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Formation of non-equilibrium alloy phases in Cr–Cu multilayered films by ion mixing
January 2000
Materials Letters 42 Ž2000. 7–11 www.elsevier.comrlocatermatlet
Formation of non-equilibrium alloy phases in Cr–Cu multilayered films by ion mixing C. Lin, G.W. Yang, Z.F. Li, B.X. Liu
)
Department of Materials Science and Engineering, Tsinghua UniÕersity, Beijing 100084, China Received 21 December 1998; accepted 20 May 1999
Abstract The Cr–Cu system is characterized by its positive heat of formation, which leads to a very limited solid solubility between the constituent metals. By ion irradiation of alternately deposited Cr–Cu multilayered films, however, non-equilibrium amorphous, face-centered-cubic Žfcc. crystalline phase and Cr-based supersaturated solid solution were formed at the Cr-rich side. The fcc phase was deduced to contain about 62 at.% of Cr and its lattice constant was determined to be 0.335 nm. The formation of the fcc phase was through a bcc–hcp–fcc transition mechanism from the Cr matrix. q 2000 Elsevier Science B.V. All rights reserved. PACS: 81.05.Bx; 81.05.Kf; 64.75.q g Keywords: Non-equilibrium alloy phase; Ion mixing; Cr–Cu alloy; Multilayered films
1. Introduction The continuously increasing need of new materials with unique properties for various applications has stimulated great interests in understanding of non-equilibrium materials in either crystalline or amorphous state. In 1954, non-equilibrium amorphous alloy films were first produced by quenching a vapor mixture of two constituent metals onto a cryogenically cooled substrate. The formed alloy films, however, were so unstable that they re-crystallized
) Corresponding author. Fax: q86-10-6277-1160; E-mail: [email protected]
even at room temperature w1x. In the early 1960s, Duwez et al. w2x brought up a liquid melt quenching technique, in which a cooling rate as high as 10 7 Krs was obtainable, to inhibit the crystallization of the liquid melt. From then on, the liquid melt quenching has been widely employed to produce amorphous alloys in both metal–metalloid and metal–metal systems. Since the 1980s, ion beam mixing ŽIM. of multilayered films has aroused enormous attention for its high effective cooling rate being in the order of 10 14 Krs as well as for the ease of controlling the experimental parameters w3x. Up to now, a great number of non-equilibrium alloy phases has been obtained by IM in more than 90 binary metal systems. The formed non-equilibrium alloy phases can be classified into five categories w4x,
00167-577Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 9 9 . 0 0 1 5 0 - 0
8
C. Lin et al.r Materials Letters 42 (2000) 7–11
i.e., Ži. amorphous alloy phases, Žii. supersaturated solid solutions, Žiii. hcp phase with enlarged lattice constants in hexagonal close-packed Žhcp.- or facecentered-cubic Žfcc.-based alloys, Živ. fcc phase in hcp-based alloys and Žv. hcp and fcc phases in body-centered-cubic Žbcc.-based alloys. The Cr–Cu system is of interest because the addition of some Cr can greatly improve the oxidation resistance of the Cu-based alloys. However, it is difficult to obtain some Cr–Cu alloys, because the Cr–Cu system has a positive heat of formation of 19 kJrmol. According to the equilibrium phase diagram, there is no intermediate compound existing and very limited terminal solid solubilities. Previous studies have shown that only supersaturated solid solutions were formed in the system by co-evaporation w5x and sputtering w6,7x, yet neither amorphous phase nor non-equilibrium crystalline phases other than supersaturated solid solution have so far been reported. In this study, the alloying behavior in the Cr–Cu system was investigated by ion beam mixing of the Cr–Cu multilayered films, and a new fcc and amorphous phases were formed.
