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The effect of scandium on the as-homogenized microstructure of 5083 alloy for extrusion
Materials Science and Engineering A280 (2000) 139 – 145 www.elsevier.com/locate/msea
The effect of scandium on the as-homogenized microstructure of 5083 alloy for extrusion Tadashi Aiura a, Nobutaka Sugawara b, Yasuhiro Miura b,* b
a Kobe Steel Ltd., Chofu Plant, Shimonoseki, 752 -0953, Japan Department of Materials Science and Engineering, Kyushu Uni6ersity, 6 -10 -1 Hakozaki, Fukuoka, Higashi-ku 812 -8581, Japan
Abstract The microstructure of 5083 alloys, standard and modified by Sc, before/after homogenizing heat treatment were examined. The optimum condition for homogenizing heat treatment and the effect of Sc were discussed. In the results of this study, the microstructure of the 5083 alloys was improved largely by the addition of Sc. As for the cast structure, DAS (dendrite arm spacing) was considerably decreased. The intermetallic compounds formed between dendrite arms became smaller in Sc modified 5083. In ordinary 5083 and Sc modified 5083 alloys, the area fraction of intermetallic compounds approaches the minimum at the homogenizing temperature around 520°C. Detailed analysis regarding the composition of existing intermetallic compounds, shows that the Sc added is mostly utilized in the Al3Sc phase, not in the intermetallic compounds other than Al3Sc. These favorable effects of modification by Sc are considered to come mainly from the fine, uniform distribution of the coherent and spherical Al3Sc precipitates. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Microstructure; Scandium (Sc); 5038 alloy
1. Introduction
2. Experimental procedure
The 5083 alloy is one of the most popular commercialized alloys, which is mostly used for ship structures due to its superior resistance against corrosion. To design this alloy, transition elements represented by Cr and Zr are normally added to obtain the fine crystallized structure. In a recent study, it was suggested that Sc might be used as a substituted modifier for Cr or Zr [1]. In this study, the effect of Sc on modifying as-cast structure and recrystallized grain structure has been explored by the investigation of an ordinary 5083 alloy after an homogenizing heat treatment process and an extrusion process. This superior behavior of Sc as structural refiner was observed.
The ordinary 5083 alloy and Sc modified 5083 (shown in Table 1) were prepared by conventional melting and semi-continuous casting process. Then ingots went to homogenizing heat treatment process. In this process, each ingots were soaked at 350, 400, 450, 500, 520, 550 and 600°C. After this process, each ingot was sliced and etched chemically on the machined surface using a 30% NaOH solution at 60°C. Following this the cut specimens were taken from the sliced ingots. These specimens were polished on the surface of cross sections and were treated by acid Keller solution to observe the microstructure.The area fraction of intermetallic compounds was analyzed using an image analyzer. The transmission electron microscope was applied to observe Al3Sc precipitates directly. An electron probe microanalyzer was also used to verify the intermetallic compounds.
* Corresponding author. Tel.: +81-92-642-3683; fax: + 81-92-6320434. E-mail address: [email protected] (Y. Miura)
0921-5093/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 9 9 ) 0 0 6 8 0 - 2
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3. Results
3.1. As-cast structure Fig. 1 shows the macro structure of each ingots etched by NaOH solution. The Sc modified 5083 has the similar macro grain structure as the ordinary 5083. The size of macro grain is B2 mm in average. Also any visual differences were not shown in macro segregation in the surface layer. This may be evidence that indicates consistency of the casting process. The specimens were taken from the three locations of the sliced ingot, then were heated at 130°C for 2 h to facilitate the precipitation of the beta Al – Mg phase to study the dendrite structure visually. They were treated on the surface by acid Keller solution. The images of microstructures of specimens are shown in Fig. 2. From the results of observations, it is obvious that the dendrite arm spacing of Sc modified 5083 (20 mm) is smaller than that of ordinary 5083 (35 mm). It is assumed that Sc modifies the solidification speed greater during the growth of dendrite structure.
3.2. The effect of Sc on the microstructure of ingots after homogenizing heat treatment The microstructure after homogenizing heat treatment is shown in Fig. 3. The Sc modified 5083 have fine dendrite structure and less second phase inter metallic compounds (dark area) than the entire 5083 examined conditions. The Sc modified fine dendrite structures
made the smaller, second phase intermetallic compounds which are formed between dendrite arms of segregated unsolved elements during the solidification phase transition. Also with rising temperature in the homogenizing heat treatment, those intermetallic compounds decrease except at 600°C where the eutectic solidification may occur in the area with a high concentration of additives. The structures of second phase intermetallic compounds were determined by EPMA analysis. It was verified that existing intermetallic compounds (second phase) are composed of Mg, Fe, Mn, Cr and Si. The results are summarized in Table 2. Sc was not detected at any of these second phase intermetallic compounds. It does not seem to form second phase intermetallic compounds with any other elements.
