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Crystallisation in Al–ETM–LTM–La metallic glasses
Intermetallics 12 (2004) 1045–1050 www.elsevier.com/locate/intermet
Crystallisation in Al –ETM –LTM –La metallic glasses Joysurya Basu*, S. Ranganathan Department of Metallurgy, Indian Institute of Science, Bangalore 560012, India Available online 19 June 2004
Abstract The addition of early and late transition metals to Al – La alloys has a strong influence on the glass forming ability and the subsequent devitrification route. Four quaternary alloys of nominal composition Al87Fe5Ni5La3, Al88Fe3Ni3La6, Al82Fe4Ni4La10, Al75Ti10Ni10La5 and a ternary alloy of nominal composition Al75Ti10Ni15 have been melt spun at 10, 20, 30 and 40 m s21 wheel speed. Amorphous alloys thus secured have been annealed at the peak temperatures as indicated by the DSC thermograms from 10 min to 2 h. The Al87Fe5Ni5La3 alloy ˚ lattice parameter. The Al88Fe3Ni3La6 alloy in the melt-spun showed in the melt-spun state precipitates of a-Al, a FCC phase with 12.5 A state shows nano-sized precipitate of a-Al in the amorphous matrix. The Al82Fe4Ni4La10 alloy after melt-spinning at 40, 30, 20 m s21 wheel speed becomes amorphous and shows precipitates of a-Al in the amorphous matrix when melt spun at 10 m s21 wheel speed. The amorphous phase in the Al82Fe4Ni4La10 alloy is stable up to 625 K and crystallises in three exothermic heat events. After heat treatment at the first crystallisation peak temperature and below the first crystallisation temperature precipitation of a-Al in the amorphous matrix can be observed. At the end of the crystallisation process very fine precipitates of Al3La and LaNi can be seen at the a-Al grain boundaries. Activation energy of crystallisation has been calculated by Kissinger and Ozawa method. Al – Ti –Ni – La becomes partially amorphous after melt-spinning. Al – Ti– Ni alloy does not produce amorphous phase after melt-spinning. Precipitation of Al3Ti phase can be observed in both the alloys. The development of different microstructures, effect of La on the glass forming ability and the stabilisation of the glassy matrix with enrichment of La and with the progress of crystallisation, establishes a possible link between Al based metallic glasses and La based bulk metallic glasses. q 2004 Elsevier Ltd. All rights reserved. Keywords: A. Composites; B. Glasses, metallic; D. Microstructure
1. Introduction Amorphous alloys in Al – TM metal system were discovered in the late seventies. The synthesis of amorphous phase in these alloy systems required a high cooling rate, there by restricting the alloy geometry to thin foils and ribbons. In order to decrease the cooling rate new alloys were searched in the aluminium poor regions in the phase diagram and that led to the discovery of La-based bulk metallic glass [1]. A glassy phase in aluminium alloys can be synthesised in binary Al –Ln, Al – TM and ternary aluminium rich Al – ETM – LTM, Al – LTM – LTM, Al –LTM – Ln and Al –Mg –Ln systems [2]. In quaternary alloys glasses form in Al –LTM – LTM –Ln [3 –5] and Al –LTM – Ln –Ln [6] systems. Apart from the glassy phase, the selection of proper alloy composition and processing route can lead to the formation of nanocrystals and quasicrystals. It has been observed that the partial crystallisation of aluminium based metallic * Corresponding author. Tel./fax: þ 91-80-3601198. E-mail address: [email protected] (J. Basu). 0966-9795/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2004.04.004
glasses leads to an enhancement of properties [7] and different aluminium based alloy systems can lead to the formation of different in situ bulk composites which have varying mechanical properties [8]. In the present study, the glass forming ability and the microstructural evolution of Al – Fe– Ni– La, Al– Ti – Ni –La and Al– Ti – Ni have been investigated. The main objective has been to find the influence of La on the glass formation and to explore the relationship of La based bulk metallic glasses with Al-based metallic glasses. 2. Experimental techniques Five alloys of nominal composition Al87Fe5Ni5La3, Al88Fe3Ni3La6, Al82Fe4Ni4La10, Al75Ti10Ni10La5, Al75Ti10Ni15 (compositions are expressed in at.%) have been melted in a vacuum induction melting furnace. Then the Al – Fe– Ni – La alloys have been melt spun with a single roller meltspinning unit in argon atmosphere with 40, 30, 20, 10 m s21 wheel speeds. The Al– Ti – Ni and Al –Ti – Ni –La alloys have been melt spun with 40 and 20 m s21 wheel speeds.