2. Experimental procedure Cr–Cu multilayered films with six bilayers were prepared by depositing alternately pure chromium and copper onto the freshly cleaved NaCl single crystals in an e-gun evaporation system with a vacuum level on the order of 10y5 Pa. The deposition rates were about 0.1–0.2 nmrs. To reduce the possible oxidation of the constituent metals, Ti was evaporated first and deposited onto a spare SiO 2 substrate to adsorb the residual oxygen in the system before depositing the multilayered samples. The total thickness of the films was about 45 nm, which was approximately the sum of the projected range Ž R p . and the projected range straggling Ž D R p . of the irradiating ions, i.e., 200 keV xenon ions employed in this study. The thickness of each layer was controlled by an in-situ quartz oscillator to obtain the designed compositions. The as-deposited Cr–Cu multilayered films were immediately moved into and stored in a dry vacuum chamber to avoid contamination after deposition before subject to ion irradiation.
Ion irradiation of the samples was conducted at room temperature with 200 keV Xe ions to doses of 1 = 10 15 , 4 = 10 15 , 7 = 10 15 and 1 = 10 16 Xeqrcm2 in an implanter with a vacuum level better than 5 = 10y4 Pa. The ion current density was confined to be less than 1 mArcm2 to minimize the beam heating effect. After irradiation, all the irradiated films together with the as-deposited ones were removed from the NaCl substrates by de-ionized water and put onto Mo grids for transmission electron microscope ŽTEM. examination and selected area diffraction ŽSAD. analysis to identify the structure. Energy dispersive spectroscopy ŽEDS. attached to the TEM was employed to determine the overall compositions of the as-deposited films. An analysis error involved in EDS was less than 5%.
3. Results and discussions EDS results showed that the real compositions of the as-deposited films are Cr84 Cu 16 , Cr 76 Cu 24 and Cr62 Cu 38 , respectively. For the Cr84 Cu 16 films, firstly an amorphous phase was detected at an irradiation dose of 1 = 10 15 Xeqrcm2 , secondly a fcc phase with lattice constant of about 0.335 nm was formed at an irradiation dose of 4 = 10 15 Xeqrcm2 , thirdly the films began to transform into Cr-based solid solution at an irradiation dose of 7 = 10 15 Xeqrcm2 , and eventually a uniform Cr-based solid solution was obtained at an irradiation dose of 1 = 10 16 Xeqrcm2 . The structural evolution emerged in the Cr84 Cu 16 films upon ion irradiation with increasing dose can therefore be summarized as follows: Cr84 Cu 16
™ ™
1=10 15
4=10 15
Cr q amorphous Cr q fcc
™
7=10 15
Cr Ž Cu .
Here, CrŽCu. means Cr-based solid solution. Fig. 1 shows the SAD patterns and the corresponding bright-field images of the Cr84 Cu 16 films taken at various irradiation doses. For the other two multilayered samples, only fcc phase was observed when the irradiation dose surpassed 4 = 10 15 Xeqrcm2 . The structural evolution
C. Lin et al.r Materials Letters 42 (2000) 7–11
9
Fig. 1. The SAD patterns and the corresponding bright-field images of the Cr84 Cu 16 multilayered films at various irradiation states, Ža. and Žb.: the as-deposited, Žc. and Žd.: irradiated to a dose of 1 = 10 15 Xeqrcm2 , Že. and Žf.: irradiated to a dose of 4 = 10 15 Xeqrcm2 , Žg. and Žh. irradiated to a dose of 7 = 10 15 Xeqrcm2 .
C. Lin et al.r Materials Letters 42 (2000) 7–11
10
observed in the Cr 76 Cu 24 and Cr62 Cu 38 multilayered films upon ion irradiation was summarized as: Cr 76 Cu 24 Cr62 Cu 38
™ ™
4=10 15 4=10
Cr q fcc
Table 1 Indexing results of the formed fcc phase in the Cr62 Cu 38 multilayered films upon 200 keV room temperature xenon ion mixing to a dose of 4=10 15 Xeqrcm2 Žfcc: as 0.335 nm. d Žnm.
Intensity
fcc Ž hkl .
