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Positron annihilation study of vacancy-type defects in stoichiometric and non-stoichiometric Al–Cu–Fe quasicrystalline alloys
Journal of Alloys and Compounds 386 (2005) 192–196
Positron annihilation study of vacancy-type defects in stoichiometric and non-stoichiometric Al–Cu–Fe quasicrystalline alloys V.S. Mikhalenkov a , E.A. Tsapko a , S.S. Polishchuk a,∗ , A.I. Ustinov b b
a G.V. Kurdjumov Institute for Metal Physics, 36 Vernadsky Str., 03142 Kyiv, Ukraine International Center of Electron Beam Technologies of E.O. Paton Electric Welding Institute, 68 Gorky Str., 03150 Kyiv, Ukraine
Received 16 April 2004; received in revised form 4 June 2004; accepted 4 June 2004
Abstract Vacancy-type defects in a non-stoichiometric quasicrystalline phase of Al–Cu–Fe coatings obtained by electron beam deposition have been studied using positron annihilation measurements. It is shown that quasicrystalline phase of non-stoichiometric composition in the coating contains more of vacancy-type defects than the stoichoimetric one in bulk specimen. Decomposition of the quasicrystalline phase during the annealing of the coatings is accompanied by the formation of microvoids, as a consequence of supply of vacancies from the icosahedral structure. The presence of vacancies which maintains the average electron concentration per unit cell at the appropriate value, may be reason of the extended compositional range of icosahedral phase in the coatings. © 2004 Elsevier B.V. All rights reserved. Keywords: Quasicrystal; Vapour deposition; Positron spectroscopy
1. Introduction In the Al–Cu–Fe system an equilibrium icosahedral phase is known to exist at room temperature in a narrow compositional region spanning a few atomic percent [1,2]. However, the metastable icosahedral phase obtained by electron beam deposition has been observed in much wider concentration range being displaced toward lower Cu and higher Fe content as related to the equilibrium composition (Fig. 1). Quasicrystals are known [3] to be electronic compounds. Therefore, the implication of the Hume-Rothery stabilization mechanism to quasicrystals suggests that the average electron concentration per atom should be preserved at a certain value. A significant deviation of the chemical composition of the icosahedral phase must cause a change of electron concentration per unit cell. It is established also that many icosahedral quasicrystals contain vacancy-type defects [4]. Some researchers [4,5] have suggested that the structural vacancies play an important role in the stabilization of the Al-based quasicrystalline structure. Particularly, Lawther et al. [5] have suggested that ∗ Corresponding author. Tel.: +380-44-4449532; fax: +380-44-2693917. E-mail address: [email protected] (S.S. Polishchuk).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.06.021
the defect quasicrystalline Al–Cu–Fe structure can be stabilized due to the presence of vacancies which maintain the average electron concentration at an appropriate level. On the other hand, a large number of non-equilibrium vacancies is known to be formed in the structure of coatings during vapour deposition [6]. Therefore, we suggest that the vacancies stabilize the quasicrystalline structure even in the case of significant deviations of the coating composition from the stoichiometric one in the icosahedral phase. In this connection, it is important to determine the occurrence of vacancy-type defects in both stoichiometric and non-stoichiometric quasicrystalline phases. The positron annihilation technique (PAT) is one of the most effective methods for detecting vacancy-type defects in crystals [7] and quasicrystals [8–10]. Particularly, it has been found using PAT [8] that the quasicrystalline Al–Cu–Fe alloys contain vacancy-type defects with sizes changing in the range from monovacancy up to sixth-vacancy clusters. In this study vacancy-type defects have been studied by measuring the angular correlation of the annihilation radiation (ACAR) in both non-stoichiometric and stoichiometric quasicrystalline Al–Cu–Fe phases (in coatings and in an Al62.5 Fe25 Fe12.5 ingot, respectively). Evolution of these defects in the structure of non-stoichiometric quasicrystalline phase in an Al62.4 Cu23.1 Fe14.5 coating in the course of ther-
V.S. Mikhalenkov et al. / Journal of Alloys and Compounds 386 (2005) 192–196
Fig. 1. The composition range of the existence of the quasicrystalline phase obtained by electron beam deposition. The symbol (䊉) denotes the icosahedral quasicrystalline phase; (䊊) denotes the tetragonal phase; (䊐) denotes the cubic phase. In the two-phase regions: ( ) denotes icosahedral + monoclinic phases; ( ) denotes icosahedral + cubic phases. The isothermal section at room temperature of the Al–Cu–Fe diagram reported by Faudot [2] has been presented for comparison.
mal annealing has been investigated also. The structure of the metastable Al–Cu–Fe quasicrystalline phase in coatings is found to contain much more vacancy-type defects than that of the annealed bulk specimen. Decomposition of the quasicrystalline structure of the coatings during annealing is accompanied by the formation of vacancy complexes. The role played by the vacancy-type defects in the stabilization of the non-stoichiometric Al–Cu–Fe icosahedral phase is discussed.