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The melt-spun alloys have been structurally characterised by JEOL JDX 8030 X-ray diffractometer in the angular range of 20 – 908 and JEOL 2000 FX II transmission electron microscope. The composition of the alloys has been confirmed with X-ray energy dispersive spectroscopy attached with the microscope. The thermal stability of the alloys has been characterised by Mettler Toledo 822E differential scanning calorimeter at 20, 30, 40, 50, 60 K min21 heating rate. In order to study the phases evolved upon crystallisation, the amorphous alloy has been heat treated in vacuum at the peak temperatures and in the supercooled liquid region for 10 min to 2 h. The partially crystallised alloys have been characterised by XRD and TEM.
3. Results The Al87Fe5Ni5La3 alloy, even after melt-spinning at 40 m s21 wheel speed, does not produce an amorphous phase. In the X-ray diffraction pattern a number of crystalline peaks can be seen, some of which can be indexed to aluminium. The transmission electron micrograph and the electron diffraction pattern for the same alloy melt spun at 40 m s21 wheel speed is given in Fig. 1. The presence of crystals with a dendritic morphology can be observed. The length of the crystals is in the range of 600– 800 nm and their width is around 200 nm. The diffraction pattern along [110] zone axis confirms that the crystal is ˚ lattice parameter. This phase closely FCC with 12.5 A matches with the Al7Fe6La phase, which has a FCC ˚ lattice parameter. The difference in structure with 12 A lattice parameter may arise due to rapid solidification or nonstoichiometry. This requires further investigation.
Fig. 1. TEM micrograph and electron diffraction pattern of the Al87Fe5Ni5La3 alloy melt spun at 40 m s21 wheel speed. Dendritic crystals with ˚ lattice parameter can be seen. 12.5 A
Fig. 2. TEM micrograph and electron diffraction pattern of the Al87Fe5Ni5La3 alloy melt spun at 40 m s21 wheel speed. A domain morphology can be seen.
Transmission electron micrograph and electron diffraction pattern from a different region of the same alloy is given in Fig. 2. In this micrograph, a domain morphology can be observed. The size of the domains varies in the range of 50– 100 nm. The diffraction pattern can be indexed to FCC ˚, [001] zone axis. The lattice parameter of the crystal is 4 A which is very close to that of aluminium. In the diffraction pattern apart from 200 and 220 principal reflections weak 110 superlattice reflections can be seen indicative of ordering. Along with that satellite spots around each reflections can be observed. The presence of the satellite spots indicates that there may be modulations in the crystal. It can be inferred that the crystal is a solid solution of aluminium and it is partially ordered. Due to the presence of different kinds of solute atoms the modulation in the d-spacings has occurred, which has given rise to the satellite spots. The alloy precipitates elongated aluminium crystallites when melt spun at 20 m s21 wheel speed. The Al88Fe3Ni3La6 alloy after melt-spinning at 40 and 20 m s21 wheel speed does not become totally amorphous. The peaks in the X-ray diffraction pattern can be indexed to aluminium. The crystallites are fine as can be confirmed from the broadening of the diffraction peaks. The transmission electron micrograph and the electron diffraction pattern of the alloy melt spun at 40 m s21 wheel speed is shown in Fig. 3. In the micrograph nearly spherical crystallites varying in the size range of 30 – 50 nm can be observed. From the diffraction pattern it can be confirmed that these are aluminium crystallites. A multiple display of the X-ray diffraction patterns of the Al82Fe4Ni4La10 alloy melt spun at 40, 30, 20, 10 m s21 wheel speeds is shown in Fig. 4. The alloy is totally
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Fig. 5. DSC thermogram of the amorphous Al82Fe4Ni4La10 alloy.
Fig. 3. TEM micrograph and electron diffraction pattern of the Al88Fe3Ni3La6 alloy melt spun at 40 m s21 wheel speed.
amorphous after melt-spinning at 40, 30, 20 m s21 wheel speed that can be confirmed from the diffraction hump. In the diffraction pattern of the alloy melt spun at 10 m s21 wheel speed, a few crystalline peaks can be observed. The peaks can be indexed to aluminium and LaNi. Aluminium crystallites varying in the size range of 50 – 100 nm can be observed under transmission electron microscope. Differential scanning calorimetric thermogram of the glassy Al82Fe4Ni4La10 alloy melt spun at 40 m s21 wheel speed is shown in Fig. 5. From the thermogram it can be confirmed that the crystallisation behaviour of the alloy is complex and it takes place in almost three overlapping heat events. In the thermogram no obvious glass transition can be observed. A minor peak shift to higher temperature side can
Fig. 4. Multiple display of XRD patterns of the Al82Fe4Ni4La10 alloy melt spun at 40, 30, 20, 10 m s21 wheel speed.
be observed with increase in heating rate. The peak temperatures of crystallisation after scanning at 60 K min21 are 625, 650, 703 and 744 K, respectively. In order to find out the activation energy for each crystallisation event activation energy is calculated by Kissinger method and Ozawa method. The activation energies for three main crystallisation events are 290, 302, 298 kJ/mole by Kissinger method and 286, 298, 295 kJ/mole by Ozawa method. Considering the assumptions and approximations of both the processes, it can be concluded that the activation energies in both the methods are in agreement with each other. A multiple display of the X-ray diffraction patterns of the amorphous Al82Fe4Ni4La10 alloy after heat treatment at 628, 648, 698 and 728 K for 10 min is given in Fig. 6. After heat treatment at 628 K a few broad peaks over the diffraction hump can be seen. These peaks can be indexed to aluminium. The nature of the peaks indicates that the crystallites are very fine in size. With the progress of crystallisation the intensity and sharpness of the peaks increases. It can be inferred from this observation that a-Al precipitates after heat treatment at 628 and 648 K. After heat treatment at 698 K a few new peaks can be observed.