0.192 0.167 0.118 0.101 0.097 0.084 0.077 0.075 0.068
Strong Strong Strong Strong Medium Weak Medium Medium Medium
111 200 220 311 222 400 331 402 422
15
fcc
The SAD patterns and the corresponding brightfield images of the Cr62 Cu 38 films at various irradiation doses are shown in Fig. 2. The indexing results of the fcc phase are listed in Table 1. It is worth mentioning that the observed diffraction lines in Fig. 2 could not match the Cr or Cu oxides or Cu–Cr–O compounds listed in the documented ASTM cards. It is also believed that the Cl and Na atoms had very minor influence on the phase formation in this system. As mentioned above, for the designed thickness of the Cr–Cu multilayered films, i.e. Ž R p q D R p ., the probability of recoiling events was minimized and thus only a trace amount of Na and Cl atoms were possible to mix into the Cr–Cu films. Even if there were some Na and Cl
Fig. 2. The SAD patterns of the Cr62 Cu 38 multilayered films at various irradiation states. Ža. As-deposited and Žb. irradiated to a dose of 4=10 15 Xeqrcm2 .
atoms in the films, they would prefer to form NaCl crystals, as the binding energy of NaCl was the highest one comparing with other possible compounds. Furthermore, the observed diffraction lines could not be attributed to the previously reported fcc Cr observed above 18508C with a lattice constant of 0.368 nm w8x. Consequently, the newly observed one was a Cr-based non-equilibrium alloy phase with a fcc structure. The next question is then how the non-equilibrium phases were formed in the Cr–Cu system upon ion irradiation and what is the structural relationship between the newly formed fcc phase and the parent constituent metals. It is well known that ion irradiation is a far from equilibrium process and includes two periods, i.e., the atomic collision cascade and relaxation periods w9x. When an irradiation ion with energy E1 collides with the targets and penetrates into the sample, some energy will be transferred to target atoms. If a target atom receives a transferred energy E2 that exceeds a displacement threshold energy Ed , the atom will be knocked out from its original lattice site and a vacancy is thus created. The knocked out atom will bombard other target atoms and a secondary collision was triggered, then the third one, and so on so forth, thus inducing an atomic collision cascade in the target. The atomic collision cascade results in the intermixing of the constituent metal atoms in the multilayered films. If the irradiation dose reaches an adequate value, the constituent metal atoms would mix uniformly and the mixture is in a highly energetic state, which would relax towards an equilib-
C. Lin et al.r Materials Letters 42 (2000) 7–11
rium state during the relaxation period. However, whether or not the mixture can reach the equilibrium state depends on the temperature and time conditions available during the relaxation period. Generally, the relaxation period is extremely short in the order of 10y1 0 to 10y9 s In such short period, the mixture cannot relax into an equilibrium state in most cases and would instead reside into some intermediate states corresponding to some non-equilibrium phases. Since the phase formation by ion irradiation is a solid–solid transformation, there should be a structural compatibility between the newly formed nonequilibrium crystalline phases and the parent metals. In our case, the newly fcc phase was thought to be formed from Cr matrix. In our previous studies, a two step transition mechanism of bcc–hcp–fcc was proposed w10x. That is first to transform bcc to hcp through a shearing mechanism. During shearing, the Ž111. bcc acted as the habit plane, while bcc acted as the shear axis. This results in a lattice parameter relationship of a hcp s Ž'3 r2. a bcc and c hcp s '2 a bcc . Then the transformation of hcp into fcc is easy to achieve by sliding of atoms on the planes parallel to the Ž0002. hcp plane along ²1100: hcp directions by a vector of 13 ²1100: hcp , resulting in a lattice parameter relationship of a fcc s '2 a hcp . Consequently, the lattice constant of fcc phase should be afcc s Ž'6 r2. a bcc . From the lattice parameter relationship, the lattice parameters of the formed fcc phase was calculated, by using the lattice constant of as-deposited Cr Ž0.273 nm., to be 0.334 nm, which is in excellent agreement with the experimental result of 0.335 nm.
