fe and Al6Fe phases. The tensile strength and elongation were also improved by rolling twice. Moreover, the electro-magnetic stirring and near-liquidus casting methods [11] also changed the size and morphology of the Fe-rich components in Al 5Fe alloys. Rare earths (RE) elements are special modifiers commonly used in aluminum alloys. However, the modification effect of the elemental RE on the deformation Al Fe alloys has not been studied extensively. Therefore, the aim of the present work is to investigate the effects of RE modification on the microstructure and mechanical properties of the Al 1Fe alloy to seek a possible route to recycle the waste aluminum scraps and to uncover the modification mechanism.
2. Materials and methods n
Corresponding author. Tel./fax: þ 86 471 6575752. E-mail address: [email protected] (Z.M. Shi).
http://dx.doi.org/10.1016/j.msea.2015.02.068 0921-5093/& 2015 Elsevier B.V. All rights reserved.
Commercial pure aluminum ingot (99.7 wt% Al, 0.12 wt% Fe, 0.14 wt% Si), and Al 10 wt% Fe master alloy were used as the raw
Z.M. Shi et al. / Materials Science & Engineering A 632 (2015) 62–71
materials. An Al RE master alloy was used as the modifier, and its composition is shown in Table 1. The aluminum ingot was melted at 750 1C in an electric furnace. The Al 10 wt% Fe master alloy was then added to form the Al 1 wt% Fe melts. To eliminate gases and inclusions in the melts, N2 was introduced to the bottom of the furnace for 5 min through a graphite pipe with a diffusive end. Subsequently, various amounts of the Al RE modifier were added to prepare five sample groups with the compositions of 0, 0.1, 0.2, 0.3 and 0.4 wt% RE respectively. When the temperature decreased to 740 1C, the melts were heated at the temperature for 5 min and then poured into a preheated steel mold (250 1C with a wall thickness of 20 mm) through a pre-heated porous ceramic filter with a pore size of 1 mm. The size of the formed ingots was 30 mm (thickness) 120 mm (height) 150 mm (width). The ingots were cooled to room temperature in the mold and sliced along the Table 1 Chemical composition of Al RE master alloy, wt%. Elements
Ce
La
Pr
Nd
Sm
ΣRE
Al
Content
7.38
1.54
0.32
0.58
0.14
9.96
Bal.
63
longitudinal symmetric plane using an electric spark method. The ingots were then treated at 480 1C for 24 h and rolled from a thickness of 15 mm to a thickness of 5 mm with 10 passes at room temperature. The ingots in different states were machined into tensile samples with a working distance of 50 mm and a cross section of 2 mm (thickness) 5 mm (width). The microstructure was examined using a metallurgical microscope (OLYMPUS-GX51) and the average size of the α-Al grains was statistically calculated by viewing 20 fields and using quantitative analysis software. The morphology of the phases was observed using a scanning electronic microscope (SEM, S3400-N, Hitachi) with an additional energy dispersive spectrometer (EDS, 7021-H, HORIBA). The surfaces of the samples used for the observation were polished and etched using an HF (5 wt%)-alcohol solution. The phase composition was inspected using an X-ray diffractometer (D/ MAX-2500/PC, PIGAKV) with CuKa radiation at 40 kV and 200 mA and at a scanning speed of 11/min. The mechanical properties were assessed using a material test machine (Jinan SHT4605) with a loading speed of 5.0 mm/min. The hardness (HB) was tested using a hardness tester (HB-3000, Huayin). Five samples from each group were used to obtain the average values of the mechanical properties.
Fig. 1. Microstructure of the Al 1Fe alloys modified with different amounts of RE in the as-cast state: (a) RE-free; (b) 0.1 wt%; (c) 0.2 wt%; (d) 0.3 wt%; (e) 0.4 wt%.
64
Z.M. Shi et al. / Materials Science & Engineering A 632 (2015) 62–71
Fig. 2. Microstructure of the Al 1Fe alloys modified with different amounts of RE in annealed state: (a) RE-free; (b) 0.1 wt%; (c) 0.2 wt%; (d) 0.3 wt%; (e) 0.4 wt%.
