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Cast bulk Zr57Ti5Al10Cu20Ni8 amorphous alloy with tendency of phase separation
MATERIALS SCIENCE & EWGRIEERIWG
ELSEVIER
Materials Scienceand Engineering A226-228 (1997) 429-433
Cast bulk Zr,,Ti5Al,0Cu,,Ni8 amorphous phase separation L.Q. Xing *, G.P. Gijrler, Institut
ftir Rnumsimulation,
DLR,
A
alloy with tendency of
D.M. Herlach
D-51 I40 Kdn,
Germany
Abstract
Zr-Ti-Al-Cu-Ni alloys show excellentglassforming ability (GFA). Amorphous cylindrical samplesof diameterfrom 8 to 20 mm were produced by castingthe Zr,,Cu,,Al,,Ni,Ti, alloy melt into a copper mould. The Zr,,Cu,,Al,,Ni,Ti, amorphousalloy shows some particular crystallization characteristics: measurementsby differential scanningcalorimetry (DSC) reveal three exothermic peaksin the DSC traces at continuousheating. The third peak of the highestpeak temperatureshifts towards lower temperaturewith the decreaseof the cooling rates at which the amorphousalloys were formed, while the first two peaksremain unchanged. Isothermal annealing near the glass transition temperature causesthe third peak shifting towards a definitive temperatureand then it becomesquite stableduring further annealing.The shift of the third peak is attributed to the tendency of phaseseparationof the alloy. 0 1997Elsevier ScienceS.A. Keywords:
Bulk amorphous alloy; Phase separation of amorphous alloy; Crystallization of amorphous alloy
1. Introduction Recently, the phase separation in amorphous alloys and compositional partitioning during crystallization attracts intense attention, because it was proposed to be associated with the production of nanocrystalline metallic materials which exhibit good magnetic and mechanical properties [l-4]. Moreover, it refers to partial crystallization of amorphous alloys, the high stability of the residual amorphous phase after partial crystallization and the high glass forming ability of some alloys [1,3,5,6-J. Two mechanisms have been proposed for the phase separation and the formation of nanocrystals. One supposes that a spinodal decomposition of the amorphous phase is the necessary precursor for the nucleation of crystals and results in a uniform distribution of crystals on a very fine length scale during primary crystallization [6,7]. Another assumes that primary crystallization occurs in a homogeneous amorphous phase, and solute partitioning during grain growth plays an important role in the evolution of nanocrystalline structure and the high stability of the residual amorphous phase against further crystallization [1,3,X]. * Corresponding
author.
0921-5093/97/$17.00 0 1997 Elsevier Science S.A. All rights reserved. . . . ..__._^___
Some previous work demonstrated the phase separation in the as-cast or annealed amorphous alloys by field ion micrography, anomalous X-ray scattering method or atom probe field ion microscopy [6,7]. The phase-separated amorphous alloys should show some characteristics on the DSC curves, for example, additional glass transition or crystallization peaks which are associated with different amorphous phases due to phase separation in the glassy state. Recently, we found an Zr,,Cu,,AllONi,Ti, alloy which exhibits very high glass forming ability [9] and some particular crystallization characteristics. By investigating the crystallization behaviour of the amorphous alloys produced under different cooling rates and after isothermal annealing, we found that these particular crystallization characteristics may be referred to the tendency of phase separation.
2. Experimental
procedure
The Zr,,Cu,OA1,,Ni,Ti, alloys, of mass between 30 and 47 g, were prepared by melting appropriate amounts of Zr, Ti, Al, Cu and Ni by electromagnetic induction in a water-cooled copper crucible under He atmosphere to form initial ingots. To obtain the amor-
430
L.Q.
