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Structural characterisation and stability of new nanoquasicrystalline Al-based alloys
Materials Science and Engineering A 375–377 (2004) 1206–1211
Structural characterisation and stability of new nanoquasicrystalline Al-based alloys M. Galano a , F. Audebert a,b,∗ , B. Cantor a , I. Stone a b
a Department of Materials, University of Oxford, Parks Road Oxford, OX1 3PH, Oxford, UK Advanced Materials Group, Facultad de Ingenier´ıa, Universidad de Buenos Aires, Paseo Colón 850, Buenos Aires 1063, Argentina
Abstract Nanoquasicrystalline Al-based alloys have been studied intensively for the last decade because of their promising mechanical properties. Recently quaternary nanoquasicrystalline alloys with improved thermal stability and better mechanical properties at elevated temperature have been reported. The present work describes an investigation into quaternary Al93 Fe3 Cr2 Z2 (at.%) (with Z = Ti, V, Nb, Ta) and ternary Al93 (Fe3 Cr2 )7 , produced by melt spinning. Structural characterisation was carried out by means of X-ray diffraction (XRD) and transmission electron microscopy (TEM), and the thermal stability and decomposition of the quasicrystalline phase were studied by means of differential scanning calorimetry (DSC). All of the alloys showed a microstructure consisting of spherical icosahedral particles embedded in an ␣-Al matrix. The fourth component produced a significant refinement of icosahedral particle size. The decomposition of the quaternary alloys was observed at higher temperatures than for the ternary alloy with a broad exothermic peak. The new alloys containing Nb and Ta showed a multimodal transformation process extending up to the onset of melting. Icosahedral particles containing Ta or Nb were observed after heat treatment at 550 ◦ C. © 2003 Elsevier B.V. All rights reserved. Keywords: Al alloys; Rapid solidification; Nanoquasicrystalline alloys; Icosahedral phase; Quasicrystals
1. Introduction Several types of Al-based nanoquasicrystalline alloys have been developed in the last decade, with microstructures of nanometer-sized icosahedral particles embedded in an ␣-Al matrix [1–3]. These new alloys have higher microstructural stability than amorphous composites [4,5]. Their mechanical strength decreases slowly with temperature, maintaining high strength relative to nanocrystalline and commercial Al alloys [6] and combining high strength and plasticity at elevated temperature. For commercial applications bulk alloys need to be produced. At present, rapid solidification processes are required to retain nanoquasicrystalline particles in the solidified alloy, therefore bulk samples are being produced by warm extrusion of atomised powders [7]. It is important to obtain nanoquasicrystalline alloys with microstructures stable to higher temperature and with a high decomposition temperature of the icosahedral phase in order to enhance pro-
∗
Corresponding author. E-mail address: [email protected] (F. Audebert).
0921-5093/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2003.10.066
cessing and improve mechanical properties at elevated temperatures. Moreover, the development of alloys with a high nucleation frequency and slow rate of growth of the icosahedral phase is an important aim in order to produce bulk Al-based nanoquasicrystalline composites by casting processes. It is known that Mn, Cr and V promote icosahedral phase formation in Al-based alloys since the icosahedral phase can be obtained in Al–Mn [8], Al–Cr [9], and Al–V [10] binary systems. It has been suggested that Fe produces icosahedral clusters in an Al-based liquid [11,12] and, depending on the alloying elements, these clusters can be stabilised to form an icosahedral quasicrystalline phase [12]. The icosahedral phase has been obtained in ternary systems containing Fe, e.g. Al–Fe–Cu [13], Al–Fe–Cr [14], Al–Fe–V [15], Al–Fe–Ta [16], Al–Fe–Ti [4] and Al–Fe–Nb [17], which shows that Fe also promotes icosahedral phase formation. Moreover, it has been observed that Fe in combination with Cr or V has a refining effect on the size of the icosahedral quasicrystalline phase [18–20]. A quaternary icosahedral phase has been observed in the Al–V–Cr–Ti and Al–Fe–Cr–(Ti,V) systems [3,19]. It is expected that a similar icosahedral phase will be formed in the Al–Fe–Cr–(Nb
M. Galano et al. / Materials Science and Engineering A 375–377 (2004) 1206–1211 Table 1 Code number, nominal chemical compositions and corrected compositions of the alloys studied Code
Nominal chemical composition (at.%)
Corrected chemical composition (at.%)
X Ti V Nb Ta
Al93 Fe4.2 Cr2.8 Al93 Fe3 Cr2 Ti2 Al93 Fe3 Cr2 V2 Al93 Fe3 Cr2 Nb2 Al93 Fe3 Cr2 Ta2
Al92.93 Fe4.24 Cr2.83 Al92.94 Fe3.02 Cr2.02 Ti2.02 Al92.95 Fe3.02 Cr2.01 V2.02 Al92.88 Fe3.05 Cr2.04 Nb2.03 Al92.94 Fe3.02 Cr2.02 Ta2.02
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electron microscopy (TEM) using JEOL 200CX and 2010 microscopes. TEM samples were prepared using a Gatan precision ion polishing system (PIPS). The thermal stability and decomposition of the microstructure were analysed by differential scanning calorimetry (DSC) using a TA Instruments 2010 thermal analyser under a dynamic Ar atmosphere. Ribbon samples were encapsulated in quartz tubes under an Ar atmosphere, followed by heat treatment for 30 min at 550 ◦ C. Heat treated samples were then characterised by XRD and TEM.
