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Effect of the substrate temperature on the microstructure and texture of Mg90Zr10 (at.%) films deposited by sputtering
Journal of Alloys and Compounds 425 (2006) 148–152
Effect of the substrate temperature on the microstructure and texture of Mg90Zr10 (at.%) films deposited by sputtering Gerardo Garc´es a,∗ , Stephan Landais b , Paloma Adeva a a
b
Department of Physical Metallurgy, CENIM, CSIC, Av. De Gregorio del Amo 8, 28040 Madrid, Spain Office National dˇıEtudes et de Recherches Aerospatiales, ONERA, BP72-29 Avenue de la Division Leclerc F-92322 Chatillon, Paris, France Received 12 April 2005; received in revised form 10 January 2006; accepted 10 January 2006 Available online 21 February 2006
Abstract The microstructure of Mg90 Zr10 (at.%) films obtained by sputtering onto copper substrate at three different temperatures (180, 320 and 350 ◦ C) has been studied. Films exhibited an intense (0 0 0 1) basal plane fibre texture with the fibre axis parallel to the growth direction. Their microstructure consisted of columnar grains growing from the copper substrate to the free surface which is typical of the zone II of the Movchan and Demchishin zone model developed for PVD materials. Nevertheless, the microstructure of films was dependent on the substrate temperature. The grain diameter increased as the substrate temperature was increased. Moreover, the dislocation density inside the grains as well as that piled-up forming sub-grain boundaries decreased as the deposition temperature increased. Although the film growth in zone II is controlled by surface diffusion the larger surface mobility of the atoms as the substrate temperature increased led to changes in the solubility of zirconium. At low substrate temperatures all zirconium was in solid solution. However, at 350 ◦ C the formation of small zirconium particles occurred at grain boundaries. © 2006 Elsevier B.V. All rights reserved. Keywords: Magnesium alloys; Sputtering; Transmission electron microscopy
1. Introduction Low mass density and high specific resistance are important requirements for materials used in the aerospace and automobile industries. Magnesium alloys present lower densities than aluminium alloys, which are commonly used in these industries. Nevertheless, their use has been limited because of their poor corrosion resistance mainly due to the presence of second phase particles acting as galvanic pairs. Many attempts are being carried out to improve this behaviour. Some of them are focused to introduce some alloying elements in solid solution that could contribute to the formation of protective layers. The corrosion behaviour of these alloys can be improved after rapid solidification processing because it produces a fine microstructure and an extension of the solid solution of alloying elements [1–4]. However, the low solubility in liquid magnesium of some elements such a titanium, zirconium, chromium and vanadium leads to synthesis of
∗
Corresponding author. E-mail address: [email protected] (G. Garc´es).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.01.015
these alloys by non-equilibrium processes avoiding the melting techniques. Physical vapour deposition (PVD) is an interesting nonequilibrium method suitable for the synthesis of solid solutions of Mg–X (X = Ti, Zr, Cr and V) [5–9] alloys. The results show that this process leads to a refinement of the microstructure, an important extension of the solid solubility limits of the alloying elements (lower than by rapid solidification) and an improvement in the corrosion resistance. Experimental evidences in Mg–Ti and Mg–Zr alloys indicate an enhancement in corrosion resistance in corrosive atmospheres and the appearance of a passive zone [10–13]. On the other hand, some authors have also proposed the use of coatings to enhance the magnesium corrosion resistance [14,15]. In these techniques, magnesium or magnesium alloys are used as evaporation source and also as substrate. Since a purification process based on the retort effect is included in the technique, the substrate is coated with deposited high purity magnesium layer. Results show an improvement of magnesium corrosion resistance without detriment to recyclability. This argument can be extended to produce Mg–Zr or Mg–Ti coatings that can improve the corrosion resistance using sputtering techniques.