3. Results 3.1. Microstructure of the samples in different states The microstructure of the RE-modified alloys in the as-cast state is shown in Fig. 1. The size of the α-Al grains first decreased and then increased with an increase in the RE content. The Al Fe phases were also refined and broken up into discontinuous networks by the modification. Moreover, the Alþ(AlFe) eutectic cells existed in the 0–0.2 wt% RE-modified alloys; 0.3 wt% and 0.4 wt% RE additions resulted in divorced eutectics. A 0.3 wt% RE addition resulted in the greatest modification effect on the alloy (Fig. 1d). Through the homogeneous annealing, the Al Fe phases were decomposed into the short flakes and particles (Fig. 2) and the subsequent rolling made them uniformly distribute in the matrix (Fig. 3). A 0.3 wt% of RE modification produced the best microstructure (Fig. 3d). The grain refinement effect of the RE modification on the α-Al grains can be confirmed by means of a statistical analysis method (Table 2). 3.2. XRD analysis of the phase composition Fig. 4 shows the XRD patterns of the unmodified and 0.3 wt% RE-modified alloys in different states. The unmodified alloy consisted
of α-Al, Al3Fe, and Al6Fe phases in the as-cast state (Fig. 4a). The addition of RE produced the Al3Ce phases (Fig. 4b). The homogeneous annealing eliminated the metastable Al6Fe phases while the Al3Ce phases remained (Fig. 4c). Further rolling decreased the diffraction intensity of the {111} planes of the α-Al crystals; indicating that the rolling yielded a more uniform microstructure. 3.3. SEM/EDS analysis of the RE distribution Fig. 5 shows the SEM images of the unmodified Al 1Fe alloy in its as-cast state. Compared with the EDS results (Fig. 6), the Fe atoms were not detected in the α-Al grains. The gray phase that forms claws and networks (points B) is the monoclinic Al3Fe (JSPD 01-1265). Moreover, the orthogonal Al6Fe (JSPD 47-1433) cannot be identified. The bright particle at point C is the Al Si Fe phase as a small amount of Si was introduced in the original aluminum ingot. Fig. 7 shows the SEM images of the as-cast alloy modified with 0.3 wt% RE. When combined with the XRD and EDS results (Figs. 4 and 8), elemental La and Ce were not detected in the α-Al grains. The bright particle at point C is the hexagonal Al3Ce phase (JSPD 19-0005), and elemental La was found to dissolve in this phase. The gray claw at point B is the Al Fe phase in which the RE elements were not found. Conversely, the bright flake of the Al Fe phase at point D contained
Z.M. Shi et al. / Materials Science & Engineering A 632 (2015) 62–71
65
Fig. 3. Microstructure of the Al 1Fe alloys modified with different amounts of RE in the rolled state: (a) RE-free; (b) 0.1 wt%; (c) 0.2 wt%; (d) 0.3 wt%; (e) 0.4 wt%.
Table 2 Statistical analysis of the size of α-Al grains. States
RE content wt%
Equivalent size of α-Al grains, μm
As-cast
0.0 0.1 0.2 0.3 0.4 0.0 0.1 0.2 0.3 0.4
85.8 79.2 47.3 76.5 42.6 76.8 23.2 74.6 44.1 75.0 88.3 78.3 46.6 75.3 43.2 74.7 25.3 76.2 46.7 77.3
Annealing
a large amount of Ce; however, La was not found in this phase. This result indicates that the distribution of the RE elements in the Al Fe phases is not uniform. The dissolution of Ce promoted the decomposition of the claw-like AlFe phases into the flakes, which is favorable to eliminate the harmful effect of the Al Fe phases on the microstructure and mechanical properties. In case of the 0.4 wt% RE modification, the white particles mainly contained the Al and Ce elements in the as-cast and annealed alloys as shown in Figs. 9 and 10. They are the aggregated Al3Ce particles,
which were not eliminated by the annealing (Fig. 10). This indicates that the extra RE addition promoted the formation and the aggregation of the Al Ce phases, which is harmful to plastic processing and the mechanical properties of the alloy. 3.4. Mechanical properties of the alloys in different states Fig. 11 shows the mechanical properties of the alloys in different states. With an increase in the RE content, the ultimate tensile strength
66
Z.M. Shi et al. / Materials Science & Engineering A 632 (2015) 62–71
Fig. 4. XRD patterns of the alloys subject to different conditions: (a) RE free, as-cast; (b) 0.3 wt% RE modified, as-cast; (c) 0.3 wt% RE modified, annealed; (d) 0.3 wt% RE modified, rolled.
Fig. 5. The morphology of Al Fe phases of the as-case Al 1Fe alloy.
(UTS) and elongation first increased and then decreased. The hardness gradually increased over the entire range. The 0.3 wt% RE modification resulted in the best comprehensive properties especially with respect to the elongation. Moreover, the alloys were greatly softened by the annealing and hardened by the rolling.
4. Discussion 4.1. Refinement of the
α-Al grains
The hypoeutectic Al 1Fe alloy is composed of α-Al and Al3Fe phases in an equilibrium solidification condition. The RE elements have greater radii (La: 0.1877 nm; Ce: 0.1825 nm) compared with Al (0.1180 nm) and Fe (0.1241 nm) [12,13]. The mismatch of RE with Al and Fe in terms of the atomic radius is greater than 40%, therefore, the RE elements can neither dissolve in the α-Al crystals nor substitute for the Al and Fe atoms.
