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Microstructures of directionally cast metals
Metallography
95
Letter a Microstructures of Directionally Cast Metals
W. W. WALKER AND E. BENN
Metallurgical Engineering Department, University of Arizona, Tucson, Arizona
Directional casting or, more properly, directional solidification of metals may be defined as control of the foundry variables in order to induce solidification that results in all the crystals in the casting being aligned in the same crystallographic direction. T h e mechanical properties of such castings are always different and sometimes superior to the properties exhibited by the randomly oriented crystal structures of conventional castings. Directional solidification has been adopted by the foundry industry, and as a result castings can now be obtained "with properties so much improved over those of ordinary castings that a new name has been coined to describe t h e m 'premium quality castings'. ''1 Design engineers can now specify "premium quality" in a commonly used aluminum alloy and be assured of over twice the strength previously possible in this alloy. ~ Directional solidification is also used commercially to produce high-quality permanent magnets in which the magnetic induction is increased by 50% .3,4 It also forms the basis for casting monocrystalline turbine blades, 5 and it is being considered for structural steel castings. ~ Despite the widespread and increasing use of this technique, the literature exhibits a dearth of photomicrographs of the typical microstructures resulting from directional solidification. T h e purpose of this note, then, is to present to other metallographers the unusual and frequently beautiful structures found in directionally solidified metals and alloys, a The submission of Technical Letters for publication is encouraged, and such Letters may be submitted to any member of the Journal Staff. As often happens, a manuscript printed in Metallography may precipitate new work, encourage further investigation, or prompt questions and/or comments. This is as it should be, and the Letters section is a fast, expedient medium by which to publish your comments. Letters need not be restricted to comments on the work published in Metallography but may contain any significant data relative to the field of metallography. All Letters are subject to review and should be in a form suitable for publication at the time of submission.
Metallography, 4 (1971) 95-100 [Letter] Copyright © 1971 by American Elsevier Publishing Company, Inc.
96
W. W. Walker and E. Benn
FIG. 1. Photomacrograph of conventionally cast, ultrapure aluminum. Magnification 1.5 × (reduced 50%). Etch: Electropolished plus Tucker's etch.
k 4"
¸¸¸¸7
J
FIe. 2. Photomacrograph of directional crystals in ultrapure aluminum ingot. Magnification approximately 1.5 × (reduced 50%). Etch: Tuckers.
Microstructures of Directionally Cast Metals
97
Fie. 3. Kellers.
Directionally solidified structure of 356 alloy. Magnification 45 × . Etch:
FIG. 4. Kellers.
Feather crystals in directionally cast 356 alloy. Magnification 45 × . Etch:
98
W. W. Walker and E. Benn
The Metallurgical Engineering Department of the University of Arizona is presently engaged in fundamental studies of directional solidification of aluminum alloys. This research is being carried out jointly with the Optical Sciences Center of the University, and it is aimed at the development of a lightweight material for astronomical mirror substrates. Present efforts involve comparison of the
r~
FIG. 5. Photomacrograph of 2024 alloy test bar. Magnification 1"5 x 66~%). Etch: Flicks.
reduced
FIG. 6, Random microstructure in conventionally cast 2024 alloy. Magnification 25 x. Etch: Kellers.
Microstructures of Directionally Cast Metals
99
!
FIG. 7. Typical feather crystals in directionally cast 2024 alloy. Magnification 25 x . Etch: Kellers.
FIG. 8. Typical feather crystals in directionally cast 2024 alloy. Magnification 45 ×. Etch : Kellers.
