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On the nature of quasicrystal phase transitions in AlCuFe alloys
July 1998
Materials Letters 36 Ž1998. 229–234
On the nature of quasicrystal phase transitions in AlCuFe alloys G. Rosas a , R. Perez
b,)
a
b
Instituto de InÕestigaciones Metalurgicas UMSNH, P.O. Box 52-B, 58000 Morelia, Mich., Mexico ´ Laboratorio de CuernaÕaca, Instituto de Fisica UNAM, P.O. Box 48-3, 62251 CuernaÕaca Mor., Mexico Received 30 August 1997; revised 23 January 1998; accepted 23 January 1998
Abstract In this investigation, a systematic study on the effects of the composition and annealed treatments in the formation of the AlCuFe icosahedral phase is carried out. The experimental results obtained from 30 different compositions under normal solidification methods and rapid solidification techniques show that the main driving force to the formation of the icosahedral phase is a cubic phase known in the past as the b phase. The b ŽAlCuFe. phase has a crystalline structure similar to the compound Al 50 Fe 50 . However, its lattice parameter is different from the parameter corresponding to the binary compound Al 50 Fe 50 , and it varies depending on the amount of copper. This gives rise to a cubic solid solution, the so-called b phase. The studied compositions were in the ranges of: AlŽ73–55 at.%., CuŽ20–25 at.%. and FeŽ10–15 at.%.. Different structural and chemical characterization techniques were employed such as: X-ray diffractometry, scanning electron microscope ŽSEM. with EDS attachments, transmission electron microscopy ŽTEM. and differential thermal analysis ŽDTA. observations. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Quasicrystal phase transition; AlCuFe alloy; b phase
1. Introduction Early investigations w1–3x have reported structural relationships between the quasicrystalline phases and the crystalline phases. In the AlCuFe system, there have been very few investigations related with the quasicrystalline phase formation at high temperatures, and also with the nature of the quasicrystalline phases obtained when the ternary alloy is produced under high solidification rates w4,5x. Furthermore, the chemical and structural behaviour of these types of alloys have not been systematically explored as a
)
Corresponding author.
function of different anneal treatments. The range of compositions and the appropriate anneal treatment of the as-cast alloys, which give rise to a single quasicrystalline phase, have not been reported in the past. Some of the compositional characteristics of the crystalline phases which co-exist with the quasicrystalline phase in the AlCuFe compound have been reported in past investigations w6,7x. However, none of their structural properties and their relationships to the quasicrystalline phase formation have been explored. The work presented in this report is related to the structural and chemical characteristics of the crystalline and quasicrystalline phases which co-exist in moderate rapidly-quenched Ž10 4 Krs. alloys of AlCuFe.
00167-577Xr98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 9 8 . 0 0 0 3 3 - 0
230
G. Rosas, R. Perez r Materials Letters 36 (1998) 229–234
2. Experimental procedure In order to carry out an investigation on the structural and chemical properties of the phases which are present in the cast AlCuFe alloys, 30 different alloy compositions were studied. These alloys were prepared in the following compositional ranges: AlŽ75–30 at.%., CuŽ40–5 at.%. and FeŽ30–7 at.%.. For the preparation of the specimens, two different casting techniques were employed. Most of the alloys were obtained using a melt-spinning approach, with an approximate solidification rate of 10 4 Krs. However, some alloy compositions were obtained using a chill–gravity-casting technique. The speci-
mens were subsequently vacuum-annealed using quartz tube containers. The anneal experiments were mainly carried-out at 700, 850 and 9008C for periods of 72 h. To characterize some of the chemical and physical properties of these materials, powder of 325 mesh were obtained from all the cast alloys. The characterization used different analytical techniques such as: X-ray diffraction patterns, differential thermal analysis ŽDTA. and transmission electron microscopy ŽTEM.. Some of the specimens were also analyzed using a scanning electron microscope ŽSEM. with an X-ray microanalysis attachment. It is also important to mention that after the annealing experiments, the specimens were water-quenched. The chemical analyses obtained were carried-out using an atomic absorption spectrophotometer, and each reported value corresponds to the average of two measurements. The uncertainty of the obtained compositional values is in the order of 1%. The meltspinning specimens were ribbons approximately 3-mm wide and 3-cm long. The chill-casting specimens were of wedge-type ingots approximately 5-cm wide and 10-cm long.
