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Formation and physical properties of Al base alloys by sputtering
Vacuum 59 (2000) 252}259
Formation and physical properties of Al base alloys by sputtering M. Naka *, T. Shibayanagi , M. Maeda , S. Zhao, H. Mori Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan Graduate Student, Osaka University, Suita 565, Japan Research Center for Ultra-High Voltage Electron Microscope, Osaka Univeristy, Suita, Osaka 567, Japan
Abstract Ti}Al and Ti}Si alloys were prepared by RF sputtering in a low-pressure argon atmosphere. The microhardness and thermal stability of these alloys were investigated in relation to the structure of alloys. In Al}Ti system, an amorphous phase and AlTi were formed in the composition range of 35}60 at%Ti and 60}80.5 at%Ti, respectively. In Al}Si system, an amorphous phase was formed for Si content of 45 at% or more. Although the microhardeness of Al}Ti alloys shows a maximum at 50 at%Ti, its value for Al}Si alloys rises with increasing Si content. These results suggest that the atomic bonding character is di!erent in both alloys, and the covalent character becomes stronger for Si rich Al}Si alloys. 2000 Elsevier Science Ltd. All rights reserved. Keywords: Sputtering; Aluminum; Titanium; Silicon; Amorphous alloys; Microhardness
1. Introduction As new applications of materials increase in engineering practice, it becomes more and more important to develop advanced materials. The enhansive new applications in engineering "elds need to make engineering materials more functional. Non-equilibrium materials possess high possibilities in industrial applications, because of their superior properties. Nano size particles, super-saturated solid solutions yield outstanding physical, chemical and mechanical properties. Non-equilibrium phases are de"ned as phases which are di!erent in their atomic arrangement from that of equilibrium phases [1,2]. For instance, a hexagonal phase at non-equilibrium state
* Corresponding author. Fax: #816-6879-8649. E-mail address: [email protected] (M. Naka). 0042-207X/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 0 ) 0 0 2 7 7 - 3
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in Au}Ge system [3], and amorphous phase which possesses a random atomic arrangement in metal}P systems [4] were reported. As a continuation of our work on Ti base structural materials [5], the present work describes progress in Al base alloys. These are Al}Ti and Al}Si alloys prepared by magnetron sputtering. In this paper we investigate the physical properties of structure, mechanical properties and the thermal stability of the non-equilibrium alloys.
2. Experimental procedure RF magnetron sputtering was used to prepare amorphous alloys in a low-pressure argon gas of 6.65 MPa. A schematic con"guration of the sputtering apparatus is presented in Fig. 1. Targets used were 100 mm in diameter and 5 mm thick and were composed of Al and Ti or Al and Si. Aluminum substrates were water cooled. The main sputtering conditions were a sputtering power and time of 600 W and 14.4 ks, respectively. The "lm thickness of the alloys prepared was 20 lm. The structure of sputtered "lms was investigated by X-ray di!ractometry. The mechanical properties and thermal stability of the "lms were measured by using a Knoop microhardness and a di!erential scanning calorimeter, respectively. The thermal stability of the "lms was also investigated by a hot stage of electron microscope.
Fig. 1. X-ray di!raction pattern of Al}Ti alloys.
