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Nanocrystallization of Fe80B20 By ball milling
NanoStructured Materials. Vol. 5. No. 4. pp. 433440.1995 Copyright Q 1995 Elsevier Science. Ltd Printed in the USA. All rights reserved 0965-9773195 $9.50 + .OO
Pergamon
09659773(95)00255-3
NANOCRYSTALLIZATION
OF FegoB2o BY BALL MILLING
GJ. Fan(2), X.P. Song(‘), M.X. Quan(2), and Z.Q. Hu(~) (t)No. 24 Laboratory and (*)National Key Lab for RSA, Institute of Metal Research, Academia Sinica, Shenyang 110015, P.R.China (Accepted March 1995) Abstract-By meansofx-raydtffraction (XRD), transmission electron microscopy(TEM), as well as dtjferential scanning calorimetry (DSC) measurements, the sequence of structural evolution by ball milling of polycrystalline and amorphous Fes&o alloys was studied. Ball milling ofpolycrystalline Fes&o alloy results in a continuous refinement of the grain size to about 13 nm and a decrease of axial ratio clafor the Fe2B phase. For the amorphous Fes&o alloy, with increasing milling time, the amorphous phase crystallizes into a-Fe and a metastable Fe3B phase, followed by the phase transformationfrom the resulting metastable Fe3 phase into a-Fe and the stable Fe2B phase, which are also nanocrystalline. The decreased axial ratio clafor Fe2B phase indicates that chemical disordering is introduced during the mechanical deformation.
INTRODUCTION Due to their unique properties, nanocrystalline materials have been widely studied. These novel materials are characterized by a high density of disordered grain boundaries which may modify their thermal and physical properties (14). These materials have promised some interesting technological applications such as production of ductile ceramics (5), effective hydrogen storage materials, etc. Nanocrystalline materials can be synthesized by a variety of different methods. A typical example is the gas condensation method which was first reported by Gleiter and coworkers (l-2). More recently, Fecht et al. have systematically studied the synthesis of nanocrystalline metals by high-energy ball milling (4). A series of bee and hcp pure elemental metals have been prepared in this manner. In addition, Eckert et al. have demonstrated that nanometer-sized fee elemental metals can also be prepared by ball milling, which have been reported to be inefficient for effective energy storage (6). It is well known that high-energy ball milling is an effective tool for large scale production and economic operation. In this paper, nanocrystalhne Fes&o alloys have been prepared by ball milling, both starting from the polycrystalline material and the amorphous alloy. The mechanism of the microstructure formation will be discussed between these synthesis methods. The deviation of lattice parameters for the Fe2B phase from the equilibrium will be interpreted with the presence of the chemical disordering in the ball milled Fe2B phase. 433
GJ FAN, XP
434
SONG,
MX QUAN
AND
ZQ Hu
EXPERIMENTAL PROCEDURES FesoEI2ualloy ingots were prepared by arc melting and were remelted several times under an argon atmosphere. The ingots were homogenized under an argon atmosphere at a temperature of 1073K for 2 h. The annealed button was crushed into small pieces and powders. The amorphous Fes&u ribbons were obtained by the melt-spin method and were cut into small pieces. The thus obtained materials were respectively sealed in an argon atmosphere and were milled in a planetary ball mill. The initial ball-to-sample mass ratio was about 1O:l.At the given milling time, a small amount of the milled material was withdrawn for XRD analysis. X-raydiffractionOCRD)patternswereperformedbyaRIGAKUX-raypowderdiffractometer using a graphite monochromator Cu-Ka radiation (h = 0.15405 nm). Transmission electron microscopy (TEM) was performed with a Phillips EM420 operated at 120 kV. Thermal analysis of the milled powders was performed using a differential scanning calorimeter (Perkin-Elmer DSC-7) at a heating rate of 20”C/min. A continuous pure argon flow was used to avoid possible oxidation.
0 U-Fe
‘1 1. IO )* .% c w
!.h
28
(degree)
Figure 1. XRD patterns for polycrystalline Fesc&c alloy after various milling times.