3. Results and discussion 3.1. Anneal experiments with alloys obtained under melt-spinning techniques Fig. 1a shows the corresponding X-ray diffraction pattern from an Al 50 Fe 50 alloy obtained using the
Fig. 1. X-ray diffraction pattern from two different types of alloys obtained under rapid solidification conditions. Ža. This pattern corresponds to an Al 50 Fe 50 alloy, showing a B2-type crystalline ˚ and Žb. phase with a lattice parameter of approximately 2.92 A; Diffraction pattern from the cubic solid solution Ž b phase.. The experimental interplanar distances, d exp ŽBragg law. are compared with the corresponding interplanar distances calculated for a cubic system Ž d cal . ŽThe angular scale is different for a and b..
Fig. 2. Lattice parameter variations of the cubic solid solution as functions of the atomic percent of Cu and Fe. The increase in these two elements give rise to increments in the lattice parameter Žrapidly-solidified material..
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melt-spinning technique. This pattern illustrates a single phase of intermetallic nature with a B2-type structure and a 0.29-nm lattice parameter. However, Fig. 1b shows the X-ray diffraction pattern which corresponds to Al 50 Cu 15 Fe 35 . This composition is very close to the composition where the icosahedral phase is usually obtained. However, the diffraction pattern indicates that the phase is of the same nature as the phase reported in Fig. 1a where the copper is
Fig. 4. DTA showing the phase transition at 8848C which is related to the transformation of the b phase to the icosahedral phase Žrapidly-solidified material..
Fig. 3. Phase transformation of the cubic solid solution Ž b phase to the icosahedral phase as a function of different annealed treatments Žrapidly-solidified material.. Ža. Single cubic solid solution Ž b phase.; Žb. Specimen obtained with the melt-spinning technique showing a mixture of two different phases: the icosahedral and the b phase; Žc. Melt-spinning specimen annealed at 8508C showing also a mixture of the icosahedral and b phase; and Žd. Single icosahedral phase.
now part of the same type of atomic lattice that has given rise to a substitutional solid solution w4x. This result can clearly be seen from the comparison Žillustrated in Fig. 1a. of the interplanar distances obtained from the Bragg law Ž d exp ., with the corresponding distances calculated for a cubic structure Ž d cal .. The solid solution obtained shows variation in the lattice parameter of the structure as a function of the atomic percent of copper and iron in the compound. The general trend in the values of this parameter is a diminution when the amount of aluminium is increased in the alloys. This is clearly illustrated in Fig. 2. It is important to point out that an increase in the atomic percent of aluminium decreases the concentration of Cu q Fe in the alloy. In the solid solution Ž b phase., both atoms ŽCu,Fe. contribute to the lattice deformation. Fig. 3 shows the structural characteristics obtained from specimens which have been vacuum-annealed at different temperatures. Thus, for example, Fig. 3b shows the X-ray diffraction pattern obtained from a specimen just after being processed under rapid solidification conditions. Fig. 3b shows the presence of two different phases. The quasicrystalline icosahedral phase, and also a crystalline phase of the B2-type which closely resembles the B2 crystalline phase obtained for the Al 50 Fe 50 and represented in Fig. 1, have been obtained in a wide range of compositions. They include Al Ž75–54 at.%., Cu Ž31–21 at.%. and Fe Ž16.5–7.5 at.%.. When the specimens are subsequently annealed at 9008C, the quasicrystalline phase disap-
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Fig. 5. Electron diffraction patterns corresponding to the icosahedral phase obtained from specimens annealed at 7008C which contain only quasicrystalline phase. Ža. fivefold, Žb. threefold, Žc. twofold, and Žd. pseudotwofold Žrapidly-solidified material..
Fig. 6. Electron diffraction patterns which correspond to the crystalline b phase obtained from specimens annealed at 9008C. Some zone axes are displayed in Ža. to Žd. Žrapidly-solidified material..