100
W. W, Walker and E. Benn
microstructures and mechanical properties of directionally solidified and normal, randomly solidified castings. Some of the typical microstructures encountered in the metallographic portion of this study are described as follows. • Initial experiments were conducted with ultrapure (99.99+ %) aluminum because it was hypothesized that directional solidification might be easier to achieve in a material that was less subject to heterogeneous nucleation. The typical random grain structure (Fig. 1) was replaced with a directional, columnar structure (Fig. 2). • Cassegrain-focus, astronomical telescopes that utilize cast aluminum alloy mirrors 7 are presently in service. Tenzalloy and 356 are the most popular alloys for this application. Directional castings of these alloys were attempted next. Figures 3 and 4 illustrate the typical, directionally solidified microstructures of 356 alloy. • It was found that directionally solidified aluminum alloy castings tend to exhibit a particular type of growth twinning known as "feather crystals. ''s, a This type of growth twinning is currently being studied at the Waseda University Casting Research Laboratory in Tokyo. t°, tz Figure 5 illustrates typical feather crystals in a directionaUy solidified 2024-alloy test bar. • Figure 6 shows the typical, randomly oriented microstructure of conventionally cast 2024. Figures 7 and 8 shows the typical directionally cast structure in 2024 alloy. These crystals are thought to be twinned feather crystals. To date it has been impossible to grow true columnar crystals in alloys. No detailed interpretation of the microstructures illustrated in this letter will be attempted at this time. Comments on these structures by other metallographers are cordially invited.
Acknowledgments This work was supported by Project THEMIS, administered by the Air Force Office of Scientific Research.
References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
M. C. Fleming, Sci. Technol. (1968) 13. Military Specification MIL-A-21180A (1967). E. M. Crank, ft. Appl. Phys., 37 (1966) 113. P. A. Naastepad, Z. Agn. Phys., 21 (1966) 104. J. Barry, Met. Progr., 90 (1966) 66. M. C. Fleming, Trans. Am. Found. Sot., 73 (1965) 1. F. F. Forbes, Appl. Opt., 7 (1968) 1907. M. Schipper and W. Roth, Z. Mettallk., 47 (1956) 78. B. Chalmers, Principles of Solidification, Wiley, New York, 1964, p. 102. S. Watanabe, V. Honma, and S. Oya, Rept. Cast. Lab. Waseda Univ., No. 20 (1969) 45. S. Miyazawa, V. Honma, and S. Oya, Rept. Cast. Res. Lab. Waseda Univ., No. 20 (1969) 55.
Accepted September 28, 1970
95
Letter a Microstructures of Directionally Cast Metals
W. W. WALKER AND E. BENN
Metallurgical Engineering Department, University of Arizona, Tucson, Arizona
Directional casting or, more properly, directional solidification of metals may be defined as control of the foundry variables in order to induce solidification that results in all the crystals in the casting being aligned in the same crystallographic direction. T h e mechanical properties of such castings are always different and sometimes superior to the properties exhibited by the randomly oriented crystal structures of conventional castings. Directional solidification has been adopted by the foundry industry, and as a result castings can now be obtained "with properties so much improved over those of ordinary castings that a new name has been coined to describe t h e m 'premium quality castings'. ''1 Design engineers can now specify "premium quality" in a commonly used aluminum alloy and be assured of over twice the strength previously possible in this alloy. ~ Directional solidification is also used commercially to produce high-quality permanent magnets in which the magnetic induction is increased by 50% .3,4 It also forms the basis for casting monocrystalline turbine blades, 5 and it is being considered for structural steel castings. ~ Despite the widespread and increasing use of this technique, the literature exhibits a dearth of photomicrographs of the typical microstructures resulting from directional solidification. T h e purpose of this note, then, is to present to other metallographers the unusual and frequently beautiful structures found in directionally solidified metals and alloys, a The submission of Technical Letters for publication is encouraged, and such Letters may be submitted to any member of the Journal Staff. As often happens, a manuscript printed in Metallography may precipitate new work, encourage further investigation, or prompt questions and/or comments. This is as it should be, and the Letters section is a fast, expedient medium by which to publish your comments. Letters need not be restricted to comments on the work published in Metallography but may contain any significant data relative to the field of metallography. All Letters are subject to review and should be in a form suitable for publication at the time of submission.
Metallography, 4 (1971) 95-100 [Letter] Copyright © 1971 by American Elsevier Publishing Company, Inc.