or Ta) system. Both elements, Nb and Ta, have higher melting points than V, Ti, Fe and Cr, and lower diffusivity should provide more stability for the microstructure. Quaternary Al93 Fe3 Cr2 Z2 (at.%) (with Z = Ti, V, Nb, Ta) and ternary Al93 (Fe3 Cr2 )7 alloys were therefore chosen in order to study icosahedral phase forming ability, and alloy microstructures and stability.
2. Experimental procedure
t-Al3Nb (200)
θ- Al(Fe,Cr)
i- phase(20/32)
t-Al3Nb (004)
i- phase (18/29)
Intensitity [a.u.]
θ- Al(Fe,Cr)
t-Al3Nb (112)
Master alloys were prepared in an arc furnace under a He atmosphere using pure elements, Al and Cr (99.99%), Fe (99.98%), and Ti, V, Nb and Ta (99.7%). The code and the nominal chemical compositions of the alloys studied are listed in Table 1 together with the corrected compositions taking into account the weight lost attributed to Al in the arc melting process. Rapidly solidified ribbons, with thicknesses between 20 and 30 m, were obtained by melt spinning at a wheel speed of 40 m/s in a reduced He atmosphere using quartz nozzles and with closed-loop controlled ejection temperature between 1200 and 1350 ◦ C depending on the alloy. The microstructures of ribbon samples were characterised by means of X-ray diffraction (XRD) in a Philips 1810 θ–2θ diffractometer using Cu K␣ radiation and by transmission
Ta Nb V
36
38
Al (200)
Al (111)
Ti
40 2θ 42
44
X 46
48
Fig. 1. X-ray diffractograms from the as melt spun alloys.
Fig. 2. Bright field TEM images of the as melt spun alloys: (a) ternary alloy an electron diffraction pattern from a near-spherical particle, (b) Ti, (c) V, (d) Nb, (e) Ta.
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3. Results and discussion The X-ray diffractograms from each of the melt spun alloys showed the presence of the ␣-Al and the icosahedral phase. The former was indexed with reflections corresponding to: (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2) and (4 0 0) planes in the range of 2θ = 10–100◦ . The icosahedral phase was indexed using Cahn’s indexation scheme [21] and the following icosahedral reflections were found in the range of 2θ = 10–100◦ : (6/9), (18/29), (20/32), (38/61) and (52/84). Additional weak X-ray peaks observed at 2θ ∼ 36.7, 43.7 and 75.5◦ could be assigned to the metastable distorted -Al13 (Cr,Fe)2–4 phase [22–24]. The Nb alloy exhibited the following reflections (0 0 2), (1 0 1), (1 1 0), (1 1 2), (0 0 4), (2 0 0), (1 1 4) and (2 2 0) corresponding to the stable tetragonal t-Al3 Nb phase. For an easy recognition of the main peaks, Fig. 1 shows an enlargement of the central part of the X-ray diffractograms where it can be seen some peaks of each phase identified including the two main icosahedral reflections. Fig. 2 shows bright field (BF) TEM images from each melt spun alloy. All the alloys exhibited a distribution of near-spherical i-phase particles, embedded in an ␣-Al matrix. A very low volume fraction of additional small particles was present for each alloy corresponding to the metastable crystalline distorted -Al13 (Cr, Fe)2–4 phase. Other authors [3,25] have reported the presence of a low volume fraction of Al23 Ti9 or Al3 Ti phases in similar melt spun and atomised alloys but these phases were not seen in the present work. A
very small fraction of small particles was observed in the BF TEM images but no reflections corresponding to the former phases have been seen in the X-ray diffractograms. Presumably, these phases are very sensitive to the cooling rate. The Nb alloy exhibited a few non spherical particles corresponding to the t-Al3 Nb phase. The TEM images in Fig. 2 show that the quaternary alloys all have smaller icosahedral particles than the ternary alloy by ∼50%. The nanoquasicrystalline quaternary alloys exhibited icosahedral particles of ∼100 nm in size compared to ∼200 nm in the ternary alloy. Fig. 3(a) shows a bright field BF TEM image of an icosahedral particle in the Nb alloy, exhibiting five-fold symmetry in the corresponding nano-beam diffraction pattern (NBDP). Nano-beam energy dispersive X-ray analysis (EDX) confirmed the presence of the four alloying elements in the icosahedral particles (see Fig. 3(b)). Small peaks corresponding to Cu and Ar were observed in the EDX spectrum, coming from the sample holder and PIPS sample preparation, respectively. A small peak can be observed at 0.28 keV corresponding to C, due to environmental contamination. Fig. 4(a) and (b) show a BF image with corresponding NBDP and EDX spectrum for the Ta alloy. The diffraction pattern also has five-fold symmetry. EDX shows that the icosahedral particles contain Ta as well as Fe, Cr and Al. Again, a small Ti peak due to the sample holder is observed. Fig. 5 shows DSC traces at a heating rate of 40 K/min from all the melt spun alloys. Each alloy showed a small exothermic peak at around 330–370 ◦ C and a main
Fig. 3. (a) Bright field TEM image from an icosahedral particle in the as melt spun Nb alloy and its corresponding electron diffraction pattern, (b) EDX spectrum from the same particle.