Fig. 6. Multiple display of XRD patterns of the amorphous Al82Fe4Ni4La10 alloy after heat treatment at different temperatures for 10 min.
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Fig. 8. Multiple display of XRD patterns of the (a) Al75Ti10Ni10La5 (b) Al75Ti10Ni15 alloy melt spun at 40 m s21 wheel speed.
Fig. 7. TEM micrograph and electron diffraction pattern of the amorphous Al82Fe4Ni4La10 alloy after heat treatment at 628 K for 2 h.
These peaks closely match with those of a-Al and LaNi phase. After heat treatment at 728 K presence of a-Al, LaNi and Al3La phase can be seen in the diffraction pattern. The nature of the diffraction pattern after heat treatment for 2 h also remains the same indicating that the phases are stable. Transmission electron micrograph and corresponding electron diffraction pattern of the amorphous alloy after heat treatment at 628 K for 2 h are given in Fig. 7. After annealing at 628 K very fine precipitates varying in the size range of 20– 50 nm can be seen in the amorphous matrix. The precipitates are spherical or polygonal in shape. After heat treatment at 728 K precipitates in the size range of 100 – 200 nm can be seen. At this stage there is no amorphous phase left in the matrix and very fine precipitates of LaNi and Al3La phase are seen to be distributed in grain boundaries of aluminium. X-ray diffraction patterns of the Al75Ti10Ni15 and Al75Ti10Ni10La5 alloy after melt-spinning at 40 m s21 wheel speed are shown in Fig. 8. The Al75Ti10Ni15 alloy even after melt-spinning at 40 m s21 wheel speed does not become amorphous. After melt-spinning precipitation of Al3Ti and a solid solution phase of NiTi can be observed. There are two allotropic modifications Al3Ti. It appears that both tetragonal and cubic modifications are present in the alloy. The Al75Ti10Ni10La5 alloy after melt-spinning at 40 m s21 wheel speed become partially amorphous. The amount of crystallinity increases with a decrease in the wheel speed. In this alloy after melt-spinning along with Al3Ti and NiTi solid solution phase, the presence of AlNi solid solution phase, La2Ni7 phase can be seen. The transmission electron micrograph and the electron diffraction pattern of the Al75Ti10Ni10La5 alloy after melt-spinning
at 40 m s21 wheel speed is shown in Fig. 9 indicating nanoprecipitates within an amorphous network.
4. Discussion Three alloys of Al– Fe –Ni – La system have been studied in which La content has been increased. From the X-ray and microscopic results it is seen that the Al87Fe5Ni5La3 alloy does not produce an amorphous phase, the Al88Fe3Ni3La6 alloy becomes partially amorphous and the Al82Fe4Ni4La10 alloy becomes totally amorphous after melt-spinning. Primarily it is observed that with the increase in La content the glass forming ability of the alloy increases. This fact can be substantiated by taking into consideration atomic radii
Fig. 9. Transmission electron micrograph and electron diffraction pattern of the Al75Ti10Ni10La5 alloy after melt-spinning at 40 m s21 wheel speed. Note the presence of amorphous phase in the intercrystalline regions.