3.3. The effect of Sc on extrusion characteristics The crystallized grain structures of specimens after extrusion shown for comparison with the optical microscope images in Fig. 4 were investigated. The 5083 alloy shows mostly recrystallized structure after extrusion process at 500°C, while Sc modified 5083 does not reveal recrystallized structure except in a surface layer [4,5]. The mechanical properties of each sample after extrusion are shown in Table 3. The Sc modified 5083 indicate about a 30% higher strength in tensile stress than ordinary 5083, it is assumed to be related with the strong fiber structure of Sc modified 5083 after extrusion process.
Table 1 Specimen chemical compositions Alloy
Si
Fe
Cu
Mn
Mg
Cr
Zn
Ti
Zr
Sc
5083 Modified 5083
0.03 0.03
0.08 0.08
Trace Trace
0.75 0.74
4.49 4.51
0.13 0.14
0.01 Trace
0.02 0.02
Trace Trace
Trace 0.20
Fig. 1. As-cast macrostructure.
a
Mn–Fe–Cr, Mn–Fe–Cr–Mg Mn–Fe–Cr, Mn–Fe–Cr–Mg
Mn–Fe–Cr, Si– Mn–Fe–Cr–Si, Mn–Fe–Cr–Mg, Mn–Fe, Mg–Si
Mn–Mg–Fe–Cr, Mn–Fe, Si – a
400°C
Mn–Mg–Fe–Cr, Mn–Fe, Si–a
Center
Compound with Si, the detail unknown.
Modified 5083
5083
Surface
As-cast
Table 2 Second phase intermetallic compounds
Mn–Fe–Cr, Mn–Fe, Mn–Fe–Cr–Si Mg–Mn–Fe–Cr, Mn–Fe, Mn–Fe–Si
520°C
Mg–Si, Mn–Fe, Mn–Cr Mg–Mn–Fe–Cr, Mn–Fe, Si–a
600°C
T. Aiura et al. / Materials Science and Engineering A280 (2000) 139–145 141
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T. Aiura et al. / Materials Science and Engineering A280 (2000) 139–145
Fig. 2. As-cast microstructure.
3.4. TEM in6estigation The image of TEM observations of specimen treated at varied temperature are showed in Fig. 5. As seen in an as-cast structure, any precipitated intermetallic com-
pounds are not detected. The sample treated at 400°C has Al3Sc with 6.7-nm diameter in average. At 520°C, the Al3Sc coherent spherical particles grow to 50 nm and at 600°C, they grow up to 200 nm where they seem to be incoherent.
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the above mechanism, further investigation has to be made.
4. Discussion
4.1. The effect of Sc modification on microstructure In the results of the observation on as-cast structure, it was found that Sc worked on modifying dendrite structure to be significantly finer. It means that solidification speed was increased in comparison with ordinary 5083. It is discussed that Sc aiddition may make the region between solidus and liquidus shallower in the Al–Mg–Sc phase diagram. This behavior eventually makes the supercooling effect weaken, resulting in a faster solidification phase transition. As reported, 0.2 mass% of Sc addition is not enough to refine the as-cast grain structure [2,3], but it is enough to modify the dendrite structure to become finer. In order to confirm
4.2. The distribution of intermetallic compounds The area fraction and number density of second phase intermetallic compounds in as-cast and as homogenized conditions are shown in Fig. 6. By the Sc addition, both area fraction and number density are decreased in as-cast condition, indicating that the growth of intermetallic compounds during solidification has been suppressed. This is considered to result from the increase in the solidification rate that can be understood from the observed refinement of the dendrite structure in Fig. 3.
Fig. 3. The microstructure after homogenizing heat treatment.
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Fig. 4. Grain structure of extrusions; cross section a: 5083 (surface layer), b: 5083 (center of ingot), c: 5083 + Sc (surface layer), d: 5083 + Sc (center of ingot).
Table 3 Mechanical properties (after extrusion)
5083 Modified 5083
T.S (Mpa)
Y.S (Mpa)
Elongation (%)
273 355
113 190
20 17
Homogenizing heat treatment facilitates the resolution of the intermetallic compounds into the matrix. Thus, the present Sc containing alloy has an advantage, over ordinary 5083, that precipitation of intermetallic compounds is less, and those finer precipitates can be easily resolved by homogenization treatment at temperatures around 500°C.
