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Synthesis and characterization of Al40Mg25Zn35 amorphous powder by rapid solidification
Powder Technology 114 Ž2001. 51–54 www.elsevier.comrlocaterpowtec
Synthesis and characterization of Al 40 Mg 25 Zn 35 amorphous powder by rapid solidification Salah-ud Din ) , S.Y. Chishti Centre of Excellence in Solid State Physics, UniÕersity of the Punjab, Quaid-e-Azam Campus, Lahore 54590, Pakistan Received 1 March 1998; accepted 1 December 1999
Abstract A laboratory method of producing amorphous powder by fine gas jet impingement of molten Al–Mg–Zn alloy is described. The powder has been characterized by X-ray diffraction and differential thermal analysis ŽDTA. techniques. The grain size of the powder varied from 5 to 80 mm . The fine grains Žsize upto ; 20 mm. are completely amorphous. However, large grains are partly amorphous. Using the DTA technique, glassy–crystalline phase transition temperature Tg is found to be 473.68C. Vickers hardness of the grains is in the range 215 to 255 kgrmm2. q 2001 Elsevier Science S.A. All rights reserved. Keywords: Synthesis; Amorphous; Solidification
1. Introduction In recent years, there have been numerous studies aimed at the production of aluminium-based rapidly solidified alloys in the form of ribbons. The need for the development techniques to produce amorphous powder is an intermediate goal for the production of large size amorphous products. Amorphous grain geometries have inherent advantage in producing good intergrain bonding during compaction and densification. Al–Mg–Zn powder could, therefore, be used in the construction of the aircraft and automobile components. One of the major techniques to produce rapidly solidified amorphous powders is gas atomization where a thin drop of melt is impinged upon by a high velocity gas w1–3x. These powders are used for making compacted planar sheets. Stempin and Wexell w4x and Rawers et al. w5x have reported compaction parameters and crystallization temperatures of Al-based compacted powder sheets. At present, a few studies are available in the literature on the fabrication of Al-based rapidly solidified powders w6,7x. This study reports on the fabrication of rapidly solidified amorphous powder of Al 40 Mg 25 Zn 35 alloy by a gas atom-
)
Corresponding author. Tel.: q92-42-4864185; fax: q92-42-5864534. E-mail address: [email protected] ŽS. Din..
ization technique and its characterization by X-ray diffraction, differential thermal analysis ŽDTA. and Vickers hardness test.
2. Experimental In order to prepare Al 40 Mg 25 Zn 35 alloy; appropriate weights of pure metallic powders ŽGoodfellow Metals, UK, purity ; 99.85%. were melted in a muffle furnace. Over-heating was avoided as zinc and magnesium would be lost by volatilization at high temperatures. The Al– Mg–Zn melt was stirred continuously with a ceramic tube in which nitrogen was passed at the rate of 1 lrmin. The bubbling of nitrogen prevented the oxidation of the alloy. After cooling the alloy, it was placed in the furnace at 4008C for 2 h for homogenization. Fig. 1 is a schematic illustration of the equipment designed for the production of atomized Al–Mg–Zn alloy. A radio frequency Žr.f.. induction heating furnace ŽModel P-5 Sr-7 of KOKUSAI Electrical, Japan. was used for melting the pre-alloyed Al–Mg–Zn ingot. An alumina tube of length 10 cm, internal diameter 1 cm was used as a crucible. The tube was fitted with a steel nozzle of conical shape at one end and an inlet for the compressed argon gas at the other end. The tube was enveloped in a graphite receptor, with a thickness of 7 cm to couple the r.f. power
0032-5910r01r$ - see front matter q 2001 Elsevier Science S.A. All rights reserved. PII: S 0 0 3 2 - 5 9 1 0 Ž 0 0 . 0 0 2 6 7 - 9
52
S. Din, S.Y. Chishtir Powder Technology 114 (2001) 51–54
Fig. 1. Systematic illustration of the atomization equipment.