G. Garc´es et al. / Journal of Alloys and Compounds 425 (2006) 148–152
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In the case of PVD magnesium deposit, the substrate temperature is an important parameter to be controlled. During the deposition, especially at low temperature, the formation of defects can induce the film brittleness. Thus, the substrate temperature must be as high as possible to minimize the defect concentration but avoiding the precipitate formation that reduces the corrosion resistance. The present paper is focused on the influence of substrate temperature on the microstructure and texture of Mg90 Zr10 (at.%) alloy deposited by sputtering. This study has been carried out by X-ray diffraction and electron microscopy. 2. Experimental procedure Films of nominal composition Mg90 Zr10 (at.%) were deposited using a Triode Sputtering technique onto copper substrates heated at 180, 320 and 350 ◦ C. The target consisted of a magnesium sheet with holes occupied by pure zirconium cylinders. The microstructural characterisation of the as-deposited film was carried out by X-ray diffraction and electron microscopy. SEM energy dispersive spectrum (EDS) measurements were used to determine the film compositions. A 15 kV electron beam was used to acquire data from deep enough in the film to obtain an average composition through the depth of the film. Films showed composition gradients and, therefore, the study was centred in the portion with composition Mg90 Zr10 (at.%). Films thickness also varied between 100 and 150 m. The metallographical samples preparation for SEM examination consisted in mechanical polishing, and etching in a solution of 10% Keller, 40% ethylene glycol and 50% water. Specimens for TEM were prepared by ion milling at liquid nitrogen temperature and examined in a JEOL JEM-2010 microscope operating at 200 kV. A TEM CM20 with a field emission gun has also been used to analyze small particles. X-ray diffraction was used for lattice parameter and texture measurements. The texture analysis was carried out by the Schulz reflection method, using a SIEMENSTM Kristalloflex D5000 diffractometer equipped with a close Eulerian cradle. The X-radiation used was -filtered Cu K␣. The orientation distribution functions (ODFs) were computed from the measure of (0 0 0 2), (1 0 1¯ 1), (1 0 1¯ 2), (1 0 1¯ 3) and (1 1 2¯ 0) pole figures by the series expansion method. The reference system was selected with the Z-axis parallel to the normal substrate. The X and Y directions were perpendicular to the Z-axis but were arbitrary on the plane parallel to the substrate.
3. Results and discussion X-ray diffraction patterns indicate an intense texture of the basal plane component in the three deposits as deduced from Fig. 1a. This figure shows the X-ray diffraction pattern of the alloy deposited at 180 ◦ C in which the (0 0 0 2) peak is only visible. This fact also agrees with the {0 0 0 2} pole figure shown in Fig. 1b. From these results it can be concluded that the films have an intense fibre texture with the fibre axis perpendicular to the substrate surface. The intensity of the fibre decreases as the substrate temperature increases. The maximum intensities are 110, 43 and 40 for the substrate temperature of 180, 315 and 350 ◦ C, respectively. Measurements of lattice parameters were carried out to evaluate the effect of zirconium atoms on the magnesium lattice. Due to the intense texture of the films, measurements were performed in samples reduced into small fragments. Results obtained were similar in the three deposits since differences were within the experimental error (a = 0.3207 nm and c = 0.51982 nm). Zirconium solid solution in the magnesium
Fig. 1. (a) X-ray diffraction pattern and (b) (0 0 0 2) pole figure of the Mg90 Zr10 alloy grown at 180 ◦ C.
