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Oxidation behavior of Al–Cu–Fe nanoquasicrystal powders
Journal of Non-Crystalline Solids 334&335 (2004) 540–543 www.elsevier.com/locate/jnoncrysol
Oxidation behavior of Al–Cu–Fe nanoquasicrystal powders V. Srinivas a, P. Barua a, T.B. Ghosh a, B.S. Murty b
b,*
a Department of Physics and Meteorology, Indian Institute of Technology, Kharagpur 721 302, India Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur 721 302, India
Abstract A nanoquasicrystalline phase was synthesized by a mechanical alloying process. The thermal stability and oxidation behavior of this phase were investigated in different environments using X-ray diffraction and X-ray photoelectron spectroscopy. The results show that the nanoquasicrystalline phase is stable up to temperatures greater than 1173 K in an inert gas atmosphere, while they are stable up to 873 K in air. The X-ray photoelectron spectroscopy results show three distinct chemical states of Al in Al–Cu–Fe, Al (3+), Al (0) and Al (hydroxide). Ó 2004 Elsevier B.V. All rights reserved. PACS: 81.65.Mq; 61.82.Bq; 61.44.Br; 81.20.Ev
1. Introduction The icosahedral Al–Cu–Fe quasicrystal is of commercial interest because it shows some excellent properties, like high hardness, low friction and good corrosion resistance [1–3]. Besides these properties, its stability at high temperatures under an oxidation atmosphere is of great importance for the application of these materials at elevated temperatures. Kang and Dubois [3] have studied the oxidation behavior of Al– Cu–Fe ingots and powders (<1 lm). They suggested that phase transformations occur because diffusion of Al to the surface depletes Al in the bulk material and also because oxygen diffuses into the bulk. All observations show that if oxidation triggers the transformations in Al–Cu–Fe, the b-phase is the end result. Wehner and Koster [4] showed that oxidation of very thin films causes a transformation to a cubic phase, whereas oxidation of thicker samples under similar conditions does not. The kinetics depend on the particle size, faster kinetics being observed for smaller particles. A single phase, i-phase, in Al70 Cu20 Fe10 , can be obtained by milling and subsequent heat-treatment of the elemental powders. Although the processing routes were
*
Corresponding author. Tel.: +91-3222 283 270; fax: +91-3222 282 280/55303. E-mail address: [email protected] (B.S. Murty). 0022-3093/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2003.12.037
different, the mechanism of the i-phase formation seems to be invariant. It is expected and visually observed that some oxidation of fine powders takes place during the transfer of the samples for post-milling studies. In particular, the smaller grain size guarantees a large specific area, and, therefore, reduces the reaction time for oxidation. This oxidation may induce a transformation of the phase at even lower temperatures for fine particles. However, oxidation is imperceptible in X-ray diffraction (XRD) patterns that are obtained from samples collected during milling, or even after annealing for long periods in an Ar atmosphere. The oxidation behavior and phase transformations were investigated with XRD, transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) measurements and are discussed in the present study. The surface chemistry dictates the phase stability, particularly in low dimensional materials, and will be important for applications in which the relevant properties of quasicrystalline (QC) and crystalline phases differ significantly. 2. Experimental details Elemental blends of Al, Cu and Fe (99.9% purity) powders of <45 lm diameter, corresponding to the nominal composition of Al70 Cu20 Fe10 , were mechanically alloyed in a high energy planetary ball mill (Intel
V. Srinivas et al. / Journal of Non-Crystalline Solids 334&335 (2004) 540–543
mill). The details of the experimental procedure are given elsewhere [5,6]. The as-milled and heat-treated samples were characterized using XRD, TEM and XPS.