The nucleation and the growth of the primary α-Al dendrites make the RE and Fe elements gradually enrich in the growth front, which produces a local compositional undercooling [14], stimulates flourishing of the α-Al dendrites, and therefore results in a grain refinement effect. However, less RE element addition resulted in its weaker enrichment while a greater addition, such as 0.4 wt% RE, yielded an aggregation of the Al Ce phases (Fig. 9). This in turn weakened the enrichment of the RE elements in the growth front. Therefore, the refinement effect is not sufficient in these conditions. A similar phenomenon was also found in the literature [15,16]. In the present test, the 0.3 wt% RE modification has the optimized effect on the refinement of the α-Al grains (Fig. 1d). 4.2. Modification of Al Fe phase morphology During the eutectic transformation at 655 1C, the eutectic Al Fe phases will precipitate in the form of particles, needles, flakes or claws, which are dependent on the Fe content and the cooling rate. In the
Z.M. Shi et al. / Materials Science & Engineering A 632 (2015) 62–71
67
Fig. 6. EDS analysis of the composition at points A, B and C in Fig. 5, (a) point A; (b) point B; (c) point C.
Fig. 7. The morphology of Al Fe and Al Ce phases of the as-cast Al 1Fe alloy modified with 0.3 wt% RE: (a) secondary electron image; (b) backscattered electron image.
68
Z.M. Shi et al. / Materials Science & Engineering A 632 (2015) 62–71
Fig. 8. EDS analysis of the composition at points A, B, C and D in Fig. 7(b), (a) point A; (b) point B; (c) point C; (d) point D.
Z.M. Shi et al. / Materials Science & Engineering A 632 (2015) 62–71
Fig. 9. Line scanning of elements in 0.4 wt% RE-modified Al 1Fe alloy in the as-cast state, (a) back scattered image; (b) Al; (c) Fe; (d) Ce.
Fig. 10. Line scanning of 0elements in 0.4 wt% RE-modified Al 1Fe alloy in the annealed state, (a) back scattered image; (b) Al; (c) Fe; (d) Ce.
69
70
Z.M. Shi et al. / Materials Science & Engineering A 632 (2015) 62–71
Fig. 11. Mechanical properties of the Al 1Fe alloys in different states: (a) ultimate tensile strength; (b) elongation; (c) hardness.
present work, lamellar eutectic cells were formed in the unmodified Al 1Fe alloy. In the RE-modified alloy, the remaining RE elements are also enriched in the growth edges of the eutectic cells, which poisons the orientation growth of the AlFe phases and increase the interfacial energy with the melts [17]. On the other hand, the Ce atoms were easier to incorporate in the Al Fe phases compared with the La (Figs. 7 and 8) as Ce has a smaller radius compared with La. The dissolution of Ce increases the lattice energy and, therefore, weakens the orientation growth of the Al Fe phases. However, the addition of RE up to 0.2 wt% is not enough to inhibit the lamellar structures. An addition of 0.3 wt% and 0.4 wt% RE yielded divorced eutectics in which the Al Fe phases tend to grow into isolated short flakes or particles. When soaking at high temperature, the Al Fe phases were further changed into small bulks and fine globular shapes for the REmodified alloys. This is because that the RE atoms located at the boundary between the eutectic cells and the α-Al grains increase the interfacial energy and the dissolution of Ce in the Al Fe phases, which weakens the Al Fe bonds and decreases their thermaldynamic stability (Oswald ripening) [18,19], Moreover, the metastable Al6Fe phases were transformed into the stable Al3Fe during
the annealing treatment. The subsequent rolling process made the Al Fe phases uniformly disperse in the matrix due to serve plastic deformation especially in the case of the 0.3 wt% RE-modified alloy.
4.3. Effect of RE modification on the mechanical properties of the alloys The variation of the mechanical properties correlates with the changes in the microstructure of the alloys. The RE modification enhanced the tensile strength and elongation of the alloys, which is due to refinement of the α-Al grains and the Al Fe phases. The addition of 0.3 wt% RE resulted in the refined microstructure, offering the highest tensile strength and elongation in these three states. Conversely, adding 0.4 wt% of RE made the Al Ce phases aggregate (Figs. 9 and 10), which coarsened the α-Al grains and worsened the distribution of the Al Fe phases; therefore, the tensile strength and elongation decreased. The slight increase in the hardness with an increase in the RE content is attributed to the increase of the hard Al Ce phases and the dispersive distribution of the Al Fe phases.
Z.M. Shi et al. / Materials Science & Engineering A 632 (2015) 62–71
5. Conclusions The mixed RE modification refined the α-Al grains and increased the divorced eutectics. The elemental La dissolved in the Al Ce phases but did not dissolve in the Al Fe phases, while the dissolution of Ce in the phases produced Al Fe phases in the form of discontinuous networks, flakes and particles. Through homogeneous annealing, the Al Fe phases were decomposed into short flakes and particles, which were dispersively distributed in the matrix by the applied rolling process. Modification with 0.3 wt% RE has the optimized effect on the microstructure and mechanical properties. An addition of 0.4 wt% RE resulted in an aggregation of the Al Ce particles, which in turn deteriorated the mechanical properties of the alloys. Acknowledgments The present work was supported by the Nature Science Foundation of Inner Mongolia of China. References [1] B. Çorlu, I. Dursun, M. Dündar, A homogenization treatment study for twin roll cast 3003 and 8006 aluminum alloys, in: Proceedings of the TMS 2009 Annual Meeting and Exhibition, San Francisco, USA, Feb 16 19, 2009.