M. Galano et al. / Materials Science and Engineering A 375–377 (2004) 1206–1211
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Fig. 4. (a) Bright field TEM image from an icosahedral particle in the as melt spun Ta alloy and its corresponding nano-beam electron diffraction pattern, (b) EDX spectrum of the same particle.
heating rate was 463 ◦ C. The DSC peaks are displaced to higher temperatures with increasing melting point of the fourth component (Ti, V, Nb, Ta). This is probably related to the decreasing diffusion coefficient of the elements in ␣-Al in the following order: Fe, Cr, Ti and V [27]. The
t: t-Al3 Nb
θ :Al-(Cr,Fe) θ
it i
^C D
V
Al
250
300
350
400
450
500
550
600
30
35
#
* ** **
#
*
Exo
D
V
#
X 200
^ ^# # #
Ti
150
C
*
C
Nb
t
^C
*
Nb
θ
** * **
Heat Flow [W/gr]
Ta
Ta
θ
*
Intensity [a.u.]
2.5 W/gr
DSC run 40 C/min
^
iθ
i t
0
D : Al3 Ti C: Al10 V : Al45 V 7
i: i-phase
: Al13 Cr 2 # : Al13 Fe 4
*
exothermic transition with a broad peak at higher temperature. In previous work [3], it was observed that for the Ti alloy the i-phase remained stable after heating beyond the first small peak and a small fraction of the distorted -Al13 (Cr,Fe)2–4 phase was precipitated. These small peaks might be related to a low level of precipitation from the supersaturated ␣-Al matrix. Ziani et al. [26] reported similar DSC traces for an Al94 Fe3 Cr3 alloy obtained by hot extrusion of rapidly solidified powders, with a peak at ∼490 ◦ C for a heating rate of 10 K/min. In the case of the ternary alloy the peak temperature for DSC traces at the same
40
Ti
D
#
X
#
Al
2θ
45
50
55
0
Temperature [ C] Fig. 5. DSC traces from the as melt spun alloys heated at 40 K/min.
Fig. 6. X-ray diffractograms from the ternary and quaternary alloys after heat treatment for 30 min at 550 ◦ C.
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M. Galano et al. / Materials Science and Engineering A 375–377 (2004) 1206–1211
Fig. 7. (a) Bright field image of an icosahedral particle in the Ta alloy after a heat treatment for 30 min at 550 ◦ C with the inset of the electron diffraction pattern of the same particle, (b) bright field image of a near-spherical particle in the Nb alloy after a heat treatment for 30 min at 550 ◦ C.
main transformations corresponding to the new nanoquasicrystalline Nb and Ta alloys have broader transformation peaks than those corresponding to the ternary, Ti and V alloys. These sluggish transformations are extended over a large temperature range up to the onset of the alloy melting. This high stability is probably related to the lower diffusivity of Nb and Ta, which slows down a diffusion controlled decomposition. The X-ray diffractograms in the range of 2θ = 10–100◦ from the heat treated ternary (X), Ti and V alloys did not show traces of the i-phase but showed peaks corresponding to the stable phases: ␣-Al, -Al13 Cr2 and -Al13 Fe4 . The heat treated Ti and V alloys, in addition showed peaks of the t-Al3 Ti phase and the -Al45 V7 and cubic Al10 V phases, respectively. The diffractograms corresponding to the heat treated Nb and Ta alloys did not show new peaks, however icosahedral, distorted -Al13 (Cr, Fe)2–4 and ␣-Al phases could be identified. Moreover, the Nb alloy also contained the t-Al3 Nb phase. In order to appreciate the main peaks of each phase indexed an enlargement of the central part of the diffractograms is shown in Fig. 6. The former results together with the DSC traces suggest that icosahedral decomposition of the X, Ti and V alloys finished after heat treatment for 30 min at 550 ◦ C, confirming the enhanced stability of the microstructure of the alloys containing Nb and Ta as a fourth alloying element. The presence of the icosahedral phase after the heat treatment was confirmed for the Ta alloy by TEM as shown by the five-fold symmetry in the selected area diffraction pattern corresponding to a near-spherical particle in Fig. 7(a). The heat treated Nb alloy also showed near-spherical particles similar to those found in the heat treated Ta alloy (Fig. 7(b)), which could be the icosahedral phase in agreement with the XRD traces (Fig. 6). The average size of the icosahedral particles after the heat treatment, in both Nb and Ta alloys, was ∼200 nm.