4. Concluding remarks By ion beam mixing of the multilayered films, non-equilibrium amorphous and fcc alloy phases were
11
indeed synthesized in the nearly immiscible Cr–Cu system at the Cr-rich side. A structural evolution sequence was found for the Cr84 Cu 16 films upon ion irradiation with increasing dose, i.e., from amorphous phase to fcc alloy phase and finally to the Cr-based supersaturated solid solution. The newly formed non-equilibrium fcc phase was deduced to be with composition of Cr62 Cu 38 and formed from Cr matrix through a bcc–hcp–fcc transformation mechanism.
Acknowledgements The authors are grateful to the National Natural Science Foundation of China. Beneficial help from the staff in the TEM laboratory of Peking University is grateful appreciated.
References w1x W. Buckel, R. Hilsch, Z. Phys. 138 Ž1954. 109. w2x P. Duwez, R.H. Williens, W. Klement, J. Appl. Phys. 31 Ž1960. 1136. w3x B.Y. Tsaur, S.S. Lau, L.S. Hung, J.W. Mayer, Nucl. Instrum. Methods 182r183 Ž1981. 67. w4x B.X. Liu, O. Jin, Phys. Status Solidi A 161 Ž1997. 3. w5x A.G. Dirks, J.J. van den Brock, J. Vac. Sci. Technol., A 3 Ž1985. 2618. w6x S.M. Shin, M.A. Ray, J.M. Rigsbee, J.E. Greene, Appl. Phys. Lett. 43 Ž1983. 249. w7x D. Mclntyre, J.-E. Sundgren, J.E. Greene, J. Appl. Phys. 64 Ž1988. 3689. w8x C.S. Barrett, T.B. Massalski, Structure of Metals: Crystallographic Methods, Principles and Data, 3rd revised edn., International Series on Materials Science and Technology, Vol. 35, Pergamon, Oxford, 1980. w9x B.X. Liu, Phys. Status Solidi A 94 Ž1986. 11. w10x Z.J. Zhang, B.X. Liu, J. Appl. Phys. 75 Ž1994. 4948.
Materials Letters 42 Ž2000. 7–11 www.elsevier.comrlocatermatlet
Formation of non-equilibrium alloy phases in Cr–Cu multilayered films by ion mixing C. Lin, G.W. Yang, Z.F. Li, B.X. Liu
)
Department of Materials Science and Engineering, Tsinghua UniÕersity, Beijing 100084, China Received 21 December 1998; accepted 20 May 1999
Abstract The Cr–Cu system is characterized by its positive heat of formation, which leads to a very limited solid solubility between the constituent metals. By ion irradiation of alternately deposited Cr–Cu multilayered films, however, non-equilibrium amorphous, face-centered-cubic Žfcc. crystalline phase and Cr-based supersaturated solid solution were formed at the Cr-rich side. The fcc phase was deduced to contain about 62 at.% of Cr and its lattice constant was determined to be 0.335 nm. The formation of the fcc phase was through a bcc–hcp–fcc transition mechanism from the Cr matrix. q 2000 Elsevier Science B.V. All rights reserved. PACS: 81.05.Bx; 81.05.Kf; 64.75.q g Keywords: Non-equilibrium alloy phase; Ion mixing; Cr–Cu alloy; Multilayered films
1. Introduction The continuously increasing need of new materials with unique properties for various applications has stimulated great interests in understanding of non-equilibrium materials in either crystalline or amorphous state. In 1954, non-equilibrium amorphous alloy films were first produced by quenching a vapor mixture of two constituent metals onto a cryogenically cooled substrate. The formed alloy films, however, were so unstable that they re-crystallized
) Corresponding author. Fax: q86-10-6277-1160; E-mail: [email protected]
even at room temperature w1x. In the early 1960s, Duwez et al. w2x brought up a liquid melt quenching technique, in which a cooling rate as high as 10 7 Krs was obtainable, to inhibit the crystallization of the liquid melt. From then on, the liquid melt quenching has been widely employed to produce amorphous alloys in both metal–metalloid and metal–metal systems. Since the 1980s, ion beam mixing ŽIM. of multilayered films has aroused enormous attention for its high effective cooling rate being in the order of 10 14 Krs as well as for the ease of controlling the experimental parameters w3x. Up to now, a great number of non-equilibrium alloy phases has been obtained by IM in more than 90 binary metal systems. The formed non-equilibrium alloy phases can be classified into five categories w4x,