J. Basu, S. Ranganathan / Intermetallics 12 (2004) 1045–1050 Table 1 Atomic radii mismatch (in %) and binary heats of mixing (kJ/mole) in Al, Fe, Ni, Ti, La systems Al Al Fe Ni Ti La
11% 13% 2% 24%
Fe
Ni
Ti
La
211
222 22
230 217 235
238 5 227 20
2.2% 13% 32%
15% 34%
22%
mismatch and heat of mixing. These values are given in Table 1. It is seen from the table that La has a high atomic radii mismatch with Al, Fe and Ni. The size of aluminium is also different from that of Fe and Ni. The heat of mixing of aluminium with Fe, Ni and La is highly negative, whereas that of Fe with La is positive. So far as atomic size is concerned, lanthanum contributes the maximum to frustrate the alloy system and it maintains a high negative heat of mixing with Al and Ni. That may be a possible reason behind the increase in glass forming ability of the alloy on lanthanum addition. Apart from the glassy phase crystalline phases and the precipitation of the solid solution phase of aluminium can also be observed in the glassy matrix. Al –Ti –Ni and Al– Ti – Ni– La alloy could not be amorphised totally. In the melt-spun state along with the predominant precipitation of Al3Ti and La2Ni7, solid solution phases of NiTi and AlNi can be seen. In the present study it is observed that Al– Fe –Ni –La is a better glass forming system than Al –Ti –Ni – La. Apart from the basic difference in atomic configuration between early transition metals and late transition metals, the atomic size difference of Ti with Al is only 2% and the heat of mixing between Ti and La is highly positive. These differences may be accounted for the lower glass forming ability of the Al – Ti – Ni –La alloy. The precipitation of Al3Ti phase during solidification and crystallisation has been reported previously [9]. Al – Ti – Ni alloy can be amorphised totally in the aluminium poor region of the phase diagram [10]. An enhancement of glass forming ability with the addition of rare earths in aluminium based ternary and germanium based alloys has been reported [11]. It is observed that addition of a number of rare earths or mischmetal does not improve the glass forming ability of the alloy significantly because atomic radii mismatch between the rare earths is not significant [12]. The crystallisation behaviour of the Al –Fe – Ni– La alloy is complex and it occurs in three steps. The alloy is thermally stable up to 625 K. At the initial stages of crystallisation nano-precipitates of aluminium are seen in the amorphous matrix. With the progress of the crystallisation process size and volume fraction of the aluminium crystallites increases. First stage of crystallisation can be attributed to the precipitation of aluminium. Second stage to the precipitation of LaNi and at the end of crystallisation precipitation of Al3La and LaNi phase at the grain
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boundaries of a-Al can be observed. The precipitation of aluminium at the initial stages of crystallisation has been reported for other aluminium based metallic glasses. From the crystallisation process it can be concluded that with the precipitation of aluminium the matrix becomes richer in lanthanum and becomes more resistant to crystallisation. At the end of crystallisation lanthanum rich precipitates are seen at the grain boundaries. The alloy is resistant to grain growth also. Even after two hours of annealing grains do not grow beyond 200 nm size that may be due to the high nucleation rate of aluminium and precipitation of Al3La and LaNi at the grain boundaries. A simple stoichiometric calculation shows that if 30 vol.% of aluminium crystallites is precipitated then the composition of the matrix shifts from 82% Al to 73% Al and 10% La to 15% La. At the end of the crystallisation process, the matrix should be even richer in La. This means that the ternary composition of the matrix shifts from one end to the other end of the phase diagram and it becomes almost similar to typical lanthanum based glasses. This can be further substantiated with the fact that the primarily crystallising phases in La-based glasses, i.e. (Ni,Al)2La, La(Al,Cu,Ni), La3(Al,Ni) [1,13] are stoichiometrically very similar to the phases precipitated in the present amorphous alloy at the end of crystallisation.
5. Conclusions Quaternary alloys of aluminium with the late transition metals Fe, Ni and with rare earth metals La can be amorphised by melt-spinning. The glass forming ability of the alloys increases with an increase in La content. Below a certain quantity of lanthanum it cannot be amorphised. The crystallisation of the glassy alloy takes place in multiple steps. At the initial stages of crystallisation nanocrystals of aluminium are precipitated and the matrix become richer in solute content. The precipitated crystallites are resistant to grain growth. The quaternary Al –Ti – Ni –La shows a lower degree of glass formation, as compound formation is preferred. Glass forming ability of Al –Ti – Ni –La is better compared to ternary Al– Ti – Ni alloy.
Acknowledgements The authors would like to acknowledge stimulating discussions with Prof. A. Inoue, Prof. H. Kimura, Prof. K. Chattopadhyay, Prof. V. Jayaram, Dr U. Ramamurty and Ms. Tripti Biswas on various occasions. Research funding from DRDO/MMT/URM/537 is gratefully acknowledged.
References [1] Inoue A, Zhang T, Masumoto T. Al –La–Ni amorphous alloy with wide supercooled liquid region. Mater Trans JIM 1989;30:965–72.
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[2] Inoue A. Amorphous, nanoquasicrystalline and nanocrystalline alloys in Al-based systems. Prog Mater Sci 1998;43:365 –520. [3] Bassim N, Kiminami CS, Kaufmann MJ. Phases formed during the crystallization of amorphous Al –Y –Ni–Co alloy. J Non-Cryst Solids 2000;273:271 –6. [4] Cardoso KR, Escorial AG, Botta FWJ. Heat treatment of amorphous Al–Fe –Nd and Al–Fe–Co –Nd alloys. J Non-Cryst Solids 2000;273: 266–70. [5] Dougherty GM, He Y, Shiflet GJ, Poon SJ. Compositional dependence of glass forming ability in Al–Fe– Ni–Gd amorphous alloys. Scr Met 1994;30:101– 6. [6] Gangopadhyay AK, Croat TK, Kelton KF. Effect of phase separation on subsequent crystallization in Al–Gd–La–Ni. Acta Mater 2000;48: 4035–43. [7] Perepezko JH, Hebert RJ. Amorphous aluminium alloys synthesis and stability. JOM 2002;54:34–9.