2. Experimental procedure Quasicrystalline coatings were produced by singlecrucible evaporation at a constant rate of feeding the Al–Cu– Fe alloy into the melt pool [11,12]. In the steady-state evaporation mode, the coating composition is close to that of the initial ingot. Coatings typically 50–150 m thick were deposited with rate of 100 nm/s at a substrate temperature of 650 ◦ C. The coating compositions were determined by X-ray fluorescent analysis in an X Unique II unit. The distribution of chemical elements across the coating thickness was measured in an Energy-200 microanalyzer, mounted on the scanning electron microscope CamScan4. Structural investigations were carried out using a DRON-4 X-ray diffractometer with Fe K␣ radiation. An Al62.5 Fe25 Fe12.5 ingot with a stable quasicrystalline phase (specimen A) was obtained by melting in an arc furnace with subsequent annealing during 72 h at a temperature of 800 ◦ C in vacuum. In the present work, coatings of the following compositions were investigated: Al60.5 Fe26.8 Fe12.7 (specimen B), Al62.4 Cu23.1 Fe14.5 (specimen C). Specimen C was annealed at 650 and 800 ◦ C to study the behaviour of vacancy-type defects in coatings during thermal annealing.
193
ACAR curves were measured at room temperature by using long-slit geometry with a resolution of 0.7 mrad in the angular range from −35 up to +20 mrad. The standard positron source 22 Na shielded from detectors was used in this work. The ACAR curves have been represented as superposition of inverted parabolas and Gaussian distributions which reflect positron annihilation by conduction electrons and by ionic cores, respectively [13]. The basic analyzed parameter was the specific contribution of the parabolic component, Sp /S, where Sp and S are the area under parabolic component and the full area of the ACAR curve, respectively. A separation of ACAR curve into the components was performed by means of the method described in [13]. The full widths at half of maximum (FWHM) of the ACAR curves and the angles Θf corresponding to the Fermi momentums were also determined. The presence of vacancy-type defects in the studied material causes the changes in the parameters of the ACAR curve until the defect concentration rises to the value when all positrons annihilate from the trapped state. Thereafter the curve is stabilized, even if the defect concentration continues to grow. Within the framework of the free electron approximation (i.e. the Fermi surface is represented by sphere), effective electron concentration Z, and the Fermi energy Ef are determined from the expressions [14]: 8πm20 c2 A Z= Θf3 d 3h3 N Ef =
m0 c2 2 Θf 2
where N is the Avogadro’s number, h the Plank’s constant, A the nuclear weight, d the density and Θf the angle between annihilation photons, which corresponds to the Fermi-momentum.
3. Results A typical microstructure of the cross section of the quasicrystalline coating obtained by electron beam deposition is presented in Fig. 2. It is seen that the layer adjacent to the substrate contains two phases whereas the top part of the coating (about 40 m) has a homogeneous structure. The chemical composition of the top part of the coating corresponds to the compound Al60.5 Fe26.8 Fe12.7 . X-ray diffraction patterns of the bulk quasicrystalline specimen A and both coatings B and C are presented in Fig. 3. Their comparison reveals that they are similar and demonstrates that the structure of the icosahedral phase in the coating is close to that obtained by slow cooling followed by an anneal. The results of the positron annihilation study of quasicrystalline coatings as deposited with various chemical compositions and the bulk specimen indicate that the Sp /S values for all studied samples are close to each other. Somewhat larger differences are observed in the values of Θf for the
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Fig. 2. Microstructure of cross section of Al60.5 Fe26.8 Fe12.7 coating (specimen B).
coatings of various compositions and for the bulk specimen. At the same time, the FWHM value of the curve for bulk specimen is significantly smaller, than that for the coatings. If the observed differences of the parameters of ACAR for the coatings and the bulk specimen are caused by the presence of vacancy-type defects, then these parameters are expected to change during annealing due to rearrangement of the defect structure in these samples. In order to check this assumption, the effect of annealing on the coating C (Al62.4 Cu23.1 Fe14.5 ) was investigated. X-ray diffraction patterns of as deposited and annealed coatings are shown in Fig. 4. It is seen that the structure of
Fig. 3. XRD-patterns of the ingot Al62.5 Fe25 Fe12.5 after anneal at Ta = 800 ◦ C during 72 h (specimen A) and coatings (specimens B and C) Fe K␣-radiation. Bragg reflections of the icosahedral phase are indexed with the scheme proposed by Cahn et al. ([15]).