4. Conclusions New melt spun nanoquasicrystalline alloys containing Nb and Ta have been obtained. Their microstructure is composed of nano-sized icosahedral particles distributed in an ␣-Al matrix. Ternary and quaternary Al–Fe–Cr–(Ti, V, Nb, Ta) alloys exhibit a distribution of icosahedral particles embedded in an ␣-Al matrix. Early transition metals (Ti, V, Nb, Ta) added to ternary Al–Fe–Cr refine the icosahedral particle size and improve the alloy thermal stability, with icosahedral phase decomposition displaced to higher temperature. Nb and Ta have a higher stabilisation effect on the microstructure than Ti and V delaying decomposition of the icosahedral phase up to the onset of melting.
Acknowledgements The work described forms part of an European Union funded Research Training Network Project on Manufacture and Characterisation of Nanostructured Al Alloys, contract no. HPRN-CT-2000-00038. F. Audebert acknowledges the financial support of the Universidad de Buenos Aires, Argentina. M. Galano acknowledges the financial support of the ORS and Clarendon Funds, and the British Embassy Korea Science and Technology Fund. The authors thank R. Doole for his help in the use of the Jeol 2010. References [1] A. Inoue, M. Watanabe, H.M. Kimura, F. Takahashi, A. Nagata, T. Masumoto, Mater. Trans. JIM 33 (1992) 723. [2] F. Schurack, J. Eckert, L. Schultz, Nanostruct. Mater. 12 (1999) 107. [3] F. Audebert, F. Prima, M. Galano, M. Tomut, P. Warren, I. Stone, B. Cantor, Mater. Trans. JIM 43 (8) (2002) 2017.
M. Galano et al. / Materials Science and Engineering A 375–377 (2004) 1206–1211 [4] A. Inoue, Nanostruct. Mater. 6 (1995) 53. [5] E.H. Buchler, E. Watanabe, N.S. Kazama, Int. J. Non-Equil. Proces. 10 (1997) 35. [6] A. Inoue, H. Kimura, T. Zhang, Mater. Sci. Eng. A 294–296 (2000) 727. [7] A. Inoue, H. Kimura, Nanostruct. Mater. 11 (1999) 221. [8] D. Schechtman, I. Blech, D. Gratias, J. Cahn, Phys. Rev. Lett. 53 (1984) 1951. [9] A. Inoue, H.M. Kimura, T. Masumoto, J. Mater. Sci. 22 (1987) 1758. [10] T. Masumoto, A. Inoue, M. Oguchi, K. Fuka Michi, K. Hiraga, M. Hirabayashi, Trans. Jpn. Inst. Met. 27 (1986) 81. [11] H. Hsieh, B. Toby, T. Egami, Y. He, S. Poon, G. Shiflet, J. Mater. Res. 5 (1990) 2807. [12] F. Audebert, B. Arcondo, D. Rodr´ıguez, H. Sirkin, J. Met. Nanocryst. Mater. 10 (2001) 155. [13] A.-P. Tsai, A. Inoue, T. Masumoto, Jpn. J. Appl. Phys. 26 (1987) L1505. [14] C. Janot, J. Pannetier, J.M. Dubois, R. Fruchart, Phys. Lett. A. 119 (1986) 309. [15] D. Skinner, V. Ramanan, M. Zedalis, N. Kim, Mater. Sci. Eng. 99 (1988) 407.
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[16] A.-P. Tsai, A. Inoue, T. Masumoto, Jpn. J. Appl. Phys. 27 (1988) L5. [17] F. Audebert, H. Sirkin, A. Garc´ıa Escorial, Phil. Mag. B. 76 (4) (1997) 483. [18] D.W. Lawther, R.A. Dunlap, D.J. Lloyd, M.E. McHenry, J. Mater. Sci. 24 (1989) 3076. [19] M. Hisamichi, M. Kimura, K. Sasamori, A. Inoue, J. Mater. Res. 17 (2000) 2737. [20] E.S. Humphreys, P.J. Warren, A. Cerezo, G.D.W. Smith, Mater. Sci. Eng. A. 270 (1999) 48. [21] J.W. Cahn, D. Shechtman, D. Gratias, J. Mater. Res. 1 (1986) 13. [22] R. Manaila, V. Florescu, A. Jianu, O. Radulescu, Phil. Mag. B 60 (1989) 589. [23] F. Audebert, R. Colaço, R. Vilar, H. Sirkin, Scripta Mater. 40 (1999) 551. [24] J. Copola, F. Audebert, S. Duhalde, J. Met. Nanocryst. Mat. 14 (14) (2002) 27. [25] H.M. Kimura, K. Sasamori, A. Inoue, Mater. Sci. Eng. A. 294–296 (2000) 168. [26] A. Ziani, G. Michot, A. Pianelli, A. Redjaimia, C.Y. Zahra, A.M. Zahra, J. Mater. Sci. 30 (1995) 2921. [27] G. Rummel, T. Zumkley, M. Eggersmann, K. Freitag, H. Meher, Z. Metallkd. 86 (2) (1995) 122.