71
[2] M. Karlík, S.M Siegl, Y. Birol, Study of the damage of AA 8006 twin-roll cast thin sheets during forming of heat exchanger fins, in: Proceedings of the 7th International Conference ICCA7, Aluminum Alloys: Their Physical and Mechanical Properties, Virginia, USA, April 9 14, 2000. [3] P. Moldovan, G. Popescu, F. Miculescu, J. Mater. Process. Technol. 153–154 (2004) 408–415. [4] Xian-jiao Xie, Phys. Test. Chem. Anal. A 41 (2005) 256–258. [5] Zhi-Ping Liu, Hui-Ming Huang, Shu-Hua Deng, Nonferrous Met. 59 (2007) 25–26 45. [6] Zhao-Yong Lu, Alum. Process. 205 (2012) 26–28. [7] Qi Zhang, Light Alloy Fabr. Technol. 32 (2004) 19–24 42. [8] Rongde Li, Jianchao Ma, Zhenping Zhou, Huishu Yu, J. Chin. Rare Earth Soc. 22 (2004) 722–724. [9] Z.P Xing, S.B Kang, H.W Kim, Scr. Mater. 45 (2001) 597–604. [10] Z.P Xing, S.B Kang, H.W Kim, Metall. Mater. Trans. A 33 (2002) 1521–1530. [11] Dajun Fu, Xiaoguang Yuan, Xihuan Liu, Hot Working Technol. 39 (2010) 58–59 106. [12] G.D. Zhou, L.Y. Duan, Basis of Structural Chemistry, Fourth ed, Beijing University Press, Beijing (2008) 15. [13] W.H. Sun, S.R. Zhang, A.Q. Hou, Behaviors of Rare Earths in Aluminum Alloys, Weapon industry Press, Beijing (1992) 58–65. [14] Man Zhu, Zengyun Jian, Lijuan Yao, Cuixia Liu, Gencang Yang, Yaohe Zhou, J. Mater. Sci. 46 (2011) 2685–2694. [15] M. Zhu, Z.G. Jian, L.J. Yao, C.X. Liu, G.C. Yang, Y.H. Zhou, J. Mater. Sci. 46 (2011) 2685–2694. [16] X.W. Yu, C.N. Cao, Z.M. Yao, J. Mater. Sci. Lett. 19 (2000) 1907–1908. [17] P. Srirangam, S. Chattopadhyay, A. Bhattacharya, S. Nag, J. Kaduk, S. Shankar, R. Banerjee, T. Shibata, Acta Mater. 65 (2014) 185–193. [18] A. Forn, M.T. Baile, E. Martin, E. Rupérez, Mater. Sci. Forum 480–481 (2005) 367–372. [19] Avraam Kalogeridis, Josef Pesicka, Eckhard Nembach, Mater. Sci. Eng. A 268 (1999) 197–201.
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Microstructure and mechanical properties of rare-earth-modified Al 1Fe binary alloys Z.M. Shi n, K. Gao, Y.T. Shi, Y. Wang School of Materials Science and Engineering, Inner Mongolia University of Technology, 010051 Hohhot, China
art ic l e i nf o
a b s t r a c t
Article history: Received 25 January 2015 Received in revised form 23 February 2015 Accepted 24 February 2015 Available online 5 March 2015
Rare-earth (RE) mischmetal modification, homogenous annealing and rolling techniques were used to improve the microstructure and mechanical properties of an Al 1Fe alloy. Al3Fe, Al6Fe and Al3Ce intermetallic phases were found in the RE-modified alloys in the as-cast state. The RE modification refined the α-Al grains, resulting in a divorced eutectic and the formation of the Al Fe phases in the forms of discontinuous networks, flakes and particles. Elemental La dissolved in the Al Ce phases but did not dissolve in the Al Fe phases. The dissolution of elemental Ce in the Al Fe phase along with homogeneous annealing caused the claw-like Al Fe phases to transform into short flakes and particles, which were uniformly distributed in the matrix by the rolling processes. A 0.3 wt% RE modification had the optimized effect on the microstructure and mechanical properties, while a 0.4 wt% RE addition resulted in the aggregation of the Al Ce particles, which then deteriorated the mechanical properties of the alloys. & 2015 Elsevier B.V. All rights reserved.