Fig. 4. XRD-patterns of coating C deposited at Ts = 650 ◦ C (a); and after subsequent annealings at Ta = 650 ◦ C during 2 h (b); Ta = 800 ◦ C, 2 h (c); Ta = 800 ◦ C, 6 h (d). Peaks of the icosahedral phase are labeled by the symbol , while those of the cubic and monoclinic phase by  and , respectively.
the as deposited coating has icosahedral symmetry (Fig. 4a). The microstructure of the top part of the cross section of the as deposited coating C is presented in Fig. 5a. It is seen that the coating is homogeneous enough and contains a small quantity of microvoids. The chemical composition corresponds to the Al62.4 Cu23.1 Fe14.5 compound and does not vary significantly with the depth. The thickness of the coating is approximately 150 m. Decomposition of the quasicrystalline phase into monoclinic and cubic phases is observed after annealing at Ta = 650 ◦ C (Fig. 4b). The analysis of the corresponding ACAR curve (see Table 1) indicates that the annealing at Ta = 650 ◦ C results in some reduction of both FWHW and Θf along with a significant increase of specific contribution of the parabolic component, Sp /S. Correspondingly, the calculated values of both the Fermi energy and the effective electronic concentration decrease. The subsequent annealing at 800 ◦ C during 2 h has not brought significant changes in the diffraction pattern (Fig. 4c). Also the parameters of the ACAR curve appear to be nearly unchanged. Further annealing at 800 ◦ C results in an almost complete transformation of the quasicrystalline phase to the crystalline phases (Fig. 4d). The microstructure of the cross section of the coating obtained after annealing
V.S. Mikhalenkov et al. / Journal of Alloys and Compounds 386 (2005) 192–196
195
Table 1 Results of ACAR measurements Specimen A B C C C
Composition Al62.5 Fe25 Fe12.5 Al60.5 Fe26.8 Fe12.7 Al62.4 Cu23.1 Fe14.5
Preparation conditions Ingot after anneal Ta =
HWHM (mrad) 800 ◦ C,
Coating as deposited Td =
t = 72 h
Sp /S
Θf (mrad)
Ef (eV)
Z (per atom)
8. 88
0.57
5.52
7.80
1.43
650 ◦ C
9.42
0.54
5.73
8.40
1.62
650 ◦ C
9.34 9.00 9.04
0.55 0.67 0.68
5.87 5.53 5.58
8.82 7.83 7.97
1.75 1.46 1.50
11.83
0.22
5.69
Coating as deposited Td = After anneal at Ta = 650 ◦ C, t = 3 h After anneal at Ta = 800 ◦ C, t = 2 h
Steel substrate
at temperature 800 ◦ C during 6 h is presented in Fig. 5b. It is seen, that the initial icosahedral phase has decomposed into the cubic and monoclinic phases. The compositions of these phases determined by microspectral analysis correspond to Al50 Cu36 Fe14 and Al70 Cu8 Fe22 , respectively. Furthermore, microvoids with sizes up to 5 m are observed in the coating. In order to consider a possible effect of the substrate on the results of the ACAR measurements a substrate without a coating was also studied. The measured FWHM, Sp /S, Θf values for the substrate are strongly different from those of the quasicrystalline specimens (see Table 1). This allows us to conclude that the observed differences in the ACAR
Fig. 5. Microstructure of coating C (a) as deposited (b) and after subsequent annealing at Ta = 800 ◦ C during 6 h (d). The cubic -phase corresponds to the composition of Al50 Cu36 Fe14 , that of the monoclinic -phase to Al70 Cu8 Fe22 .
parameters of the coatings and the bulk specimens are not caused by a contribution of the substrate. Thus, it has been established, that ACAR curves for the non-stoichiometric quasicrystalline coatings and the bulk specimen of stoichiometric composition are essentially different. The decomposition of the quasicrystalline phase in the coatings during annealing is accompanied by a change in concentration of the vacancy-type defects in the structure.