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3. Results and discussion 3.1. X-ray diwraction analysis of Al}Ti alloys The structure of the sputtered alloys was analyzed by X-ray di!ractometry. Fig. 1 shows X-ray di!raction patterns of Al-17.6 at%Ti, Al-52.1 at%Ti, Al-80.5 at%Ti and Al-90.3 at%Ti alloys. Al}Ti alloys with Ti content up to 32.4 at%Ti yield X-ray di!raction pattern of an fcc phase of Al solid solution dissolving Ti. Although Al Ti and Al Ti intermetallics exist at corresponding Ti content in the equilibrium Al}Ti phase diagram, Al solid solution saturated with Ti is formed by suppressing the formation of those intermetallics in the non-equilibrium sputtering state. The Al}Ti sputtered alloys with Ti content from 35}58.7 at% show few di!use X-ray di!raction patterns around 2h"393 for the structure of the amorphous phase. These results indicate that the amorphous phase possesses local atomic arrangements. Al}Ti alloys with Ti content of 60 at% or more are crystalline phase. Al}Ti alloys with Ti content from 60}85 at% and alloys with Ti content of 85 at% or more are the crystal structure of Ti Al and hcp Ti solid solution saturated with Al, respectively. Mixed regions with crystalline phases at the equilibrium state become a single phase region, including crystalline or amorphous phase by sputtering. The formation range of sputtered phases in Al}Ti alloys is superimposed on Al}Ti phase diagram [6] of Fig. 2. 3.2. Mechanical properties of Al}Ti sputtered alloys Fig. 3 shows the relationship between microhardness and Ti content in Al}Ti sputtered alloys. In the Al-rich side, the microhardness of Al}Ti sputtered alloys increases continuously with increasing
Fig. 2. Structure of Al}Ti sputtered alloys.
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Fig. 3. Microhardness of Al}Ti sputtered alloys.
Fig. 4. Crystallization temperature of Al}Ti alloys plotted against Ti content.
Ti content in the compositional range from Al crystalline solid solution to amorphous phase. The increase in microhardness with Ti content up to 32.5 at% arises from the dissolving e!ect of large amounts of Ti in the FCC crystalline Al}Ti alloys. The microhardness of Al}Ti alloys increases successively even in the range of amorphous phases with Ti content from 35 at%Ti to 60 at%Ti. This tendency of the microhardness for the amorphous phase means that the amorphous phase can be thought of as a kind of solid solution. The microhardness of amorphous Al}Ti alloys yields
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a maximum at 50 at%Ti around the composition of Al}Ti intermetallic. This compositional dependence suggests that the local atomic arrangement in the amorphous Al}Ti alloy possesses a similar structure to the Al}Ti intermetallic. 3.3. Thermal stability of Al}Ti alloys The thermal stability for crystallization of Al}Ti amorphous sputtered alloys was determined by measuring di!erential scanning calorimetry (DSC) curves. The crystallization temperature and exothermic heat were obtained from the exothermic beginning point and the area of exothermic peak. Fig. 4 shows the dependence of the crystallization temperature and exothermic heat for Al}Ti alloys on the Ti content. The temperature of Al}Ti alloys increases from 555 K for Al-58.7 at% to 781 K for Al-58.7 at%Ti., and the temperature of the alloys rises sharply. The thermal stability of amorphous Al}Ti alloys is improved with increasing Ti content. The exothermic heat decreases with increasing Ti content in the alloys. Thus, the energy di!erence between the amorphous phase and the crystallized phase becomes smaller. 3.4. Structural analysis of Al}Si sputtered alloys Fig. 5 shows the X-ray di!raction patterns of Al}Si sputtered alloys. Al-6.9 at%Si alloy is a solid solution containing a large concentration of Si. Since Al dissolves only the Si content of 2 at% in the equilibrium state, the sputtered alloy is a non-equilibrium phase. Al}Si alloys with Si content up to 42.3 at% are composed of mixed phases of Al solid solution and Si. Al}Si alloys with Si content of 55 at% or more constitute an amorphous single phase which shows di!used X-ray di!raction patterns. When the alloy possesses excess Si atoms, a ratio greater than Si/Al of 0.5, the
Fig. 5. X-ray di!raction pattern of Al}Si sputtered alloys.