Rh thermocouple of 0.1-mm diameter with an open hot junction surrounded by a ceramic double hole sheath of 2-mm diameter was inserted into the hole in the copper tube. The thermocouple was connected to a data acquisition system, which allowed up to 300 measurements per second Žtime interval 0.0033 s.. The data was displayed as a cooling curve. The powder size was calculated from the S.E.M. micrographs by the line intercept method. Vickers microhardness test indentations were made at 100-g load. Contact time between the indenter and the sample was kept as 15 s. Five indentations at the same load were averaged to yield the average microhardness. X-ray diffraction analysis of as-cast Al 55 Mg 20 Zn 25 alloy and powder was performed using a Rigaku X.R.D.r MAX-11A diffractometer. The measurements were taken
to the alumina tube. The steel nozzle extruded from its bottom. This system was placed vertically and concentric to the output coil of the r.f. induction heating furnace. A fine jet of argon gas was made to impinge at a pressure of around 7 MPa on the droplets of molten alloy oozing from the orifice of the alumina crucible for atomization. An electronic meter, capable of measuring the gas speed, was assembled in the laboratory. The circuit used for this meter was the modified version of a digital speedometer designed by Knight w8x. The speed of the argon jet was maintained in the range 50–100 mrs. The quenching rate of the molten alloy depended upon the speed of the gas, jet impingement distance, and mass flow rate of alloy. The method employed for measuring the cooling rate was similar to the one employed by Liu and Fredriksson w9x. It involved collection of atomized powder in a copper tube of 5-mm diameter placed very close to the nozzle. A Pt–13%
Fig. 2. The phase diagram of Al–Mg–Zn system Žreproduced from Ref. w10x. Phase nomenclature: bs Al 8 Mg 5 ; gs Al 12 Mg 17 ; s s ŽAl,Zn. 2 Mg; T s ŽAl,Zn.49 Mg 32 ; fs Al 2 Mg 5 Zn 2 . The point APB corresponds to the composition Al 40 Mg 25 Zn 35 .
Fig. 3. Ža. Indexed X-ray diffraction pattern of as-cast alloy Al 40 Mg 25 Zn 35 . Žb. X-ray diffraction pattern of the rapidly solidified powder sample Žgrain size 20–30 mm.. Note the disappearance of several peaks seen in Ža. and broadening of the remaining peaks indicating amorphization.
S. Din, S.Y. Chishtir Powder Technology 114 (2001) 51–54
Fig. 4. DTA curve of Al 40 Mg 25 Zn 35 powder showing the glassy-crystallization phase change temperature Tg , indicated by an exotherm at 473.68C.
at room temperature using CuK a alpha radiation Ž l s ˚ .. Scanning electron micrographs ŽSEMs. of the 1.54178 A powder were taken using a high resolution scanning electron microscope JSM 35 CG, which was operated at 25 keV in a secondary electron image mode. Simultaneous thermal analyzer NETSCH STA-429 was used to record the phase transitions of the rapidly quenched powder. The test specimen was heated to 5208C at the rate of 108Crmin using an Al 2 O 3 crucible in the thermal analyser.
3. Results Several phase diagrams of Al–Mg–Zn system w10–12x are available. The phase diagram shown in Fig. 1 has been reproduced from Ref. w12x. The composition Al 40 Mg 25 Zn 35 is represented by point ‘P’ in this diagram and this lies in T phase wformula ŽAl,Zn.49 Mg 32 x, A s B phase wformula ŽAl,Zn. 2 Mgx and Al phase, which is in agreement with the
53
Fig. 6. Histogram of grain size of amorphous powder.