Xilzg et al. /Materials
Science
asd Engineering
phous alloys under different cooling rates, the ingots were remelted by electromagnetic levitation under He atmosphere and allowed to drop into different copper moulds. One of the cylindrical copper moulds is of an outer diameter of 45 mm and an inner casting hole of 8 mm diameter, and another is of the size of 47 and 20 mm, respectively. For higher cooling rates, the melt was quenched into 1 mm thick sheets by copper mould. The moulds were inserted into a water cooled copper holder, but only the bottoms of the moulds were in direct contact with the holder. The amorphous state of the alloys was checked by X-ray diffractometry, differential scanning calorimetry, high resolution transmission electron microscopy (HRTEM, Topcon 002B) at 200 kV, and scanning electron and optical microscopy @EM, OM). For optical microscopy, the as-prepared samples were etched using a solution of 40 ml HNO, + 4 ml HF + 25 g CrO, + 70 ml H,O. The crystallization enthalpy of the amorphous samples was measured by integrating the area of the respective crystallization peaks in the DSC traces. To test the glass forming ability, some of the samples prepared by the above method were remelted in a quartz tube under He atmosphere, heated up to temperatures between 1370 and 1470 K and maintained at these temperatures for about 1 min. Subsequently, the liquid alloys were injected into a wedge-shaped copper mold by He gas of a pressure ranging from 150 to 180
A226-228
(1997)
429-433
cuKa
Zr~~Ti~hl~oCuzoNi8
Cross-section of the cylinder of 20 m m diameter
20
I
1
30
40
,
1
I
50 GO Degrees (20)
70
80
90
Fig. 2. X-ray diffraction pattern of the cross section of the Zr,,Ti,Al&u,,,Ni, amorphous cylindrical sample of 20 mm diameter cast into water cooled copper mould.
mbar. The wedge-shaped samples were cut from the center, mechanically polished and etched with the solution mentioned above at a temperature of about 350 K. After etching, the samples showed distinct contrast revealing the crystalline regions (dark appearance), the mixed regions of the amorphous and the crystalline phase (dark/bright appearance) and the amorphous regions (bright appearance). Comparison of the maximum thickness of the bright areas [amorphous phase) of the samples allows a qualitative evaluation of the GFA for the alloys of different composition. The effectiveness of this method was confirmed by checking the pieces from the corresponding regions with X-ray diffraction, DSC and comparing the crystallization enthalpy.
3. Results Fig. 1 shows a photograph of six as-prepared wedgeshaped samples of different compositions. After etching the samples the amorphous regions appear bright while / Zrs?TisAlloCuzoNi8
Fig. 1. Comparison of the relative GFA of some alloys. The width of the samples (perpendicular to the photo) are 15 mm. The dark regions are crystals. The arrows ‘ -+’ indicate the top of the boundaries between the amorphous regions and the crystalline regions, where the small mixed regions of amorphous phase and crystalline phase appear. The compositions of the alloys (correspondingly from left to right in the photograph).
IL. 550
I .,.I( GO0
0.33 IUS
,,,,t 650
Il.,..
700
750
1,
800
1,.
a50
1, I,,
900
I
1I
950
Tcmpcraturc (K)
Fig. 3. DSC curves of the Zr,,Ti,Al,,Cu,,h’i, under different cooling conditions.
amorphous alloys cast
L.Q.
Xing
et al. /Materials
Science
and Engineering
A226-228
(1997)
431
429-433
Zrj7TigA110Cu20Nig minutes
(d)120
5 9 $ Z 2 z 3
I.I.II. 1
1.33 Kk
600
650
Glass transition
T Annealing
at 723 K for 40 minutes
.~,.,.,I.,,,,,,,,,,,,,,
700
750
800
8.50
900
950
Temperature (K)
Fig. 4. The crystallization peak changes of the Zr,,Ti,Al,,Cu,,Ni, amorphous alloys after isothermal annealing at 673 K for various times. The specimens are from the 1 mm thick sheet for (a) and (b) and from the 8 mm diameter cylinder for (c) and (d).