Table 1 Volume fraction of the i-phase obtained from the sample annealed in different environments Heat-treated for 4 h (K)
3. Results and discussion Prior to the oxidation investigations, samples were annealed for 4 h at different temperatures, 623–1173 K, in an Ar atmosphere after MA for 40 h. As shown in the Fig. 1, when the sample is annealed at 623 K the cubic solid solution, Al(Cu,Fe) (the b-phase) and the i-phase are observed. The amount of b-phase is suppressed when the sample is heat-treated at 723 and 873 K, while annealing at higher temperatures (1073 and 1173 K) causes the amount of the b-phase to increase. It is interesting to note that the i-phase is stable even up to 1173 K. However, secondary impurity phases such as the b-phase and Al2 O3 also exist at this temperature. On closer observation it can be seen that the i-phase XRD peaks become sharper at higher annealing temperatures indicating a nearly perfect F-type QC structure. The volume fractions of the i-phase as a function of annealing conditions are listed in Table 1. It is clear from the table that the sample annealed at 873 K for 4 h in an Ar atmosphere has the highest volume fraction of i-phase. Although the volume fraction decreases, a perfect icosahedral order develops at elevated temperatures. Fig. 2 shows the TEM micrograph of the as-milled sample (for 40 h); the estimated particle diameter is about 15–25 nm with a narrow distribution of particles sizes. Unequivocal support for the presence of the iphase comes from the electron micro-diffraction pattern,
ARGON
Al2O3 i-phase β -phase
541
623 723 873 973 1073
Volume fraction of the i-phasea Argon
Air
0.66 0.85 1.00 0.76 0.72
0.51 0.80 0.73 0.44 –
a The volume fraction of the i-phase in the as-milled condition is 0.45.
Fig. 2. TEM micrograph after 40 h of MA.
which shows a five-fold symmetry pattern, depicted in Fig. 3, obtained from grains of the QC sample. Due to the higher surface to volume ratio, the chemical reaction between the QC material and the environment is higher. In order to study this under ambient conditions, the milled samples were annealed in air. Fig. 4 shows the XRD patterns from the 40 h milled sample that was heat-treated for 4 h at different
Intensity (arb. units)
1173K
1073K 873K
723K 623K
30
45
60 75 2θ (deg.)
90
Fig. 1. XRD patterns of the Al70 Cu20 Fe10 alloy annealed at different temperatures for 4 h in Ar after 40 h of MA.
Fig. 3. Electron micro-beam diffraction pattern of the Al70 Cu20 Fe10 alloy after 40 h of MA and subsequent annealing at 873 K for 4 h.
542
V. Srinivas et al. / Journal of Non-Crystalline Solids 334&335 (2004) 540–543
(b)
AIR
Al2O3
β-phase i-phase
Intensity (arb. units)
773K/2 Days (a) 1073K 973K 873K 723K 623K
30
45
60 2θ (deg.)
75
90
Fig. 4. XRD patterns of the sample annealed at different temperatures (a) for 4 h and (b) 2 days in air.
temperatures in air. It is interesting to note that when the sample was annealed even for two days at temperatures below 773 K, no trace of oxidation was observed, within the resolution possible with XRD and an almost single phase QC was observed. As revealed by the XRD line profile, the sample annealed in Ar at 873 K consists of the i-phase, which is severely phason-strained while an additional b-phase was observed for samples annealed in air. This observation is in agreement with those of Kang and Dubois [4], where the experiments were carried out in air on particles of 1 lm diameter. However, the transformation temperature for these grains is close to 1040 K. On the other hand, such a transformation is not observed for the ingot samples that were annealed in air. From these observations, it could be speculated that a preferential oxidation of Al in the samples is a possible cause for this transformation. Due to the formation of a thin layer of Al2 O3 on the surface of the particles, a gradual depletion of Al in i-phase composition occurs. This process eventually leads to the formation of the bphase and Al2 O3 . Indeed, this is the case when the sample is annealed at 1073 K in air. In order to understand the oxidation at the microscopic level and to estimate the chemical state of the present composition, XPS measurements were carried out on these samples. A large number of XPS spectra have been recorded e.g., during several hours of milling and on subsequent heat-treatments. From the wide-scan XPS spectra the ratio of oxygen to aluminum content is estimated to be of the order of 1.5. Hence, it appears that the surfaces of these samples are covered with Al2 O3 . The high resolution Al 2p photoelectron line obtained from the 40 h milled sample was used to sep-