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Fig. 6. TEM bright image (a) and electron di!raction pattern (b) of Al-54.1 at%Si alloys.
amorphous structure becomes stable in the Al}Si alloys. Fig. 6 shows TEM bright image and electron di!raction pattern of Al-54.1 at%Si alloy. The electron di!raction pattern shows the pattern of crystalline Si phase. These results indicate that amorphous and crystalline Si phases co-exist in this alloy. Fig. 7 represents the structure of sputtered Al}Si alloys with the corresponding binary phase diagram [7]. Al}Ti alloys with Si content up to 45 at% are composed of Al solid solution and crystalline Si. The alloys with Si content from 45 at% to 55 at% are mixed phases of amorphous phase and crystalline Si phase. The amorphous single phase is formed at Si content of 50 at% or more. Fig. 8 shows the microhardness for the Al}Ti alloys. The microhardness of the alloys increases from 100 Hk for pure Al to 500 Hk for Al-40 at%Si. The value of 500 Hk is higher than 130 Hk for Al-13 at%Si Silumin alloy with Mg treatment. The microhardness of the amorphous phase increases as the Si content increases upto 55 at% or more, and reaches 1150 Hk for pure Si. This increase in the hardness of the Al}Si alloys with Si content of 55 at% or more arises from the increase in the number of Si to Si bonding in the amorphous phase. The covalent bonding of Si to Si is known to be strong. 3.5. Thermal stability of Al}Si alloys Since Al}Si sputtered alloys are in a non-equilibrium state, the alloys transform to an equilibrium state. The thermal stability of the alloys was investigated using a di!erential thermal calorimeter. Amorphous Al}Si alloys crystallize to a mixture of Al and Si with exothermic heat during heating. Fig. 9 shows the Si content dependence of the crystallization of Al}Si amorphous alloys. The exothermic crystallization takes place in two stages. In the "rst peak amorphous phases transform to mixed phases of Al and Si, and in the second peak the mixed phases cause the grain
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Fig. 7. Structure of Al}Si sputtered alloys.
Fig. 8. Microhardness of Al}Si sputtered alloys.
growth. The crystallization temperatures in two stages de"nitely shift to higher temperatures with increasing Si content. In other words, the thermal stability of amorphous Si drastically reduces with increasing Al content in amorphous Si. This harmful e!ect is realized for an amorphous phase formed by a di!usion process of the Al/Si interface.
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Fig. 9. Crystallization temperature of Al}Si alloys.
4. Conclusion Al}Ti and Al}Si non-equilibrium alloys were prepared by sputtering in a low-pressure argon atmosphere. In the Al-rich or Ti-rich alloy side, super-saturated solid solutions with Ti or Al were formed. Near Ti Al composition, an intermetallic super-saturated with Ti or Al is formed. An amorphous phase, which possesses a random atomic arrangement was formed at Ti content from 35.0 at%Ti}58.7 at%. Mixed phases of Al and Si, and amorphous single phases were formed at Si contents up to 432.3 at%, and Si content of 54 at% or more, respectively. Although the microhardness of Al}Ti amorphous alloys shows a maximum near the Al}Ti intermetallic, the microhardness of Al}Si amorphous alloys yields a monotonic increase with an increase in Si content. These di!erent tendencies of microhardness for Al}Ti and Al}Si alloys suggest that the bonding character in both alloys is di!erent, and the covalent character becomes stronger in Si-rich Al}Si alloys. At a crystallization temperature amorphous phase crystallizes to crystalline phases. The crystallization temperatures and thermal stability of both Al}Ti and Al}Si alloys rise with increasing Ti and Si content, respectively.
References [1] Naka M, Shibayanagi T. J High Temp Soc 1998;24:131. [2] Cohen M, Keath BH, Meharabian M. Proceedings of the Second International Conference on Rapid Solidi"cation Processing, Claiter Publishing, Reston, Virginia, U.S.A., 1980:1. [3] Duwetz P. ASM 1967;60:607. [4] Naka M, Inoue A, Masumoto T. Sci Rep Research Inst Tohoku Univ 1981;A-26:343. [5] Naka M, Shibayanagi T, Maeda M, Ogata Y. Proceedings of the First International Conference on Advances in Applied Plasma Science 1997, p. 123. [6] Alloy Phase Diagrams, ASM Int 1992; 3:52. [7] Alloy Phase Diagrams, ASM Int 1992.