lattice provokes a small contraction of the magnesium lattice (aMg = 0.321 nm and cMg = 0.5209 nm [16]). Since both elements (magnesium and zirconium) have the same crystalline structure and their lattice parameters are very close, the addition of the zirconium has low effects on the lattice parameter. This is in agreement with the observation in PVD Mg–Zr alloys [9,17,18]. Fig. 2 shows the grain structure throughout the thickness in the case of the alloy deposited at 180 and 350 ◦ C. The film microstructure is similar for the three substrate temperatures and consists of columnar grains orientated in the film growth direction. The grain size increases with increasing substrate temperature. The columnar grain structure in the growth direction as shown in Fig. 2 is typical of the zone II of the Movchan, Demchishin and Thornton model developed for PVD materials [19,20], which is defined by 0.3 < Ts/Tm < 0.5–0.6, where Ts and Tm are the substrate and melting temperature, respectively. The authors of this model explained that the adatoms, which arrive to the surface, have energy to diffuse only along the surface. Then, the growth of films is controlled by surface diffusion. The grain growth process has been described in terms of surface recrystallization [19], evolutionary selection [21] and granular epitaxy [22]. This high adatom mobility leads to a dense columnar grain structure in the growth direc-
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Fig. 2. Microstructure through the thickness of the Mg90 Zr10 alloy deposited at: (a) 180 ◦ C, (b) 320 ◦ C and (c) 350 ◦ C.
tion. The increase in the substrate temperature results in an enhancement of surface mobility and an increase in the grain size [22,23]. The formation of a fibre texture of the basal plane component perpendicular to the grain growth direction during the growth of the film can be explained by the minimisation of the surface energy of the island formed in the earliest stages of deposit formation. These islands tend to form with the densest atomic plane parallel to the substrate to minimise their interfacial free
energy, and this energy is related to the densest atomic planes. In the case of hcp metal the basal plane is the closest atomic plane inducing this texture. Mg–Zr thick films produced by EBPVD also present a similar texture [17,24,25]. However, the fibre intensity in the case of sputtering is higher with respect to evaporation. On the other hand, although the three Mg90 Zr10 (at.%) films have been grown at the same characteristic zone (II), some differences between them are observed.
Fig. 3. Bright field image of the Mg90 Zr10 alloy deposited at 180 ◦ C: (a) grain structure of the alloy, (b) grain boundaries and (c) sub-grain boundary.
G. Garc´es et al. / Journal of Alloys and Compounds 425 (2006) 148–152
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Fig. 4. SAD of: (a) two neighbour grains at the [0 0 0 1] zone axis and (b) two neighbour subgrains (inside the grain) at the [0 0 0 1] zone axis.
From below: Samples were examined by TEM to study in more detail the structure of these magnesium films. Bright field images at different magnifications of the alloy deposited at 180 ◦ C are shown in Fig. 3. Inside the grains, a high dislocation density is observed (Fig. 3a). From below: Sub-grain inside the grain is resolved (Fig. 3b and c). A pile-up of dislocations forming the sub-grain boundary is clearly observed. The electron diffraction patterns of the above images taken in the grain and sub-grain boundaries, at the [0 0 0 1] zone axis, respectively, are presented in Fig. 4. The two grains of Fig. 3b are orientated in the same direction but rotated with respect to the [0 0 0 1] direction. This observation is in agreement with the fibre texture of axis normal to the deposit growth direction. Throughout the SAD from the two neighbour grains (Fig. 4a), it is possible to measure the disorientation angle. The calculated angle between the grains is around 15◦ . The SAD pattern of the [0 0 0 1] zone axis obtained in the sub-grain boundary (Fig. 4b) also shows that subgrains are rotated around the [0 0 0 1] direction, the disorientation angle being about 2◦ . In the case of the sample grown at 320 ◦ C (Fig. 5), the grain structure is totally different. The dislocation density inside the grain is lower than that of the alloy grown at lower temperature. This dislocation structure seems to indicate that a recovery process takes place during deposition. The recovery process can be due to two mechanisms acting simultaneously. On one hand, the recovery can take place through dislocations movement and aniquilation. Furthermore, because the neighbour grain and subgrain, oriented with a strong (0 0 0 1) fibre texture, are separated by low angle boundaries, the recovery can occur by grain and subgrain rotation along the [0 0 0 1] direction. This agrees with the low or almost null density of dislocation pile-up observed in this film.