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pears completely and only the crystalline B2-type phase remains. This is illustrated in Fig. 3a. The specimens annealed at 8508C show a diminution of the crystalline phase ŽFig. 3c., and when the specimens are annealed at 7008C only, the icosahedral phase is obtained ŽFig. 3d.. This phase transformation can also be obtained through differential thermal analysis studies. This is illustrated in Fig. 4 where at 8848C, a peak is obtained corresponding to the transformation of the cubic b phase to the icosahedral phase, as is also illustrated in Fig. 3. Electron diffraction patterns obtained from these types of annealed alloys were also obtained. Thus, for example, Fig. 5 shows the diffraction patterns obtained from specimens annealed at 7008C. Fig. 5 illustrates some of the more common ‘zone axes’ related to the icosahedral phase. Furthermore, some diffraction patterns have also been obtained from the crystalline solid solution. These diffraction patterns are illustrated in Fig. 6a to d. 3.2. Anneal experiments with alloys obtained under normal casting techniques Fig. 7a shows the microstructure obtained from an alloy of Al 58 Cu 30 Fe12 which has been produced with a normal casting method and subsequently annealed at 9008C for 8 h. In Fig. 7a, two different interphase regions Žzone 1 and zone 2. can be seen clearly. The chemical analysis carried out through EDS techniques from the zone 1 region shows the presence of two different phases: the Al 3 Fe phase ŽAl 70.7 , Cu 3.7 , Fe 25.4 at.%, black region,. and a ternary phase with a composition similar to the composition of the icosahedral phase ŽAl 60.1 , Cu 25.4 , Fe14.5 at.%, gray region.. On the other hand, the chemical analysis obtained from the zone 2 region indicates the presence of two different phases: the quasicrystalline phase Žgray region. and the solid solution compound ŽAl 50.6 , Cu 45.3 , Fe 4.0 at.%, light gray region.. Therefore, zone 1 illustrates the transformation of Al 3 Fe phase, which is the commonly found phase in these types of alloys obtained under normal casting, to the quasicrystalline ternary phase. Furthermore, the second transformation region Žzone 2. illustrates the transition of the quasicrystalline phase to the solid solution compound. The same kind of phase transitions have been obtained from the
Fig. 7. SEM images obtained from specimens produced under normal casting methods. Ža. Annealed at 9008C, and illustrating two different phase transformation regions: zone 1, transition from Al 3 Fe to the icosahedral phase; zone 2, transition from the icosahedral phase to the b phase; and Žb. Small regions which correspond to the tetragonal Al 7 Cu 2 Fe obtained after annealing at 7008C.
melt-spinning specimens under annealed processes. These transformations have been clearly indicated by the X-ray diffraction results presented in Section 5 ŽFig. 3.. Finally, Fig. 7b shows the results when an alloy of Al 70 Cu 20 Fe10 is obtained under normal casting and is subsequently annealed at 7008C for 150 h. This composition is very close to the quasicrystalline phase composition. However, in this case, single crystalline structures can be seen with an analyzed composition of approximately Al 7 Cu 2 Fe. This composition resembles the tetragonal ternary phase commonly found in these alloys w7x.
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4. Discussion The TEM results and the experimental results obtained from X-ray diffraction patterns strongly suggest decomposition on heating of the cubic solid solution Ž b phase. into an icosahedral phase at temperatures in the order of 8848C ŽFig. 3.. Therefore, the b phase is strongly related to the formation of the quasicrystalline phase. Similar results were obtained for the chill-casting and the melt-spinning Ž10 4 Krs. alloys. These results suggest that the icosahedral phase is the product of a peritactic reaction of the type:
b Al Ž Cu,Fe . q liq ™ Icosahedral Ž AlCuFe. which was initially suggested by Bradley and Goldschmidt w4x. Furthermore, the study of approximately 30 different compositions has shown that the solubility of Cu and Fe in the b phase is very wide. It covers a range of AlFe up to Al 30 Cu 40 Fe 30 and Al 70 Fe 25 Cu. It is interesting to point out that past investigations have reported the presence of new crystalline phases which are found in coexistence with the icosahedral phase of AlCuFe w8x. This study indicates that the new crystalline phases reported correspond to the crystalline solid solution Ž b phase. with particular solubilities of Cu and Fe.
Annealing experiments of the as-cast alloys suggest a phase transformation of the cubic solid solution Ž b phase. into the icosahedral phase ŽFig. 3.. Some of the crystalline phases which coexist with the icosahedral phase and have been reported in the past as new crystalline phases correspond to the cubic solid solution Ž b phase. with particular solubilities of Cu and Fe. The specimens obtained under rapid solidification or normal casting give rise to the same type of phase transformations when annealed under similar temperature conditions. The increment of Cu and Fe in the solid solution gives rise to an increment in the lattice parameter of the cubic b structure.