the crystalline part appears dark. The maximum thickness of the amorphous regions of the wedge shaped samples gives an approximate evaluation of the relative GFA of the different alloys (arrows in Fig. 1). The addition of a few at.% Ti to the Zr-Al-Cu-Ni alloy system increases the glass forming ability remarkably. Several compositions of the quinternary alloy show much higher GFA than the quarternary alloy Zr,,Cu,,,Ni,,Al,., which was reported to have excellent GFA [lo]. Amorphous cylindrical samples of 20 mm diameter were formed by casting the Zr,,Ti,Al,,Cu,,Ni, alloy in the cylindrical mold. Fig. 2 exhibits the X-ray diffraction pattern of the cross section of the 20 mm diameter sample. The pattern consists only of one big broad peak and one small broad peak and no sharp diffraction peaks corresponding to crystalline phases appear. The crystallization enthalpy measured by DSC was 67 J g- ’ for sample pieces cut from the 20 mm diameter cylinder (both from the center and near the surface), 69 J g - ’ for the sample piece taken from the 8 mm diameter cylinder, and 68 J g- ’ for the 1 mm thick sheet. The consistency of these values indicates that the 20 mm diameter cylinder is amorphous within the accuracy limit of the DSC measurements of + 3%. Fig. 3 shows the DSC curves of the sample pieces cut from the center of the 20 mm diameter cylinder (a), from the 8 mm diameter cylinder (b) and from the 1 mm thick sheet (c), respectively. It is assumed that the thickness of the samples correlate inversely with the cooling rate. The peak temperatures of the first two exothermic peaks have no distinct change with the different cooling rates, at which the amorphous alloys were formed, but the third exothermic peak shifts to lower temperature with the decrease of the cooling rates. The peak temperatures of the third peaks are 922 K for the 1 mm thick sheet, 900 K for the 8 mm diameter cylinder and 858 K for the 20 mm diameter cylinder.
470
520
570
620
670
720
770
Temperature
820
870
920
970
(K)
Fig. 5. DSC curve of the Zr,,Ti,Al,,Cu,,Ni, annealing at 723 K for 40 min.
amorphous alloy after
Fig. 4 shows the DSC curves of the amorphous alloys taken from the 1 mm thick sheet and the 8 mm diameter cylinder after isothermal annealing at 673 K for various annealing times. The first exothermic peak vanishes quickly, as shown in Fig. 4(a) and (b). The second exothermic peak vanishes slowly with further annealing, and the peak temperature shifts to higher temperature, as shown in Fig. 4(c) and (d). It is interesting to note that the third peak shifts to about 895 K before the second peak begins to change distinctly upon initial annealing near the glass transition temperature. The shifted third peak then becomes very stable, without any change of peak position and the crystallization enthalpy during further annealing which causes the second peak to vanish gradually and shift to higher temperature, as shown in Fig. 4(c) and (d). This is contrary to the normal crystallization sequence of amorphous alloys. Isothermal annealing or partial crystallization usually cause the crystallization peaks of lower temperatures to change before any change of the crystallization peaks of higher temperatures [7].
/ CoKe ZrnTisAlloCuzoNig
30
40
50
60 Degrees
70
80
90
(2%
Fig, 6. X-ray diffraction patterns of the Zr,,Ti,A1,,Cu,,Ni, phous alloy after heating up to 823 and 998 K, separatell.
amor-
432
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Xing et al. /Materials
Science
and Engineering
A226-228
(1997)
429-433
Fig. 7. High resolution transmission microscopy of the Zr,,Ti,Al,,Cu,,,Ni, amorphous alloy after annealing at 723 K for 40 min. Insert shows an electron diffraction pattern revealimg the existence of amorphous phase (rings) and crystalline phase (spots).
4. Discussion For a first analysis we assume that the third exothermid peak in the DSC traces as visible in Figs. 3 and 4 corresponds to a crystallization event of an amorphous phase. The amorphous alloy Zr,,Ti,Al,,Cu,ONi, was annealed at 723 K for 40 min to eliminate the first two exothermic peaks. Afterwards, the annealed specimen was heated with a rate of 1.33 K s- ’ above the third exothermic peak, as shown in Fig. 5. There is an endothermic reaction at a temperature of about 770 K below the temperature of the exothermic peak at 950 K, which may refer to a glass transition. For a further check, the amorphous alloy was heated up to 823 and 998 K, separately. After each heating procedure at the both different temperatures the sample state was examined by X-ray diffraction at ambient temperature. Fig. 6 shows the X-ray diffraction patterns obtained from the samples annealed at 823 K (upper part of Fig. 6) and annealed at 998 K (lower part of Fig. 6). The upper diffraction pattern in Fig. 6 reveals a diffraction pattern in which the spectrum typical for amorphous alloys is superposed by diffraction lines of crystalline phases while the lower diffraction pattern of Fig. 6 shows clearly the spectrum of a fully crystallized sample. From the comparison of the X-ray measurements of the differently annealed samples we conclude that the specimen annealed at a temperature of 823 K (being below the temperature of the third