4. Discussion It is well known, that vacancies and their clusters or other defects with a deficiency in ionic density serve as positron trapping centers in quasicrystals [8–10]. The results of the present studies show that the ACAR curves in quasicrystalline Al–Cu–Fe coatings are characterized by almost the same contributions of a parabolic component Sp /S, as in the annealed stoichiometric quasicrystalline Al62.5 Cu25 Fe12.5 alloy. This fact indicates the occurrence of vacancy-type defects in the structure of the coatings B, C as well as in the structure of the annealed ingot A. The larger values of FWHM of the ACAR curve in the coatings B and C, as compared with the annealed ingot A, can be caused by a smaller size of the vacancy clusters in the coatings [16]. Furthermore, the same values Sp /S and larger values of FWHM for the coating in comparison with that for the ingot A imply that the number of vacancy-type defects (of small size) in the structure of the coatings is larger. The larger values of Θf in the specimens B and C as compared with the specimen A suggest a larger effective electronic concentration (e/a) and Fermi energy Ef in icosahedral structure of coatings. The annealing of the specimens at 650 ◦ C results in an increase of the contribution of Sp /S of the ACAR curve (see Table 1). This fact testifies to a rise of the contribution of the trapped positrons at the expense of an increase of either the defect concentration or their size. The reduction of HFWM can be interpreted as a consequence of growth of the size of vacancy clusters [16] as compared to the as deposited coating structure. As this change in the structure coincides with the transformation of the quasicrystalline phase into the crystalline ones, it can be accompanied by the formation of disordered boundaries containing microscopic cavities. Simultaneously the concentration of the defects in the grains
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may decrease. As a result of this process, the defects in the boundaries between the microcrystallites become more effective trapping centres for positrons. Subsequent annealing at 800 ◦ C results in the coagulation of the vacancy clusters in the interphase boundaries and the formation of a microvoids. It should be noted also that thermal activation of phason disorder has occurred in the temperature range from 600 to 700 ◦ C in addition to the observed decomposition of the metastable quasicrystalline phase (Fig. 2b). This can affect the internal structural vacancies in the quasicrystalline phase [10]. One should keep in mind, that the effective electron concentration estimated in the present work, provides information on the electron concentration inside of vacancy-type defect and cannot be directly compared with the average electron concentration. Nevertheless, the change of the average electron concentration per atom correlates with the change of the electron concentration in the defects. Therefore, it is reasonable to assume, that the reduction of average electron concentration during annealing can be connected with a supply of vacancies from the quasicrystalline phase. Such behaviour of electron concentration can cause destabilization of the quasicrystalline phase and result in its decomposition.
5. Conclusions The comparison of the parameters of the ACAR curves in quasicrystalline structures of non-stoichiometric and stoichiometric composition (in coatings and bulk specimen, respectively) shows that the structure of coatings contains a higher concentration of smaller vacancy-type defects. An annealing of the non-stoichiometric quasicrystalline structure of the Al62.4 Cu23.1 Fe14.5 coating is accompanied by a reduction of the concentration of vacancy-type defects. The suggestion is put forward that this effect is responsible for destabilization of the quasicrystalline phase in coatings which decomposes into the crystalline cubic -phase and the monoclinic -phase. During further annealing, vacancies or their clusters move to the interphase boundaries, forming microvoids, which do not affect essentially the parameters of the ACAR measurements.
The experimental results lead us to suggest, that the stabilization of the Al–Cu–Fe quasicrystalline phase of non-stoichiometric composition is caused by the presence of vacancies which maintain the average electron concentration per unit cell at the appropriate value. To receive additional information on the atomic environment of the vacancy-type defects in the structure of the quasicrystalline coatings we plan to perform positron lifetime measurements.