arate the chemical states of Al in the nanopowder. The separate peaks clearly show the presence of three distinct states of Al:Al 2p (I) at 74.77 eV identified as the component corresponding to Al2 O3 (Al 3+), Al 2p (II) at 73.1 eV corresponding to elemental Al (Al (0)), and the peak at 76.63 eV corresponding to the formation of Al hydroxide. These observations were further confirmed by a high resolution narrow scan of the Al 2p peaks from all of the samples [6]. Since the contribution from elemental Al can be seen in the spectra, it appears that thickness, which is the the oxide layer is within 40 A detection limit of the XPS technique employed here. In order to explore the presence of elemental Fe and Cu, the samples were further sputter-cleaned for 25 min with an argon ion current of 40 lA at 8 kV. The resolved Al spectra show the presence of three distinct states of Al similar to the parent material. However, the elemental Al (0) contribution increases considerably indicating the removal of an oxide layer. The O 1s spectra recorded for the 40 h milled sample and the corresponding etched sample shows a minor decrease of width after sputter cleaning. The presence of elemental Fe and Cu could not be conclusively proved from this study. In order to get quantitative results from the narrow scan spectra, the photoelectron peak areas of Al 2p (II), Cu 2p3/2 and Fe 2p3/2 were measured. The relative sensitivity factors of these elements were obtained from Briggs and Seah [7]. The estimated at.% for most of the samples studied are Al (II) ¼ 62 at.%, Cu ¼ 25 at.%, Fe ¼ 13 at.%. These quantitative results are correct to within an accuracy of 10 at.%. The uptake of oxygen remains nearly constant, even when annealing the sample at 873 K for 4 h. Hence, it can be concluded that the samples are immediately covered with thin layers of Al2 O3 . As mentioned earlier, the Al 2p (II) signal is present, which indicates that the average oxide layer thickness is 6 4 nm. It can be inferred from a rough estimate of the sputtering time, current and energy that the oxide layer thickness lies within 5 nm, which confirms the observations made by Kang and Dubois [3].
4. Conclusions XRD and TEM measurements suggest that the nanoQC Al70 Cu20 Fe10 materials could be obtained through a combination of MA and subsequent heattreatment in an inert gas atmosphere. Thermal treatment experiments suggest that the QC phase is stable up to temperature greater than 1173 K, in an inert gas atmosphere. On the other hand, the nanoQC materials are stable up to 873 K in air, while they transform to the b-phase and Al2 O3 on high temperature annealing in air. Three distinct chemical states for the Al [Al (3+), Al (0), Al (hydroxide)] were identified in the Al–Cu–Fe
V. Srinivas et al. / Journal of Non-Crystalline Solids 334&335 (2004) 540–543
nanoQC powder. The oxide thickness is of the order of a few nanometers and a long-time exposure to atmospheric conditions has little effect on the chemical state of the nanoQC materials. References [1] K.B. Kim, S.H. Kim, W.T. Kim, D.H. Kim, K.T. Hong, Mater. Sci. Eng. A304–306 (2001) 822.
543
[2] C. Dong, M. de Boissieu, J.M. Dubois, J. Pannetier, C. Janot, J. Mater. Sci. Lett. 8 (1989) 827. [3] S.S. Kang, J.M. Dubois, J. Mater. Res. 10 (1995) 1071. [4] B.I. Wehner, U. Koster, Oxid. Met. 54 (2000) 445. [5] P. Barua, B.S. Murty, B.K. Mathur, V. Srinivas, J. Appl. Phys. 91 (2002) 5353. [6] P. Barua, V. Srinivas, S. Dhabol, T.B. Ghosh, J. Mater. Res. 17 (2002) 1892. [7] D. Briggs, M.P. Seah (Eds.), Practical Surface Analysis by Auger and XPS, John Wiley, 1983, p. 511.