exothermic peak) remains partially amorphous while annealing at 998 K leads to the complete crystallization of amorphous phase. In addition to X-ray investigations high resolution transmission electron microscopy (HRTEM) was employed to study the heat treated samples. Fig. 7 exhibits the microstructure of an alloy annealed at 723 K for 40 min, which was observed by HRTEM analysis. The microstructure shows inclusions of single-crystalline grains of periodic order in size of a few nanometers (arrow denoted crystal) which are surrounded by the disordered (no periodicity detectable) amorphous phase (arrow denoted amorphous) quite similar as observed in rapidly quenched Fe-M-3 alloys consisting of nanocrystals embedded in an amorphous matrix [2]. From these observations we conclude that the third exothermic peak in the DSC traces corresponds to a crystallization event of a residual amorphous phase. The question arises on the variation of the crystallization temperature of the third peak with the change of cooling rate and annealing conditions. One possible explanation may be that the associated amorphous phase varies its composition. It is well known that the crystallization temperatures of metallic glasses depend on their compositions [ll]. Such a compositional change could occur via the phase separation in the amorphous alloy as observed in similar systems of high GFA [6,7]. It is presumed that the undercooled Zr,,Ti,Al,,Cu,,Ni, alloy has a tendency of phase sepa-
L.Q.
Xirzg et al. /Materials
Science
ration, with one amorphous phase being associated to the third peak in the DSC curves. The composition of the residual amorphous phase can be attributed to the decomposition of the amorphous alloy by primary crystallyzation (in case of the amorphous alloy produced under very high cooling rate) or to the combined effects of the phase separation during casting and the decomposition of the matrix phase by primary crystallyzation (in case of the amorphous alloy produced under relatively low cooling rate). Under low cooling rates the phase separation will be more developed with the result that the separated phase has a distinct composition difference from the matrix phase. (But the composition of the matrix phase may not change very much because its large volume fraction). When the as-cast amorphous alloys is scaned by DSC, the combination effects of the decomposition of the matrix amorphous phase and the phase separation during casting may result in a residual amorphous phase of varying compositions with the cooling rates of casting. When the as-cast amorphous alloys are subjected to the isothermal annealing, The composition of the residual amorphous phase may develop towards a equilibrium state by combining the composition of the separated phase and the decomposition of the matrix phase, with the result that the residual amophase phase tends to a de6nite crystallization temperature. As shown in Fig. 4, the third peaks shift towards about 895 K when the amorphous alloy is annealed at 673 K.
5. Conclusion The glass forming ability of Zr-Ti-AL-Cu-Ni alloys has been investigated. This qinternary alloy system shows a very high glass folming ability. Amorphous cylinders of Zr,,Ti,Al,,Cu,,Ni, in diameters up to 20 mm were prepared by casting the melt into a water cooled copper mould under the atmosphere. The Zr,,Ti,Al,,Cu,,Ni, amorphous alloy shows some pecu-
and D?gineering
A226-228
(1997)
433
429-433
liar crystallization behaviour which is attributed to its tendency of phase separation. The phase separation is marked by the variation of the crystallization peaks in the DSC curves with varying the cooling rates of casting and the annealing conditions.
Acknowledgements The authors thank J.P. Chevalier and M. Cornet for their assistance and discussion in the research, P. Ochin for his assistance in alloy preparation and J. DevaudRzepski for the assistance of the electron microscopy. This work has been financially supported by the Alexander-von-Humboldt Stiftung.
References [I] A.R. Yavari and 0. Drbohlav, Mater.
Trans.
JIM,
36 (1995)
896.
[2] A. Makino, A. Inoue and T. Masumoto, Mate?. (1995)
Trans.
JIM,
36
924.
[3] N. Shiga, F. Kogiku and M. Yukumoto, IMater. Trans. JIM, 36 (1995) 962. [4] N. Mattern, M. Miiller, C. Stiller and A. Danzig, Mater. Sci. Ei~g., A179/Al80
(1994)
473.
[5] X.D. Liu, K. Lu, B.Z. Ding and Z.Q. Hu, Mater. Al 79JA180
(1994)
Sci.
Eng.,
386.