References [1] M. Quiquandon, A. Quivy, J. Devaud, F. Faudot, S. Lefèbvre, M. Bessière, Y. Calvayrac, J. Phys.: Condens. Matter 8 (1996) 2487– 2512. [2] F. Faudot, Anal. Chim. Fr. 18 (1993) 445–456. [3] C. Dong, A. Perrot, J.M. Dubois, E. Belin, Mater. Sci. Forum 150–151 (1994) 403–416. [4] K. Sato, Y. Takahashi, H. Uchiyama, I. Kanazawa, R. Tamura, R. Kimura, F. Komori, R. Suzuki, T. Ohdaira, S. Takeuchi, Phys. Rev. B 59 (1999) 6712–6716. [5] D.W. Lawther, G. Beydaghyan, R.A. Dunlap, J. Phys.: Condens. Matter 2 (1990) 6239–6244. [6] L.S. Palatnik, M.Ya. Fuks, V.M. Kosevich, Mechanism of the Formation and the Substructure of Condensed Films, Nauka, Moscow, 1972. [7] V.S. Mikhalenkov, Metallofizika 5 (6) (1983) 44–59. [8] R. Chidambaram, M.K. Sanyal, P.M.G. Nambissan, P. Sen, J. Phys.: Condens. Matter 2 (1990) 9941–9947. [9] R. Chidambaram, M.K. Sanyal, V.S. Raghunathan, P.M.G. Nambissan, P. Sen, Phys. Rev. B 48 (1993) 3030–3035. [10] D.W. Lawther, R.A. Dunlap, Phys. Rev. B 49 (1994) 3183– 3189. [11] A.I. Ustinov, B.A. Movchan, S.S. Polishchuk, Metallofiz. Noveishie. Technol. 24 (2002) 365–374. [12] A.I. Ustinov, S.S. Polischuk, Scripta Mater. 47 (2002) 881–886. [13] A.A. Adamenko, I.Ya. Dechtyar, V.S. Mikhalenkov, DAN USSR 173 (1967) 1291–1294. [14] V.S. Mikhalenkov, Study of electron structure of metals and alloys using positrons, in: V.V. Nemoshkalenko (Ed.), The Electron Structure and Properties of Metals and Alloys, Naukova Dumka, Kiev, 1988, pp. 108–133. [15] J.W. Cahn, D. Shechtman, D. Cratias, J. Mater. Res. 1 (1986) 13. [16] P. Hautojarvi, J. Heinio, M. Manninen, R. Nieminen, Philos. Mag. 35 (1977) 973–981.
Positron annihilation study of vacancy-type defects in stoichiometric and non-stoichiometric Al–Cu–Fe quasicrystalline alloys V.S. Mikhalenkov a , E.A. Tsapko a , S.S. Polishchuk a,∗ , A.I. Ustinov b b
a G.V. Kurdjumov Institute for Metal Physics, 36 Vernadsky Str., 03142 Kyiv, Ukraine International Center of Electron Beam Technologies of E.O. Paton Electric Welding Institute, 68 Gorky Str., 03150 Kyiv, Ukraine
Received 16 April 2004; received in revised form 4 June 2004; accepted 4 June 2004
Abstract Vacancy-type defects in a non-stoichiometric quasicrystalline phase of Al–Cu–Fe coatings obtained by electron beam deposition have been studied using positron annihilation measurements. It is shown that quasicrystalline phase of non-stoichiometric composition in the coating contains more of vacancy-type defects than the stoichoimetric one in bulk specimen. Decomposition of the quasicrystalline phase during the annealing of the coatings is accompanied by the formation of microvoids, as a consequence of supply of vacancies from the icosahedral structure. The presence of vacancies which maintains the average electron concentration per unit cell at the appropriate value, may be reason of the extended compositional range of icosahedral phase in the coatings. © 2004 Elsevier B.V. All rights reserved. Keywords: Quasicrystal; Vapour deposition; Positron spectroscopy
1. Introduction In the Al–Cu–Fe system an equilibrium icosahedral phase is known to exist at room temperature in a narrow compositional region spanning a few atomic percent [1,2]. However, the metastable icosahedral phase obtained by electron beam deposition has been observed in much wider concentration range being displaced toward lower Cu and higher Fe content as related to the equilibrium composition (Fig. 1). Quasicrystals are known [3] to be electronic compounds. Therefore, the implication of the Hume-Rothery stabilization mechanism to quasicrystals suggests that the average electron concentration per atom should be preserved at a certain value. A significant deviation of the chemical composition of the icosahedral phase must cause a change of electron concentration per unit cell. It is established also that many icosahedral quasicrystals contain vacancy-type defects [4]. Some researchers [4,5] have suggested that the structural vacancies play an important role in the stabilization of the Al-based quasicrystalline structure. Particularly, Lawther et al. [5] have suggested that ∗ Corresponding author. Tel.: +380-44-4449532; fax: +380-44-2693917. E-mail address: [email protected] (S.S. Polishchuk).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.06.021
the defect quasicrystalline Al–Cu–Fe structure can be stabilized due to the presence of vacancies which maintain the average electron concentration at an appropriate level. On the other hand, a large number of non-equilibrium vacancies is known to be formed in the structure of coatings during vapour deposition [6]. Therefore, we suggest that the vacancies stabilize the quasicrystalline structure even in the case of significant deviations of the coating composition from the stoichiometric one in the icosahedral phase. In this connection, it is important to determine the occurrence of vacancy-type defects in both stoichiometric and non-stoichiometric quasicrystalline phases. The positron annihilation technique (PAT) is one of the most effective methods for detecting vacancy-type defects in crystals [7] and quasicrystals [8–10]. Particularly, it has been found using PAT [8] that the quasicrystalline Al–Cu–Fe alloys contain vacancy-type defects with sizes changing in the range from monovacancy up to sixth-vacancy clusters. In this study vacancy-type defects have been studied by measuring the angular correlation of the annihilation radiation (ACAR) in both non-stoichiometric and stoichiometric quasicrystalline Al–Cu–Fe phases (in coatings and in an Al62.5 Fe25 Fe12.5 ingot, respectively). Evolution of these defects in the structure of non-stoichiometric quasicrystalline phase in an Al62.4 Cu23.1 Fe14.5 coating in the course of ther-
V.S. Mikhalenkov et al. / Journal of Alloys and Compounds 386 (2005) 192–196
Fig. 1. The composition range of the existence of the quasicrystalline phase obtained by electron beam deposition. The symbol (䊉) denotes the icosahedral quasicrystalline phase; (䊊) denotes the tetragonal phase; (䊐) denotes the cubic phase. In the two-phase regions: ( ) denotes icosahedral + monoclinic phases; ( ) denotes icosahedral + cubic phases. The isothermal section at room temperature of the Al–Cu–Fe diagram reported by Faudot [2] has been presented for comparison.