Structural characterisation and stability of new nanoquasicrystalline Al-based alloys M. Galano a , F. Audebert a,b,∗ , B. Cantor a , I. Stone a b
a Department of Materials, University of Oxford, Parks Road Oxford, OX1 3PH, Oxford, UK Advanced Materials Group, Facultad de Ingenier´ıa, Universidad de Buenos Aires, Paseo Colón 850, Buenos Aires 1063, Argentina
Abstract Nanoquasicrystalline Al-based alloys have been studied intensively for the last decade because of their promising mechanical properties. Recently quaternary nanoquasicrystalline alloys with improved thermal stability and better mechanical properties at elevated temperature have been reported. The present work describes an investigation into quaternary Al93 Fe3 Cr2 Z2 (at.%) (with Z = Ti, V, Nb, Ta) and ternary Al93 (Fe3 Cr2 )7 , produced by melt spinning. Structural characterisation was carried out by means of X-ray diffraction (XRD) and transmission electron microscopy (TEM), and the thermal stability and decomposition of the quasicrystalline phase were studied by means of differential scanning calorimetry (DSC). All of the alloys showed a microstructure consisting of spherical icosahedral particles embedded in an ␣-Al matrix. The fourth component produced a significant refinement of icosahedral particle size. The decomposition of the quaternary alloys was observed at higher temperatures than for the ternary alloy with a broad exothermic peak. The new alloys containing Nb and Ta showed a multimodal transformation process extending up to the onset of melting. Icosahedral particles containing Ta or Nb were observed after heat treatment at 550 ◦ C. © 2003 Elsevier B.V. All rights reserved. Keywords: Al alloys; Rapid solidification; Nanoquasicrystalline alloys; Icosahedral phase; Quasicrystals
1. Introduction Several types of Al-based nanoquasicrystalline alloys have been developed in the last decade, with microstructures of nanometer-sized icosahedral particles embedded in an ␣-Al matrix [1–3]. These new alloys have higher microstructural stability than amorphous composites [4,5]. Their mechanical strength decreases slowly with temperature, maintaining high strength relative to nanocrystalline and commercial Al alloys [6] and combining high strength and plasticity at elevated temperature. For commercial applications bulk alloys need to be produced. At present, rapid solidification processes are required to retain nanoquasicrystalline particles in the solidified alloy, therefore bulk samples are being produced by warm extrusion of atomised powders [7]. It is important to obtain nanoquasicrystalline alloys with microstructures stable to higher temperature and with a high decomposition temperature of the icosahedral phase in order to enhance pro-
∗
Corresponding author. E-mail address: [email protected] (F. Audebert).
0921-5093/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2003.10.066
cessing and improve mechanical properties at elevated temperatures. Moreover, the development of alloys with a high nucleation frequency and slow rate of growth of the icosahedral phase is an important aim in order to produce bulk Al-based nanoquasicrystalline composites by casting processes. It is known that Mn, Cr and V promote icosahedral phase formation in Al-based alloys since the icosahedral phase can be obtained in Al–Mn [8], Al–Cr [9], and Al–V [10] binary systems. It has been suggested that Fe produces icosahedral clusters in an Al-based liquid [11,12] and, depending on the alloying elements, these clusters can be stabilised to form an icosahedral quasicrystalline phase [12]. The icosahedral phase has been obtained in ternary systems containing Fe, e.g. Al–Fe–Cu [13], Al–Fe–Cr [14], Al–Fe–V [15], Al–Fe–Ta [16], Al–Fe–Ti [4] and Al–Fe–Nb [17], which shows that Fe also promotes icosahedral phase formation. Moreover, it has been observed that Fe in combination with Cr or V has a refining effect on the size of the icosahedral quasicrystalline phase [18–20]. A quaternary icosahedral phase has been observed in the Al–V–Cr–Ti and Al–Fe–Cr–(Ti,V) systems [3,19]. It is expected that a similar icosahedral phase will be formed in the Al–Fe–Cr–(Nb
M. Galano et al. / Materials Science and Engineering A 375–377 (2004) 1206–1211 Table 1 Code number, nominal chemical compositions and corrected compositions of the alloys studied Code
Nominal chemical composition (at.%)
Corrected chemical composition (at.%)
X Ti V Nb Ta
Al93 Fe4.2 Cr2.8 Al93 Fe3 Cr2 Ti2 Al93 Fe3 Cr2 V2 Al93 Fe3 Cr2 Nb2 Al93 Fe3 Cr2 Ta2
Al92.93 Fe4.24 Cr2.83 Al92.94 Fe3.02 Cr2.02 Ti2.02 Al92.95 Fe3.02 Cr2.01 V2.02 Al92.88 Fe3.05 Cr2.04 Nb2.03 Al92.94 Fe3.02 Cr2.02 Ta2.02
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electron microscopy (TEM) using JEOL 200CX and 2010 microscopes. TEM samples were prepared using a Gatan precision ion polishing system (PIPS). The thermal stability and decomposition of the microstructure were analysed by differential scanning calorimetry (DSC) using a TA Instruments 2010 thermal analyser under a dynamic Ar atmosphere. Ribbon samples were encapsulated in quartz tubes under an Ar atmosphere, followed by heat treatment for 30 min at 550 ◦ C. Heat treated samples were then characterised by XRD and TEM.