Oxidation behavior of Al–Cu–Fe nanoquasicrystal powders V. Srinivas a, P. Barua a, T.B. Ghosh a, B.S. Murty b
b,*
a Department of Physics and Meteorology, Indian Institute of Technology, Kharagpur 721 302, India Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur 721 302, India
Abstract A nanoquasicrystalline phase was synthesized by a mechanical alloying process. The thermal stability and oxidation behavior of this phase were investigated in different environments using X-ray diffraction and X-ray photoelectron spectroscopy. The results show that the nanoquasicrystalline phase is stable up to temperatures greater than 1173 K in an inert gas atmosphere, while they are stable up to 873 K in air. The X-ray photoelectron spectroscopy results show three distinct chemical states of Al in Al–Cu–Fe, Al (3+), Al (0) and Al (hydroxide). Ó 2004 Elsevier B.V. All rights reserved. PACS: 81.65.Mq; 61.82.Bq; 61.44.Br; 81.20.Ev
1. Introduction The icosahedral Al–Cu–Fe quasicrystal is of commercial interest because it shows some excellent properties, like high hardness, low friction and good corrosion resistance [1–3]. Besides these properties, its stability at high temperatures under an oxidation atmosphere is of great importance for the application of these materials at elevated temperatures. Kang and Dubois [3] have studied the oxidation behavior of Al– Cu–Fe ingots and powders (<1 lm). They suggested that phase transformations occur because diffusion of Al to the surface depletes Al in the bulk material and also because oxygen diffuses into the bulk. All observations show that if oxidation triggers the transformations in Al–Cu–Fe, the b-phase is the end result. Wehner and Koster [4] showed that oxidation of very thin films causes a transformation to a cubic phase, whereas oxidation of thicker samples under similar conditions does not. The kinetics depend on the particle size, faster kinetics being observed for smaller particles. A single phase, i-phase, in Al70 Cu20 Fe10 , can be obtained by milling and subsequent heat-treatment of the elemental powders. Although the processing routes were
*
Corresponding author. Tel.: +91-3222 283 270; fax: +91-3222 282 280/55303. E-mail address: [email protected] (B.S. Murty). 0022-3093/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2003.12.037
different, the mechanism of the i-phase formation seems to be invariant. It is expected and visually observed that some oxidation of fine powders takes place during the transfer of the samples for post-milling studies. In particular, the smaller grain size guarantees a large specific area, and, therefore, reduces the reaction time for oxidation. This oxidation may induce a transformation of the phase at even lower temperatures for fine particles. However, oxidation is imperceptible in X-ray diffraction (XRD) patterns that are obtained from samples collected during milling, or even after annealing for long periods in an Ar atmosphere. The oxidation behavior and phase transformations were investigated with XRD, transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) measurements and are discussed in the present study. The surface chemistry dictates the phase stability, particularly in low dimensional materials, and will be important for applications in which the relevant properties of quasicrystalline (QC) and crystalline phases differ significantly. 2. Experimental details Elemental blends of Al, Cu and Fe (99.9% purity) powders of <45 lm diameter, corresponding to the nominal composition of Al70 Cu20 Fe10 , were mechanically alloyed in a high energy planetary ball mill (Intel
V. Srinivas et al. / Journal of Non-Crystalline Solids 334&335 (2004) 540–543
mill). The details of the experimental procedure are given elsewhere [5,6]. The as-milled and heat-treated samples were characterized using XRD, TEM and XPS.