[8] Schurack F, Eckert J, Schultz L. Synthesis and mechanical properties of as cast quasicrystal reinforced Al-alloys. Acta Mater 2001;49: 1351–61. [9] Kimura H, Sasamori K, Inoue A. New amorphous alloys in Al–Ti binary system. Mater Trans JIM 1998;39:773– 7. [10] Davies HA. In: Luborsky FE, editor. Amorphous metallic alloys, vol. 8. London: Butterworth; 1983. [11] Louzguine DV, Inoue A. Influence of rare earth metals on formation range and structure of amorphous phase in Ge–Al –Cr–RE system. Mater Trans JIM 1999;40:485–90. [12] Louzguine DV, Yavari AR, Inoue A. Mischmetal as an alloying addition to the amorphous materials and glass formers. J Non-Cryst Solids 2003;316:255 –60. [13] Inoue A, Yamaguchi H, Zhang T, Masumoto T. Al – La – Cu amorphous alloys with a wide supercooled liquid region. Mater Trans JIM 1990;31:104–9.
Crystallisation in Al –ETM –LTM –La metallic glasses Joysurya Basu*, S. Ranganathan Department of Metallurgy, Indian Institute of Science, Bangalore 560012, India Available online 19 June 2004
Abstract The addition of early and late transition metals to Al – La alloys has a strong influence on the glass forming ability and the subsequent devitrification route. Four quaternary alloys of nominal composition Al87Fe5Ni5La3, Al88Fe3Ni3La6, Al82Fe4Ni4La10, Al75Ti10Ni10La5 and a ternary alloy of nominal composition Al75Ti10Ni15 have been melt spun at 10, 20, 30 and 40 m s21 wheel speed. Amorphous alloys thus secured have been annealed at the peak temperatures as indicated by the DSC thermograms from 10 min to 2 h. The Al87Fe5Ni5La3 alloy ˚ lattice parameter. The Al88Fe3Ni3La6 alloy in the melt-spun showed in the melt-spun state precipitates of a-Al, a FCC phase with 12.5 A state shows nano-sized precipitate of a-Al in the amorphous matrix. The Al82Fe4Ni4La10 alloy after melt-spinning at 40, 30, 20 m s21 wheel speed becomes amorphous and shows precipitates of a-Al in the amorphous matrix when melt spun at 10 m s21 wheel speed. The amorphous phase in the Al82Fe4Ni4La10 alloy is stable up to 625 K and crystallises in three exothermic heat events. After heat treatment at the first crystallisation peak temperature and below the first crystallisation temperature precipitation of a-Al in the amorphous matrix can be observed. At the end of the crystallisation process very fine precipitates of Al3La and LaNi can be seen at the a-Al grain boundaries. Activation energy of crystallisation has been calculated by Kissinger and Ozawa method. Al – Ti –Ni – La becomes partially amorphous after melt-spinning. Al – Ti– Ni alloy does not produce amorphous phase after melt-spinning. Precipitation of Al3Ti phase can be observed in both the alloys. The development of different microstructures, effect of La on the glass forming ability and the stabilisation of the glassy matrix with enrichment of La and with the progress of crystallisation, establishes a possible link between Al based metallic glasses and La based bulk metallic glasses. q 2004 Elsevier Ltd. All rights reserved. Keywords: A. Composites; B. Glasses, metallic; D. Microstructure
1. Introduction Amorphous alloys in Al – TM metal system were discovered in the late seventies. The synthesis of amorphous phase in these alloy systems required a high cooling rate, there by restricting the alloy geometry to thin foils and ribbons. In order to decrease the cooling rate new alloys were searched in the aluminium poor regions in the phase diagram and that led to the discovery of La-based bulk metallic glass [1]. A glassy phase in aluminium alloys can be synthesised in binary Al –Ln, Al – TM and ternary aluminium rich Al – ETM – LTM, Al – LTM – LTM, Al –LTM – Ln and Al –Mg –Ln systems [2]. In quaternary alloys glasses form in Al –LTM – LTM –Ln [3 –5] and Al –LTM – Ln –Ln [6] systems. Apart from the glassy phase, the selection of proper alloy composition and processing route can lead to the formation of nanocrystals and quasicrystals. It has been observed that the partial crystallisation of aluminium based metallic * Corresponding author. Tel./fax: þ 91-80-3601198. E-mail address: [email protected] (J. Basu). 0966-9795/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2004.04.004
glasses leads to an enhancement of properties [7] and different aluminium based alloy systems can lead to the formation of different in situ bulk composites which have varying mechanical properties [8]. In the present study, the glass forming ability and the microstructural evolution of Al – Fe– Ni– La, Al– Ti – Ni –La and Al– Ti – Ni have been investigated. The main objective has been to find the influence of La on the glass formation and to explore the relationship of La based bulk metallic glasses with Al-based metallic glasses. 2. Experimental techniques Five alloys of nominal composition Al87Fe5Ni5La3, Al88Fe3Ni3La6, Al82Fe4Ni4La10, Al75Ti10Ni10La5, Al75Ti10Ni15 (compositions are expressed in at.%) have been melted in a vacuum induction melting furnace. Then the Al – Fe– Ni – La alloys have been melt spun with a single roller meltspinning unit in argon atmosphere with 40, 30, 20, 10 m s21 wheel speeds. The Al– Ti – Ni and Al –Ti – Ni –La alloys have been melt spun with 40 and 20 m s21 wheel speeds.