00167-577Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 9 9 . 0 0 1 5 0 - 0
8
C. Lin et al.r Materials Letters 42 (2000) 7–11
i.e., Ži. amorphous alloy phases, Žii. supersaturated solid solutions, Žiii. hcp phase with enlarged lattice constants in hexagonal close-packed Žhcp.- or facecentered-cubic Žfcc.-based alloys, Živ. fcc phase in hcp-based alloys and Žv. hcp and fcc phases in body-centered-cubic Žbcc.-based alloys. The Cr–Cu system is of interest because the addition of some Cr can greatly improve the oxidation resistance of the Cu-based alloys. However, it is difficult to obtain some Cr–Cu alloys, because the Cr–Cu system has a positive heat of formation of 19 kJrmol. According to the equilibrium phase diagram, there is no intermediate compound existing and very limited terminal solid solubilities. Previous studies have shown that only supersaturated solid solutions were formed in the system by co-evaporation w5x and sputtering w6,7x, yet neither amorphous phase nor non-equilibrium crystalline phases other than supersaturated solid solution have so far been reported. In this study, the alloying behavior in the Cr–Cu system was investigated by ion beam mixing of the Cr–Cu multilayered films, and a new fcc and amorphous phases were formed.
2. Experimental procedure Cr–Cu multilayered films with six bilayers were prepared by depositing alternately pure chromium and copper onto the freshly cleaved NaCl single crystals in an e-gun evaporation system with a vacuum level on the order of 10y5 Pa. The deposition rates were about 0.1–0.2 nmrs. To reduce the possible oxidation of the constituent metals, Ti was evaporated first and deposited onto a spare SiO 2 substrate to adsorb the residual oxygen in the system before depositing the multilayered samples. The total thickness of the films was about 45 nm, which was approximately the sum of the projected range Ž R p . and the projected range straggling Ž D R p . of the irradiating ions, i.e., 200 keV xenon ions employed in this study. The thickness of each layer was controlled by an in-situ quartz oscillator to obtain the designed compositions. The as-deposited Cr–Cu multilayered films were immediately moved into and stored in a dry vacuum chamber to avoid contamination after deposition before subject to ion irradiation.
Ion irradiation of the samples was conducted at room temperature with 200 keV Xe ions to doses of 1 = 10 15 , 4 = 10 15 , 7 = 10 15 and 1 = 10 16 Xeqrcm2 in an implanter with a vacuum level better than 5 = 10y4 Pa. The ion current density was confined to be less than 1 mArcm2 to minimize the beam heating effect. After irradiation, all the irradiated films together with the as-deposited ones were removed from the NaCl substrates by de-ionized water and put onto Mo grids for transmission electron microscope ŽTEM. examination and selected area diffraction ŽSAD. analysis to identify the structure. Energy dispersive spectroscopy ŽEDS. attached to the TEM was employed to determine the overall compositions of the as-deposited films. An analysis error involved in EDS was less than 5%.
3. Results and discussions EDS results showed that the real compositions of the as-deposited films are Cr84 Cu 16 , Cr 76 Cu 24 and Cr62 Cu 38 , respectively. For the Cr84 Cu 16 films, firstly an amorphous phase was detected at an irradiation dose of 1 = 10 15 Xeqrcm2 , secondly a fcc phase with lattice constant of about 0.335 nm was formed at an irradiation dose of 4 = 10 15 Xeqrcm2 , thirdly the films began to transform into Cr-based solid solution at an irradiation dose of 7 = 10 15 Xeqrcm2 , and eventually a uniform Cr-based solid solution was obtained at an irradiation dose of 1 = 10 16 Xeqrcm2 . The structural evolution emerged in the Cr84 Cu 16 films upon ion irradiation with increasing dose can therefore be summarized as follows: Cr84 Cu 16
™ ™
1=10 15
4=10 15
Cr q amorphous Cr q fcc
™
7=10 15
Cr Ž Cu .