Table 1 Volume fraction of the i-phase obtained from the sample annealed in different environments Heat-treated for 4 h (K)
3. Results and discussion Prior to the oxidation investigations, samples were annealed for 4 h at different temperatures, 623–1173 K, in an Ar atmosphere after MA for 40 h. As shown in the Fig. 1, when the sample is annealed at 623 K the cubic solid solution, Al(Cu,Fe) (the b-phase) and the i-phase are observed. The amount of b-phase is suppressed when the sample is heat-treated at 723 and 873 K, while annealing at higher temperatures (1073 and 1173 K) causes the amount of the b-phase to increase. It is interesting to note that the i-phase is stable even up to 1173 K. However, secondary impurity phases such as the b-phase and Al2 O3 also exist at this temperature. On closer observation it can be seen that the i-phase XRD peaks become sharper at higher annealing temperatures indicating a nearly perfect F-type QC structure. The volume fractions of the i-phase as a function of annealing conditions are listed in Table 1. It is clear from the table that the sample annealed at 873 K for 4 h in an Ar atmosphere has the highest volume fraction of i-phase. Although the volume fraction decreases, a perfect icosahedral order develops at elevated temperatures. Fig. 2 shows the TEM micrograph of the as-milled sample (for 40 h); the estimated particle diameter is about 15–25 nm with a narrow distribution of particles sizes. Unequivocal support for the presence of the iphase comes from the electron micro-diffraction pattern,
ARGON
Al2O3 i-phase β -phase
541
623 723 873 973 1073
Volume fraction of the i-phasea Argon
Air
0.66 0.85 1.00 0.76 0.72
0.51 0.80 0.73 0.44 –
a The volume fraction of the i-phase in the as-milled condition is 0.45.
Fig. 2. TEM micrograph after 40 h of MA.
which shows a five-fold symmetry pattern, depicted in Fig. 3, obtained from grains of the QC sample. Due to the higher surface to volume ratio, the chemical reaction between the QC material and the environment is higher. In order to study this under ambient conditions, the milled samples were annealed in air. Fig. 4 shows the XRD patterns from the 40 h milled sample that was heat-treated for 4 h at different
Intensity (arb. units)
1173K
1073K 873K
723K 623K
30
45
60 75 2θ (deg.)
90
Fig. 1. XRD patterns of the Al70 Cu20 Fe10 alloy annealed at different temperatures for 4 h in Ar after 40 h of MA.
Fig. 3. Electron micro-beam diffraction pattern of the Al70 Cu20 Fe10 alloy after 40 h of MA and subsequent annealing at 873 K for 4 h.
542
V. Srinivas et al. / Journal of Non-Crystalline Solids 334&335 (2004) 540–543
(b)
AIR
Al2O3
β-phase i-phase
Intensity (arb. units)
773K/2 Days (a) 1073K 973K 873K 723K 623K
30
45
60 2θ (deg.)
75
90
Fig. 4. XRD patterns of the sample annealed at different temperatures (a) for 4 h and (b) 2 days in air.
temperatures in air. It is interesting to note that when the sample was annealed even for two days at temperatures below 773 K, no trace of oxidation was observed, within the resolution possible with XRD and an almost single phase QC was observed. As revealed by the XRD line profile, the sample annealed in Ar at 873 K consists of the i-phase, which is severely phason-strained while an additional b-phase was observed for samples annealed in air. This observation is in agreement with those of Kang and Dubois [4], where the experiments were carried out in air on particles of 1 lm diameter. However, the transformation temperature for these grains is close to 1040 K. On the other hand, such a transformation is not observed for the ingot samples that were annealed in air. From these observations, it could be speculated that a preferential oxidation of Al in the samples is a possible cause for this transformation. Due to the formation of a thin layer of Al2 O3 on the surface of the particles, a gradual depletion of Al in i-phase composition occurs. This process eventually leads to the formation of the bphase and Al2 O3 . Indeed, this is the case when the sample is annealed at 1073 K in air. In order to understand the oxidation at the microscopic level and to estimate the chemical state of the present composition, XPS measurements were carried out on these samples. A large number of XPS spectra have been recorded e.g., during several hours of milling and on subsequent heat-treatments. From the wide-scan XPS spectra the ratio of oxygen to aluminum content is estimated to be of the order of 1.5. Hence, it appears that the surfaces of these samples are covered with Al2 O3 . The high resolution Al 2p photoelectron line obtained from the 40 h milled sample was used to sep-