Keywords: Mechanical characterization Microanalysis X-ray diffraction Aluminum alloys Casting Grain refinement
1. Introduction Aluminum alloys are increasingly being used in the fields of transportation and architecture due to the conceptualization of lightweight manufacturing. Conversely, a tremendous amount of wasted aluminum products need to be recycled for use. In these products, the increased Fe content is a primary problem. In most cases, the Fe content in wrought aluminum alloys is limited to a smaller amount because the Al Fe phases present are in the form of needles, flakes and lath shapes when the content is greater than approximately 0.1 wt%. Conversely, in 8xxx series wrought aluminum alloys, the Fe content can be as great as 2.0 wt% (8006), and these alloys are only used to prepare some unimportant products, such as bottle caps, heat radiators and foils. Çorlu et al. [1], Karlík et al. [2], Moldovan et al. [3] and Xie [4] studied the effects of heat treatment and rolling on the microstructure and mechanical properties of AA8006 and AA8011 alloys. It was found that the coarse Al Fe Mn, Al Fe Si and Al FeMn Si phases serve as crack sourced and extensively damaged the plastic processing properties. Therefore, many methods have been used to eliminate the negative effects of the Al Fe intermetallic phases by reducing their size and changing their morphology and distribution. Liu et al. [5], Lu [6] and Zhang [7] treated the 8011 alloy melts using Al Ti B, Al Ti C and Al 5Ti 1B grain refiners,
respectively. Although the α-Al grains were refined to some extent, the mechanical properties were not enhanced because limited grain refinement cannot improve the distribution and the morphology of the coarse Al Fe phases. Li et al. [8] found that 0.4 wt% La-rich rare earth mixtures had a good modification effect on the Al3Fe phases in an Al 2Fe alloy. The addition of 0.6 wt% rare earth inhibited the preferred growth of eutectic cells. Li et al. [8] and Xing et al. [9,10] prepared an AA8011 alloy with fine α-Al grains using accumulative rolling at 650 1C. Annealing at 200 1C resulted in further refined α-Al grains and the α-AlFeSi, Al3Fe and Al6Fe phases. The tensile strength and elongation were also improved by rolling twice. Moreover, the electro-magnetic stirring and near-liquidus casting methods [11] also changed the size and morphology of the Fe-rich components in Al 5Fe alloys. Rare earths (RE) elements are special modifiers commonly used in aluminum alloys. However, the modification effect of the elemental RE on the deformation Al Fe alloys has not been studied extensively. Therefore, the aim of the present work is to investigate the effects of RE modification on the microstructure and mechanical properties of the Al 1Fe alloy to seek a possible route to recycle the waste aluminum scraps and to uncover the modification mechanism.
2. Materials and methods n
Corresponding author. Tel./fax: þ 86 471 6575752. E-mail address: [email protected] (Z.M. Shi).
http://dx.doi.org/10.1016/j.msea.2015.02.068 0921-5093/& 2015 Elsevier B.V. All rights reserved.
Commercial pure aluminum ingot (99.7 wt% Al, 0.12 wt% Fe, 0.14 wt% Si), and Al 10 wt% Fe master alloy were used as the raw
Z.M. Shi et al. / Materials Science & Engineering A 632 (2015) 62–71
materials. An Al RE master alloy was used as the modifier, and its composition is shown in Table 1. The aluminum ingot was melted at 750 1C in an electric furnace. The Al 10 wt% Fe master alloy was then added to form the Al 1 wt% Fe melts. To eliminate gases and inclusions in the melts, N2 was introduced to the bottom of the furnace for 5 min through a graphite pipe with a diffusive end. Subsequently, various amounts of the Al RE modifier were added to prepare five sample groups with the compositions of 0, 0.1, 0.2, 0.3 and 0.4 wt% RE respectively. When the temperature decreased to 740 1C, the melts were heated at the temperature for 5 min and then poured into a preheated steel mold (250 1C with a wall thickness of 20 mm) through a pre-heated porous ceramic filter with a pore size of 1 mm. The size of the formed ingots was 30 mm (thickness) 120 mm (height) 150 mm (width). The ingots were cooled to room temperature in the mold and sliced along the Table 1 Chemical composition of Al RE master alloy, wt%. Elements
Ce
La
Pr
Nd
Sm
ΣRE
Al
Content
7.38
1.54
0.32
0.58
0.14
9.96
Bal.
63
longitudinal symmetric plane using an electric spark method. The ingots were then treated at 480 1C for 24 h and rolled from a thickness of 15 mm to a thickness of 5 mm with 10 passes at room temperature. The ingots in different states were machined into tensile samples with a working distance of 50 mm and a cross section of 2 mm (thickness) 5 mm (width). The microstructure was examined using a metallurgical microscope (OLYMPUS-GX51) and the average size of the α-Al grains was statistically calculated by viewing 20 fields and using quantitative analysis software. The morphology of the phases was observed using a scanning electronic microscope (SEM, S3400-N, Hitachi) with an additional energy dispersive spectrometer (EDS, 7021-H, HORIBA). The surfaces of the samples used for the observation were polished and etched using an HF (5 wt%)-alcohol solution. The phase composition was inspected using an X-ray diffractometer (D/ MAX-2500/PC, PIGAKV) with CuKa radiation at 40 kV and 200 mA and at a scanning speed of 11/min. The mechanical properties were assessed using a material test machine (Jinan SHT4605) with a loading speed of 5.0 mm/min. The hardness (HB) was tested using a hardness tester (HB-3000, Huayin). Five samples from each group were used to obtain the average values of the mechanical properties.