[6] R. Busch, S. Schneider, A. Peker and W.L. Johnson, Appl. Phys. Lett., 67 (1995) 1544. [7] M. Matsuura, T. Fukunaga and U. Mizutani, Mater. Sci. Eng., A179/A180 (1994) 464. [8] K. Hono, Y. Zhang, A. Inoue and T. Sakurai, Mater. Trans. JIM,
36 (1995)
909.
[9] L.Q. Xing, P. O&in, M. Harmelin, F. Faudot and J. Bigot, 9th International Conference on Liquid and Amorphous Metals, Aug. 27-Sep. 1, 1995, Chicago, USA. J. Non-crystalline Solids: Z&-/207 (1996) 597. [lo] A, Inoue, T. Zhang, N. Nishiyama, K. Ohba and T. Masumoto, Mater.
[ll]
Trans.
JIM,
34 (1993)
1234.
T. Zhang, A. Inoue and T. Masumoto, Mater. (1991) 1005.
Trans.
JIM,
32
ELSEVIER
Materials Scienceand Engineering A226-228 (1997) 429-433
Cast bulk Zr,,Ti5Al,0Cu,,Ni8 amorphous phase separation L.Q. Xing *, G.P. Gijrler, Institut
ftir Rnumsimulation,
DLR,
A
alloy with tendency of
D.M. Herlach
D-51 I40 Kdn,
Germany
Abstract
Zr-Ti-Al-Cu-Ni alloys show excellentglassforming ability (GFA). Amorphous cylindrical samplesof diameterfrom 8 to 20 mm were produced by castingthe Zr,,Cu,,Al,,Ni,Ti, alloy melt into a copper mould. The Zr,,Cu,,Al,,Ni,Ti, amorphousalloy shows some particular crystallization characteristics: measurementsby differential scanningcalorimetry (DSC) reveal three exothermic peaksin the DSC traces at continuousheating. The third peak of the highestpeak temperatureshifts towards lower temperaturewith the decreaseof the cooling rates at which the amorphousalloys were formed, while the first two peaksremain unchanged. Isothermal annealing near the glass transition temperature causesthe third peak shifting towards a definitive temperatureand then it becomesquite stableduring further annealing.The shift of the third peak is attributed to the tendency of phaseseparationof the alloy. 0 1997Elsevier ScienceS.A. Keywords:
Bulk amorphous alloy; Phase separation of amorphous alloy; Crystallization of amorphous alloy
1. Introduction Recently, the phase separation in amorphous alloys and compositional partitioning during crystallization attracts intense attention, because it was proposed to be associated with the production of nanocrystalline metallic materials which exhibit good magnetic and mechanical properties [l-4]. Moreover, it refers to partial crystallization of amorphous alloys, the high stability of the residual amorphous phase after partial crystallization and the high glass forming ability of some alloys [1,3,5,6-J. Two mechanisms have been proposed for the phase separation and the formation of nanocrystals. One supposes that a spinodal decomposition of the amorphous phase is the necessary precursor for the nucleation of crystals and results in a uniform distribution of crystals on a very fine length scale during primary crystallization [6,7]. Another assumes that primary crystallization occurs in a homogeneous amorphous phase, and solute partitioning during grain growth plays an important role in the evolution of nanocrystalline structure and the high stability of the residual amorphous phase against further crystallization [1,3,X]. * Corresponding
author.
0921-5093/97/$17.00 0 1997 Elsevier Science S.A. All rights reserved. . . . ..__._^___
Some previous work demonstrated the phase separation in the as-cast or annealed amorphous alloys by field ion micrography, anomalous X-ray scattering method or atom probe field ion microscopy [6,7]. The phase-separated amorphous alloys should show some characteristics on the DSC curves, for example, additional glass transition or crystallization peaks which are associated with different amorphous phases due to phase separation in the glassy state. Recently, we found an Zr,,Cu,,AllONi,Ti, alloy which exhibits very high glass forming ability [9] and some particular crystallization characteristics. By investigating the crystallization behaviour of the amorphous alloys produced under different cooling rates and after isothermal annealing, we found that these particular crystallization characteristics may be referred to the tendency of phase separation.