Here, CrŽCu. means Cr-based solid solution. Fig. 1 shows the SAD patterns and the corresponding bright-field images of the Cr84 Cu 16 films taken at various irradiation doses. For the other two multilayered samples, only fcc phase was observed when the irradiation dose surpassed 4 = 10 15 Xeqrcm2 . The structural evolution
C. Lin et al.r Materials Letters 42 (2000) 7–11
9
Fig. 1. The SAD patterns and the corresponding bright-field images of the Cr84 Cu 16 multilayered films at various irradiation states, Ža. and Žb.: the as-deposited, Žc. and Žd.: irradiated to a dose of 1 = 10 15 Xeqrcm2 , Že. and Žf.: irradiated to a dose of 4 = 10 15 Xeqrcm2 , Žg. and Žh. irradiated to a dose of 7 = 10 15 Xeqrcm2 .
C. Lin et al.r Materials Letters 42 (2000) 7–11
10
observed in the Cr 76 Cu 24 and Cr62 Cu 38 multilayered films upon ion irradiation was summarized as: Cr 76 Cu 24 Cr62 Cu 38
™ ™
4=10 15 4=10
Cr q fcc
Table 1 Indexing results of the formed fcc phase in the Cr62 Cu 38 multilayered films upon 200 keV room temperature xenon ion mixing to a dose of 4=10 15 Xeqrcm2 Žfcc: as 0.335 nm. d Žnm.
Intensity
fcc Ž hkl .
0.192 0.167 0.118 0.101 0.097 0.084 0.077 0.075 0.068
Strong Strong Strong Strong Medium Weak Medium Medium Medium
111 200 220 311 222 400 331 402 422
15
fcc
The SAD patterns and the corresponding brightfield images of the Cr62 Cu 38 films at various irradiation doses are shown in Fig. 2. The indexing results of the fcc phase are listed in Table 1. It is worth mentioning that the observed diffraction lines in Fig. 2 could not match the Cr or Cu oxides or Cu–Cr–O compounds listed in the documented ASTM cards. It is also believed that the Cl and Na atoms had very minor influence on the phase formation in this system. As mentioned above, for the designed thickness of the Cr–Cu multilayered films, i.e. Ž R p q D R p ., the probability of recoiling events was minimized and thus only a trace amount of Na and Cl atoms were possible to mix into the Cr–Cu films. Even if there were some Na and Cl
Fig. 2. The SAD patterns of the Cr62 Cu 38 multilayered films at various irradiation states. Ža. As-deposited and Žb. irradiated to a dose of 4=10 15 Xeqrcm2 .