!‘hOCRfSTAWZATlON
OF
FemBmBY h_L hhLlMG
435
RESULTS AND DISCUSSION Figure 1 shows the XRD patterns for polycrystalline FesoBzo alloy after various milting times. The equilibrium phases, a-Fe and FezB, can be clearly observed for the unmilled sample. After 4 h of milling, the XRD patterns for both a-Fe and FQB diffraction lines broaden, due to the refinement of the grain size and the introduction of the lattice strain by severe mechanical deformation. After 20 h of milling, the (022) and (130) XRD lines for Fe2B phase merge into a single line. 80 h of milling did not give rise to pronounced differences in the XRD patterns compared with that of the 20 h milled sample. The Scherrer formula is applied to estimate the grain size of a-Fe by using the Fe(200) diffraction line after the correction for instrumental broadening and the separation of Kal and Ka2. Figure 2 shows the obtained results which reveal that the initial 40 h of milling leads to a continuous refmement of the grain size and then saturates to about 9 nm. Further refinement of the grain size seems to be impossible. This behavior can be understood according to the well known Hall-Petch relationship, which sets a limit to the reduction of the grain size achieved by plastic deformation during ball milling. We expect that the Fe&320 pre-alloyed ingot can be vitrified if subjected to high-energy ball milling, since Fes(&o alloy can be easily vitrified by a melt-spin method. However, there is no trace of an amorphous phase in the XRD patterns, even after a 80 h of milling.
30 0 25-
0
z s 'G -E 5
z
20 0
15 -
0
0
Sk u
10 -
0 I
o
10
I
I
20 30
I
I
40
50
I
0 I
60 70
MillingTime W Figure 2. Variation of grain size with milling time.
I
80
436
GJ FAN, XP SONG, MX
QUAN AND
ZO Hu
0 U-Fe
5 tTi
0
f .-
0 0
E
al +
c
w
25
85
5s 26Idegree)
Figure 3. XRD patterns for amorphous Fe8&o
alloy after various milling times.
Figure 3 shows the XRD patterns for amorphous Fee&n alloy after various milling times. After 4 h of milling, the amorphous phase crystallizes into a-Fe and a metastable FeaB phase. By increasing the milling time to 6 h, the stable Fe2B phase can be observed in the XRD patterns. This can be attributed to the decompositional products of the metastable FesB phase. The metastable FesB phase may undergo a phase transformation into o-Fe and Fe2B during mechanical attrition. After 20 h of milling, the equilibrium phase of a-Fe and Fe2B were obtained by mechanically driven crystallization. However, during a usual thermal crystallization process, the amorphous Fes&u alloy crystallizes into a-Fe and a metastable FesB. Accordingly, one can conclude that some type of defect has been introduced during mechanical attrition which kinetically favors the phase transformation from Fe3B into a-Fe and Fe2B. Figure 4 shows the DSC curve of the amorphous FesoB2u alloy. A sharp exothermal peak was observed. The crystallization temperature and the heat release are 467.4”C and 134.6 J/g, respectively. Trudeau et cd. have reported that amorphous alloys will undergo crystallization during high-energy ball milling (7). They have shown that the crystallization behavior due to a
NANOCRYSTALLIZATION OF
Fe&,,
Temperature
BY BALL MILLING
437
i°C1
Figure 4. DSC curve of the melt-spin amorphous Fes&u
alloy.