2. Experimental
procedure
The Zr,,Cu,OA1,,Ni,Ti, alloys, of mass between 30 and 47 g, were prepared by melting appropriate amounts of Zr, Ti, Al, Cu and Ni by electromagnetic induction in a water-cooled copper crucible under He atmosphere to form initial ingots. To obtain the amor-
430
L.Q.
Xilzg et al. /Materials
Science
asd Engineering
phous alloys under different cooling rates, the ingots were remelted by electromagnetic levitation under He atmosphere and allowed to drop into different copper moulds. One of the cylindrical copper moulds is of an outer diameter of 45 mm and an inner casting hole of 8 mm diameter, and another is of the size of 47 and 20 mm, respectively. For higher cooling rates, the melt was quenched into 1 mm thick sheets by copper mould. The moulds were inserted into a water cooled copper holder, but only the bottoms of the moulds were in direct contact with the holder. The amorphous state of the alloys was checked by X-ray diffractometry, differential scanning calorimetry, high resolution transmission electron microscopy (HRTEM, Topcon 002B) at 200 kV, and scanning electron and optical microscopy @EM, OM). For optical microscopy, the as-prepared samples were etched using a solution of 40 ml HNO, + 4 ml HF + 25 g CrO, + 70 ml H,O. The crystallization enthalpy of the amorphous samples was measured by integrating the area of the respective crystallization peaks in the DSC traces. To test the glass forming ability, some of the samples prepared by the above method were remelted in a quartz tube under He atmosphere, heated up to temperatures between 1370 and 1470 K and maintained at these temperatures for about 1 min. Subsequently, the liquid alloys were injected into a wedge-shaped copper mold by He gas of a pressure ranging from 150 to 180
A226-228
(1997)
429-433
cuKa
Zr~~Ti~hl~oCuzoNi8
Cross-section of the cylinder of 20 m m diameter
20
I
1
30
40
,
1
I
50 GO Degrees (20)
70
80
90
Fig. 2. X-ray diffraction pattern of the cross section of the Zr,,Ti,Al&u,,,Ni, amorphous cylindrical sample of 20 mm diameter cast into water cooled copper mould.
mbar. The wedge-shaped samples were cut from the center, mechanically polished and etched with the solution mentioned above at a temperature of about 350 K. After etching, the samples showed distinct contrast revealing the crystalline regions (dark appearance), the mixed regions of the amorphous and the crystalline phase (dark/bright appearance) and the amorphous regions (bright appearance). Comparison of the maximum thickness of the bright areas [amorphous phase) of the samples allows a qualitative evaluation of the GFA for the alloys of different composition. The effectiveness of this method was confirmed by checking the pieces from the corresponding regions with X-ray diffraction, DSC and comparing the crystallization enthalpy.
3. Results Fig. 1 shows a photograph of six as-prepared wedgeshaped samples of different compositions. After etching the samples the amorphous regions appear bright while / Zrs?TisAlloCuzoNi8
Fig. 1. Comparison of the relative GFA of some alloys. The width of the samples (perpendicular to the photo) are 15 mm. The dark regions are crystals. The arrows ‘ -+’ indicate the top of the boundaries between the amorphous regions and the crystalline regions, where the small mixed regions of amorphous phase and crystalline phase appear. The compositions of the alloys (correspondingly from left to right in the photograph).
IL. 550
I .,.I( GO0
0.33 IUS
,,,,t 650
Il.,..
700
750
1,
800
1,.
a50
1, I,,
900
I
1I
950
Tcmpcraturc (K)
Fig. 3. DSC curves of the Zr,,Ti,Al,,Cu,,h’i, under different cooling conditions.
amorphous alloys cast
L.Q.
et al. /Materials
Science
and Engineering
A226-228
(1997)
431
429-433
Zrj7TigA110Cu20Nig minutes
(d)120
5 9 $ Z 2 z 3
I.I.II. 1
1.33 Kk
600
650
Glass transition
T Annealing
at 723 K for 40 minutes
.~,.,.,I.,,,,,,,,,,,,,,
700
750
800
8.50
900
950
Temperature (K)
Fig. 4. The crystallization peak changes of the Zr,,Ti,Al,,Cu,,Ni, amorphous alloys after isothermal annealing at 673 K for various times. The specimens are from the 1 mm thick sheet for (a) and (b) and from the 8 mm diameter cylinder for (c) and (d).