Fig. 5. Dark field image of the Mg90 Zr10 alloy deposited at 320 ◦ C.
Fig. 6 presents the microstructure of the alloy deposited at 350 ◦ C showing very small particles decorating the grain boundaries. The microanalysis of these particles indicated they have higher zirconium content than the matrix (Fig. 6b). From these results and the analysis of the equilibrium phase diagrams, it can be concluded that particles are pure zirconium. The presence of particle indicates that temperature is not low enough to induce the total solid solution of zirconium atoms. Nevertheless, some zirconium is present in solid solution according to the lattice parameter measurements. The formation of these zirconium particles should be produced during the film growth since at this high substrate temperature the surface mobility of atoms should be larger. It can be assumed that when the zirconium atoms arrive to the surface of the growing deposit they can diffuse easily at the grain boundaries, forming zirconium agglomerates.
Fig. 6. (a) Bright field image of the Mg90 Zr10 alloy deposited at 350 ◦ C. (b) Overlapped EDS spectra of the matrix and particles at grain boundaries.
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4. Conclusions The microstructure of Mg90 Zr10 (at.%) films obtained by sputtering at three substrate temperatures (180, 320 and 350 ◦ C) have been studied. The main conclusions are: 1. The three deposits grow with an intense (0 0 0 1) basal plane fibre texture with the fibre axis parallel to the deposit growth direction. Their microstructure consists of columnar grain growing from the copper substrate to the free surface which is typical of the zone II of the Movchan, Demchishin and Thornton zone model. 2. The grain diameter increases with the substrate temperature. Moreover, the dislocation density inside the grains as well as that piled-up forming sub-grain boundaries decreases as deposition temperature increases. 3. At low substrates temperatures all zirconium content is in solid solution. However, at 350 ◦ C the formation of small zirconium particles took place at grain boundaries. Acknowledgements The authors (GG) would like to thank the Ministry of Science and Technology for a contract within the Ramon y Cajal Programme. We gratefully acknowledge the support of the CICYT MAT 981620-CE. References [1] F. Sommer, F. Hehmann, H. Jones, R.G. Edyvean, J. Mater. Sci. 24 (1989) 2369–2379. [2] S. Krishnamurthy, K. Khobaid, E. Robertson, F.H. Froes, Mater. Sci. Eng. 99 (1988) 507–511. [3] F. Sommer, F. Hehmann, H. Jones, J. Less-Common Met. 159 (1990) 237–259.
[4] D.S. Ahmed, R.G.J. Edyvean, C.M. Sellars, H. Jones, Mater. Sci. Technol. 6 (1990) 469–474. [5] S.B. Dodd, R.W. Gardiner, Proceedings of the Third International Magnesium Conference, The Institute of Metals, Manchester, 1996, p. 271. [6] S.B. Dodd, S. Morris, R.W. Gardiner, Proceedings of Magnesium Alloys and Their Applications, Werkstoff-Informationgesellschaft, Frankfurt, 1998, p. 375. [7] K.R. Baldwin, D.J. Bray, G.D. Howard, R.W. Gardiner, Mater. Sci. Technol. 12 (1996) 937–943. [8] S. Diplas, P. Tsakiropoulos, R.M.D. Brydson, Mater. Sci. Technol. 14 (1998) 689–698. [9] S. Diplas, P. Tsakiropoulos, R.M.D. Brysond, Mater. Sci. Technol. 15 (1999) 1349–1357. [10] C.B. Baliga, P. Tsakiropoulos, S.B. Dodd, R.W. Gardiner, Proceedings of the Third International Magnesium Conference, The Institute of Metals, Manchester, 1996, p. 627. [11] R. Subramanian, S. Sirca, J. Mazumder, J. Mater. Sci. 26 (1991) 951– 956. [12] S. Diplas, P. Tsakiropoulos, R.M.D. Brydson, Mater. Sci. Technol. 15 (1999) 1359–1372. [13] G. Neite, K. Kubota, K. Higashi, F. Hehmann, Materials Science and Technology, vol. 8, VHC, Weinheim, 1996, p. 191. [14] S. Fukumoto, K. Sugahara, A. Yamamoto, H. Tsubakino, Mater. Trans. 