4.3. The correlation of Sc with second phase intermetallic compounds on creation Cr is often utilized for the purpose of refining recrystallized grain structures. From the results of EPMA analysis, it was found that Cr was one of the elements
forming a second phase intermetallic compounds during solidification. However, Sc does not form any second phase intermetallic compounds during solidification. Sc is hardly wasted as second phase intermetallic compounds. As a result, it can be said that Sc is more effective refiner of recrystallized grain structure in comparison with Cr or Zr that tends to segregate in second phase intermetallic compounds.
5. Summary (1) A 0.2 mass% Sc addition is able to refine the dendrite structure of 5083 alloy. The second phase intermetallic compounds are also refined by Sc addition. (2) In 0.2 mass% Sc modified 5083, Sc does not form any second phase intermetallic compounds with other alloying elements such as Fe, Mn and Cr. (3) The alloy microstructure with intermetallic compounds was varied by homogenizing at different temperatures between 350 and 600°C. The area fraction of intermetallic compounds was smallest at 520°C for both Sc free and Sc containing alloys.
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Fig. 5. TEM images of Al3Sc precipitates a: as-cast, b: 400°C, c: 520°C, d: 600°C.
Acknowledgements The authors express their gratitude to Kobe Steel for supplying the billet and extrusion materials for examination. Financial support by The Light Metals Educational Foundation, Osaka, Japan, is also acknowledged. References
Fig. 6. The area fraction of second phase intermetallic compounds. .
[1] M.E. Drits, S.G. Pavlenko, L.S. Toropova, Yu.G. Bykov, L.B. Ber, Sov. Phys. Dokl. 26 (1981) 344. [2] L.K. Lamikov, G.V. Samsonov, Sov. Non-Ferr. Met. Res. 8 (1964) 79. [3] A.F. Norman, P.B. Prangnell, R.S. McEwen, Acta Mater. 46 (1998) 5715 – 5732. [4] Y. Miura, Mater. Sci. Forum 217 – 222 (1996) 505 – 510. [5] Y.W. Riddle, H.G. Paris, T.H. Sanders Jr., Proc. ICAA 6, Japan Inst. Light Metals 2 (1998) 1179 – 1184.
The effect of scandium on the as-homogenized microstructure of 5083 alloy for extrusion Tadashi Aiura a, Nobutaka Sugawara b, Yasuhiro Miura b,* b
a Kobe Steel Ltd., Chofu Plant, Shimonoseki, 752 -0953, Japan Department of Materials Science and Engineering, Kyushu Uni6ersity, 6 -10 -1 Hakozaki, Fukuoka, Higashi-ku 812 -8581, Japan
Abstract The microstructure of 5083 alloys, standard and modified by Sc, before/after homogenizing heat treatment were examined. The optimum condition for homogenizing heat treatment and the effect of Sc were discussed. In the results of this study, the microstructure of the 5083 alloys was improved largely by the addition of Sc. As for the cast structure, DAS (dendrite arm spacing) was considerably decreased. The intermetallic compounds formed between dendrite arms became smaller in Sc modified 5083. In ordinary 5083 and Sc modified 5083 alloys, the area fraction of intermetallic compounds approaches the minimum at the homogenizing temperature around 520°C. Detailed analysis regarding the composition of existing intermetallic compounds, shows that the Sc added is mostly utilized in the Al3Sc phase, not in the intermetallic compounds other than Al3Sc. These favorable effects of modification by Sc are considered to come mainly from the fine, uniform distribution of the coherent and spherical Al3Sc precipitates. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Microstructure; Scandium (Sc); 5038 alloy
1. Introduction
2. Experimental procedure
The 5083 alloy is one of the most popular commercialized alloys, which is mostly used for ship structures due to its superior resistance against corrosion. To design this alloy, transition elements represented by Cr and Zr are normally added to obtain the fine crystallized structure. In a recent study, it was suggested that Sc might be used as a substituted modifier for Cr or Zr [1]. In this study, the effect of Sc on modifying as-cast structure and recrystallized grain structure has been explored by the investigation of an ordinary 5083 alloy after an homogenizing heat treatment process and an extrusion process. This superior behavior of Sc as structural refiner was observed.