thermal process and the one induced by high energy ball milling can be very different. The structural evolution during ball milling cannot be associated with a local effective temperature rise at the collision site which causes the crystallization. One may estimate that the local effective temperature rise is lower than the crystallization temperature of an amorphous phase. Our experiments seem to prove this idea also. However, high-energy ball milling can cause the local microstructural changes in amorphous Fes&oalloy during the first 4 h of milling, which alter the mechanism of crystallization of an amorphous alloy compared with a thermal process (8). Figure 5 shows the TEM images for polycrystalline Fes&e alloy after 80 h of milling and amorphous FesuBzo alloy after 20 h of milling. It can be seen that the crystallization of an amorphous alloy gives almost the same small grain size as that of the ball milled ingot alloy, which is in agreement of the XRD results (see Figure 2). The mechanism of the grain size reduction during ball milling has been previously described as follows (4): the early stage of milling leads to the formation of the shear bands which consist of a dense of network of dislocations. When dislocations accumulate to a certain density by continuous plastic deformation, the crystal disintegrates into subgrains separated by low-angle grain boundaries with neighboring subgrains. The orientation of the subgrains with respect to each other may ultimately become completely random, which is characteristic of a nanocrystalline structure. In the case of amorphous alloy during ball milling, the mechanism of the nanocrystalline structural formation may differ from that
GJ FAN, XP SONG, MX
438
QUAN AND
ZQ Hu
Figure 5. TEM images for (a) polycrystalline Feg$zo alloy after 80 h of milling and (b) amorphous Fes&o alloy after 20 h of milling.
I-I _ 4.24
.
n
m
04
4.22 z
5.13
W
5.12 .=
U .-
t m
0
0
n
5.11 iI
I
I
I
I
I
I
I
I
lo
20
30
40
50
60
70
80
MillingTime(h) Figure 6. Variation of the lattice parameter a and c for the tetragonal structured Fe2B phase with milling time (I a axis, 0 c axis).
NANOCRYSTALLIUTION OF
Fe,&
BY BALL MILLING
439
of ball milling the polycrystalline materials, as described above. The crystallization in the amorphous matrix requires a nucleation and growth process. On the other hand, the mechanical deformation during ball milling can also reduce the grain size, even when crystal growth occurs. After a certain amount of milling, the dynamical equilibrium between the grain growth and the fracture of the crystallite will be established, which limits the grain size to about 12 nm (9). The smaller grain size after mechanically induced crystallization can also be ascribed to the large amount of nucleation sites existing in the amorphous matrix. Figure 6 shows the lattice parameter a and c for tetragonal structured FezB phase versus milling time. It is noticed that the a axis expands and the c axis shrinks rapidly after the first stage of milling for 40 h, then both saturate at constant values at the later stage of milling. It is interesting that the refinement of the grain size and the change of the lattice parameter occur almost at the same time range during ball milling. The variation of the lattice parameter with milling time as shown in Figure 6 indicates that chemical disordering has occurred in the tetragonal structured Fe2B phase. It has been well known that ball milling can lead to chemical disordering (10-14). Koch e? al. have reported that the ordered L12-Ni3Al compound may lose its LRO after a certain milling time (10). However, an amorphous phase was also evidenced after the disappearance of the superlattice reflections. They concluded that the energy stored in the nanocrystalline grain boundary can act as a driven force for the partial amorphization. Partial amorphization of the Ni3Al during mechanical milling was also observed by Benamueur and Yavari (14). They explain this result by the existence of a two-phase region in the nonequilibrium phase diagram, corresponding to the free-energy curve of the fee solid solution and of the undercooled liquid (amorphous phase).
0.835Cl 0.830-n 0.825- q ,, 2p
0.8200
0.8150.810-
0
0
q
I
I
I
I
I
I
I
I
10
20
30
40
50
60
70
80
MillingTime(h) Figure 7. Variation of axial ratio c/u for the tetragonal structured Fe2B phase with milling time.
440
GJ FAN, XP SONG, MX QUANANDZQ Hu
On the other hand, Zhou et al. have demonstrated that the atomic disorder should be considered as the major source of the energy storage. Accordingly, the type of energy stored during ball milling remains controversial. The degree of disorder of the Fe2B phase can be followed by measuring the tetragonality of the unit cell, i.e. the axial ratio between c and a. Figure 7 displays the variation of axial ratio c/u with the milling time. The perfectly ordered Fe2B phase has an axial ratio of 0.83 15. After 80 h of milling, the axial ratio c/u decreases by about 2.4%, indicating that chemical disordering has indeed been introduced into the Fe2B phase.