Fig. 1. Microstructure of the Al 1Fe alloys modified with different amounts of RE in the as-cast state: (a) RE-free; (b) 0.1 wt%; (c) 0.2 wt%; (d) 0.3 wt%; (e) 0.4 wt%.
64
Z.M. Shi et al. / Materials Science & Engineering A 632 (2015) 62–71
Fig. 2. Microstructure of the Al 1Fe alloys modified with different amounts of RE in annealed state: (a) RE-free; (b) 0.1 wt%; (c) 0.2 wt%; (d) 0.3 wt%; (e) 0.4 wt%.
3. Results 3.1. Microstructure of the samples in different states The microstructure of the RE-modified alloys in the as-cast state is shown in Fig. 1. The size of the α-Al grains first decreased and then increased with an increase in the RE content. The Al Fe phases were also refined and broken up into discontinuous networks by the modification. Moreover, the Alþ(AlFe) eutectic cells existed in the 0–0.2 wt% RE-modified alloys; 0.3 wt% and 0.4 wt% RE additions resulted in divorced eutectics. A 0.3 wt% RE addition resulted in the greatest modification effect on the alloy (Fig. 1d). Through the homogeneous annealing, the Al Fe phases were decomposed into the short flakes and particles (Fig. 2) and the subsequent rolling made them uniformly distribute in the matrix (Fig. 3). A 0.3 wt% of RE modification produced the best microstructure (Fig. 3d). The grain refinement effect of the RE modification on the α-Al grains can be confirmed by means of a statistical analysis method (Table 2). 3.2. XRD analysis of the phase composition Fig. 4 shows the XRD patterns of the unmodified and 0.3 wt% RE-modified alloys in different states. The unmodified alloy consisted
of α-Al, Al3Fe, and Al6Fe phases in the as-cast state (Fig. 4a). The addition of RE produced the Al3Ce phases (Fig. 4b). The homogeneous annealing eliminated the metastable Al6Fe phases while the Al3Ce phases remained (Fig. 4c). Further rolling decreased the diffraction intensity of the {111} planes of the α-Al crystals; indicating that the rolling yielded a more uniform microstructure. 3.3. SEM/EDS analysis of the RE distribution Fig. 5 shows the SEM images of the unmodified Al 1Fe alloy in its as-cast state. Compared with the EDS results (Fig. 6), the Fe atoms were not detected in the α-Al grains. The gray phase that forms claws and networks (points B) is the monoclinic Al3Fe (JSPD 01-1265). Moreover, the orthogonal Al6Fe (JSPD 47-1433) cannot be identified. The bright particle at point C is the Al Si Fe phase as a small amount of Si was introduced in the original aluminum ingot. Fig. 7 shows the SEM images of the as-cast alloy modified with 0.3 wt% RE. When combined with the XRD and EDS results (Figs. 4 and 8), elemental La and Ce were not detected in the α-Al grains. The bright particle at point C is the hexagonal Al3Ce phase (JSPD 19-0005), and elemental La was found to dissolve in this phase. The gray claw at point B is the Al Fe phase in which the RE elements were not found. Conversely, the bright flake of the Al Fe phase at point D contained
Z.M. Shi et al. / Materials Science & Engineering A 632 (2015) 62–71
65
Fig. 3. Microstructure of the Al 1Fe alloys modified with different amounts of RE in the rolled state: (a) RE-free; (b) 0.1 wt%; (c) 0.2 wt%; (d) 0.3 wt%; (e) 0.4 wt%.
Table 2 Statistical analysis of the size of α-Al grains. States
RE content wt%
Equivalent size of α-Al grains, μm
As-cast
0.0 0.1 0.2 0.3 0.4 0.0 0.1 0.2 0.3 0.4
85.8 79.2 47.3 76.5 42.6 76.8 23.2 74.6 44.1 75.0 88.3 78.3 46.6 75.3 43.2 74.7 25.3 76.2 46.7 77.3
Annealing
a large amount of Ce; however, La was not found in this phase. This result indicates that the distribution of the RE elements in the Al Fe phases is not uniform. The dissolution of Ce promoted the decomposition of the claw-like AlFe phases into the flakes, which is favorable to eliminate the harmful effect of the Al Fe phases on the microstructure and mechanical properties. In case of the 0.4 wt% RE modification, the white particles mainly contained the Al and Ce elements in the as-cast and annealed alloys as shown in Figs. 9 and 10. They are the aggregated Al3Ce particles,
which were not eliminated by the annealing (Fig. 10). This indicates that the extra RE addition promoted the formation and the aggregation of the Al Ce phases, which is harmful to plastic processing and the mechanical properties of the alloy. 3.4. Mechanical properties of the alloys in different states Fig. 11 shows the mechanical properties of the alloys in different states. With an increase in the RE content, the ultimate tensile strength
66
Z.M. Shi et al. / Materials Science & Engineering A 632 (2015) 62–71
Fig. 4. XRD patterns of the alloys subject to different conditions: (a) RE free, as-cast; (b) 0.3 wt% RE modified, as-cast; (c) 0.3 wt% RE modified, annealed; (d) 0.3 wt% RE modified, rolled.