arate the chemical states of Al in the nanopowder. The separate peaks clearly show the presence of three distinct states of Al:Al 2p (I) at 74.77 eV identified as the component corresponding to Al2 O3 (Al 3+), Al 2p (II) at 73.1 eV corresponding to elemental Al (Al (0)), and the peak at 76.63 eV corresponding to the formation of Al hydroxide. These observations were further confirmed by a high resolution narrow scan of the Al 2p peaks from all of the samples [6]. Since the contribution from elemental Al can be seen in the spectra, it appears that thickness, which is the the oxide layer is within 40 A detection limit of the XPS technique employed here. In order to explore the presence of elemental Fe and Cu, the samples were further sputter-cleaned for 25 min with an argon ion current of 40 lA at 8 kV. The resolved Al spectra show the presence of three distinct states of Al similar to the parent material. However, the elemental Al (0) contribution increases considerably indicating the removal of an oxide layer. The O 1s spectra recorded for the 40 h milled sample and the corresponding etched sample shows a minor decrease of width after sputter cleaning. The presence of elemental Fe and Cu could not be conclusively proved from this study. In order to get quantitative results from the narrow scan spectra, the photoelectron peak areas of Al 2p (II), Cu 2p3/2 and Fe 2p3/2 were measured. The relative sensitivity factors of these elements were obtained from Briggs and Seah [7]. The estimated at.% for most of the samples studied are Al (II) ¼ 62 at.%, Cu ¼ 25 at.%, Fe ¼ 13 at.%. These quantitative results are correct to within an accuracy of 10 at.%. The uptake of oxygen remains nearly constant, even when annealing the sample at 873 K for 4 h. Hence, it can be concluded that the samples are immediately covered with thin layers of Al2 O3 . As mentioned earlier, the Al 2p (II) signal is present, which indicates that the average oxide layer thickness is 6 4 nm. It can be inferred from a rough estimate of the sputtering time, current and energy that the oxide layer thickness lies within 5 nm, which confirms the observations made by Kang and Dubois [3].
4. Conclusions XRD and TEM measurements suggest that the nanoQC Al70 Cu20 Fe10 materials could be obtained through a combination of MA and subsequent heattreatment in an inert gas atmosphere. Thermal treatment experiments suggest that the QC phase is stable up to temperature greater than 1173 K, in an inert gas atmosphere. On the other hand, the nanoQC materials are stable up to 873 K in air, while they transform to the b-phase and Al2 O3 on high temperature annealing in air. Three distinct chemical states for the Al [Al (3+), Al (0), Al (hydroxide)] were identified in the Al–Cu–Fe
V. Srinivas et al. / Journal of Non-Crystalline Solids 334&335 (2004) 540–543
nanoQC powder. The oxide thickness is of the order of a few nanometers and a long-time exposure to atmospheric conditions has little effect on the chemical state of the nanoQC materials. References [1] K.B. Kim, S.H. Kim, W.T. Kim, D.H. Kim, K.T. Hong, Mater. Sci. Eng. A304–306 (2001) 822.
543
[2] C. Dong, M. de Boissieu, J.M. Dubois, J. Pannetier, C. Janot, J. Mater. Sci. Lett. 8 (1989) 827. [3] S.S. Kang, J.M. Dubois, J. Mater. Res. 10 (1995) 1071. [4] B.I. Wehner, U. Koster, Oxid. Met. 54 (2000) 445. [5] P. Barua, B.S. Murty, B.K. Mathur, V. Srinivas, J. Appl. Phys. 91 (2002) 5353. [6] P. Barua, V. Srinivas, S. Dhabol, T.B. Ghosh, J. Mater. Res. 17 (2002) 1892. [7] D. Briggs, M.P. Seah (Eds.), Practical Surface Analysis by Auger and XPS, John Wiley, 1983, p. 511.