Acknowledgements The authors would like to thank J.L. Albarran, O. Flores, A. Gonzalez, R. Guardian and A. Arizmendi ŽIFUNAM., for technical help. G. Rosas would like to thank the Universidad Michoacana ŽUMSNH., Univesidad de Morelos ŽUAEM. and CONACYT for financial support.
References 5. Conclusions The results illustrated in this report indicate that the crystalline phase Ž b phase. is related to the quasicrystalline phase formation. Depending on the ternary composition of the AlCuFe alloys and on the subsequent annealing treatment, the crystalline phase obtained Ž b type. displays variations in the lattice parameter Ži.e., the Al 50 Fe 50 lattice parameter.. The experimental results also show that the icosahedral phase is formed at about 8848C in ranges of composition Al Ž75–54 at.%., Cu Ž31–21 at.%. and Fe Ž16.5–7.5 at.%..
w1x Z. Zhang, N.C. Li, Scripta Metall. 24 Ž1990. 1329. w2x F.W. Gayle, A.J. Shapiro, F.S. Biancaniello, W.J. Boettinger, Met. Trans. 23A Ž1992. 2409. w3x D. Gratias, Y. Calvayrac, J. Devaud-Rzepski, F. Faudot, M. Harmelin, A. Quivy, P.A. Bancel, J. Non-Cryst. Solids 153– 154 Ž1993. 482. w4x A.J. Bradley, H.J. Goldschmidt, J. Inst. Met. 65 Ž1939. 389. w5x C. Dong, J.M. Dubois, M. de Boissieu, C. Janot, J. Phys. Condens. Matter 2 Ž1990. 6339. w6x G. Rosas, R. Perez, Scripta Metall. et Mater., in press. w7x A.P. Tsai, A. Inoue, T. Masumoto, J. Mater Sci. Lett. 6 Ž1987. 1403. w8x Y.F. Cheng, M.J. Hui, F.H. Li, Philos. Mag. Lett. 63 Ž1. Ž1991. 49.
Materials Letters 36 Ž1998. 229–234
On the nature of quasicrystal phase transitions in AlCuFe alloys G. Rosas a , R. Perez
b,)
a
b
Instituto de InÕestigaciones Metalurgicas UMSNH, P.O. Box 52-B, 58000 Morelia, Mich., Mexico ´ Laboratorio de CuernaÕaca, Instituto de Fisica UNAM, P.O. Box 48-3, 62251 CuernaÕaca Mor., Mexico Received 30 August 1997; revised 23 January 1998; accepted 23 January 1998
Abstract In this investigation, a systematic study on the effects of the composition and annealed treatments in the formation of the AlCuFe icosahedral phase is carried out. The experimental results obtained from 30 different compositions under normal solidification methods and rapid solidification techniques show that the main driving force to the formation of the icosahedral phase is a cubic phase known in the past as the b phase. The b ŽAlCuFe. phase has a crystalline structure similar to the compound Al 50 Fe 50 . However, its lattice parameter is different from the parameter corresponding to the binary compound Al 50 Fe 50 , and it varies depending on the amount of copper. This gives rise to a cubic solid solution, the so-called b phase. The studied compositions were in the ranges of: AlŽ73–55 at.%., CuŽ20–25 at.%. and FeŽ10–15 at.%.. Different structural and chemical characterization techniques were employed such as: X-ray diffractometry, scanning electron microscope ŽSEM. with EDS attachments, transmission electron microscopy ŽTEM. and differential thermal analysis ŽDTA. observations. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Quasicrystal phase transition; AlCuFe alloy; b phase
1. Introduction Early investigations w1–3x have reported structural relationships between the quasicrystalline phases and the crystalline phases. In the AlCuFe system, there have been very few investigations related with the quasicrystalline phase formation at high temperatures, and also with the nature of the quasicrystalline phases obtained when the ternary alloy is produced under high solidification rates w4,5x. Furthermore, the chemical and structural behaviour of these types of alloys have not been systematically explored as a
)
Corresponding author.
function of different anneal treatments. The range of compositions and the appropriate anneal treatment of the as-cast alloys, which give rise to a single quasicrystalline phase, have not been reported in the past. Some of the compositional characteristics of the crystalline phases which co-exist with the quasicrystalline phase in the AlCuFe compound have been reported in past investigations w6,7x. However, none of their structural properties and their relationships to the quasicrystalline phase formation have been explored. The work presented in this report is related to the structural and chemical characteristics of the crystalline and quasicrystalline phases which co-exist in moderate rapidly-quenched Ž10 4 Krs. alloys of AlCuFe.