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The melt-spun alloys have been structurally characterised by JEOL JDX 8030 X-ray diffractometer in the angular range of 20 – 908 and JEOL 2000 FX II transmission electron microscope. The composition of the alloys has been confirmed with X-ray energy dispersive spectroscopy attached with the microscope. The thermal stability of the alloys has been characterised by Mettler Toledo 822E differential scanning calorimeter at 20, 30, 40, 50, 60 K min21 heating rate. In order to study the phases evolved upon crystallisation, the amorphous alloy has been heat treated in vacuum at the peak temperatures and in the supercooled liquid region for 10 min to 2 h. The partially crystallised alloys have been characterised by XRD and TEM.
3. Results The Al87Fe5Ni5La3 alloy, even after melt-spinning at 40 m s21 wheel speed, does not produce an amorphous phase. In the X-ray diffraction pattern a number of crystalline peaks can be seen, some of which can be indexed to aluminium. The transmission electron micrograph and the electron diffraction pattern for the same alloy melt spun at 40 m s21 wheel speed is given in Fig. 1. The presence of crystals with a dendritic morphology can be observed. The length of the crystals is in the range of 600– 800 nm and their width is around 200 nm. The diffraction pattern along [110] zone axis confirms that the crystal is ˚ lattice parameter. This phase closely FCC with 12.5 A matches with the Al7Fe6La phase, which has a FCC ˚ lattice parameter. The difference in structure with 12 A lattice parameter may arise due to rapid solidification or nonstoichiometry. This requires further investigation.
Fig. 1. TEM micrograph and electron diffraction pattern of the Al87Fe5Ni5La3 alloy melt spun at 40 m s21 wheel speed. Dendritic crystals with ˚ lattice parameter can be seen. 12.5 A
Fig. 2. TEM micrograph and electron diffraction pattern of the Al87Fe5Ni5La3 alloy melt spun at 40 m s21 wheel speed. A domain morphology can be seen.
Transmission electron micrograph and electron diffraction pattern from a different region of the same alloy is given in Fig. 2. In this micrograph, a domain morphology can be observed. The size of the domains varies in the range of 50– 100 nm. The diffraction pattern can be indexed to FCC ˚, [001] zone axis. The lattice parameter of the crystal is 4 A which is very close to that of aluminium. In the diffraction pattern apart from 200 and 220 principal reflections weak 110 superlattice reflections can be seen indicative of ordering. Along with that satellite spots around each reflections can be observed. The presence of the satellite spots indicates that there may be modulations in the crystal. It can be inferred that the crystal is a solid solution of aluminium and it is partially ordered. Due to the presence of different kinds of solute atoms the modulation in the d-spacings has occurred, which has given rise to the satellite spots. The alloy precipitates elongated aluminium crystallites when melt spun at 20 m s21 wheel speed. The Al88Fe3Ni3La6 alloy after melt-spinning at 40 and 20 m s21 wheel speed does not become totally amorphous. The peaks in the X-ray diffraction pattern can be indexed to aluminium. The crystallites are fine as can be confirmed from the broadening of the diffraction peaks. The transmission electron micrograph and the electron diffraction pattern of the alloy melt spun at 40 m s21 wheel speed is shown in Fig. 3. In the micrograph nearly spherical crystallites varying in the size range of 30 – 50 nm can be observed. From the diffraction pattern it can be confirmed that these are aluminium crystallites. A multiple display of the X-ray diffraction patterns of the Al82Fe4Ni4La10 alloy melt spun at 40, 30, 20, 10 m s21 wheel speeds is shown in Fig. 4. The alloy is totally
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Fig. 5. DSC thermogram of the amorphous Al82Fe4Ni4La10 alloy.