other phase studies ŽFig. 2.. X-ray diffraction pattern of as-cast Al 40 Mg 25 Zn 35 alloy is shown in Fig. 3Ža.. The pattern was indexed and compared with the Joint Committee Powder Diffraction Standards ŽJCPDS, Pennsylvania, USA. Powder Diffraction File No. 31-24. It was found that the observed and calculated d-values of peaks are in good agreement and are very close to those given in the powder diffraction file. Comparison of X-ray diffraction pattern of as-cast alloy and the powder is shown in Fig. 3Ža,b.. It clearly shows that the intensity of reflections is stronger in as-cast alloy and the peak width at half-maximum has increased for the powder sample. The DTA curve of powder in Fig. 4 shows endotherms at 473.68C. This is the glassy phase transition temperature, Tg . A SEM of the powder is shown in Fig. 5. The grain size varied from 5 to 80 mm. The shape of the grains also varied. Some grains were spherical and the rest were of irregular shape. The histogram of the grains, worked out from the SEM, is shown in Fig. 6. X-ray powder diffraction analysis showed the grains larger than 20 mm were partly amorphous and crystalline, whereas, the grains less than this size were completely amorphous. The quenching rate was estimated from the slope of the cooling curve as 10 5 Krs, depending upon the pressure of the impinging gas. This is the critical cooling rate required to retain an amorphous phase in alloys w13x. Vickers hardness of the lateral cross-sections of small grain size Ž10–20 mm. and large Ž20–80 mm. powder samples Al 40 Mg 25 Zn 35 were in the range 215 to 255 kgrmm2 . The Vickers hardness thus depends upon the phase of the surface Žamorphous or amorphous q crystalline..
4. Conclusions
Fig. 5. SEM of Al 40 Mg 25 Zn 35 powder sample.
The Al–Mg–Zn alloy was rapidly solidified into powder by the argon gas impingement at 7 MPa pressure. The
54
S. Din, S.Y. Chishtir Powder Technology 114 (2001) 51–54
glass transition temperature, Tg , of the powder was 473.68C. The grain size of the resulting powder was in the range of 5–80 mm. The Vickers hardness of small grains Žsize - 20 mm. was found to be less than the hardness of large grains Žsize ) 20 mm. by 16%. The small grains were completely amorphous, whereas, the large grains were partially transformed Žamorphous and crystalline..
w4x w5x w6x w7x w8x w9x w10x
References w11x w1x J.K. Beddow, The Production of Metal Powders by Atomization, Heydons & Sons, London, 1980. w2x A. Lawley, Int. J. Powder Metall. Powder Technol. 13 Ž1977. 169. w3x E. Klar, W.M. Shafer, in: J.J. Burke, V. Wiess ŽEds.., Powder
w12x w13x
Metallurgy for High Performance Applications, Syracuse Univ. Press, Syracuse, 1972, p. 72. J.L. Stempin, D.R. Wexell, U.S. Patent, 4298382 Ž1981.. J. Rawers, W. Sauer, R. German, J. Mater. Sci. Lett. 16 Ž1993. L327. S.A. Miller, R.J. Murphy, Scr. Metall. 13 Ž1979. 673. K. Ohtera, K. Kita, H. Nagahama, A. Inoue, T. Masumoto, J. Mater. Sci. Eng. A 179 Ž1994. 592. S. Knight, Everyday Pract. Electronics ŽU.K.. 22–28 Ž1993. 566. J. Liu, Fredriksson, Solidification processing, Proc. 3rd Int. Conf. The Institute of Metals, London Ž1987. 198. L.A. Wiley, Metals Handbook — Metallogr., Struct. and Phase Diagr. vol. 8 ASM, Ohio, 1973, p. 397. P.E. Dronen, N. Ryum, Metall. Mater. Trans. A ŽPhys. Metall. Mater. Sci.. 25A Ž3. Ž1994. 521. H. Liang, S.L. Chen, Y.A. Chang, Metall. Mater. Trans. A ŽPhys. Metall. Mater. Sci.. 28 Ž9. Ž1997. 1725. H. Ishii, M. Naka, T. Masumoto, Sci. Rep. Res. Inst., Tohoku Univ. 29 Ž1981. 343.