Formation and physical properties of Al base alloys by sputtering M. Naka *, T. Shibayanagi , M. Maeda , S. Zhao, H. Mori Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan Graduate Student, Osaka University, Suita 565, Japan Research Center for Ultra-High Voltage Electron Microscope, Osaka Univeristy, Suita, Osaka 567, Japan
Abstract Ti}Al and Ti}Si alloys were prepared by RF sputtering in a low-pressure argon atmosphere. The microhardness and thermal stability of these alloys were investigated in relation to the structure of alloys. In Al}Ti system, an amorphous phase and AlTi were formed in the composition range of 35}60 at%Ti and 60}80.5 at%Ti, respectively. In Al}Si system, an amorphous phase was formed for Si content of 45 at% or more. Although the microhardeness of Al}Ti alloys shows a maximum at 50 at%Ti, its value for Al}Si alloys rises with increasing Si content. These results suggest that the atomic bonding character is di!erent in both alloys, and the covalent character becomes stronger for Si rich Al}Si alloys. 2000 Elsevier Science Ltd. All rights reserved. Keywords: Sputtering; Aluminum; Titanium; Silicon; Amorphous alloys; Microhardness
1. Introduction As new applications of materials increase in engineering practice, it becomes more and more important to develop advanced materials. The enhansive new applications in engineering "elds need to make engineering materials more functional. Non-equilibrium materials possess high possibilities in industrial applications, because of their superior properties. Nano size particles, super-saturated solid solutions yield outstanding physical, chemical and mechanical properties. Non-equilibrium phases are de"ned as phases which are di!erent in their atomic arrangement from that of equilibrium phases [1,2]. For instance, a hexagonal phase at non-equilibrium state
* Corresponding author. Fax: #816-6879-8649. E-mail address: [email protected] (M. Naka). 0042-207X/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 0 ) 0 0 2 7 7 - 3
M. Naka et al. / Vacuum 59 (2000) 252}259
253
in Au}Ge system [3], and amorphous phase which possesses a random atomic arrangement in metal}P systems [4] were reported. As a continuation of our work on Ti base structural materials [5], the present work describes progress in Al base alloys. These are Al}Ti and Al}Si alloys prepared by magnetron sputtering. In this paper we investigate the physical properties of structure, mechanical properties and the thermal stability of the non-equilibrium alloys.
2. Experimental procedure RF magnetron sputtering was used to prepare amorphous alloys in a low-pressure argon gas of 6.65 MPa. A schematic con"guration of the sputtering apparatus is presented in Fig. 1. Targets used were 100 mm in diameter and 5 mm thick and were composed of Al and Ti or Al and Si. Aluminum substrates were water cooled. The main sputtering conditions were a sputtering power and time of 600 W and 14.4 ks, respectively. The "lm thickness of the alloys prepared was 20 lm. The structure of sputtered "lms was investigated by X-ray di!ractometry. The mechanical properties and thermal stability of the "lms were measured by using a Knoop microhardness and a di!erential scanning calorimeter, respectively. The thermal stability of the "lms was also investigated by a hot stage of electron microscope.
Fig. 1. X-ray di!raction pattern of Al}Ti alloys.