or Ta) system. Both elements, Nb and Ta, have higher melting points than V, Ti, Fe and Cr, and lower diffusivity should provide more stability for the microstructure. Quaternary Al93 Fe3 Cr2 Z2 (at.%) (with Z = Ti, V, Nb, Ta) and ternary Al93 (Fe3 Cr2 )7 alloys were therefore chosen in order to study icosahedral phase forming ability, and alloy microstructures and stability.
2. Experimental procedure
t-Al3Nb (200)
θ- Al(Fe,Cr)
i- phase(20/32)
t-Al3Nb (004)
i- phase (18/29)
Intensitity [a.u.]
θ- Al(Fe,Cr)
t-Al3Nb (112)
Master alloys were prepared in an arc furnace under a He atmosphere using pure elements, Al and Cr (99.99%), Fe (99.98%), and Ti, V, Nb and Ta (99.7%). The code and the nominal chemical compositions of the alloys studied are listed in Table 1 together with the corrected compositions taking into account the weight lost attributed to Al in the arc melting process. Rapidly solidified ribbons, with thicknesses between 20 and 30 m, were obtained by melt spinning at a wheel speed of 40 m/s in a reduced He atmosphere using quartz nozzles and with closed-loop controlled ejection temperature between 1200 and 1350 ◦ C depending on the alloy. The microstructures of ribbon samples were characterised by means of X-ray diffraction (XRD) in a Philips 1810 θ–2θ diffractometer using Cu K␣ radiation and by transmission
Ta Nb V
36
38
Al (200)
Al (111)
Ti
40 2θ 42
44
X 46
48
Fig. 1. X-ray diffractograms from the as melt spun alloys.
Fig. 2. Bright field TEM images of the as melt spun alloys: (a) ternary alloy an electron diffraction pattern from a near-spherical particle, (b) Ti, (c) V, (d) Nb, (e) Ta.
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M. Galano et al. / Materials Science and Engineering A 375–377 (2004) 1206–1211
3. Results and discussion The X-ray diffractograms from each of the melt spun alloys showed the presence of the ␣-Al and the icosahedral phase. The former was indexed with reflections corresponding to: (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2) and (4 0 0) planes in the range of 2θ = 10–100◦ . The icosahedral phase was indexed using Cahn’s indexation scheme [21] and the following icosahedral reflections were found in the range of 2θ = 10–100◦ : (6/9), (18/29), (20/32), (38/61) and (52/84). Additional weak X-ray peaks observed at 2θ ∼ 36.7, 43.7 and 75.5◦ could be assigned to the metastable distorted -Al13 (Cr,Fe)2–4 phase [22–24]. The Nb alloy exhibited the following reflections (0 0 2), (1 0 1), (1 1 0), (1 1 2), (0 0 4), (2 0 0), (1 1 4) and (2 2 0) corresponding to the stable tetragonal t-Al3 Nb phase. For an easy recognition of the main peaks, Fig. 1 shows an enlargement of the central part of the X-ray diffractograms where it can be seen some peaks of each phase identified including the two main icosahedral reflections. Fig. 2 shows bright field (BF) TEM images from each melt spun alloy. All the alloys exhibited a distribution of near-spherical i-phase particles, embedded in an ␣-Al matrix. A very low volume fraction of additional small particles was present for each alloy corresponding to the metastable crystalline distorted -Al13 (Cr, Fe)2–4 phase. Other authors [3,25] have reported the presence of a low volume fraction of Al23 Ti9 or Al3 Ti phases in similar melt spun and atomised alloys but these phases were not seen in the present work. A
very small fraction of small particles was observed in the BF TEM images but no reflections corresponding to the former phases have been seen in the X-ray diffractograms. Presumably, these phases are very sensitive to the cooling rate. The Nb alloy exhibited a few non spherical particles corresponding to the t-Al3 Nb phase. The TEM images in Fig. 2 show that the quaternary alloys all have smaller icosahedral particles than the ternary alloy by ∼50%. The nanoquasicrystalline quaternary alloys exhibited icosahedral particles of ∼100 nm in size compared to ∼200 nm in the ternary alloy. Fig. 3(a) shows a bright field BF TEM image of an icosahedral particle in the Nb alloy, exhibiting five-fold symmetry in the corresponding nano-beam diffraction pattern (NBDP). Nano-beam energy dispersive X-ray analysis (EDX) confirmed the presence of the four alloying elements in the icosahedral particles (see Fig. 3(b)). Small peaks corresponding to Cu and Ar were observed in the EDX spectrum, coming from the sample holder and PIPS sample preparation, respectively. A small peak can be observed at 0.28 keV corresponding to C, due to environmental contamination. Fig. 4(a) and (b) show a BF image with corresponding NBDP and EDX spectrum for the Ta alloy. The diffraction pattern also has five-fold symmetry. EDX shows that the icosahedral particles contain Ta as well as Fe, Cr and Al. Again, a small Ti peak due to the sample holder is observed. Fig. 5 shows DSC traces at a heating rate of 40 K/min from all the melt spun alloys. Each alloy showed a small exothermic peak at around 330–370 ◦ C and a main
Fig. 3. (a) Bright field TEM image from an icosahedral particle in the as melt spun Nb alloy and its corresponding electron diffraction pattern, (b) EDX spectrum from the same particle.
