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Preparation of W–Al alloys by mechanical alloying
Journal of Alloys and Compounds 347 (2002) 228–230
L
www.elsevier.com / locate / jallcom
Preparation of W–Al alloys by mechanical alloying H.G. Tang, X.F. Ma*, W. Zhao, X.W. Yan, R.J. Hong Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China Received 12 November 2001; accepted 7 March 2002
Abstract W12x Al x (x50–0.86) alloys were synthesized by mechanically alloying the pure metal powder mixtures at designated compositions by conventional high-energy ball milling. The W–Al alloys were stable under high pressure and high temperature. The alloys were lighter than W. The hardness and oxidation resistance of the alloys was greatly improved compared to both W and Al. 2002 Elsevier Science B.V. All rights reserved. Keywords: Mechanical alloying; W–Al alloys; Hardness; Density; Oxidation resistance
1. Introduction
2. Experimental
Tungsten has a variety of special applications that depend on its unique properties [1]. With the development of the science, it needs an even wider range of applications. But there are several shortcomings which limit its utility. W–Al alloys are expected to show improved mechanical properties compared with W. Owing to the very large difference between the melting points of tungsten (3683 K) and aluminum (933 K), and also the density of tungsten (19.25 g / cm 3 ) and aluminum (2.70 g / cm 3 ), the alloying of such incompatible elements by melting may present considerable problems. But these may be overcome by mechanical alloying. Mechanical alloying (MA) [2–4] is a high-energy ball milling technique and it utilizes various types of milling machines in which a blend of different powders is subjected to highly energetic compressive forces. By the repeated fracture and cold welding of the constituent powder particles, it is possible to make alloys from normally immiscible components. A large number of studies reported in the literature have dealt with Al–Ni, Al–Ti and Al–Fe alloys [5–7]. One group has reported on the W–Al system by mechanical alloying. However, they suggested that Al dissolves into the W (bcc) lattice at less than 50% [8]. MA extends the solid solubility from 50 to 86% in the as milled alloys of our present study.
Elemental powders of tungsten (2200 mesh, 99.8% purity) and aluminum (2200 mesh, 99.5% purity) were used as starting materials. The powders were sealed (Al must be added in step by step when x.0.5) under an argon atmosphere in steel vials with ball-to-powder weight ratios of 30:1–15:1, milled for various times from 20 to 120 h. Alloy powders were sintered in the cell of a 638MN cubic anvil press (DS-029C) under high pressure and high temperature. X-ray diffraction (XRD) analyses were performed on a Rigaku D/ max-IIB X-ray diffractometer with CuKa radia˚ at room temperature. The scan speed tion ( l51.54178 A) was 48 / min. A Vickers indenter with a load of 50 kg was employed to determine the hardness of the samples. The density of the samples was determined by the Archimedes principle using water. Oxidation tests were conducted in air using a high temperature thermal analysis system.
*Corresponding author. Fax: 186-431-5685653. E-mail address: [email protected] (X.F. Ma).
3. Results and discussion
3.1. X-ray diffractometry The XRD patterns of the mechanically alloyed powders of one characteristic composition for various milling times are presented in Fig. 1. As the milling time increased, both the W and Al peaks broadened. The Al peak intensities continuously decreased as the milling time increased and
0925-8388 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 02 )00760-0
H.G. Tang et al. / Journal of Alloys and Compounds 347 (2002) 228–230
Fig. 1. XRD patterns of the mechanically alloyed W0.5 Al 0.5 powders after various milling times.
disappeared completely for the sample milled for 20 h. A solid solution of W0.5 Al 0.5 was obtained. Fig. 2 shows the XRD patterns of the mechanically alloyed powders of several compositions for various milling times. Only W peaks can be seen in the figure (x50–0.86) while the Al peaks disappear, and both the Al and W peaks are visible in the diffraction patterns (x50.9). It is clear that a solid solution of Al in W was obtained and the solubility reached 86%. A longer milling time was needed with the increase in aluminum. Alloy powders of different compositions were sintered under high pressure and temperature. The temperature of the press was set between 1200 and 1600 8C; the pressure at 4 GPa. Alloy tablets were obtained. Fig. 3 shows the
229
Fig. 3. XRD patterns of the ball-milled W0.2 Al 0.8 (a) and the same material after sintering at 1200 8C for 5 min (b).