Fig. 3. TEM micrograph and electron diffraction pattern of the Al88Fe3Ni3La6 alloy melt spun at 40 m s21 wheel speed.
amorphous after melt-spinning at 40, 30, 20 m s21 wheel speed that can be confirmed from the diffraction hump. In the diffraction pattern of the alloy melt spun at 10 m s21 wheel speed, a few crystalline peaks can be observed. The peaks can be indexed to aluminium and LaNi. Aluminium crystallites varying in the size range of 50 – 100 nm can be observed under transmission electron microscope. Differential scanning calorimetric thermogram of the glassy Al82Fe4Ni4La10 alloy melt spun at 40 m s21 wheel speed is shown in Fig. 5. From the thermogram it can be confirmed that the crystallisation behaviour of the alloy is complex and it takes place in almost three overlapping heat events. In the thermogram no obvious glass transition can be observed. A minor peak shift to higher temperature side can
Fig. 4. Multiple display of XRD patterns of the Al82Fe4Ni4La10 alloy melt spun at 40, 30, 20, 10 m s21 wheel speed.
be observed with increase in heating rate. The peak temperatures of crystallisation after scanning at 60 K min21 are 625, 650, 703 and 744 K, respectively. In order to find out the activation energy for each crystallisation event activation energy is calculated by Kissinger method and Ozawa method. The activation energies for three main crystallisation events are 290, 302, 298 kJ/mole by Kissinger method and 286, 298, 295 kJ/mole by Ozawa method. Considering the assumptions and approximations of both the processes, it can be concluded that the activation energies in both the methods are in agreement with each other. A multiple display of the X-ray diffraction patterns of the amorphous Al82Fe4Ni4La10 alloy after heat treatment at 628, 648, 698 and 728 K for 10 min is given in Fig. 6. After heat treatment at 628 K a few broad peaks over the diffraction hump can be seen. These peaks can be indexed to aluminium. The nature of the peaks indicates that the crystallites are very fine in size. With the progress of crystallisation the intensity and sharpness of the peaks increases. It can be inferred from this observation that a-Al precipitates after heat treatment at 628 and 648 K. After heat treatment at 698 K a few new peaks can be observed.
Fig. 6. Multiple display of XRD patterns of the amorphous Al82Fe4Ni4La10 alloy after heat treatment at different temperatures for 10 min.
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Fig. 8. Multiple display of XRD patterns of the (a) Al75Ti10Ni10La5 (b) Al75Ti10Ni15 alloy melt spun at 40 m s21 wheel speed.
Fig. 7. TEM micrograph and electron diffraction pattern of the amorphous Al82Fe4Ni4La10 alloy after heat treatment at 628 K for 2 h.
These peaks closely match with those of a-Al and LaNi phase. After heat treatment at 728 K presence of a-Al, LaNi and Al3La phase can be seen in the diffraction pattern. The nature of the diffraction pattern after heat treatment for 2 h also remains the same indicating that the phases are stable. Transmission electron micrograph and corresponding electron diffraction pattern of the amorphous alloy after heat treatment at 628 K for 2 h are given in Fig. 7. After annealing at 628 K very fine precipitates varying in the size range of 20– 50 nm can be seen in the amorphous matrix. The precipitates are spherical or polygonal in shape. After heat treatment at 728 K precipitates in the size range of 100 – 200 nm can be seen. At this stage there is no amorphous phase left in the matrix and very fine precipitates of LaNi and Al3La phase are seen to be distributed in grain boundaries of aluminium. X-ray diffraction patterns of the Al75Ti10Ni15 and Al75Ti10Ni10La5 alloy after melt-spinning at 40 m s21 wheel speed are shown in Fig. 8. The Al75Ti10Ni15 alloy even after melt-spinning at 40 m s21 wheel speed does not become amorphous. After melt-spinning precipitation of Al3Ti and a solid solution phase of NiTi can be observed. There are two allotropic modifications Al3Ti. It appears that both tetragonal and cubic modifications are present in the alloy. The Al75Ti10Ni10La5 alloy after melt-spinning at 40 m s21 wheel speed become partially amorphous. The amount of crystallinity increases with a decrease in the wheel speed. In this alloy after melt-spinning along with Al3Ti and NiTi solid solution phase, the presence of AlNi solid solution phase, La2Ni7 phase can be seen. The transmission electron micrograph and the electron diffraction pattern of the Al75Ti10Ni10La5 alloy after melt-spinning
at 40 m s21 wheel speed is shown in Fig. 9 indicating nanoprecipitates within an amorphous network.
4. Discussion Three alloys of Al– Fe –Ni – La system have been studied in which La content has been increased. From the X-ray and microscopic results it is seen that the Al87Fe5Ni5La3 alloy does not produce an amorphous phase, the Al88Fe3Ni3La6 alloy becomes partially amorphous and the Al82Fe4Ni4La10 alloy becomes totally amorphous after melt-spinning. Primarily it is observed that with the increase in La content the glass forming ability of the alloy increases. This fact can be substantiated by taking into consideration atomic radii
Fig. 9. Transmission electron micrograph and electron diffraction pattern of the Al75Ti10Ni10La5 alloy after melt-spinning at 40 m s21 wheel speed. Note the presence of amorphous phase in the intercrystalline regions.