The ordinary 5083 alloy and Sc modified 5083 (shown in Table 1) were prepared by conventional melting and semi-continuous casting process. Then ingots went to homogenizing heat treatment process. In this process, each ingots were soaked at 350, 400, 450, 500, 520, 550 and 600°C. After this process, each ingot was sliced and etched chemically on the machined surface using a 30% NaOH solution at 60°C. Following this the cut specimens were taken from the sliced ingots. These specimens were polished on the surface of cross sections and were treated by acid Keller solution to observe the microstructure.The area fraction of intermetallic compounds was analyzed using an image analyzer. The transmission electron microscope was applied to observe Al3Sc precipitates directly. An electron probe microanalyzer was also used to verify the intermetallic compounds.
* Corresponding author. Tel.: +81-92-642-3683; fax: + 81-92-6320434. E-mail address: [email protected] (Y. Miura)
0921-5093/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 9 9 ) 0 0 6 8 0 - 2
T. Aiura et al. / Materials Science and Engineering A280 (2000) 139–145
140
3. Results
3.1. As-cast structure Fig. 1 shows the macro structure of each ingots etched by NaOH solution. The Sc modified 5083 has the similar macro grain structure as the ordinary 5083. The size of macro grain is B2 mm in average. Also any visual differences were not shown in macro segregation in the surface layer. This may be evidence that indicates consistency of the casting process. The specimens were taken from the three locations of the sliced ingot, then were heated at 130°C for 2 h to facilitate the precipitation of the beta Al – Mg phase to study the dendrite structure visually. They were treated on the surface by acid Keller solution. The images of microstructures of specimens are shown in Fig. 2. From the results of observations, it is obvious that the dendrite arm spacing of Sc modified 5083 (20 mm) is smaller than that of ordinary 5083 (35 mm). It is assumed that Sc modifies the solidification speed greater during the growth of dendrite structure.
3.2. The effect of Sc on the microstructure of ingots after homogenizing heat treatment The microstructure after homogenizing heat treatment is shown in Fig. 3. The Sc modified 5083 have fine dendrite structure and less second phase inter metallic compounds (dark area) than the entire 5083 examined conditions. The Sc modified fine dendrite structures
made the smaller, second phase intermetallic compounds which are formed between dendrite arms of segregated unsolved elements during the solidification phase transition. Also with rising temperature in the homogenizing heat treatment, those intermetallic compounds decrease except at 600°C where the eutectic solidification may occur in the area with a high concentration of additives. The structures of second phase intermetallic compounds were determined by EPMA analysis. It was verified that existing intermetallic compounds (second phase) are composed of Mg, Fe, Mn, Cr and Si. The results are summarized in Table 2. Sc was not detected at any of these second phase intermetallic compounds. It does not seem to form second phase intermetallic compounds with any other elements.
3.3. The effect of Sc on extrusion characteristics The crystallized grain structures of specimens after extrusion shown for comparison with the optical microscope images in Fig. 4 were investigated. The 5083 alloy shows mostly recrystallized structure after extrusion process at 500°C, while Sc modified 5083 does not reveal recrystallized structure except in a surface layer [4,5]. The mechanical properties of each sample after extrusion are shown in Table 3. The Sc modified 5083 indicate about a 30% higher strength in tensile stress than ordinary 5083, it is assumed to be related with the strong fiber structure of Sc modified 5083 after extrusion process.
Table 1 Specimen chemical compositions Alloy
Si
Fe
Cu
Mn
Mg
Cr
Zn
Ti
Zr
Sc
5083 Modified 5083
0.03 0.03
0.08 0.08
Trace Trace
0.75 0.74
4.49 4.51
0.13 0.14
0.01 Trace
0.02 0.02
Trace Trace
Trace 0.20
Fig. 1. As-cast macrostructure.
a
Mn–Fe–Cr, Mn–Fe–Cr–Mg Mn–Fe–Cr, Mn–Fe–Cr–Mg
Mn–Fe–Cr, Si– Mn–Fe–Cr–Si, Mn–Fe–Cr–Mg, Mn–Fe, Mg–Si
Mn–Mg–Fe–Cr, Mn–Fe, Si – a
400°C
Mn–Mg–Fe–Cr, Mn–Fe, Si–a
Center
Compound with Si, the detail unknown.
Modified 5083
5083
Surface
As-cast
Table 2 Second phase intermetallic compounds
Mn–Fe–Cr, Mn–Fe, Mn–Fe–Cr–Si Mg–Mn–Fe–Cr, Mn–Fe, Mn–Fe–Si
520°C
Mg–Si, Mn–Fe, Mn–Cr Mg–Mn–Fe–Cr, Mn–Fe, Si–a
600°C
T. Aiura et al. / Materials Science and Engineering A280 (2000) 139–145 141
142
T. Aiura et al. / Materials Science and Engineering A280 (2000) 139–145
Fig. 2. As-cast microstructure.