SUMMARY Nanocrystalline Fes&u alloys were synthesized by ball milling both from polycrystalline and amorphous alloys. The mechanism of nanocrystallization for both cases was present. The reduction of the grain size for polycrystahine Fes&c alloy may originate from the introduction of a large amount of dislocations by mechanical deformation: whereas the nanocrystallization of the amorphous FeseB;?ualloy during ball milling can be attributed to the presence of large amount of nucleation sites in the amorphous matrix and the establishment of dynamic equilibrium between the grain growth and the fracture of the crystallites. The observed decrease in the axial ratio c/a for tetragonal structured Fe2B phase can be interpreted with the occurrence of the chemical disordering during ball milling. REFERENCES 1.
2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
Phys. Lett. Am, 356 (1984). R. Birringer, H. Gleiter, HP Blein, and P. Marquardt, X. Zhu, R. Bit-ringer, U. Herr, and H. Gleiter, Phys. Rev. B 3,9085 (1987). L. Schulz, J. Wecker, and E. Hellstern, J. Appl. Phys. 6l, 3583 (1987). H.J. Fecht, E. Hellstem, Z. Pu, and W.L. Johnson, Metall. Trans. A 2l, 2333 (1990). J. Karch, R. Bit-ringer, and H. Gleiter, Nature 330,556 (1987). J. Eckert, J.C. Holzer, C.E. KriIl , and W.L. Johnson, Mater. Sci. Forum 88_90,505 (1992). M.L. Trudeau, R. Schulz, D. Dussault, and A.Van Neste, Phys. Rev. Lett. @,99 (1990). G.J. Fan, M.X. Quan, and Z.Q. Hu, for publication in Appl. Phys. Lett. M.L. Trudeau, J.Y. Huot, R. Schulz, D. Dussanh, A. Van Neste, and G. L’Esperance, Phys. Rev. B 45,4626 (1992). J.S.C. Jang and C.C. Koch, J. Mater. Res. 5,498 (1990). L.M. Di, G.F. Zhou, and H. Bakker, Phys. Rev. B 47,489O (1993). G.F. Zhou and H. Bakker, Acta Metall. et Mater. 42, 3009 (1994). G.P. Zhou and H. Bakker, Jntermetallics 2, 103 (1994). T. Benarneur and A.R. Yavari, J. Mater. Res. I,2971 (1992).
Pergamon
09659773(95)00255-3
NANOCRYSTALLIZATION
OF FegoB2o BY BALL MILLING
GJ. Fan(2), X.P. Song(‘), M.X. Quan(2), and Z.Q. Hu(~) (t)No. 24 Laboratory and (*)National Key Lab for RSA, Institute of Metal Research, Academia Sinica, Shenyang 110015, P.R.China (Accepted March 1995) Abstract-By meansofx-raydtffraction (XRD), transmission electron microscopy(TEM), as well as dtjferential scanning calorimetry (DSC) measurements, the sequence of structural evolution by ball milling of polycrystalline and amorphous Fes&o alloys was studied. Ball milling ofpolycrystalline Fes&o alloy results in a continuous refinement of the grain size to about 13 nm and a decrease of axial ratio clafor the Fe2B phase. For the amorphous Fes&o alloy, with increasing milling time, the amorphous phase crystallizes into a-Fe and a metastable Fe3B phase, followed by the phase transformationfrom the resulting metastable Fe3 phase into a-Fe and the stable Fe2B phase, which are also nanocrystalline. The decreased axial ratio clafor Fe2B phase indicates that chemical disordering is introduced during the mechanical deformation.