J. Basu, S. Ranganathan / Intermetallics 12 (2004) 1045–1050 Table 1 Atomic radii mismatch (in %) and binary heats of mixing (kJ/mole) in Al, Fe, Ni, Ti, La systems Al Al Fe Ni Ti La
11% 13% 2% 24%
Fe
Ni
Ti
La
211
222 22
230 217 235
238 5 227 20
2.2% 13% 32%
15% 34%
22%
mismatch and heat of mixing. These values are given in Table 1. It is seen from the table that La has a high atomic radii mismatch with Al, Fe and Ni. The size of aluminium is also different from that of Fe and Ni. The heat of mixing of aluminium with Fe, Ni and La is highly negative, whereas that of Fe with La is positive. So far as atomic size is concerned, lanthanum contributes the maximum to frustrate the alloy system and it maintains a high negative heat of mixing with Al and Ni. That may be a possible reason behind the increase in glass forming ability of the alloy on lanthanum addition. Apart from the glassy phase crystalline phases and the precipitation of the solid solution phase of aluminium can also be observed in the glassy matrix. Al –Ti –Ni and Al– Ti – Ni– La alloy could not be amorphised totally. In the melt-spun state along with the predominant precipitation of Al3Ti and La2Ni7, solid solution phases of NiTi and AlNi can be seen. In the present study it is observed that Al– Fe –Ni –La is a better glass forming system than Al –Ti –Ni – La. Apart from the basic difference in atomic configuration between early transition metals and late transition metals, the atomic size difference of Ti with Al is only 2% and the heat of mixing between Ti and La is highly positive. These differences may be accounted for the lower glass forming ability of the Al – Ti – Ni –La alloy. The precipitation of Al3Ti phase during solidification and crystallisation has been reported previously [9]. Al – Ti – Ni alloy can be amorphised totally in the aluminium poor region of the phase diagram [10]. An enhancement of glass forming ability with the addition of rare earths in aluminium based ternary and germanium based alloys has been reported [11]. It is observed that addition of a number of rare earths or mischmetal does not improve the glass forming ability of the alloy significantly because atomic radii mismatch between the rare earths is not significant [12]. The crystallisation behaviour of the Al –Fe – Ni– La alloy is complex and it occurs in three steps. The alloy is thermally stable up to 625 K. At the initial stages of crystallisation nano-precipitates of aluminium are seen in the amorphous matrix. With the progress of the crystallisation process size and volume fraction of the aluminium crystallites increases. First stage of crystallisation can be attributed to the precipitation of aluminium. Second stage to the precipitation of LaNi and at the end of crystallisation precipitation of Al3La and LaNi phase at the grain
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boundaries of a-Al can be observed. The precipitation of aluminium at the initial stages of crystallisation has been reported for other aluminium based metallic glasses. From the crystallisation process it can be concluded that with the precipitation of aluminium the matrix becomes richer in lanthanum and becomes more resistant to crystallisation. At the end of crystallisation lanthanum rich precipitates are seen at the grain boundaries. The alloy is resistant to grain growth also. Even after two hours of annealing grains do not grow beyond 200 nm size that may be due to the high nucleation rate of aluminium and precipitation of Al3La and LaNi at the grain boundaries. A simple stoichiometric calculation shows that if 30 vol.% of aluminium crystallites is precipitated then the composition of the matrix shifts from 82% Al to 73% Al and 10% La to 15% La. At the end of the crystallisation process, the matrix should be even richer in La. This means that the ternary composition of the matrix shifts from one end to the other end of the phase diagram and it becomes almost similar to typical lanthanum based glasses. This can be further substantiated with the fact that the primarily crystallising phases in La-based glasses, i.e. (Ni,Al)2La, La(Al,Cu,Ni), La3(Al,Ni) [1,13] are stoichiometrically very similar to the phases precipitated in the present amorphous alloy at the end of crystallisation.
5. Conclusions Quaternary alloys of aluminium with the late transition metals Fe, Ni and with rare earth metals La can be amorphised by melt-spinning. The glass forming ability of the alloys increases with an increase in La content. Below a certain quantity of lanthanum it cannot be amorphised. The crystallisation of the glassy alloy takes place in multiple steps. At the initial stages of crystallisation nanocrystals of aluminium are precipitated and the matrix become richer in solute content. The precipitated crystallites are resistant to grain growth. The quaternary Al –Ti – Ni –La shows a lower degree of glass formation, as compound formation is preferred. Glass forming ability of Al –Ti – Ni –La is better compared to ternary Al– Ti – Ni alloy.
Acknowledgements The authors would like to acknowledge stimulating discussions with Prof. A. Inoue, Prof. H. Kimura, Prof. K. Chattopadhyay, Prof. V. Jayaram, Dr U. Ramamurty and Ms. Tripti Biswas on various occasions. Research funding from DRDO/MMT/URM/537 is gratefully acknowledged.
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