44 (2003) 518–523. [15] H. Tsubakino, A. Yamamoto, S. Fukumoto, A. Watanabe, K. Sugahara, H. Inoue, Mater. Trans. 44 (2003) 504–510. [16] C.S. Robert, Magnesium and its Alloys, John Wiley, New York, 1960, p. 18. [17] G. Garces, J.M. Antonaz, P. P´erez, J.M. Bad´ıa, S.B. Dodd, S. Morris, P. Adeva, Z. Metall. 94 (2003) 880–885. [18] G. Garces, P. P´erez, P. Adeva, Rev. Metal. Madrid 39 (2003) 17–24. [19] B.A. Movchan, A.V. Demchishin, Phys. Met. Metallogr. 28 (1969) 653–660. [20] J.A. Thornton, Rev. Mater. Sci. 7 (1977) 239–260. [21] A. Van der Drift, Philips Res. Rep. 22 (1967) 267–275. [22] C.R.M. Grovenor, H.T.G. Hertzell, D.A. Smith, Acta Metall. 32 (1984) 773–781. [23] B. Cantor, W. Cahn, Acta Metall. 24 (1976) 845–852. [24] G. Garc´es, M.C. Cristina, M. Torralba, P. Adeva, J. Alloys Compd. 309 (2000) 229–238. [25] G. Garces, P. Adeva, J. Mater. Res. 17 (2002) 614–619.
Effect of the substrate temperature on the microstructure and texture of Mg90Zr10 (at.%) films deposited by sputtering Gerardo Garc´es a,∗ , Stephan Landais b , Paloma Adeva a a
b
Department of Physical Metallurgy, CENIM, CSIC, Av. De Gregorio del Amo 8, 28040 Madrid, Spain Office National dˇıEtudes et de Recherches Aerospatiales, ONERA, BP72-29 Avenue de la Division Leclerc F-92322 Chatillon, Paris, France Received 12 April 2005; received in revised form 10 January 2006; accepted 10 January 2006 Available online 21 February 2006
Abstract The microstructure of Mg90 Zr10 (at.%) films obtained by sputtering onto copper substrate at three different temperatures (180, 320 and 350 ◦ C) has been studied. Films exhibited an intense (0 0 0 1) basal plane fibre texture with the fibre axis parallel to the growth direction. Their microstructure consisted of columnar grains growing from the copper substrate to the free surface which is typical of the zone II of the Movchan and Demchishin zone model developed for PVD materials. Nevertheless, the microstructure of films was dependent on the substrate temperature. The grain diameter increased as the substrate temperature was increased. Moreover, the dislocation density inside the grains as well as that piled-up forming sub-grain boundaries decreased as the deposition temperature increased. Although the film growth in zone II is controlled by surface diffusion the larger surface mobility of the atoms as the substrate temperature increased led to changes in the solubility of zirconium. At low substrate temperatures all zirconium was in solid solution. However, at 350 ◦ C the formation of small zirconium particles occurred at grain boundaries. © 2006 Elsevier B.V. All rights reserved. Keywords: Magnesium alloys; Sputtering; Transmission electron microscopy
1. Introduction Low mass density and high specific resistance are important requirements for materials used in the aerospace and automobile industries. Magnesium alloys present lower densities than aluminium alloys, which are commonly used in these industries. Nevertheless, their use has been limited because of their poor corrosion resistance mainly due to the presence of second phase particles acting as galvanic pairs. Many attempts are being carried out to improve this behaviour. Some of them are focused to introduce some alloying elements in solid solution that could contribute to the formation of protective layers. The corrosion behaviour of these alloys can be improved after rapid solidification processing because it produces a fine microstructure and an extension of the solid solution of alloying elements [1–4]. However, the low solubility in liquid magnesium of some elements such a titanium, zirconium, chromium and vanadium leads to synthesis of
∗
Corresponding author. E-mail address: [email protected] (G. Garc´es).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.01.015