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3. Results and discussion 3.1. X-ray diwraction analysis of Al}Ti alloys The structure of the sputtered alloys was analyzed by X-ray di!ractometry. Fig. 1 shows X-ray di!raction patterns of Al-17.6 at%Ti, Al-52.1 at%Ti, Al-80.5 at%Ti and Al-90.3 at%Ti alloys. Al}Ti alloys with Ti content up to 32.4 at%Ti yield X-ray di!raction pattern of an fcc phase of Al solid solution dissolving Ti. Although Al Ti and Al Ti intermetallics exist at corresponding Ti content in the equilibrium Al}Ti phase diagram, Al solid solution saturated with Ti is formed by suppressing the formation of those intermetallics in the non-equilibrium sputtering state. The Al}Ti sputtered alloys with Ti content from 35}58.7 at% show few di!use X-ray di!raction patterns around 2h"393 for the structure of the amorphous phase. These results indicate that the amorphous phase possesses local atomic arrangements. Al}Ti alloys with Ti content of 60 at% or more are crystalline phase. Al}Ti alloys with Ti content from 60}85 at% and alloys with Ti content of 85 at% or more are the crystal structure of Ti Al and hcp Ti solid solution saturated with Al, respectively. Mixed regions with crystalline phases at the equilibrium state become a single phase region, including crystalline or amorphous phase by sputtering. The formation range of sputtered phases in Al}Ti alloys is superimposed on Al}Ti phase diagram [6] of Fig. 2. 3.2. Mechanical properties of Al}Ti sputtered alloys Fig. 3 shows the relationship between microhardness and Ti content in Al}Ti sputtered alloys. In the Al-rich side, the microhardness of Al}Ti sputtered alloys increases continuously with increasing
Fig. 2. Structure of Al}Ti sputtered alloys.
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Fig. 3. Microhardness of Al}Ti sputtered alloys.
Fig. 4. Crystallization temperature of Al}Ti alloys plotted against Ti content.
Ti content in the compositional range from Al crystalline solid solution to amorphous phase. The increase in microhardness with Ti content up to 32.5 at% arises from the dissolving e!ect of large amounts of Ti in the FCC crystalline Al}Ti alloys. The microhardness of Al}Ti alloys increases successively even in the range of amorphous phases with Ti content from 35 at%Ti to 60 at%Ti. This tendency of the microhardness for the amorphous phase means that the amorphous phase can be thought of as a kind of solid solution. The microhardness of amorphous Al}Ti alloys yields
256
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a maximum at 50 at%Ti around the composition of Al}Ti intermetallic. This compositional dependence suggests that the local atomic arrangement in the amorphous Al}Ti alloy possesses a similar structure to the Al}Ti intermetallic. 3.3. Thermal stability of Al}Ti alloys The thermal stability for crystallization of Al}Ti amorphous sputtered alloys was determined by measuring di!erential scanning calorimetry (DSC) curves. The crystallization temperature and exothermic heat were obtained from the exothermic beginning point and the area of exothermic peak. Fig. 4 shows the dependence of the crystallization temperature and exothermic heat for Al}Ti alloys on the Ti content. The temperature of Al}Ti alloys increases from 555 K for Al-58.7 at% to 781 K for Al-58.7 at%Ti., and the temperature of the alloys rises sharply. The thermal stability of amorphous Al}Ti alloys is improved with increasing Ti content. The exothermic heat decreases with increasing Ti content in the alloys. Thus, the energy di!erence between the amorphous phase and the crystallized phase becomes smaller. 3.4. Structural analysis of Al}Si sputtered alloys Fig. 5 shows the X-ray di!raction patterns of Al}Si sputtered alloys. Al-6.9 at%Si alloy is a solid solution containing a large concentration of Si. Since Al dissolves only the Si content of 2 at% in the equilibrium state, the sputtered alloy is a non-equilibrium phase. Al}Si alloys with Si content up to 42.3 at% are composed of mixed phases of Al solid solution and Si. Al}Si alloys with Si content of 55 at% or more constitute an amorphous single phase which shows di!used X-ray di!raction patterns. When the alloy possesses excess Si atoms, a ratio greater than Si/Al of 0.5, the
Fig. 5. X-ray di!raction pattern of Al}Si sputtered alloys.