00167-577Xr98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 9 8 . 0 0 0 3 3 - 0
230
G. Rosas, R. Perez r Materials Letters 36 (1998) 229–234
2. Experimental procedure In order to carry out an investigation on the structural and chemical properties of the phases which are present in the cast AlCuFe alloys, 30 different alloy compositions were studied. These alloys were prepared in the following compositional ranges: AlŽ75–30 at.%., CuŽ40–5 at.%. and FeŽ30–7 at.%.. For the preparation of the specimens, two different casting techniques were employed. Most of the alloys were obtained using a melt-spinning approach, with an approximate solidification rate of 10 4 Krs. However, some alloy compositions were obtained using a chill–gravity-casting technique. The speci-
mens were subsequently vacuum-annealed using quartz tube containers. The anneal experiments were mainly carried-out at 700, 850 and 9008C for periods of 72 h. To characterize some of the chemical and physical properties of these materials, powder of 325 mesh were obtained from all the cast alloys. The characterization used different analytical techniques such as: X-ray diffraction patterns, differential thermal analysis ŽDTA. and transmission electron microscopy ŽTEM.. Some of the specimens were also analyzed using a scanning electron microscope ŽSEM. with an X-ray microanalysis attachment. It is also important to mention that after the annealing experiments, the specimens were water-quenched. The chemical analyses obtained were carried-out using an atomic absorption spectrophotometer, and each reported value corresponds to the average of two measurements. The uncertainty of the obtained compositional values is in the order of 1%. The meltspinning specimens were ribbons approximately 3-mm wide and 3-cm long. The chill-casting specimens were of wedge-type ingots approximately 5-cm wide and 10-cm long.
3. Results and discussion 3.1. Anneal experiments with alloys obtained under melt-spinning techniques Fig. 1a shows the corresponding X-ray diffraction pattern from an Al 50 Fe 50 alloy obtained using the
Fig. 1. X-ray diffraction pattern from two different types of alloys obtained under rapid solidification conditions. Ža. This pattern corresponds to an Al 50 Fe 50 alloy, showing a B2-type crystalline ˚ and Žb. phase with a lattice parameter of approximately 2.92 A; Diffraction pattern from the cubic solid solution Ž b phase.. The experimental interplanar distances, d exp ŽBragg law. are compared with the corresponding interplanar distances calculated for a cubic system Ž d cal . ŽThe angular scale is different for a and b..
Fig. 2. Lattice parameter variations of the cubic solid solution as functions of the atomic percent of Cu and Fe. The increase in these two elements give rise to increments in the lattice parameter Žrapidly-solidified material..
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melt-spinning technique. This pattern illustrates a single phase of intermetallic nature with a B2-type structure and a 0.29-nm lattice parameter. However, Fig. 1b shows the X-ray diffraction pattern which corresponds to Al 50 Cu 15 Fe 35 . This composition is very close to the composition where the icosahedral phase is usually obtained. However, the diffraction pattern indicates that the phase is of the same nature as the phase reported in Fig. 1a where the copper is
Fig. 4. DTA showing the phase transition at 8848C which is related to the transformation of the b phase to the icosahedral phase Žrapidly-solidified material..