Fig. 5. The morphology of Al Fe phases of the as-case Al 1Fe alloy.
(UTS) and elongation first increased and then decreased. The hardness gradually increased over the entire range. The 0.3 wt% RE modification resulted in the best comprehensive properties especially with respect to the elongation. Moreover, the alloys were greatly softened by the annealing and hardened by the rolling.
4. Discussion 4.1. Refinement of the
α-Al grains
The hypoeutectic Al 1Fe alloy is composed of α-Al and Al3Fe phases in an equilibrium solidification condition. The RE elements have greater radii (La: 0.1877 nm; Ce: 0.1825 nm) compared with Al (0.1180 nm) and Fe (0.1241 nm) [12,13]. The mismatch of RE with Al and Fe in terms of the atomic radius is greater than 40%, therefore, the RE elements can neither dissolve in the α-Al crystals nor substitute for the Al and Fe atoms.
The nucleation and the growth of the primary α-Al dendrites make the RE and Fe elements gradually enrich in the growth front, which produces a local compositional undercooling [14], stimulates flourishing of the α-Al dendrites, and therefore results in a grain refinement effect. However, less RE element addition resulted in its weaker enrichment while a greater addition, such as 0.4 wt% RE, yielded an aggregation of the Al Ce phases (Fig. 9). This in turn weakened the enrichment of the RE elements in the growth front. Therefore, the refinement effect is not sufficient in these conditions. A similar phenomenon was also found in the literature [15,16]. In the present test, the 0.3 wt% RE modification has the optimized effect on the refinement of the α-Al grains (Fig. 1d). 4.2. Modification of Al Fe phase morphology During the eutectic transformation at 655 1C, the eutectic Al Fe phases will precipitate in the form of particles, needles, flakes or claws, which are dependent on the Fe content and the cooling rate. In the
Z.M. Shi et al. / Materials Science & Engineering A 632 (2015) 62–71
67
Fig. 6. EDS analysis of the composition at points A, B and C in Fig. 5, (a) point A; (b) point B; (c) point C.
Fig. 7. The morphology of Al Fe and Al Ce phases of the as-cast Al 1Fe alloy modified with 0.3 wt% RE: (a) secondary electron image; (b) backscattered electron image.
68
Z.M. Shi et al. / Materials Science & Engineering A 632 (2015) 62–71
Fig. 8. EDS analysis of the composition at points A, B, C and D in Fig. 7(b), (a) point A; (b) point B; (c) point C; (d) point D.
Z.M. Shi et al. / Materials Science & Engineering A 632 (2015) 62–71
Fig. 9. Line scanning of elements in 0.4 wt% RE-modified Al 1Fe alloy in the as-cast state, (a) back scattered image; (b) Al; (c) Fe; (d) Ce.
Fig. 10. Line scanning of 0elements in 0.4 wt% RE-modified Al 1Fe alloy in the annealed state, (a) back scattered image; (b) Al; (c) Fe; (d) Ce.
69
70
Z.M. Shi et al. / Materials Science & Engineering A 632 (2015) 62–71
Fig. 11. Mechanical properties of the Al 1Fe alloys in different states: (a) ultimate tensile strength; (b) elongation; (c) hardness.
present work, lamellar eutectic cells were formed in the unmodified Al 1Fe alloy. In the RE-modified alloy, the remaining RE elements are also enriched in the growth edges of the eutectic cells, which poisons the orientation growth of the AlFe phases and increase the interfacial energy with the melts [17]. On the other hand, the Ce atoms were easier to incorporate in the Al Fe phases compared with the La (Figs. 7 and 8) as Ce has a smaller radius compared with La. The dissolution of Ce increases the lattice energy and, therefore, weakens the orientation growth of the Al Fe phases. However, the addition of RE up to 0.2 wt% is not enough to inhibit the lamellar structures. An addition of 0.3 wt% and 0.4 wt% RE yielded divorced eutectics in which the Al Fe phases tend to grow into isolated short flakes or particles. When soaking at high temperature, the Al Fe phases were further changed into small bulks and fine globular shapes for the REmodified alloys. This is because that the RE atoms located at the boundary between the eutectic cells and the α-Al grains increase the interfacial energy and the dissolution of Ce in the Al Fe phases, which weakens the Al Fe bonds and decreases their thermaldynamic stability (Oswald ripening) [18,19], Moreover, the metastable Al6Fe phases were transformed into the stable Al3Fe during
the annealing treatment. The subsequent rolling process made the Al Fe phases uniformly disperse in the matrix due to serve plastic deformation especially in the case of the 0.3 wt% RE-modified alloy.