XRD patterns of the ball-milled W0.2 Al 0.8 and the same material after sintering at 1200 8C for 5 min. The tests on the W0.2 Al 0.8 prove that the Al peaks in the XRD pattern are not present at 1200 8C. The peaks are sharper and the intensities increase remarkably. This indicates that the crystallinity of the alloy W0.2 Al 0.8 has been improved and it is still stable under high pressure and temperature. The Hume–Rothery rule suggests that good solubility (5 at.% at moderate temperatures) requires that the metallic radii of the solvent and solute should differ by less than 15% and the electronegativity difference between the elements should not be too large, typically within a range of 60.4 [9]. The metallic radii of W and Al differ by 4.5% ˚ and that of Al is 1.431 A). ˚ The (the radii of W is 1.370 A electronegativities differ by 0.1 (W is 1.6 and Al is 1.5). Both conditions of the Hume–Rothery rules are satisfied, so the solid solubility of Al in W can be extended by MA under nonequilibrium conditions. The alloying can be ascribed to the fact that during the milling process, aluminum is embedded in the tungsten matrix and the aluminum atoms displace the positions of tungsten.
3.2. Hardness and density
Fig. 2. XRD patterns of mechanically alloyed powders as a function of Al content. Milling time for x50.5–0.67 was 20 h, for x50.8–0.83 was 80 h and for x50.86–0.9 was 120 h.
Table 1 shows the micro vicker-hardness of the tablets. The result shows that the hardnesses of the alloys have greatly improved. The values are about 3 times as big as W. When the aluminum atoms are dissolved in the tungsten matrix, the W–Al system can exhibit a new degree of hardness. Table 2 shows the densities of the alloys. They are all lighter than pure W. The measured density is more consistent with a substitutional solid than an interstitial solid solution [10], so the values of the density decreased remarkably with the increasing of the concentration of Al.
H.G. Tang et al. / Journal of Alloys and Compounds 347 (2002) 228–230
230
Table 1 Hardness of several compositions of the alloys compared with standard values of W
Hv (kg / mm 2 )
W
W0.5 Al 0.5
W0.33 Al 0.67
W0.25 Al 0.75
W0.17 Al 0.83
W0.14 Al 0.86
394
1027
927
1137
1137
1090
Table 2 Density of several compositions of the alloys
r (g / cm 3 )
W
W0.5 Al 0.5
W0.33 Al 0.67
W0.25 Al 0.75
W0.17 Al 0.83
W0.14 Al 0.86
19.25
10.25
10.02
6.78
4.83
4.54
3.3. Oxidation resistance Oxidation tests were conducted on W0.5 Al 0.5 at 300 and 600 8C. The oxidation resistance of the composite at 300 8C was fairly good and was comparable to that of W and Al. Both W and Al have become oxides at 300 8C, while W0.5 Al 0.5 shows little oxidation. The oxidation of W0.5 Al 0.5 becomes significant at about 600 8C. We consider that the oxide films were formed on the surface of the crystallite during the process of MA, so the oxidation resistance of the alloys has been much improved.
4. Conclusion In our present work W12x Al x (x50–0.86) alloys were prepared by MA. The alloys are lightened products. In
addition, the hardness and oxidation resistance of the alloys are greatly improved compared to W and Al.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
C.L. Briant, Adv. Mater. Process. 11 (1998) 29–31. J.S. Benjamin, Metall. Trans. 1 (1970) 2943. J.S. Benjamin, Sci. Am. 234 (1976) 40. P.S. Gilman, J.S. Benjamin, Mech.-Alloy, Ann. Rev. Mater. Sci. 13 (1983) 279. R.L. Valier, Nanostruct. Mater. 6 (1993) 73. C. Suryanarayano, Int. Mater. Rev. 40 (1995) 41. M.A. Morris, D.G. Morros, Mater. Sci. Forum 271 (1992) 88–90. Y.F. Oyang, Sci. China (A) 30 (2000) 49–53. C. Bansal, Z.Q. Gao, L.B. Hong, B. Fultz, J. Appl. Phys. 76 (1994) 5961. A. Hightower, J. Alloy Comp. 252 (1997) 238–244.