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Fig. 6. TEM bright image (a) and electron di!raction pattern (b) of Al-54.1 at%Si alloys.
amorphous structure becomes stable in the Al}Si alloys. Fig. 6 shows TEM bright image and electron di!raction pattern of Al-54.1 at%Si alloy. The electron di!raction pattern shows the pattern of crystalline Si phase. These results indicate that amorphous and crystalline Si phases co-exist in this alloy. Fig. 7 represents the structure of sputtered Al}Si alloys with the corresponding binary phase diagram [7]. Al}Ti alloys with Si content up to 45 at% are composed of Al solid solution and crystalline Si. The alloys with Si content from 45 at% to 55 at% are mixed phases of amorphous phase and crystalline Si phase. The amorphous single phase is formed at Si content of 50 at% or more. Fig. 8 shows the microhardness for the Al}Ti alloys. The microhardness of the alloys increases from 100 Hk for pure Al to 500 Hk for Al-40 at%Si. The value of 500 Hk is higher than 130 Hk for Al-13 at%Si Silumin alloy with Mg treatment. The microhardness of the amorphous phase increases as the Si content increases upto 55 at% or more, and reaches 1150 Hk for pure Si. This increase in the hardness of the Al}Si alloys with Si content of 55 at% or more arises from the increase in the number of Si to Si bonding in the amorphous phase. The covalent bonding of Si to Si is known to be strong. 3.5. Thermal stability of Al}Si alloys Since Al}Si sputtered alloys are in a non-equilibrium state, the alloys transform to an equilibrium state. The thermal stability of the alloys was investigated using a di!erential thermal calorimeter. Amorphous Al}Si alloys crystallize to a mixture of Al and Si with exothermic heat during heating. Fig. 9 shows the Si content dependence of the crystallization of Al}Si amorphous alloys. The exothermic crystallization takes place in two stages. In the "rst peak amorphous phases transform to mixed phases of Al and Si, and in the second peak the mixed phases cause the grain
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Fig. 7. Structure of Al}Si sputtered alloys.
Fig. 8. Microhardness of Al}Si sputtered alloys.
growth. The crystallization temperatures in two stages de"nitely shift to higher temperatures with increasing Si content. In other words, the thermal stability of amorphous Si drastically reduces with increasing Al content in amorphous Si. This harmful e!ect is realized for an amorphous phase formed by a di!usion process of the Al/Si interface.
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Fig. 9. Crystallization temperature of Al}Si alloys.
4. Conclusion Al}Ti and Al}Si non-equilibrium alloys were prepared by sputtering in a low-pressure argon atmosphere. In the Al-rich or Ti-rich alloy side, super-saturated solid solutions with Ti or Al were formed. Near Ti Al composition, an intermetallic super-saturated with Ti or Al is formed. An amorphous phase, which possesses a random atomic arrangement was formed at Ti content from 35.0 at%Ti}58.7 at%. Mixed phases of Al and Si, and amorphous single phases were formed at Si contents up to 432.3 at%, and Si content of 54 at% or more, respectively. Although the microhardness of Al}Ti amorphous alloys shows a maximum near the Al}Ti intermetallic, the microhardness of Al}Si amorphous alloys yields a monotonic increase with an increase in Si content. These di!erent tendencies of microhardness for Al}Ti and Al}Si alloys suggest that the bonding character in both alloys is di!erent, and the covalent character becomes stronger in Si-rich Al}Si alloys. At a crystallization temperature amorphous phase crystallizes to crystalline phases. The crystallization temperatures and thermal stability of both Al}Ti and Al}Si alloys rise with increasing Ti and Si content, respectively.
References [1] Naka M, Shibayanagi T. J High Temp Soc 1998;24:131. [2] Cohen M, Keath BH, Meharabian M. Proceedings of the Second International Conference on Rapid Solidi"cation Processing, Claiter Publishing, Reston, Virginia, U.S.A., 1980:1. [3] Duwetz P. ASM 1967;60:607. [4] Naka M, Inoue A, Masumoto T. Sci Rep Research Inst Tohoku Univ 1981;A-26:343. [5] Naka M, Shibayanagi T, Maeda M, Ogata Y. Proceedings of the First International Conference on Advances in Applied Plasma Science 1997, p. 123. [6] Alloy Phase Diagrams, ASM Int 1992; 3:52. [7] Alloy Phase Diagrams, ASM Int 1992.