INTRODUCTION Due to their unique properties, nanocrystalline materials have been widely studied. These novel materials are characterized by a high density of disordered grain boundaries which may modify their thermal and physical properties (14). These materials have promised some interesting technological applications such as production of ductile ceramics (5), effective hydrogen storage materials, etc. Nanocrystalline materials can be synthesized by a variety of different methods. A typical example is the gas condensation method which was first reported by Gleiter and coworkers (l-2). More recently, Fecht et al. have systematically studied the synthesis of nanocrystalline metals by high-energy ball milling (4). A series of bee and hcp pure elemental metals have been prepared in this manner. In addition, Eckert et al. have demonstrated that nanometer-sized fee elemental metals can also be prepared by ball milling, which have been reported to be inefficient for effective energy storage (6). It is well known that high-energy ball milling is an effective tool for large scale production and economic operation. In this paper, nanocrystalhne Fes&o alloys have been prepared by ball milling, both starting from the polycrystalline material and the amorphous alloy. The mechanism of the microstructure formation will be discussed between these synthesis methods. The deviation of lattice parameters for the Fe2B phase from the equilibrium will be interpreted with the presence of the chemical disordering in the ball milled Fe2B phase. 433
GJ FAN, XP
434
SONG,
MX QUAN
AND
ZQ Hu
EXPERIMENTAL PROCEDURES FesoEI2ualloy ingots were prepared by arc melting and were remelted several times under an argon atmosphere. The ingots were homogenized under an argon atmosphere at a temperature of 1073K for 2 h. The annealed button was crushed into small pieces and powders. The amorphous Fes&u ribbons were obtained by the melt-spin method and were cut into small pieces. The thus obtained materials were respectively sealed in an argon atmosphere and were milled in a planetary ball mill. The initial ball-to-sample mass ratio was about 1O:l.At the given milling time, a small amount of the milled material was withdrawn for XRD analysis. X-raydiffractionOCRD)patternswereperformedbyaRIGAKUX-raypowderdiffractometer using a graphite monochromator Cu-Ka radiation (h = 0.15405 nm). Transmission electron microscopy (TEM) was performed with a Phillips EM420 operated at 120 kV. Thermal analysis of the milled powders was performed using a differential scanning calorimeter (Perkin-Elmer DSC-7) at a heating rate of 20”C/min. A continuous pure argon flow was used to avoid possible oxidation.
0 U-Fe
‘1 1. IO )* .% c w
!.h
28
(degree)
Figure 1. XRD patterns for polycrystalline Fesc&c alloy after various milling times.
!‘hOCRfSTAWZATlON
OF
FemBmBY h_L hhLlMG
435
RESULTS AND DISCUSSION Figure 1 shows the XRD patterns for polycrystalline FesoBzo alloy after various milting times. The equilibrium phases, a-Fe and FezB, can be clearly observed for the unmilled sample. After 4 h of milling, the XRD patterns for both a-Fe and FQB diffraction lines broaden, due to the refinement of the grain size and the introduction of the lattice strain by severe mechanical deformation. After 20 h of milling, the (022) and (130) XRD lines for Fe2B phase merge into a single line. 80 h of milling did not give rise to pronounced differences in the XRD patterns compared with that of the 20 h milled sample. The Scherrer formula is applied to estimate the grain size of a-Fe by using the Fe(200) diffraction line after the correction for instrumental broadening and the separation of Kal and Ka2. Figure 2 shows the obtained results which reveal that the initial 40 h of milling leads to a continuous refmement of the grain size and then saturates to about 9 nm. Further refinement of the grain size seems to be impossible. This behavior can be understood according to the well known Hall-Petch relationship, which sets a limit to the reduction of the grain size achieved by plastic deformation during ball milling. We expect that the Fe&320 pre-alloyed ingot can be vitrified if subjected to high-energy ball milling, since Fes(&o alloy can be easily vitrified by a melt-spin method. However, there is no trace of an amorphous phase in the XRD patterns, even after a 80 h of milling.
30 0 25-
0
z s 'G -E 5
z
20 0
15 -
0
0
Sk u
10 -
0 I
o
10
I
I
20 30
I
I
40
50
I
0 I
60 70
MillingTime W Figure 2. Variation of grain size with milling time.
I
80
436
GJ FAN, XP SONG, MX
QUAN AND
ZO Hu
0 U-Fe
5 tTi
0
f .-
0 0
E
al +
c
w
25
85
5s 26Idegree)
Figure 3. XRD patterns for amorphous Fe8&o
alloy after various milling times.