4.3. Effect of RE modification on the mechanical properties of the alloys The variation of the mechanical properties correlates with the changes in the microstructure of the alloys. The RE modification enhanced the tensile strength and elongation of the alloys, which is due to refinement of the α-Al grains and the Al Fe phases. The addition of 0.3 wt% RE resulted in the refined microstructure, offering the highest tensile strength and elongation in these three states. Conversely, adding 0.4 wt% of RE made the Al Ce phases aggregate (Figs. 9 and 10), which coarsened the α-Al grains and worsened the distribution of the Al Fe phases; therefore, the tensile strength and elongation decreased. The slight increase in the hardness with an increase in the RE content is attributed to the increase of the hard Al Ce phases and the dispersive distribution of the Al Fe phases.
Z.M. Shi et al. / Materials Science & Engineering A 632 (2015) 62–71
5. Conclusions The mixed RE modification refined the α-Al grains and increased the divorced eutectics. The elemental La dissolved in the Al Ce phases but did not dissolve in the Al Fe phases, while the dissolution of Ce in the phases produced Al Fe phases in the form of discontinuous networks, flakes and particles. Through homogeneous annealing, the Al Fe phases were decomposed into short flakes and particles, which were dispersively distributed in the matrix by the applied rolling process. Modification with 0.3 wt% RE has the optimized effect on the microstructure and mechanical properties. An addition of 0.4 wt% RE resulted in an aggregation of the Al Ce particles, which in turn deteriorated the mechanical properties of the alloys. Acknowledgments The present work was supported by the Nature Science Foundation of Inner Mongolia of China. References [1] B. Çorlu, I. Dursun, M. Dündar, A homogenization treatment study for twin roll cast 3003 and 8006 aluminum alloys, in: Proceedings of the TMS 2009 Annual Meeting and Exhibition, San Francisco, USA, Feb 16 19, 2009.
71
[2] M. Karlík, S.M Siegl, Y. Birol, Study of the damage of AA 8006 twin-roll cast thin sheets during forming of heat exchanger fins, in: Proceedings of the 7th International Conference ICCA7, Aluminum Alloys: Their Physical and Mechanical Properties, Virginia, USA, April 9 14, 2000. [3] P. Moldovan, G. Popescu, F. Miculescu, J. Mater. Process. Technol. 153–154 (2004) 408–415. [4] Xian-jiao Xie, Phys. Test. Chem. Anal. A 41 (2005) 256–258. [5] Zhi-Ping Liu, Hui-Ming Huang, Shu-Hua Deng, Nonferrous Met. 59 (2007) 25–26 45. [6] Zhao-Yong Lu, Alum. Process. 205 (2012) 26–28. [7] Qi Zhang, Light Alloy Fabr. Technol. 32 (2004) 19–24 42. [8] Rongde Li, Jianchao Ma, Zhenping Zhou, Huishu Yu, J. Chin. Rare Earth Soc. 22 (2004) 722–724. [9] Z.P Xing, S.B Kang, H.W Kim, Scr. Mater. 45 (2001) 597–604. [10] Z.P Xing, S.B Kang, H.W Kim, Metall. Mater. Trans. A 33 (2002) 1521–1530. [11] Dajun Fu, Xiaoguang Yuan, Xihuan Liu, Hot Working Technol. 39 (2010) 58–59 106. [12] G.D. Zhou, L.Y. Duan, Basis of Structural Chemistry, Fourth ed, Beijing University Press, Beijing (2008) 15. [13] W.H. Sun, S.R. Zhang, A.Q. Hou, Behaviors of Rare Earths in Aluminum Alloys, Weapon industry Press, Beijing (1992) 58–65. [14] Man Zhu, Zengyun Jian, Lijuan Yao, Cuixia Liu, Gencang Yang, Yaohe Zhou, J. Mater. Sci. 46 (2011) 2685–2694. [15] M. Zhu, Z.G. Jian, L.J. Yao, C.X. Liu, G.C. Yang, Y.H. Zhou, J. Mater. Sci. 46 (2011) 2685–2694. [16] X.W. Yu, C.N. Cao, Z.M. Yao, J. Mater. Sci. Lett. 19 (2000) 1907–1908. [17] P. Srirangam, S. Chattopadhyay, A. Bhattacharya, S. Nag, J. Kaduk, S. Shankar, R. Banerjee, T. Shibata, Acta Mater. 65 (2014) 185–193. [18] A. Forn, M.T. Baile, E. Martin, E. Rupérez, Mater. Sci. Forum 480–481 (2005) 367–372. [19] Avraam Kalogeridis, Josef Pesicka, Eckhard Nembach, Mater. Sci. Eng. A 268 (1999) 197–201.