these alloys by non-equilibrium processes avoiding the melting techniques. Physical vapour deposition (PVD) is an interesting nonequilibrium method suitable for the synthesis of solid solutions of Mg–X (X = Ti, Zr, Cr and V) [5–9] alloys. The results show that this process leads to a refinement of the microstructure, an important extension of the solid solubility limits of the alloying elements (lower than by rapid solidification) and an improvement in the corrosion resistance. Experimental evidences in Mg–Ti and Mg–Zr alloys indicate an enhancement in corrosion resistance in corrosive atmospheres and the appearance of a passive zone [10–13]. On the other hand, some authors have also proposed the use of coatings to enhance the magnesium corrosion resistance [14,15]. In these techniques, magnesium or magnesium alloys are used as evaporation source and also as substrate. Since a purification process based on the retort effect is included in the technique, the substrate is coated with deposited high purity magnesium layer. Results show an improvement of magnesium corrosion resistance without detriment to recyclability. This argument can be extended to produce Mg–Zr or Mg–Ti coatings that can improve the corrosion resistance using sputtering techniques.
G. Garc´es et al. / Journal of Alloys and Compounds 425 (2006) 148–152
149
In the case of PVD magnesium deposit, the substrate temperature is an important parameter to be controlled. During the deposition, especially at low temperature, the formation of defects can induce the film brittleness. Thus, the substrate temperature must be as high as possible to minimize the defect concentration but avoiding the precipitate formation that reduces the corrosion resistance. The present paper is focused on the influence of substrate temperature on the microstructure and texture of Mg90 Zr10 (at.%) alloy deposited by sputtering. This study has been carried out by X-ray diffraction and electron microscopy. 2. Experimental procedure Films of nominal composition Mg90 Zr10 (at.%) were deposited using a Triode Sputtering technique onto copper substrates heated at 180, 320 and 350 ◦ C. The target consisted of a magnesium sheet with holes occupied by pure zirconium cylinders. The microstructural characterisation of the as-deposited film was carried out by X-ray diffraction and electron microscopy. SEM energy dispersive spectrum (EDS) measurements were used to determine the film compositions. A 15 kV electron beam was used to acquire data from deep enough in the film to obtain an average composition through the depth of the film. Films showed composition gradients and, therefore, the study was centred in the portion with composition Mg90 Zr10 (at.%). Films thickness also varied between 100 and 150 m. The metallographical samples preparation for SEM examination consisted in mechanical polishing, and etching in a solution of 10% Keller, 40% ethylene glycol and 50% water. Specimens for TEM were prepared by ion milling at liquid nitrogen temperature and examined in a JEOL JEM-2010 microscope operating at 200 kV. A TEM CM20 with a field emission gun has also been used to analyze small particles. X-ray diffraction was used for lattice parameter and texture measurements. The texture analysis was carried out by the Schulz reflection method, using a SIEMENSTM Kristalloflex D5000 diffractometer equipped with a close Eulerian cradle. The X-radiation used was -filtered Cu K␣. The orientation distribution functions (ODFs) were computed from the measure of (0 0 0 2), (1 0 1¯ 1), (1 0 1¯ 2), (1 0 1¯ 3) and (1 1 2¯ 0) pole figures by the series expansion method. The reference system was selected with the Z-axis parallel to the normal substrate. The X and Y directions were perpendicular to the Z-axis but were arbitrary on the plane parallel to the substrate.