the crystalline part appears dark. The maximum thickness of the amorphous regions of the wedge shaped samples gives an approximate evaluation of the relative GFA of the different alloys (arrows in Fig. 1). The addition of a few at.% Ti to the Zr-Al-Cu-Ni alloy system increases the glass forming ability remarkably. Several compositions of the quinternary alloy show much higher GFA than the quarternary alloy Zr,,Cu,,,Ni,,Al,., which was reported to have excellent GFA [lo]. Amorphous cylindrical samples of 20 mm diameter were formed by casting the Zr,,Ti,Al,,Cu,,Ni, alloy in the cylindrical mold. Fig. 2 exhibits the X-ray diffraction pattern of the cross section of the 20 mm diameter sample. The pattern consists only of one big broad peak and one small broad peak and no sharp diffraction peaks corresponding to crystalline phases appear. The crystallization enthalpy measured by DSC was 67 J g- ’ for sample pieces cut from the 20 mm diameter cylinder (both from the center and near the surface), 69 J g - ’ for the sample piece taken from the 8 mm diameter cylinder, and 68 J g- ’ for the 1 mm thick sheet. The consistency of these values indicates that the 20 mm diameter cylinder is amorphous within the accuracy limit of the DSC measurements of + 3%. Fig. 3 shows the DSC curves of the sample pieces cut from the center of the 20 mm diameter cylinder (a), from the 8 mm diameter cylinder (b) and from the 1 mm thick sheet (c), respectively. It is assumed that the thickness of the samples correlate inversely with the cooling rate. The peak temperatures of the first two exothermic peaks have no distinct change with the different cooling rates, at which the amorphous alloys were formed, but the third exothermic peak shifts to lower temperature with the decrease of the cooling rates. The peak temperatures of the third peaks are 922 K for the 1 mm thick sheet, 900 K for the 8 mm diameter cylinder and 858 K for the 20 mm diameter cylinder.
470
520
570
620
670
720
770
Temperature
820
870
920
970
(K)
Fig. 5. DSC curve of the Zr,,Ti,Al,,Cu,,Ni, annealing at 723 K for 40 min.
amorphous alloy after
Fig. 4 shows the DSC curves of the amorphous alloys taken from the 1 mm thick sheet and the 8 mm diameter cylinder after isothermal annealing at 673 K for various annealing times. The first exothermic peak vanishes quickly, as shown in Fig. 4(a) and (b). The second exothermic peak vanishes slowly with further annealing, and the peak temperature shifts to higher temperature, as shown in Fig. 4(c) and (d). It is interesting to note that the third peak shifts to about 895 K before the second peak begins to change distinctly upon initial annealing near the glass transition temperature. The shifted third peak then becomes very stable, without any change of peak position and the crystallization enthalpy during further annealing which causes the second peak to vanish gradually and shift to higher temperature, as shown in Fig. 4(c) and (d). This is contrary to the normal crystallization sequence of amorphous alloys. Isothermal annealing or partial crystallization usually cause the crystallization peaks of lower temperatures to change before any change of the crystallization peaks of higher temperatures [7].
/ CoKe ZrnTisAlloCuzoNig
30
40
50
60 Degrees
70
80
90
(2%
Fig, 6. X-ray diffraction patterns of the Zr,,Ti,A1,,Cu,,Ni, phous alloy after heating up to 823 and 998 K, separatell.
amor-
432
L.Q.
Xing et al. /Materials
Science
and Engineering
A226-228
(1997)
429-433
Fig. 7. High resolution transmission microscopy of the Zr,,Ti,Al,,Cu,,,Ni, amorphous alloy after annealing at 723 K for 40 min. Insert shows an electron diffraction pattern revealimg the existence of amorphous phase (rings) and crystalline phase (spots).