Synthesis and characterization of Al 40 Mg 25 Zn 35 amorphous powder by rapid solidification Salah-ud Din ) , S.Y. Chishti Centre of Excellence in Solid State Physics, UniÕersity of the Punjab, Quaid-e-Azam Campus, Lahore 54590, Pakistan Received 1 March 1998; accepted 1 December 1999
Abstract A laboratory method of producing amorphous powder by fine gas jet impingement of molten Al–Mg–Zn alloy is described. The powder has been characterized by X-ray diffraction and differential thermal analysis ŽDTA. techniques. The grain size of the powder varied from 5 to 80 mm . The fine grains Žsize upto ; 20 mm. are completely amorphous. However, large grains are partly amorphous. Using the DTA technique, glassy–crystalline phase transition temperature Tg is found to be 473.68C. Vickers hardness of the grains is in the range 215 to 255 kgrmm2. q 2001 Elsevier Science S.A. All rights reserved. Keywords: Synthesis; Amorphous; Solidification
1. Introduction In recent years, there have been numerous studies aimed at the production of aluminium-based rapidly solidified alloys in the form of ribbons. The need for the development techniques to produce amorphous powder is an intermediate goal for the production of large size amorphous products. Amorphous grain geometries have inherent advantage in producing good intergrain bonding during compaction and densification. Al–Mg–Zn powder could, therefore, be used in the construction of the aircraft and automobile components. One of the major techniques to produce rapidly solidified amorphous powders is gas atomization where a thin drop of melt is impinged upon by a high velocity gas w1–3x. These powders are used for making compacted planar sheets. Stempin and Wexell w4x and Rawers et al. w5x have reported compaction parameters and crystallization temperatures of Al-based compacted powder sheets. At present, a few studies are available in the literature on the fabrication of Al-based rapidly solidified powders w6,7x. This study reports on the fabrication of rapidly solidified amorphous powder of Al 40 Mg 25 Zn 35 alloy by a gas atom-
)
Corresponding author. Tel.: q92-42-4864185; fax: q92-42-5864534. E-mail address: [email protected] ŽS. Din..
ization technique and its characterization by X-ray diffraction, differential thermal analysis ŽDTA. and Vickers hardness test.
2. Experimental In order to prepare Al 40 Mg 25 Zn 35 alloy; appropriate weights of pure metallic powders ŽGoodfellow Metals, UK, purity ; 99.85%. were melted in a muffle furnace. Over-heating was avoided as zinc and magnesium would be lost by volatilization at high temperatures. The Al– Mg–Zn melt was stirred continuously with a ceramic tube in which nitrogen was passed at the rate of 1 lrmin. The bubbling of nitrogen prevented the oxidation of the alloy. After cooling the alloy, it was placed in the furnace at 4008C for 2 h for homogenization. Fig. 1 is a schematic illustration of the equipment designed for the production of atomized Al–Mg–Zn alloy. A radio frequency Žr.f.. induction heating furnace ŽModel P-5 Sr-7 of KOKUSAI Electrical, Japan. was used for melting the pre-alloyed Al–Mg–Zn ingot. An alumina tube of length 10 cm, internal diameter 1 cm was used as a crucible. The tube was fitted with a steel nozzle of conical shape at one end and an inlet for the compressed argon gas at the other end. The tube was enveloped in a graphite receptor, with a thickness of 7 cm to couple the r.f. power
0032-5910r01r$ - see front matter q 2001 Elsevier Science S.A. All rights reserved. PII: S 0 0 3 2 - 5 9 1 0 Ž 0 0 . 0 0 2 6 7 - 9
52
S. Din, S.Y. Chishtir Powder Technology 114 (2001) 51–54
Fig. 1. Systematic illustration of the atomization equipment.