3. Results and discussion X-ray diffraction patterns indicate an intense texture of the basal plane component in the three deposits as deduced from Fig. 1a. This figure shows the X-ray diffraction pattern of the alloy deposited at 180 ◦ C in which the (0 0 0 2) peak is only visible. This fact also agrees with the {0 0 0 2} pole figure shown in Fig. 1b. From these results it can be concluded that the films have an intense fibre texture with the fibre axis perpendicular to the substrate surface. The intensity of the fibre decreases as the substrate temperature increases. The maximum intensities are 110, 43 and 40 for the substrate temperature of 180, 315 and 350 ◦ C, respectively. Measurements of lattice parameters were carried out to evaluate the effect of zirconium atoms on the magnesium lattice. Due to the intense texture of the films, measurements were performed in samples reduced into small fragments. Results obtained were similar in the three deposits since differences were within the experimental error (a = 0.3207 nm and c = 0.51982 nm). Zirconium solid solution in the magnesium
Fig. 1. (a) X-ray diffraction pattern and (b) (0 0 0 2) pole figure of the Mg90 Zr10 alloy grown at 180 ◦ C.
lattice provokes a small contraction of the magnesium lattice (aMg = 0.321 nm and cMg = 0.5209 nm [16]). Since both elements (magnesium and zirconium) have the same crystalline structure and their lattice parameters are very close, the addition of the zirconium has low effects on the lattice parameter. This is in agreement with the observation in PVD Mg–Zr alloys [9,17,18]. Fig. 2 shows the grain structure throughout the thickness in the case of the alloy deposited at 180 and 350 ◦ C. The film microstructure is similar for the three substrate temperatures and consists of columnar grains orientated in the film growth direction. The grain size increases with increasing substrate temperature. The columnar grain structure in the growth direction as shown in Fig. 2 is typical of the zone II of the Movchan, Demchishin and Thornton model developed for PVD materials [19,20], which is defined by 0.3 < Ts/Tm < 0.5–0.6, where Ts and Tm are the substrate and melting temperature, respectively. The authors of this model explained that the adatoms, which arrive to the surface, have energy to diffuse only along the surface. Then, the growth of films is controlled by surface diffusion. The grain growth process has been described in terms of surface recrystallization [19], evolutionary selection [21] and granular epitaxy [22]. This high adatom mobility leads to a dense columnar grain structure in the growth direc-
150
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Fig. 2. Microstructure through the thickness of the Mg90 Zr10 alloy deposited at: (a) 180 ◦ C, (b) 320 ◦ C and (c) 350 ◦ C.
tion. The increase in the substrate temperature results in an enhancement of surface mobility and an increase in the grain size [22,23]. The formation of a fibre texture of the basal plane component perpendicular to the grain growth direction during the growth of the film can be explained by the minimisation of the surface energy of the island formed in the earliest stages of deposit formation. These islands tend to form with the densest atomic plane parallel to the substrate to minimise their interfacial free
energy, and this energy is related to the densest atomic planes. In the case of hcp metal the basal plane is the closest atomic plane inducing this texture. Mg–Zr thick films produced by EBPVD also present a similar texture [17,24,25]. However, the fibre intensity in the case of sputtering is higher with respect to evaporation. On the other hand, although the three Mg90 Zr10 (at.%) films have been grown at the same characteristic zone (II), some differences between them are observed.
Fig. 3. Bright field image of the Mg90 Zr10 alloy deposited at 180 ◦ C: (a) grain structure of the alloy, (b) grain boundaries and (c) sub-grain boundary.
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Fig. 4. SAD of: (a) two neighbour grains at the [0 0 0 1] zone axis and (b) two neighbour subgrains (inside the grain) at the [0 0 0 1] zone axis.