L
www.elsevier.com / locate / jallcom
Preparation of W–Al alloys by mechanical alloying H.G. Tang, X.F. Ma*, W. Zhao, X.W. Yan, R.J. Hong Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China Received 12 November 2001; accepted 7 March 2002
Abstract W12x Al x (x50–0.86) alloys were synthesized by mechanically alloying the pure metal powder mixtures at designated compositions by conventional high-energy ball milling. The W–Al alloys were stable under high pressure and high temperature. The alloys were lighter than W. The hardness and oxidation resistance of the alloys was greatly improved compared to both W and Al. 2002 Elsevier Science B.V. All rights reserved. Keywords: Mechanical alloying; W–Al alloys; Hardness; Density; Oxidation resistance
1. Introduction
2. Experimental
Tungsten has a variety of special applications that depend on its unique properties [1]. With the development of the science, it needs an even wider range of applications. But there are several shortcomings which limit its utility. W–Al alloys are expected to show improved mechanical properties compared with W. Owing to the very large difference between the melting points of tungsten (3683 K) and aluminum (933 K), and also the density of tungsten (19.25 g / cm 3 ) and aluminum (2.70 g / cm 3 ), the alloying of such incompatible elements by melting may present considerable problems. But these may be overcome by mechanical alloying. Mechanical alloying (MA) [2–4] is a high-energy ball milling technique and it utilizes various types of milling machines in which a blend of different powders is subjected to highly energetic compressive forces. By the repeated fracture and cold welding of the constituent powder particles, it is possible to make alloys from normally immiscible components. A large number of studies reported in the literature have dealt with Al–Ni, Al–Ti and Al–Fe alloys [5–7]. One group has reported on the W–Al system by mechanical alloying. However, they suggested that Al dissolves into the W (bcc) lattice at less than 50% [8]. MA extends the solid solubility from 50 to 86% in the as milled alloys of our present study.
Elemental powders of tungsten (2200 mesh, 99.8% purity) and aluminum (2200 mesh, 99.5% purity) were used as starting materials. The powders were sealed (Al must be added in step by step when x.0.5) under an argon atmosphere in steel vials with ball-to-powder weight ratios of 30:1–15:1, milled for various times from 20 to 120 h. Alloy powders were sintered in the cell of a 638MN cubic anvil press (DS-029C) under high pressure and high temperature. X-ray diffraction (XRD) analyses were performed on a Rigaku D/ max-IIB X-ray diffractometer with CuKa radia˚ at room temperature. The scan speed tion ( l51.54178 A) was 48 / min. A Vickers indenter with a load of 50 kg was employed to determine the hardness of the samples. The density of the samples was determined by the Archimedes principle using water. Oxidation tests were conducted in air using a high temperature thermal analysis system.
*Corresponding author. Fax: 186-431-5685653. E-mail address: [email protected] (X.F. Ma).
3. Results and discussion
3.1. X-ray diffractometry The XRD patterns of the mechanically alloyed powders of one characteristic composition for various milling times are presented in Fig. 1. As the milling time increased, both the W and Al peaks broadened. The Al peak intensities continuously decreased as the milling time increased and
0925-8388 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 02 )00760-0
H.G. Tang et al. / Journal of Alloys and Compounds 347 (2002) 228–230
Fig. 1. XRD patterns of the mechanically alloyed W0.5 Al 0.5 powders after various milling times.
disappeared completely for the sample milled for 20 h. A solid solution of W0.5 Al 0.5 was obtained. Fig. 2 shows the XRD patterns of the mechanically alloyed powders of several compositions for various milling times. Only W peaks can be seen in the figure (x50–0.86) while the Al peaks disappear, and both the Al and W peaks are visible in the diffraction patterns (x50.9). It is clear that a solid solution of Al in W was obtained and the solubility reached 86%. A longer milling time was needed with the increase in aluminum. Alloy powders of different compositions were sintered under high pressure and temperature. The temperature of the press was set between 1200 and 1600 8C; the pressure at 4 GPa. Alloy tablets were obtained. Fig. 3 shows the
229
Fig. 3. XRD patterns of the ball-milled W0.2 Al 0.8 (a) and the same material after sintering at 1200 8C for 5 min (b).