Fig. 3. Phase transformation of the cubic solid solution Ž b phase to the icosahedral phase as a function of different annealed treatments Žrapidly-solidified material.. Ža. Single cubic solid solution Ž b phase.; Žb. Specimen obtained with the melt-spinning technique showing a mixture of two different phases: the icosahedral and the b phase; Žc. Melt-spinning specimen annealed at 8508C showing also a mixture of the icosahedral and b phase; and Žd. Single icosahedral phase.
now part of the same type of atomic lattice that has given rise to a substitutional solid solution w4x. This result can clearly be seen from the comparison Žillustrated in Fig. 1a. of the interplanar distances obtained from the Bragg law Ž d exp ., with the corresponding distances calculated for a cubic structure Ž d cal .. The solid solution obtained shows variation in the lattice parameter of the structure as a function of the atomic percent of copper and iron in the compound. The general trend in the values of this parameter is a diminution when the amount of aluminium is increased in the alloys. This is clearly illustrated in Fig. 2. It is important to point out that an increase in the atomic percent of aluminium decreases the concentration of Cu q Fe in the alloy. In the solid solution Ž b phase., both atoms ŽCu,Fe. contribute to the lattice deformation. Fig. 3 shows the structural characteristics obtained from specimens which have been vacuum-annealed at different temperatures. Thus, for example, Fig. 3b shows the X-ray diffraction pattern obtained from a specimen just after being processed under rapid solidification conditions. Fig. 3b shows the presence of two different phases. The quasicrystalline icosahedral phase, and also a crystalline phase of the B2-type which closely resembles the B2 crystalline phase obtained for the Al 50 Fe 50 and represented in Fig. 1, have been obtained in a wide range of compositions. They include Al Ž75–54 at.%., Cu Ž31–21 at.%. and Fe Ž16.5–7.5 at.%.. When the specimens are subsequently annealed at 9008C, the quasicrystalline phase disap-
232
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Fig. 5. Electron diffraction patterns corresponding to the icosahedral phase obtained from specimens annealed at 7008C which contain only quasicrystalline phase. Ža. fivefold, Žb. threefold, Žc. twofold, and Žd. pseudotwofold Žrapidly-solidified material..
Fig. 6. Electron diffraction patterns which correspond to the crystalline b phase obtained from specimens annealed at 9008C. Some zone axes are displayed in Ža. to Žd. Žrapidly-solidified material..
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pears completely and only the crystalline B2-type phase remains. This is illustrated in Fig. 3a. The specimens annealed at 8508C show a diminution of the crystalline phase ŽFig. 3c., and when the specimens are annealed at 7008C only, the icosahedral phase is obtained ŽFig. 3d.. This phase transformation can also be obtained through differential thermal analysis studies. This is illustrated in Fig. 4 where at 8848C, a peak is obtained corresponding to the transformation of the cubic b phase to the icosahedral phase, as is also illustrated in Fig. 3. Electron diffraction patterns obtained from these types of annealed alloys were also obtained. Thus, for example, Fig. 5 shows the diffraction patterns obtained from specimens annealed at 7008C. Fig. 5 illustrates some of the more common ‘zone axes’ related to the icosahedral phase. Furthermore, some diffraction patterns have also been obtained from the crystalline solid solution. These diffraction patterns are illustrated in Fig. 6a to d. 3.2. Anneal experiments with alloys obtained under normal casting techniques Fig. 7a shows the microstructure obtained from an alloy of Al 58 Cu 30 Fe12 which has been produced with a normal casting method and subsequently annealed at 9008C for 8 h. In Fig. 7a, two different interphase regions Žzone 1 and zone 2. can be seen clearly. The chemical analysis carried out through EDS techniques from the zone 1 region shows the presence of two different phases: the Al 3 Fe phase ŽAl 70.7 , Cu 3.7 , Fe 25.4 at.%, black region,. and a ternary phase with a composition similar to the composition of the icosahedral phase ŽAl 60.1 , Cu 25.4 , Fe14.5 at.%, gray region.. On the other hand, the chemical analysis obtained from the zone 2 region indicates the presence of two different phases: the quasicrystalline phase Žgray region. and the solid solution compound ŽAl 50.6 , Cu 45.3 , Fe 4.0 at.%, light gray region.. Therefore, zone 1 illustrates the transformation of Al 3 Fe phase, which is the commonly found phase in these types of alloys obtained under normal casting, to the quasicrystalline ternary phase. Furthermore, the second transformation region Žzone 2. illustrates the transition of the quasicrystalline phase to the solid solution compound. The same kind of phase transitions have been obtained from the
Fig. 7. SEM images obtained from specimens produced under normal casting methods. Ža. Annealed at 9008C, and illustrating two different phase transformation regions: zone 1, transition from Al 3 Fe to the icosahedral phase; zone 2, transition from the icosahedral phase to the b phase; and Žb. Small regions which correspond to the tetragonal Al 7 Cu 2 Fe obtained after annealing at 7008C.
melt-spinning specimens under annealed processes. These transformations have been clearly indicated by the X-ray diffraction results presented in Section 5 ŽFig. 3.. Finally, Fig. 7b shows the results when an alloy of Al 70 Cu 20 Fe10 is obtained under normal casting and is subsequently annealed at 7008C for 150 h. This composition is very close to the quasicrystalline phase composition. However, in this case, single crystalline structures can be seen with an analyzed composition of approximately Al 7 Cu 2 Fe. This composition resembles the tetragonal ternary phase commonly found in these alloys w7x.