Figure 3 shows the XRD patterns for amorphous Fee&n alloy after various milling times. After 4 h of milling, the amorphous phase crystallizes into a-Fe and a metastable FeaB phase. By increasing the milling time to 6 h, the stable Fe2B phase can be observed in the XRD patterns. This can be attributed to the decompositional products of the metastable FesB phase. The metastable FesB phase may undergo a phase transformation into o-Fe and Fe2B during mechanical attrition. After 20 h of milling, the equilibrium phase of a-Fe and Fe2B were obtained by mechanically driven crystallization. However, during a usual thermal crystallization process, the amorphous Fes&u alloy crystallizes into a-Fe and a metastable FesB. Accordingly, one can conclude that some type of defect has been introduced during mechanical attrition which kinetically favors the phase transformation from Fe3B into a-Fe and Fe2B. Figure 4 shows the DSC curve of the amorphous FesoB2u alloy. A sharp exothermal peak was observed. The crystallization temperature and the heat release are 467.4”C and 134.6 J/g, respectively. Trudeau et cd. have reported that amorphous alloys will undergo crystallization during high-energy ball milling (7). They have shown that the crystallization behavior due to a
NANOCRYSTALLIZATION OF
Fe&,,
Temperature
BY BALL MILLING
437
i°C1
Figure 4. DSC curve of the melt-spin amorphous Fes&u
alloy.
thermal process and the one induced by high energy ball milling can be very different. The structural evolution during ball milling cannot be associated with a local effective temperature rise at the collision site which causes the crystallization. One may estimate that the local effective temperature rise is lower than the crystallization temperature of an amorphous phase. Our experiments seem to prove this idea also. However, high-energy ball milling can cause the local microstructural changes in amorphous Fes&oalloy during the first 4 h of milling, which alter the mechanism of crystallization of an amorphous alloy compared with a thermal process (8). Figure 5 shows the TEM images for polycrystalline Fes&e alloy after 80 h of milling and amorphous FesuBzo alloy after 20 h of milling. It can be seen that the crystallization of an amorphous alloy gives almost the same small grain size as that of the ball milled ingot alloy, which is in agreement of the XRD results (see Figure 2). The mechanism of the grain size reduction during ball milling has been previously described as follows (4): the early stage of milling leads to the formation of the shear bands which consist of a dense of network of dislocations. When dislocations accumulate to a certain density by continuous plastic deformation, the crystal disintegrates into subgrains separated by low-angle grain boundaries with neighboring subgrains. The orientation of the subgrains with respect to each other may ultimately become completely random, which is characteristic of a nanocrystalline structure. In the case of amorphous alloy during ball milling, the mechanism of the nanocrystalline structural formation may differ from that
GJ FAN, XP SONG, MX
438
QUAN AND
ZQ Hu
Figure 5. TEM images for (a) polycrystalline Feg$zo alloy after 80 h of milling and (b) amorphous Fes&o alloy after 20 h of milling.
I-I _ 4.24
.
n
m
04
4.22 z
5.13
W
5.12 .=
U .-
t m
0
0
n
5.11 iI
I
I
I
I
I
I
I
I
lo
20
30
40
50
60
70
80
MillingTime(h) Figure 6. Variation of the lattice parameter a and c for the tetragonal structured Fe2B phase with milling time (I a axis, 0 c axis).
NANOCRYSTALLIUTION OF
Fe,&
BY BALL MILLING
439
of ball milling the polycrystalline materials, as described above. The crystallization in the amorphous matrix requires a nucleation and growth process. On the other hand, the mechanical deformation during ball milling can also reduce the grain size, even when crystal growth occurs. After a certain amount of milling, the dynamical equilibrium between the grain growth and the fracture of the crystallite will be established, which limits the grain size to about 12 nm (9). The smaller grain size after mechanically induced crystallization can also be ascribed to the large amount of nucleation sites existing in the amorphous matrix. Figure 6 shows the lattice parameter a and c for tetragonal structured FezB phase versus milling time. It is noticed that the a axis expands and the c axis shrinks rapidly after the first stage of milling for 40 h, then both saturate at constant values at the later stage of milling. It is interesting that the refinement of the grain size and the change of the lattice parameter occur almost at the same time range during ball milling. The variation of the lattice parameter with milling time as shown in Figure 6 indicates that chemical disordering has occurred in the tetragonal structured Fe2B phase. It has been well known that ball milling can lead to chemical disordering (10-14). Koch e? al. have reported that the ordered L12-Ni3Al compound may lose its LRO after a certain milling time (10). However, an amorphous phase was also evidenced after the disappearance of the superlattice reflections. They concluded that the energy stored in the nanocrystalline grain boundary can act as a driven force for the partial amorphization. Partial amorphization of the Ni3Al during mechanical milling was also observed by Benamueur and Yavari (14). They explain this result by the existence of a two-phase region in the nonequilibrium phase diagram, corresponding to the free-energy curve of the fee solid solution and of the undercooled liquid (amorphous phase).