atoms in the films, they would prefer to form NaCl crystals, as the binding energy of NaCl was the highest one comparing with other possible compounds. Furthermore, the observed diffraction lines could not be attributed to the previously reported fcc Cr observed above 18508C with a lattice constant of 0.368 nm w8x. Consequently, the newly observed one was a Cr-based non-equilibrium alloy phase with a fcc structure. The next question is then how the non-equilibrium phases were formed in the Cr–Cu system upon ion irradiation and what is the structural relationship between the newly formed fcc phase and the parent constituent metals. It is well known that ion irradiation is a far from equilibrium process and includes two periods, i.e., the atomic collision cascade and relaxation periods w9x. When an irradiation ion with energy E1 collides with the targets and penetrates into the sample, some energy will be transferred to target atoms. If a target atom receives a transferred energy E2 that exceeds a displacement threshold energy Ed , the atom will be knocked out from its original lattice site and a vacancy is thus created. The knocked out atom will bombard other target atoms and a secondary collision was triggered, then the third one, and so on so forth, thus inducing an atomic collision cascade in the target. The atomic collision cascade results in the intermixing of the constituent metal atoms in the multilayered films. If the irradiation dose reaches an adequate value, the constituent metal atoms would mix uniformly and the mixture is in a highly energetic state, which would relax towards an equilib-
C. Lin et al.r Materials Letters 42 (2000) 7–11
rium state during the relaxation period. However, whether or not the mixture can reach the equilibrium state depends on the temperature and time conditions available during the relaxation period. Generally, the relaxation period is extremely short in the order of 10y1 0 to 10y9 s In such short period, the mixture cannot relax into an equilibrium state in most cases and would instead reside into some intermediate states corresponding to some non-equilibrium phases. Since the phase formation by ion irradiation is a solid–solid transformation, there should be a structural compatibility between the newly formed nonequilibrium crystalline phases and the parent metals. In our case, the newly fcc phase was thought to be formed from Cr matrix. In our previous studies, a two step transition mechanism of bcc–hcp–fcc was proposed w10x. That is first to transform bcc to hcp through a shearing mechanism. During shearing, the Ž111. bcc acted as the habit plane, while bcc acted as the shear axis. This results in a lattice parameter relationship of a hcp s Ž'3 r2. a bcc and c hcp s '2 a bcc . Then the transformation of hcp into fcc is easy to achieve by sliding of atoms on the planes parallel to the Ž0002. hcp plane along ²1100: hcp directions by a vector of 13 ²1100: hcp , resulting in a lattice parameter relationship of a fcc s '2 a hcp . Consequently, the lattice constant of fcc phase should be afcc s Ž'6 r2. a bcc . From the lattice parameter relationship, the lattice parameters of the formed fcc phase was calculated, by using the lattice constant of as-deposited Cr Ž0.273 nm., to be 0.334 nm, which is in excellent agreement with the experimental result of 0.335 nm.
4. Concluding remarks By ion beam mixing of the multilayered films, non-equilibrium amorphous and fcc alloy phases were
11
indeed synthesized in the nearly immiscible Cr–Cu system at the Cr-rich side. A structural evolution sequence was found for the Cr84 Cu 16 films upon ion irradiation with increasing dose, i.e., from amorphous phase to fcc alloy phase and finally to the Cr-based supersaturated solid solution. The newly formed non-equilibrium fcc phase was deduced to be with composition of Cr62 Cu 38 and formed from Cr matrix through a bcc–hcp–fcc transformation mechanism.
Acknowledgements The authors are grateful to the National Natural Science Foundation of China. Beneficial help from the staff in the TEM laboratory of Peking University is grateful appreciated.
References w1x W. Buckel, R. Hilsch, Z. Phys. 138 Ž1954. 109. w2x P. Duwez, R.H. Williens, W. Klement, J. Appl. Phys. 31 Ž1960. 1136. w3x B.Y. Tsaur, S.S. Lau, L.S. Hung, J.W. Mayer, Nucl. Instrum. Methods 182r183 Ž1981. 67. w4x B.X. Liu, O. Jin, Phys. Status Solidi A 161 Ž1997. 3. w5x A.G. Dirks, J.J. van den Brock, J. Vac. Sci. Technol., A 3 Ž1985. 2618. w6x S.M. Shin, M.A. Ray, J.M. Rigsbee, J.E. Greene, Appl. Phys. Lett. 43 Ž1983. 249. w7x D. Mclntyre, J.-E. Sundgren, J.E. Greene, J. Appl. Phys. 64 Ž1988. 3689. w8x C.S. Barrett, T.B. Massalski, Structure of Metals: Crystallographic Methods, Principles and Data, 3rd revised edn., International Series on Materials Science and Technology, Vol. 35, Pergamon, Oxford, 1980. w9x B.X. Liu, Phys. Status Solidi A 94 Ž1986. 11. w10x Z.J. Zhang, B.X. Liu, J. Appl. Phys. 75 Ž1994. 4948.