3.4. TEM in6estigation The image of TEM observations of specimen treated at varied temperature are showed in Fig. 5. As seen in an as-cast structure, any precipitated intermetallic com-
pounds are not detected. The sample treated at 400°C has Al3Sc with 6.7-nm diameter in average. At 520°C, the Al3Sc coherent spherical particles grow to 50 nm and at 600°C, they grow up to 200 nm where they seem to be incoherent.
T. Aiura et al. / Materials Science and Engineering A280 (2000) 139–145
143
the above mechanism, further investigation has to be made.
4. Discussion
4.1. The effect of Sc modification on microstructure In the results of the observation on as-cast structure, it was found that Sc worked on modifying dendrite structure to be significantly finer. It means that solidification speed was increased in comparison with ordinary 5083. It is discussed that Sc aiddition may make the region between solidus and liquidus shallower in the Al–Mg–Sc phase diagram. This behavior eventually makes the supercooling effect weaken, resulting in a faster solidification phase transition. As reported, 0.2 mass% of Sc addition is not enough to refine the as-cast grain structure [2,3], but it is enough to modify the dendrite structure to become finer. In order to confirm
4.2. The distribution of intermetallic compounds The area fraction and number density of second phase intermetallic compounds in as-cast and as homogenized conditions are shown in Fig. 6. By the Sc addition, both area fraction and number density are decreased in as-cast condition, indicating that the growth of intermetallic compounds during solidification has been suppressed. This is considered to result from the increase in the solidification rate that can be understood from the observed refinement of the dendrite structure in Fig. 3.
Fig. 3. The microstructure after homogenizing heat treatment.
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Fig. 4. Grain structure of extrusions; cross section a: 5083 (surface layer), b: 5083 (center of ingot), c: 5083 + Sc (surface layer), d: 5083 + Sc (center of ingot).
Table 3 Mechanical properties (after extrusion)
5083 Modified 5083
T.S (Mpa)
Y.S (Mpa)
Elongation (%)
273 355
113 190
20 17
Homogenizing heat treatment facilitates the resolution of the intermetallic compounds into the matrix. Thus, the present Sc containing alloy has an advantage, over ordinary 5083, that precipitation of intermetallic compounds is less, and those finer precipitates can be easily resolved by homogenization treatment at temperatures around 500°C.
4.3. The correlation of Sc with second phase intermetallic compounds on creation Cr is often utilized for the purpose of refining recrystallized grain structures. From the results of EPMA analysis, it was found that Cr was one of the elements
forming a second phase intermetallic compounds during solidification. However, Sc does not form any second phase intermetallic compounds during solidification. Sc is hardly wasted as second phase intermetallic compounds. As a result, it can be said that Sc is more effective refiner of recrystallized grain structure in comparison with Cr or Zr that tends to segregate in second phase intermetallic compounds.
5. Summary (1) A 0.2 mass% Sc addition is able to refine the dendrite structure of 5083 alloy. The second phase intermetallic compounds are also refined by Sc addition. (2) In 0.2 mass% Sc modified 5083, Sc does not form any second phase intermetallic compounds with other alloying elements such as Fe, Mn and Cr. (3) The alloy microstructure with intermetallic compounds was varied by homogenizing at different temperatures between 350 and 600°C. The area fraction of intermetallic compounds was smallest at 520°C for both Sc free and Sc containing alloys.
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Fig. 5. TEM images of Al3Sc precipitates a: as-cast, b: 400°C, c: 520°C, d: 600°C.
Acknowledgements The authors express their gratitude to Kobe Steel for supplying the billet and extrusion materials for examination. Financial support by The Light Metals Educational Foundation, Osaka, Japan, is also acknowledged. References
Fig. 6. The area fraction of second phase intermetallic compounds. .
[1] M.E. Drits, S.G. Pavlenko, L.S. Toropova, Yu.G. Bykov, L.B. Ber, Sov. Phys. Dokl. 26 (1981) 344. [2] L.K. Lamikov, G.V. Samsonov, Sov. Non-Ferr. Met. Res. 8 (1964) 79. [3] A.F. Norman, P.B. Prangnell, R.S. McEwen, Acta Mater. 46 (1998) 5715 – 5732. [4] Y. Miura, Mater. Sci. Forum 217 – 222 (1996) 505 – 510. [5] Y.W. Riddle, H.G. Paris, T.H. Sanders Jr., Proc. ICAA 6, Japan Inst. Light Metals 2 (1998) 1179 – 1184.