Rh thermocouple of 0.1-mm diameter with an open hot junction surrounded by a ceramic double hole sheath of 2-mm diameter was inserted into the hole in the copper tube. The thermocouple was connected to a data acquisition system, which allowed up to 300 measurements per second Žtime interval 0.0033 s.. The data was displayed as a cooling curve. The powder size was calculated from the S.E.M. micrographs by the line intercept method. Vickers microhardness test indentations were made at 100-g load. Contact time between the indenter and the sample was kept as 15 s. Five indentations at the same load were averaged to yield the average microhardness. X-ray diffraction analysis of as-cast Al 55 Mg 20 Zn 25 alloy and powder was performed using a Rigaku X.R.D.r MAX-11A diffractometer. The measurements were taken
to the alumina tube. The steel nozzle extruded from its bottom. This system was placed vertically and concentric to the output coil of the r.f. induction heating furnace. A fine jet of argon gas was made to impinge at a pressure of around 7 MPa on the droplets of molten alloy oozing from the orifice of the alumina crucible for atomization. An electronic meter, capable of measuring the gas speed, was assembled in the laboratory. The circuit used for this meter was the modified version of a digital speedometer designed by Knight w8x. The speed of the argon jet was maintained in the range 50–100 mrs. The quenching rate of the molten alloy depended upon the speed of the gas, jet impingement distance, and mass flow rate of alloy. The method employed for measuring the cooling rate was similar to the one employed by Liu and Fredriksson w9x. It involved collection of atomized powder in a copper tube of 5-mm diameter placed very close to the nozzle. A Pt–13%
Fig. 2. The phase diagram of Al–Mg–Zn system Žreproduced from Ref. w10x. Phase nomenclature: bs Al 8 Mg 5 ; gs Al 12 Mg 17 ; s s ŽAl,Zn. 2 Mg; T s ŽAl,Zn.49 Mg 32 ; fs Al 2 Mg 5 Zn 2 . The point APB corresponds to the composition Al 40 Mg 25 Zn 35 .
Fig. 3. Ža. Indexed X-ray diffraction pattern of as-cast alloy Al 40 Mg 25 Zn 35 . Žb. X-ray diffraction pattern of the rapidly solidified powder sample Žgrain size 20–30 mm.. Note the disappearance of several peaks seen in Ža. and broadening of the remaining peaks indicating amorphization.
S. Din, S.Y. Chishtir Powder Technology 114 (2001) 51–54
Fig. 4. DTA curve of Al 40 Mg 25 Zn 35 powder showing the glassy-crystallization phase change temperature Tg , indicated by an exotherm at 473.68C.
at room temperature using CuK a alpha radiation Ž l s ˚ .. Scanning electron micrographs ŽSEMs. of the 1.54178 A powder were taken using a high resolution scanning electron microscope JSM 35 CG, which was operated at 25 keV in a secondary electron image mode. Simultaneous thermal analyzer NETSCH STA-429 was used to record the phase transitions of the rapidly quenched powder. The test specimen was heated to 5208C at the rate of 108Crmin using an Al 2 O 3 crucible in the thermal analyser.
3. Results Several phase diagrams of Al–Mg–Zn system w10–12x are available. The phase diagram shown in Fig. 1 has been reproduced from Ref. w12x. The composition Al 40 Mg 25 Zn 35 is represented by point ‘P’ in this diagram and this lies in T phase wformula ŽAl,Zn.49 Mg 32 x, A s B phase wformula ŽAl,Zn. 2 Mgx and Al phase, which is in agreement with the
53
Fig. 6. Histogram of grain size of amorphous powder.