96
W. W. Walker and E. Benn
FIG. 1. Photomacrograph of conventionally cast, ultrapure aluminum. Magnification 1.5 × (reduced 50%). Etch: Electropolished plus Tucker's etch.
k 4"
¸¸¸¸7
J
FIe. 2. Photomacrograph of directional crystals in ultrapure aluminum ingot. Magnification approximately 1.5 × (reduced 50%). Etch: Tuckers.
Microstructures of Directionally Cast Metals
97
Fie. 3. Kellers.
Directionally solidified structure of 356 alloy. Magnification 45 × . Etch:
FIG. 4. Kellers.
Feather crystals in directionally cast 356 alloy. Magnification 45 × . Etch:
98
W. W. Walker and E. Benn
The Metallurgical Engineering Department of the University of Arizona is presently engaged in fundamental studies of directional solidification of aluminum alloys. This research is being carried out jointly with the Optical Sciences Center of the University, and it is aimed at the development of a lightweight material for astronomical mirror substrates. Present efforts involve comparison of the
r~
FIG. 5. Photomacrograph of 2024 alloy test bar. Magnification 1"5 x 66~%). Etch: Flicks.
reduced
FIG. 6, Random microstructure in conventionally cast 2024 alloy. Magnification 25 x. Etch: Kellers.
Microstructures of Directionally Cast Metals
99
!
FIG. 7. Typical feather crystals in directionally cast 2024 alloy. Magnification 25 x . Etch: Kellers.
FIG. 8. Typical feather crystals in directionally cast 2024 alloy. Magnification 45 ×. Etch : Kellers.
100
W. W, Walker and E. Benn
microstructures and mechanical properties of directionally solidified and normal, randomly solidified castings. Some of the typical microstructures encountered in the metallographic portion of this study are described as follows. • Initial experiments were conducted with ultrapure (99.99+ %) aluminum because it was hypothesized that directional solidification might be easier to achieve in a material that was less subject to heterogeneous nucleation. The typical random grain structure (Fig. 1) was replaced with a directional, columnar structure (Fig. 2). • Cassegrain-focus, astronomical telescopes that utilize cast aluminum alloy mirrors 7 are presently in service. Tenzalloy and 356 are the most popular alloys for this application. Directional castings of these alloys were attempted next. Figures 3 and 4 illustrate the typical, directionally solidified microstructures of 356 alloy. • It was found that directionally solidified aluminum alloy castings tend to exhibit a particular type of growth twinning known as "feather crystals. ''s, a This type of growth twinning is currently being studied at the Waseda University Casting Research Laboratory in Tokyo. t°, tz Figure 5 illustrates typical feather crystals in a directionaUy solidified 2024-alloy test bar. • Figure 6 shows the typical, randomly oriented microstructure of conventionally cast 2024. Figures 7 and 8 shows the typical directionally cast structure in 2024 alloy. These crystals are thought to be twinned feather crystals. To date it has been impossible to grow true columnar crystals in alloys. No detailed interpretation of the microstructures illustrated in this letter will be attempted at this time. Comments on these structures by other metallographers are cordially invited.
Acknowledgments This work was supported by Project THEMIS, administered by the Air Force Office of Scientific Research.
References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
M. C. Fleming, Sci. Technol. (1968) 13. Military Specification MIL-A-21180A (1967). E. M. Crank, ft. Appl. Phys., 37 (1966) 113. P. A. Naastepad, Z. Agn. Phys., 21 (1966) 104. J. Barry, Met. Progr., 90 (1966) 66. M. C. Fleming, Trans. Am. Found. Sot., 73 (1965) 1. F. F. Forbes, Appl. Opt., 7 (1968) 1907. M. Schipper and W. Roth, Z. Mettallk., 47 (1956) 78. B. Chalmers, Principles of Solidification, Wiley, New York, 1964, p. 102. S. Watanabe, V. Honma, and S. Oya, Rept. Cast. Lab. Waseda Univ., No. 20 (1969) 45. S. Miyazawa, V. Honma, and S. Oya, Rept. Cast. Res. Lab. Waseda Univ., No. 20 (1969) 55.
Accepted September 28, 1970