M. Galano et al. / Materials Science and Engineering A 375–377 (2004) 1206–1211
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Fig. 4. (a) Bright field TEM image from an icosahedral particle in the as melt spun Ta alloy and its corresponding nano-beam electron diffraction pattern, (b) EDX spectrum of the same particle.
heating rate was 463 ◦ C. The DSC peaks are displaced to higher temperatures with increasing melting point of the fourth component (Ti, V, Nb, Ta). This is probably related to the decreasing diffusion coefficient of the elements in ␣-Al in the following order: Fe, Cr, Ti and V [27]. The
t: t-Al3 Nb
θ :Al-(Cr,Fe) θ
it i
^C D
V
Al
250
300
350
400
450
500
550
600
30
35
#
* ** **
#
*
Exo
D
V
#
X 200
^ ^# # #
Ti
150
C
*
C
Nb
t
^C
*
Nb
θ
** * **
Heat Flow [W/gr]
Ta
Ta
θ
*
Intensity [a.u.]
2.5 W/gr
DSC run 40 C/min
^
iθ
i t
0
D : Al3 Ti C: Al10 V : Al45 V 7
i: i-phase
: Al13 Cr 2 # : Al13 Fe 4
*
exothermic transition with a broad peak at higher temperature. In previous work [3], it was observed that for the Ti alloy the i-phase remained stable after heating beyond the first small peak and a small fraction of the distorted -Al13 (Cr,Fe)2–4 phase was precipitated. These small peaks might be related to a low level of precipitation from the supersaturated ␣-Al matrix. Ziani et al. [26] reported similar DSC traces for an Al94 Fe3 Cr3 alloy obtained by hot extrusion of rapidly solidified powders, with a peak at ∼490 ◦ C for a heating rate of 10 K/min. In the case of the ternary alloy the peak temperature for DSC traces at the same
40
Ti
D
#
X
#
Al
2θ
45
50
55
0
Temperature [ C] Fig. 5. DSC traces from the as melt spun alloys heated at 40 K/min.
Fig. 6. X-ray diffractograms from the ternary and quaternary alloys after heat treatment for 30 min at 550 ◦ C.
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Fig. 7. (a) Bright field image of an icosahedral particle in the Ta alloy after a heat treatment for 30 min at 550 ◦ C with the inset of the electron diffraction pattern of the same particle, (b) bright field image of a near-spherical particle in the Nb alloy after a heat treatment for 30 min at 550 ◦ C.
main transformations corresponding to the new nanoquasicrystalline Nb and Ta alloys have broader transformation peaks than those corresponding to the ternary, Ti and V alloys. These sluggish transformations are extended over a large temperature range up to the onset of the alloy melting. This high stability is probably related to the lower diffusivity of Nb and Ta, which slows down a diffusion controlled decomposition. The X-ray diffractograms in the range of 2θ = 10–100◦ from the heat treated ternary (X), Ti and V alloys did not show traces of the i-phase but showed peaks corresponding to the stable phases: ␣-Al, -Al13 Cr2 and -Al13 Fe4 . The heat treated Ti and V alloys, in addition showed peaks of the t-Al3 Ti phase and the -Al45 V7 and cubic Al10 V phases, respectively. The diffractograms corresponding to the heat treated Nb and Ta alloys did not show new peaks, however icosahedral, distorted -Al13 (Cr, Fe)2–4 and ␣-Al phases could be identified. Moreover, the Nb alloy also contained the t-Al3 Nb phase. In order to appreciate the main peaks of each phase indexed an enlargement of the central part of the diffractograms is shown in Fig. 6. The former results together with the DSC traces suggest that icosahedral decomposition of the X, Ti and V alloys finished after heat treatment for 30 min at 550 ◦ C, confirming the enhanced stability of the microstructure of the alloys containing Nb and Ta as a fourth alloying element. The presence of the icosahedral phase after the heat treatment was confirmed for the Ta alloy by TEM as shown by the five-fold symmetry in the selected area diffraction pattern corresponding to a near-spherical particle in Fig. 7(a). The heat treated Nb alloy also showed near-spherical particles similar to those found in the heat treated Ta alloy (Fig. 7(b)), which could be the icosahedral phase in agreement with the XRD traces (Fig. 6). The average size of the icosahedral particles after the heat treatment, in both Nb and Ta alloys, was ∼200 nm.