From below: Samples were examined by TEM to study in more detail the structure of these magnesium films. Bright field images at different magnifications of the alloy deposited at 180 ◦ C are shown in Fig. 3. Inside the grains, a high dislocation density is observed (Fig. 3a). From below: Sub-grain inside the grain is resolved (Fig. 3b and c). A pile-up of dislocations forming the sub-grain boundary is clearly observed. The electron diffraction patterns of the above images taken in the grain and sub-grain boundaries, at the [0 0 0 1] zone axis, respectively, are presented in Fig. 4. The two grains of Fig. 3b are orientated in the same direction but rotated with respect to the [0 0 0 1] direction. This observation is in agreement with the fibre texture of axis normal to the deposit growth direction. Throughout the SAD from the two neighbour grains (Fig. 4a), it is possible to measure the disorientation angle. The calculated angle between the grains is around 15◦ . The SAD pattern of the [0 0 0 1] zone axis obtained in the sub-grain boundary (Fig. 4b) also shows that subgrains are rotated around the [0 0 0 1] direction, the disorientation angle being about 2◦ . In the case of the sample grown at 320 ◦ C (Fig. 5), the grain structure is totally different. The dislocation density inside the grain is lower than that of the alloy grown at lower temperature. This dislocation structure seems to indicate that a recovery process takes place during deposition. The recovery process can be due to two mechanisms acting simultaneously. On one hand, the recovery can take place through dislocations movement and aniquilation. Furthermore, because the neighbour grain and subgrain, oriented with a strong (0 0 0 1) fibre texture, are separated by low angle boundaries, the recovery can occur by grain and subgrain rotation along the [0 0 0 1] direction. This agrees with the low or almost null density of dislocation pile-up observed in this film.
Fig. 5. Dark field image of the Mg90 Zr10 alloy deposited at 320 ◦ C.
Fig. 6 presents the microstructure of the alloy deposited at 350 ◦ C showing very small particles decorating the grain boundaries. The microanalysis of these particles indicated they have higher zirconium content than the matrix (Fig. 6b). From these results and the analysis of the equilibrium phase diagrams, it can be concluded that particles are pure zirconium. The presence of particle indicates that temperature is not low enough to induce the total solid solution of zirconium atoms. Nevertheless, some zirconium is present in solid solution according to the lattice parameter measurements. The formation of these zirconium particles should be produced during the film growth since at this high substrate temperature the surface mobility of atoms should be larger. It can be assumed that when the zirconium atoms arrive to the surface of the growing deposit they can diffuse easily at the grain boundaries, forming zirconium agglomerates.
Fig. 6. (a) Bright field image of the Mg90 Zr10 alloy deposited at 350 ◦ C. (b) Overlapped EDS spectra of the matrix and particles at grain boundaries.
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4. Conclusions The microstructure of Mg90 Zr10 (at.%) films obtained by sputtering at three substrate temperatures (180, 320 and 350 ◦ C) have been studied. The main conclusions are: 1. The three deposits grow with an intense (0 0 0 1) basal plane fibre texture with the fibre axis parallel to the deposit growth direction. Their microstructure consists of columnar grain growing from the copper substrate to the free surface which is typical of the zone II of the Movchan, Demchishin and Thornton zone model. 2. The grain diameter increases with the substrate temperature. Moreover, the dislocation density inside the grains as well as that piled-up forming sub-grain boundaries decreases as deposition temperature increases. 3. At low substrates temperatures all zirconium content is in solid solution. However, at 350 ◦ C the formation of small zirconium particles took place at grain boundaries. Acknowledgements The authors (GG) would like to thank the Ministry of Science and Technology for a contract within the Ramon y Cajal Programme. We gratefully acknowledge the support of the CICYT MAT 981620-CE. References [1] F. Sommer, F. Hehmann, H. Jones, R.G. Edyvean, J. Mater. Sci. 24 (1989) 2369–2379. [2] S. Krishnamurthy, K. Khobaid, E. Robertson, F.H. Froes, Mater. Sci. Eng. 99 (1988) 507–511. [3] F. Sommer, F. Hehmann, H. Jones, J. Less-Common Met. 159 (1990) 237–259.
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