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4. Discussion The TEM results and the experimental results obtained from X-ray diffraction patterns strongly suggest decomposition on heating of the cubic solid solution Ž b phase. into an icosahedral phase at temperatures in the order of 8848C ŽFig. 3.. Therefore, the b phase is strongly related to the formation of the quasicrystalline phase. Similar results were obtained for the chill-casting and the melt-spinning Ž10 4 Krs. alloys. These results suggest that the icosahedral phase is the product of a peritactic reaction of the type:
b Al Ž Cu,Fe . q liq ™ Icosahedral Ž AlCuFe. which was initially suggested by Bradley and Goldschmidt w4x. Furthermore, the study of approximately 30 different compositions has shown that the solubility of Cu and Fe in the b phase is very wide. It covers a range of AlFe up to Al 30 Cu 40 Fe 30 and Al 70 Fe 25 Cu. It is interesting to point out that past investigations have reported the presence of new crystalline phases which are found in coexistence with the icosahedral phase of AlCuFe w8x. This study indicates that the new crystalline phases reported correspond to the crystalline solid solution Ž b phase. with particular solubilities of Cu and Fe.
Annealing experiments of the as-cast alloys suggest a phase transformation of the cubic solid solution Ž b phase. into the icosahedral phase ŽFig. 3.. Some of the crystalline phases which coexist with the icosahedral phase and have been reported in the past as new crystalline phases correspond to the cubic solid solution Ž b phase. with particular solubilities of Cu and Fe. The specimens obtained under rapid solidification or normal casting give rise to the same type of phase transformations when annealed under similar temperature conditions. The increment of Cu and Fe in the solid solution gives rise to an increment in the lattice parameter of the cubic b structure.
Acknowledgements The authors would like to thank J.L. Albarran, O. Flores, A. Gonzalez, R. Guardian and A. Arizmendi ŽIFUNAM., for technical help. G. Rosas would like to thank the Universidad Michoacana ŽUMSNH., Univesidad de Morelos ŽUAEM. and CONACYT for financial support.
References 5. Conclusions The results illustrated in this report indicate that the crystalline phase Ž b phase. is related to the quasicrystalline phase formation. Depending on the ternary composition of the AlCuFe alloys and on the subsequent annealing treatment, the crystalline phase obtained Ž b type. displays variations in the lattice parameter Ži.e., the Al 50 Fe 50 lattice parameter.. The experimental results also show that the icosahedral phase is formed at about 8848C in ranges of composition Al Ž75–54 at.%., Cu Ž31–21 at.%. and Fe Ž16.5–7.5 at.%..
w1x Z. Zhang, N.C. Li, Scripta Metall. 24 Ž1990. 1329. w2x F.W. Gayle, A.J. Shapiro, F.S. Biancaniello, W.J. Boettinger, Met. Trans. 23A Ž1992. 2409. w3x D. Gratias, Y. Calvayrac, J. Devaud-Rzepski, F. Faudot, M. Harmelin, A. Quivy, P.A. Bancel, J. Non-Cryst. Solids 153– 154 Ž1993. 482. w4x A.J. Bradley, H.J. Goldschmidt, J. Inst. Met. 65 Ž1939. 389. w5x C. Dong, J.M. Dubois, M. de Boissieu, C. Janot, J. Phys. Condens. Matter 2 Ž1990. 6339. w6x G. Rosas, R. Perez, Scripta Metall. et Mater., in press. w7x A.P. Tsai, A. Inoue, T. Masumoto, J. Mater Sci. Lett. 6 Ž1987. 1403. w8x Y.F. Cheng, M.J. Hui, F.H. Li, Philos. Mag. Lett. 63 Ž1. Ž1991. 49.