4. Discussion For a first analysis we assume that the third exothermid peak in the DSC traces as visible in Figs. 3 and 4 corresponds to a crystallization event of an amorphous phase. The amorphous alloy Zr,,Ti,Al,,Cu,ONi, was annealed at 723 K for 40 min to eliminate the first two exothermic peaks. Afterwards, the annealed specimen was heated with a rate of 1.33 K s- ’ above the third exothermic peak, as shown in Fig. 5. There is an endothermic reaction at a temperature of about 770 K below the temperature of the exothermic peak at 950 K, which may refer to a glass transition. For a further check, the amorphous alloy was heated up to 823 and 998 K, separately. After each heating procedure at the both different temperatures the sample state was examined by X-ray diffraction at ambient temperature. Fig. 6 shows the X-ray diffraction patterns obtained from the samples annealed at 823 K (upper part of Fig. 6) and annealed at 998 K (lower part of Fig. 6). The upper diffraction pattern in Fig. 6 reveals a diffraction pattern in which the spectrum typical for amorphous alloys is superposed by diffraction lines of crystalline phases while the lower diffraction pattern of Fig. 6 shows clearly the spectrum of a fully crystallized sample. From the comparison of the X-ray measurements of the differently annealed samples we conclude that the specimen annealed at a temperature of 823 K (being below the temperature of the third
exothermic peak) remains partially amorphous while annealing at 998 K leads to the complete crystallization of amorphous phase. In addition to X-ray investigations high resolution transmission electron microscopy (HRTEM) was employed to study the heat treated samples. Fig. 7 exhibits the microstructure of an alloy annealed at 723 K for 40 min, which was observed by HRTEM analysis. The microstructure shows inclusions of single-crystalline grains of periodic order in size of a few nanometers (arrow denoted crystal) which are surrounded by the disordered (no periodicity detectable) amorphous phase (arrow denoted amorphous) quite similar as observed in rapidly quenched Fe-M-3 alloys consisting of nanocrystals embedded in an amorphous matrix [2]. From these observations we conclude that the third exothermic peak in the DSC traces corresponds to a crystallization event of a residual amorphous phase. The question arises on the variation of the crystallization temperature of the third peak with the change of cooling rate and annealing conditions. One possible explanation may be that the associated amorphous phase varies its composition. It is well known that the crystallization temperatures of metallic glasses depend on their compositions [ll]. Such a compositional change could occur via the phase separation in the amorphous alloy as observed in similar systems of high GFA [6,7]. It is presumed that the undercooled Zr,,Ti,Al,,Cu,,Ni, alloy has a tendency of phase sepa-
L.Q.
Xirzg et al. /Materials
Science
ration, with one amorphous phase being associated to the third peak in the DSC curves. The composition of the residual amorphous phase can be attributed to the decomposition of the amorphous alloy by primary crystallyzation (in case of the amorphous alloy produced under very high cooling rate) or to the combined effects of the phase separation during casting and the decomposition of the matrix phase by primary crystallyzation (in case of the amorphous alloy produced under relatively low cooling rate). Under low cooling rates the phase separation will be more developed with the result that the separated phase has a distinct composition difference from the matrix phase. (But the composition of the matrix phase may not change very much because its large volume fraction). When the as-cast amorphous alloys is scaned by DSC, the combination effects of the decomposition of the matrix amorphous phase and the phase separation during casting may result in a residual amorphous phase of varying compositions with the cooling rates of casting. When the as-cast amorphous alloys are subjected to the isothermal annealing, The composition of the residual amorphous phase may develop towards a equilibrium state by combining the composition of the separated phase and the decomposition of the matrix phase, with the result that the residual amophase phase tends to a de6nite crystallization temperature. As shown in Fig. 4, the third peaks shift towards about 895 K when the amorphous alloy is annealed at 673 K.
5. Conclusion The glass forming ability of Zr-Ti-AL-Cu-Ni alloys has been investigated. This qinternary alloy system shows a very high glass folming ability. Amorphous cylinders of Zr,,Ti,Al,,Cu,,Ni, in diameters up to 20 mm were prepared by casting the melt into a water cooled copper mould under the atmosphere. The Zr,,Ti,Al,,Cu,,Ni, amorphous alloy shows some pecu-
and D?gineering
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liar crystallization behaviour which is attributed to its tendency of phase separation. The phase separation is marked by the variation of the crystallization peaks in the DSC curves with varying the cooling rates of casting and the annealing conditions.
Acknowledgements The authors thank J.P. Chevalier and M. Cornet for their assistance and discussion in the research, P. Ochin for his assistance in alloy preparation and J. DevaudRzepski for the assistance of the electron microscopy. This work has been financially supported by the Alexander-von-Humboldt Stiftung.
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