mal annealing has been investigated also. The structure of the metastable Al–Cu–Fe quasicrystalline phase in coatings is found to contain much more vacancy-type defects than that of the annealed bulk specimen. Decomposition of the quasicrystalline structure of the coatings during annealing is accompanied by the formation of vacancy complexes. The role played by the vacancy-type defects in the stabilization of the non-stoichiometric Al–Cu–Fe icosahedral phase is discussed.
2. Experimental procedure Quasicrystalline coatings were produced by singlecrucible evaporation at a constant rate of feeding the Al–Cu– Fe alloy into the melt pool [11,12]. In the steady-state evaporation mode, the coating composition is close to that of the initial ingot. Coatings typically 50–150 m thick were deposited with rate of 100 nm/s at a substrate temperature of 650 ◦ C. The coating compositions were determined by X-ray fluorescent analysis in an X Unique II unit. The distribution of chemical elements across the coating thickness was measured in an Energy-200 microanalyzer, mounted on the scanning electron microscope CamScan4. Structural investigations were carried out using a DRON-4 X-ray diffractometer with Fe K␣ radiation. An Al62.5 Fe25 Fe12.5 ingot with a stable quasicrystalline phase (specimen A) was obtained by melting in an arc furnace with subsequent annealing during 72 h at a temperature of 800 ◦ C in vacuum. In the present work, coatings of the following compositions were investigated: Al60.5 Fe26.8 Fe12.7 (specimen B), Al62.4 Cu23.1 Fe14.5 (specimen C). Specimen C was annealed at 650 and 800 ◦ C to study the behaviour of vacancy-type defects in coatings during thermal annealing.
193
ACAR curves were measured at room temperature by using long-slit geometry with a resolution of 0.7 mrad in the angular range from −35 up to +20 mrad. The standard positron source 22 Na shielded from detectors was used in this work. The ACAR curves have been represented as superposition of inverted parabolas and Gaussian distributions which reflect positron annihilation by conduction electrons and by ionic cores, respectively [13]. The basic analyzed parameter was the specific contribution of the parabolic component, Sp /S, where Sp and S are the area under parabolic component and the full area of the ACAR curve, respectively. A separation of ACAR curve into the components was performed by means of the method described in [13]. The full widths at half of maximum (FWHM) of the ACAR curves and the angles Θf corresponding to the Fermi momentums were also determined. The presence of vacancy-type defects in the studied material causes the changes in the parameters of the ACAR curve until the defect concentration rises to the value when all positrons annihilate from the trapped state. Thereafter the curve is stabilized, even if the defect concentration continues to grow. Within the framework of the free electron approximation (i.e. the Fermi surface is represented by sphere), effective electron concentration Z, and the Fermi energy Ef are determined from the expressions [14]: 8πm20 c2 A Z= Θf3 d 3h3 N Ef =
m0 c2 2 Θf 2
where N is the Avogadro’s number, h the Plank’s constant, A the nuclear weight, d the density and Θf the angle between annihilation photons, which corresponds to the Fermi-momentum.