4. Conclusions New melt spun nanoquasicrystalline alloys containing Nb and Ta have been obtained. Their microstructure is composed of nano-sized icosahedral particles distributed in an ␣-Al matrix. Ternary and quaternary Al–Fe–Cr–(Ti, V, Nb, Ta) alloys exhibit a distribution of icosahedral particles embedded in an ␣-Al matrix. Early transition metals (Ti, V, Nb, Ta) added to ternary Al–Fe–Cr refine the icosahedral particle size and improve the alloy thermal stability, with icosahedral phase decomposition displaced to higher temperature. Nb and Ta have a higher stabilisation effect on the microstructure than Ti and V delaying decomposition of the icosahedral phase up to the onset of melting.
Acknowledgements The work described forms part of an European Union funded Research Training Network Project on Manufacture and Characterisation of Nanostructured Al Alloys, contract no. HPRN-CT-2000-00038. F. Audebert acknowledges the financial support of the Universidad de Buenos Aires, Argentina. M. Galano acknowledges the financial support of the ORS and Clarendon Funds, and the British Embassy Korea Science and Technology Fund. The authors thank R. Doole for his help in the use of the Jeol 2010. References [1] A. Inoue, M. Watanabe, H.M. Kimura, F. Takahashi, A. Nagata, T. Masumoto, Mater. Trans. JIM 33 (1992) 723. [2] F. Schurack, J. Eckert, L. Schultz, Nanostruct. Mater. 12 (1999) 107. [3] F. Audebert, F. Prima, M. Galano, M. Tomut, P. Warren, I. Stone, B. Cantor, Mater. Trans. JIM 43 (8) (2002) 2017.
M. Galano et al. / Materials Science and Engineering A 375–377 (2004) 1206–1211 [4] A. Inoue, Nanostruct. Mater. 6 (1995) 53. [5] E.H. Buchler, E. Watanabe, N.S. Kazama, Int. J. Non-Equil. Proces. 10 (1997) 35. [6] A. Inoue, H. Kimura, T. Zhang, Mater. Sci. Eng. A 294–296 (2000) 727. [7] A. Inoue, H. Kimura, Nanostruct. Mater. 11 (1999) 221. [8] D. Schechtman, I. Blech, D. Gratias, J. Cahn, Phys. Rev. Lett. 53 (1984) 1951. [9] A. Inoue, H.M. Kimura, T. Masumoto, J. Mater. Sci. 22 (1987) 1758. [10] T. Masumoto, A. Inoue, M. Oguchi, K. Fuka Michi, K. Hiraga, M. Hirabayashi, Trans. Jpn. Inst. Met. 27 (1986) 81. [11] H. Hsieh, B. Toby, T. Egami, Y. He, S. Poon, G. Shiflet, J. Mater. Res. 5 (1990) 2807. [12] F. Audebert, B. Arcondo, D. Rodr´ıguez, H. Sirkin, J. Met. Nanocryst. Mater. 10 (2001) 155. [13] A.-P. Tsai, A. Inoue, T. Masumoto, Jpn. J. Appl. Phys. 26 (1987) L1505. [14] C. Janot, J. Pannetier, J.M. Dubois, R. Fruchart, Phys. Lett. A. 119 (1986) 309. [15] D. Skinner, V. Ramanan, M. Zedalis, N. Kim, Mater. Sci. Eng. 99 (1988) 407.
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[16] A.-P. Tsai, A. Inoue, T. Masumoto, Jpn. J. Appl. Phys. 27 (1988) L5. [17] F. Audebert, H. Sirkin, A. Garc´ıa Escorial, Phil. Mag. B. 76 (4) (1997) 483. [18] D.W. Lawther, R.A. Dunlap, D.J. Lloyd, M.E. McHenry, J. Mater. Sci. 24 (1989) 3076. [19] M. Hisamichi, M. Kimura, K. Sasamori, A. Inoue, J. Mater. Res. 17 (2000) 2737. [20] E.S. Humphreys, P.J. Warren, A. Cerezo, G.D.W. Smith, Mater. Sci. Eng. A. 270 (1999) 48. [21] J.W. Cahn, D. Shechtman, D. Gratias, J. Mater. Res. 1 (1986) 13. [22] R. Manaila, V. Florescu, A. Jianu, O. Radulescu, Phil. Mag. B 60 (1989) 589. [23] F. Audebert, R. Colaço, R. Vilar, H. Sirkin, Scripta Mater. 40 (1999) 551. [24] J. Copola, F. Audebert, S. Duhalde, J. Met. Nanocryst. Mat. 14 (14) (2002) 27. [25] H.M. Kimura, K. Sasamori, A. Inoue, Mater. Sci. Eng. A. 294–296 (2000) 168. [26] A. Ziani, G. Michot, A. Pianelli, A. Redjaimia, C.Y. Zahra, A.M. Zahra, J. Mater. Sci. 30 (1995) 2921. [27] G. Rummel, T. Zumkley, M. Eggersmann, K. Freitag, H. Meher, Z. Metallkd. 86 (2) (1995) 122.