XRD patterns of the ball-milled W0.2 Al 0.8 and the same material after sintering at 1200 8C for 5 min. The tests on the W0.2 Al 0.8 prove that the Al peaks in the XRD pattern are not present at 1200 8C. The peaks are sharper and the intensities increase remarkably. This indicates that the crystallinity of the alloy W0.2 Al 0.8 has been improved and it is still stable under high pressure and temperature. The Hume–Rothery rule suggests that good solubility (5 at.% at moderate temperatures) requires that the metallic radii of the solvent and solute should differ by less than 15% and the electronegativity difference between the elements should not be too large, typically within a range of 60.4 [9]. The metallic radii of W and Al differ by 4.5% ˚ and that of Al is 1.431 A). ˚ The (the radii of W is 1.370 A electronegativities differ by 0.1 (W is 1.6 and Al is 1.5). Both conditions of the Hume–Rothery rules are satisfied, so the solid solubility of Al in W can be extended by MA under nonequilibrium conditions. The alloying can be ascribed to the fact that during the milling process, aluminum is embedded in the tungsten matrix and the aluminum atoms displace the positions of tungsten.
3.2. Hardness and density
Fig. 2. XRD patterns of mechanically alloyed powders as a function of Al content. Milling time for x50.5–0.67 was 20 h, for x50.8–0.83 was 80 h and for x50.86–0.9 was 120 h.
Table 1 shows the micro vicker-hardness of the tablets. The result shows that the hardnesses of the alloys have greatly improved. The values are about 3 times as big as W. When the aluminum atoms are dissolved in the tungsten matrix, the W–Al system can exhibit a new degree of hardness. Table 2 shows the densities of the alloys. They are all lighter than pure W. The measured density is more consistent with a substitutional solid than an interstitial solid solution [10], so the values of the density decreased remarkably with the increasing of the concentration of Al.
H.G. Tang et al. / Journal of Alloys and Compounds 347 (2002) 228–230
230
Table 1 Hardness of several compositions of the alloys compared with standard values of W
Hv (kg / mm 2 )
W
W0.5 Al 0.5
W0.33 Al 0.67
W0.25 Al 0.75
W0.17 Al 0.83
W0.14 Al 0.86
394
1027
927
1137
1137
1090
Table 2 Density of several compositions of the alloys
r (g / cm 3 )
W
W0.5 Al 0.5
W0.33 Al 0.67
W0.25 Al 0.75
W0.17 Al 0.83
W0.14 Al 0.86
19.25
10.25
10.02
6.78
4.83
4.54
3.3. Oxidation resistance Oxidation tests were conducted on W0.5 Al 0.5 at 300 and 600 8C. The oxidation resistance of the composite at 300 8C was fairly good and was comparable to that of W and Al. Both W and Al have become oxides at 300 8C, while W0.5 Al 0.5 shows little oxidation. The oxidation of W0.5 Al 0.5 becomes significant at about 600 8C. We consider that the oxide films were formed on the surface of the crystallite during the process of MA, so the oxidation resistance of the alloys has been much improved.
4. Conclusion In our present work W12x Al x (x50–0.86) alloys were prepared by MA. The alloys are lightened products. In
addition, the hardness and oxidation resistance of the alloys are greatly improved compared to W and Al.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
C.L. Briant, Adv. Mater. Process. 11 (1998) 29–31. J.S. Benjamin, Metall. Trans. 1 (1970) 2943. J.S. Benjamin, Sci. Am. 234 (1976) 40. P.S. Gilman, J.S. Benjamin, Mech.-Alloy, Ann. Rev. Mater. Sci. 13 (1983) 279. R.L. Valier, Nanostruct. Mater. 6 (1993) 73. C. Suryanarayano, Int. Mater. Rev. 40 (1995) 41. M.A. Morris, D.G. Morros, Mater. Sci. Forum 271 (1992) 88–90. Y.F. Oyang, Sci. China (A) 30 (2000) 49–53. C. Bansal, Z.Q. Gao, L.B. Hong, B. Fultz, J. Appl. Phys. 76 (1994) 5961. A. Hightower, J. Alloy Comp. 252 (1997) 238–244.