3. Results A typical microstructure of the cross section of the quasicrystalline coating obtained by electron beam deposition is presented in Fig. 2. It is seen that the layer adjacent to the substrate contains two phases whereas the top part of the coating (about 40 m) has a homogeneous structure. The chemical composition of the top part of the coating corresponds to the compound Al60.5 Fe26.8 Fe12.7 . X-ray diffraction patterns of the bulk quasicrystalline specimen A and both coatings B and C are presented in Fig. 3. Their comparison reveals that they are similar and demonstrates that the structure of the icosahedral phase in the coating is close to that obtained by slow cooling followed by an anneal. The results of the positron annihilation study of quasicrystalline coatings as deposited with various chemical compositions and the bulk specimen indicate that the Sp /S values for all studied samples are close to each other. Somewhat larger differences are observed in the values of Θf for the
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Fig. 2. Microstructure of cross section of Al60.5 Fe26.8 Fe12.7 coating (specimen B).
coatings of various compositions and for the bulk specimen. At the same time, the FWHM value of the curve for bulk specimen is significantly smaller, than that for the coatings. If the observed differences of the parameters of ACAR for the coatings and the bulk specimen are caused by the presence of vacancy-type defects, then these parameters are expected to change during annealing due to rearrangement of the defect structure in these samples. In order to check this assumption, the effect of annealing on the coating C (Al62.4 Cu23.1 Fe14.5 ) was investigated. X-ray diffraction patterns of as deposited and annealed coatings are shown in Fig. 4. It is seen that the structure of
Fig. 3. XRD-patterns of the ingot Al62.5 Fe25 Fe12.5 after anneal at Ta = 800 ◦ C during 72 h (specimen A) and coatings (specimens B and C) Fe K␣-radiation. Bragg reflections of the icosahedral phase are indexed with the scheme proposed by Cahn et al. ([15]).
Fig. 4. XRD-patterns of coating C deposited at Ts = 650 ◦ C (a); and after subsequent annealings at Ta = 650 ◦ C during 2 h (b); Ta = 800 ◦ C, 2 h (c); Ta = 800 ◦ C, 6 h (d). Peaks of the icosahedral phase are labeled by the symbol , while those of the cubic and monoclinic phase by  and , respectively.
the as deposited coating has icosahedral symmetry (Fig. 4a). The microstructure of the top part of the cross section of the as deposited coating C is presented in Fig. 5a. It is seen that the coating is homogeneous enough and contains a small quantity of microvoids. The chemical composition corresponds to the Al62.4 Cu23.1 Fe14.5 compound and does not vary significantly with the depth. The thickness of the coating is approximately 150 m. Decomposition of the quasicrystalline phase into monoclinic and cubic phases is observed after annealing at Ta = 650 ◦ C (Fig. 4b). The analysis of the corresponding ACAR curve (see Table 1) indicates that the annealing at Ta = 650 ◦ C results in some reduction of both FWHW and Θf along with a significant increase of specific contribution of the parabolic component, Sp /S. Correspondingly, the calculated values of both the Fermi energy and the effective electronic concentration decrease. The subsequent annealing at 800 ◦ C during 2 h has not brought significant changes in the diffraction pattern (Fig. 4c). Also the parameters of the ACAR curve appear to be nearly unchanged. Further annealing at 800 ◦ C results in an almost complete transformation of the quasicrystalline phase to the crystalline phases (Fig. 4d). The microstructure of the cross section of the coating obtained after annealing
V.S. Mikhalenkov et al. / Journal of Alloys and Compounds 386 (2005) 192–196
195
Table 1 Results of ACAR measurements Specimen A B C C C
Composition Al62.5 Fe25 Fe12.5 Al60.5 Fe26.8 Fe12.7 Al62.4 Cu23.1 Fe14.5
Preparation conditions Ingot after anneal Ta =
HWHM (mrad) 800 ◦ C,
Coating as deposited Td =
t = 72 h
Sp /S
Θf (mrad)
Ef (eV)
Z (per atom)
8. 88
0.57
5.52
7.80
1.43
650 ◦ C
9.42
0.54
5.73
8.40
1.62
650 ◦ C
9.34 9.00 9.04
0.55 0.67 0.68
5.87 5.53 5.58
8.82 7.83 7.97
1.75 1.46 1.50
11.83
0.22
5.69
Coating as deposited Td = After anneal at Ta = 650 ◦ C, t = 3 h After anneal at Ta = 800 ◦ C, t = 2 h
Steel substrate
at temperature 800 ◦ C during 6 h is presented in Fig. 5b. It is seen, that the initial icosahedral phase has decomposed into the cubic and monoclinic phases. The compositions of these phases determined by microspectral analysis correspond to Al50 Cu36 Fe14 and Al70 Cu8 Fe22 , respectively. Furthermore, microvoids with sizes up to 5 m are observed in the coating. In order to consider a possible effect of the substrate on the results of the ACAR measurements a substrate without a coating was also studied. The measured FWHM, Sp /S, Θf values for the substrate are strongly different from those of the quasicrystalline specimens (see Table 1). This allows us to conclude that the observed differences in the ACAR
Fig. 5. Microstructure of coating C (a) as deposited (b) and after subsequent annealing at Ta = 800 ◦ C during 6 h (d). The cubic -phase corresponds to the composition of Al50 Cu36 Fe14 , that of the monoclinic -phase to Al70 Cu8 Fe22 .