0.835Cl 0.830-n 0.825- q ,, 2p
0.8200
0.8150.810-
0
0
q
I
I
I
I
I
I
I
I
10
20
30
40
50
60
70
80
MillingTime(h) Figure 7. Variation of axial ratio c/u for the tetragonal structured Fe2B phase with milling time.
440
GJ FAN, XP SONG, MX QUANANDZQ Hu
On the other hand, Zhou et al. have demonstrated that the atomic disorder should be considered as the major source of the energy storage. Accordingly, the type of energy stored during ball milling remains controversial. The degree of disorder of the Fe2B phase can be followed by measuring the tetragonality of the unit cell, i.e. the axial ratio between c and a. Figure 7 displays the variation of axial ratio c/u with the milling time. The perfectly ordered Fe2B phase has an axial ratio of 0.83 15. After 80 h of milling, the axial ratio c/u decreases by about 2.4%, indicating that chemical disordering has indeed been introduced into the Fe2B phase.
SUMMARY Nanocrystalline Fes&u alloys were synthesized by ball milling both from polycrystalline and amorphous alloys. The mechanism of nanocrystallization for both cases was present. The reduction of the grain size for polycrystahine Fes&c alloy may originate from the introduction of a large amount of dislocations by mechanical deformation: whereas the nanocrystallization of the amorphous FeseB;?ualloy during ball milling can be attributed to the presence of large amount of nucleation sites in the amorphous matrix and the establishment of dynamic equilibrium between the grain growth and the fracture of the crystallites. The observed decrease in the axial ratio c/a for tetragonal structured Fe2B phase can be interpreted with the occurrence of the chemical disordering during ball milling. REFERENCES 1.
2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
Phys. Lett. Am, 356 (1984). R. Birringer, H. Gleiter, HP Blein, and P. Marquardt, X. Zhu, R. Bit-ringer, U. Herr, and H. Gleiter, Phys. Rev. B 3,9085 (1987). L. Schulz, J. Wecker, and E. Hellstern, J. Appl. Phys. 6l, 3583 (1987). H.J. Fecht, E. Hellstem, Z. Pu, and W.L. Johnson, Metall. Trans. A 2l, 2333 (1990). J. Karch, R. Bit-ringer, and H. Gleiter, Nature 330,556 (1987). J. Eckert, J.C. Holzer, C.E. KriIl , and W.L. Johnson, Mater. Sci. Forum 88_90,505 (1992). M.L. Trudeau, R. Schulz, D. Dussault, and A.Van Neste, Phys. Rev. Lett. @,99 (1990). G.J. Fan, M.X. Quan, and Z.Q. Hu, for publication in Appl. Phys. Lett. M.L. Trudeau, J.Y. Huot, R. Schulz, D. Dussanh, A. Van Neste, and G. L’Esperance, Phys. Rev. B 45,4626 (1992). J.S.C. Jang and C.C. Koch, J. Mater. Res. 5,498 (1990). L.M. Di, G.F. Zhou, and H. Bakker, Phys. Rev. B 47,489O (1993). G.F. Zhou and H. Bakker, Acta Metall. et Mater. 42, 3009 (1994). G.P. Zhou and H. Bakker, Jntermetallics 2, 103 (1994). T. Benarneur and A.R. Yavari, J. Mater. Res. I,2971 (1992).