other phase studies ŽFig. 2.. X-ray diffraction pattern of as-cast Al 40 Mg 25 Zn 35 alloy is shown in Fig. 3Ža.. The pattern was indexed and compared with the Joint Committee Powder Diffraction Standards ŽJCPDS, Pennsylvania, USA. Powder Diffraction File No. 31-24. It was found that the observed and calculated d-values of peaks are in good agreement and are very close to those given in the powder diffraction file. Comparison of X-ray diffraction pattern of as-cast alloy and the powder is shown in Fig. 3Ža,b.. It clearly shows that the intensity of reflections is stronger in as-cast alloy and the peak width at half-maximum has increased for the powder sample. The DTA curve of powder in Fig. 4 shows endotherms at 473.68C. This is the glassy phase transition temperature, Tg . A SEM of the powder is shown in Fig. 5. The grain size varied from 5 to 80 mm. The shape of the grains also varied. Some grains were spherical and the rest were of irregular shape. The histogram of the grains, worked out from the SEM, is shown in Fig. 6. X-ray powder diffraction analysis showed the grains larger than 20 mm were partly amorphous and crystalline, whereas, the grains less than this size were completely amorphous. The quenching rate was estimated from the slope of the cooling curve as 10 5 Krs, depending upon the pressure of the impinging gas. This is the critical cooling rate required to retain an amorphous phase in alloys w13x. Vickers hardness of the lateral cross-sections of small grain size Ž10–20 mm. and large Ž20–80 mm. powder samples Al 40 Mg 25 Zn 35 were in the range 215 to 255 kgrmm2 . The Vickers hardness thus depends upon the phase of the surface Žamorphous or amorphous q crystalline..
4. Conclusions
Fig. 5. SEM of Al 40 Mg 25 Zn 35 powder sample.
The Al–Mg–Zn alloy was rapidly solidified into powder by the argon gas impingement at 7 MPa pressure. The
54
S. Din, S.Y. Chishtir Powder Technology 114 (2001) 51–54
glass transition temperature, Tg , of the powder was 473.68C. The grain size of the resulting powder was in the range of 5–80 mm. The Vickers hardness of small grains Žsize - 20 mm. was found to be less than the hardness of large grains Žsize ) 20 mm. by 16%. The small grains were completely amorphous, whereas, the large grains were partially transformed Žamorphous and crystalline..
w4x w5x w6x w7x w8x w9x w10x
References w11x w1x J.K. Beddow, The Production of Metal Powders by Atomization, Heydons & Sons, London, 1980. w2x A. Lawley, Int. J. Powder Metall. Powder Technol. 13 Ž1977. 169. w3x E. Klar, W.M. Shafer, in: J.J. Burke, V. Wiess ŽEds.., Powder
w12x w13x
Metallurgy for High Performance Applications, Syracuse Univ. Press, Syracuse, 1972, p. 72. J.L. Stempin, D.R. Wexell, U.S. Patent, 4298382 Ž1981.. J. Rawers, W. Sauer, R. German, J. Mater. Sci. Lett. 16 Ž1993. L327. S.A. Miller, R.J. Murphy, Scr. Metall. 13 Ž1979. 673. K. Ohtera, K. Kita, H. Nagahama, A. Inoue, T. Masumoto, J. Mater. Sci. Eng. A 179 Ž1994. 592. S. Knight, Everyday Pract. Electronics ŽU.K.. 22–28 Ž1993. 566. J. Liu, Fredriksson, Solidification processing, Proc. 3rd Int. Conf. The Institute of Metals, London Ž1987. 198. L.A. Wiley, Metals Handbook — Metallogr., Struct. and Phase Diagr. vol. 8 ASM, Ohio, 1973, p. 397. P.E. Dronen, N. Ryum, Metall. Mater. Trans. A ŽPhys. Metall. Mater. Sci.. 25A Ž3. Ž1994. 521. H. Liang, S.L. Chen, Y.A. Chang, Metall. Mater. Trans. A ŽPhys. Metall. Mater. Sci.. 28 Ž9. Ž1997. 1725. H. Ishii, M. Naka, T. Masumoto, Sci. Rep. Res. Inst., Tohoku Univ. 29 Ž1981. 343.