parameters of the coatings and the bulk specimens are not caused by a contribution of the substrate. Thus, it has been established, that ACAR curves for the non-stoichiometric quasicrystalline coatings and the bulk specimen of stoichiometric composition are essentially different. The decomposition of the quasicrystalline phase in the coatings during annealing is accompanied by a change in concentration of the vacancy-type defects in the structure.
4. Discussion It is well known, that vacancies and their clusters or other defects with a deficiency in ionic density serve as positron trapping centers in quasicrystals [8–10]. The results of the present studies show that the ACAR curves in quasicrystalline Al–Cu–Fe coatings are characterized by almost the same contributions of a parabolic component Sp /S, as in the annealed stoichiometric quasicrystalline Al62.5 Cu25 Fe12.5 alloy. This fact indicates the occurrence of vacancy-type defects in the structure of the coatings B, C as well as in the structure of the annealed ingot A. The larger values of FWHM of the ACAR curve in the coatings B and C, as compared with the annealed ingot A, can be caused by a smaller size of the vacancy clusters in the coatings [16]. Furthermore, the same values Sp /S and larger values of FWHM for the coating in comparison with that for the ingot A imply that the number of vacancy-type defects (of small size) in the structure of the coatings is larger. The larger values of Θf in the specimens B and C as compared with the specimen A suggest a larger effective electronic concentration (e/a) and Fermi energy Ef in icosahedral structure of coatings. The annealing of the specimens at 650 ◦ C results in an increase of the contribution of Sp /S of the ACAR curve (see Table 1). This fact testifies to a rise of the contribution of the trapped positrons at the expense of an increase of either the defect concentration or their size. The reduction of HFWM can be interpreted as a consequence of growth of the size of vacancy clusters [16] as compared to the as deposited coating structure. As this change in the structure coincides with the transformation of the quasicrystalline phase into the crystalline ones, it can be accompanied by the formation of disordered boundaries containing microscopic cavities. Simultaneously the concentration of the defects in the grains
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may decrease. As a result of this process, the defects in the boundaries between the microcrystallites become more effective trapping centres for positrons. Subsequent annealing at 800 ◦ C results in the coagulation of the vacancy clusters in the interphase boundaries and the formation of a microvoids. It should be noted also that thermal activation of phason disorder has occurred in the temperature range from 600 to 700 ◦ C in addition to the observed decomposition of the metastable quasicrystalline phase (Fig. 2b). This can affect the internal structural vacancies in the quasicrystalline phase [10]. One should keep in mind, that the effective electron concentration estimated in the present work, provides information on the electron concentration inside of vacancy-type defect and cannot be directly compared with the average electron concentration. Nevertheless, the change of the average electron concentration per atom correlates with the change of the electron concentration in the defects. Therefore, it is reasonable to assume, that the reduction of average electron concentration during annealing can be connected with a supply of vacancies from the quasicrystalline phase. Such behaviour of electron concentration can cause destabilization of the quasicrystalline phase and result in its decomposition.
5. Conclusions The comparison of the parameters of the ACAR curves in quasicrystalline structures of non-stoichiometric and stoichiometric composition (in coatings and bulk specimen, respectively) shows that the structure of coatings contains a higher concentration of smaller vacancy-type defects. An annealing of the non-stoichiometric quasicrystalline structure of the Al62.4 Cu23.1 Fe14.5 coating is accompanied by a reduction of the concentration of vacancy-type defects. The suggestion is put forward that this effect is responsible for destabilization of the quasicrystalline phase in coatings which decomposes into the crystalline cubic -phase and the monoclinic -phase. During further annealing, vacancies or their clusters move to the interphase boundaries, forming microvoids, which do not affect essentially the parameters of the ACAR measurements.
The experimental results lead us to suggest, that the stabilization of the Al–Cu–Fe quasicrystalline phase of non-stoichiometric composition is caused by the presence of vacancies which maintain the average electron concentration per unit cell at the appropriate value. To receive additional information on the atomic environment of the vacancy-type defects in the structure of the quasicrystalline coatings we plan to perform positron lifetime measurements.
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