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The influence of quench temperature on the structure and crystallization of glassy NiZr2
Journal of Non-CrystaUineSolids 61 & 62 (1984) 469474 North-Holland, Amsterdam
469
THE INFLUENCE OF QUENCH TEMPERATURE ON THE STRUCTURE AND CRYSTALLIZATION OF GLASSY NiZr 2
J.L. WALTER General Electric Research and Development Center, Schenectady,
Z. ALTOUNI~N and J.O.
N.Y. 12301.
STROH-OLSEN
R u t h e r f o r d P h y s i c s B u i l d i n g , McGill U n i v e r s i t y , M o n t r e a l , Quebec, Canada, H3A 2T8.
3600 U n i v e r s i t y S t r e e t ,
The crystallization characteristics of glassy NiZr2 are very sensitive to quench temperature. Samples quenched just above tile melt show a single peak in DSC which splits into two peaks as the quench temperature is raised to several hundred degrees above the melt. In all cases the crystallization product is the same and electron microprobe, transmission electron microscopy and electron diffraction show the same microstructure in the glassy phase down to a scale of 1 nm with no evidence for phase separation. Studies of crystallization kinetics show that the differences in DSC are caused by differences in nucleation and growth rates ; we believe these are the outcome of different degrees of chemical short range order frozen in from the different quench temperatures.
i. INTRODUCTION A recent study I on Ni-Zr glasses in the range 33 to 37 at.% Zr concluded that phase separation occurred in these alloys : two diffuse haloes were seen in transmission electron micrographs
(TEM) and two crystallization peaks in
differential scanning calorimetry (DSC).
These results were in conflict with
those of other studies 2'3 where, at the composition 33 at.% Zr,
a single
diffuse halo was seen in TEM with a polymorphous transformation into crystalline NiZr 2. In an effort to resolve these differences and to answer the important question of whether phase separation occurs we undertook a detailed study of the crystallization of Ni33Zr67 prepared by quenching from different temperatures.
2. EXPERIMENTAL METHODS The glasses were prepared by melt spinning under high purity helium at 50 kPa pressure with all the manufacturing parameters carefully monitored so that the only difference between ribbons was the temperature from which they were quenched.
Nine ribbons were made with the quench temperature covering the
range 1150 K to 1450 K.
Crystallization temperatures,
0022-3093/84/$03.00 ©Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
(Tx) , heats of
J.L. Walter et al. / The influence o f quench temperature
470
crystallization,
activation energies,
(E), and kinetics of crystallization
were all measured by a calibrated Perkin-Elmer DSC-2C under high purity argon. Structural information both before, during and after crystallization was obtained by TEM, electron and X-ray diffraction.
Compositional homogeneity
was examined by electron beam microprobe analysis on the scale of i~.
3, RESULTS AND DISCUSSION Representative DSC scans at a heating rate of 40 K/min. are shown in figure I.
Ribbons quenched from low temperatures show only a single peak which
starts to separate into two peaks as the quench temperature increases. crystallization temperature also increases with quench temperature.
ribbon #I T x = 680 K, while for ribbon #4 Txl = 692 K and Tx2 = 712 K. #i shows the same DSC characteristics same as in reference I.
The
For Ribbon
as in references 2 and 3 ; ribbon #4 the
These two ribbons therefore were selected for ex-
haustive analysis by TEM and crystallization kinetics.
Through measurements
of superconducting transition temperatures of all nine ribbons 4 in the as-made and thermally relaxed state we have eliminated the possibility that different quenching temperatures merely introduce different degrees of thermal relaxation.
'692
r~2
w I
1
I L 660
I
I
I
I I I I 700 T(K)
I 740
FIGURE 1 DSC thermograms of some representative ribbons heated at 40 K/min.
FIGURE 2 TEH of ribbon #i with electron diffraction pattern inset.
ZL. Walter et al. / The influence o f quench temperature
471
Electron beam microprobe measurements made on transverse and longitudinal sections of both ribbons, showed no difference in composition and no variations in composition from surface to surface or from surface to centre of each ribbon.
As made the samples had an oxide layer of perhaps I0 nm thick-
ness which was removed by abrasion. crystallization characteristics.
The abraded samples showed no change in
Samples were thinned for TEM in an electro-
lyte consisting of perchloric acid in methanol.
The ribbons contained some
deep pits where oxide films could be seen in the thinned TEM samples.
Figure
2 shows a TEM and electron diffraction pattern of the as made ribbon #i. The speckles (about 1 nm diameter) are probably a surface artifact of the electropolishing of this reactive alloy and are not a reflection of phase separation or demixing since the electron diffraction patterns are the same for all parts of the surface and show only a single principal halo at 0.238 nm, and a secondary ring at 0.143 nm.
Ribbon #4 shows an essentially identical
microstructure with the same speckles and the same electron diffraction pattern though the d-values corresponding to the principal and secondary ring are slightly larger : 0.242 and 0.149 nm, respectively. no evidence for phase separation.
Once again there is
We are able, however, to reproduce the
results of reference 1 by deliberately incorporating one of the abovementioned pits into the area of diffraction. a d-value of 0.249 nm was observed.
In this case an inner ring with
We conclude that there is no phase
separation in glassy NiZr 2 and that, down to the scale of 1 nm, the microstructure is independent of quench temperature. To study the differences in crystallization, samples of ribbon #I and #4 were heated in the DSC at 40 K/min. to 675 K and 692 K respectively, where 25% crystallization has occurred, then rapidly cooled (see figure I).
Figure
3, a TEM and electron diffraction pattern for ribbon #1 shows an area of highly striated, elongated NiZr 2 crystals averaging 0.2 to 0.3~m in length. Figure 4 shows an area of ribbon #4 which indicates, in spite of the position in the DSC thermogram, that the extent of crystallization is less than in ribbon #i and that the NiZr 2 crystals are smaller also, averaging about 0.1~m in diameter.
Another sample of ribbon #4 was heated to 712 K (see figure l)
corresponding to about 75% crystallization.
TEM analysis confirms that the
sample is indeed about three quarters crystallized with large crystals about 0.25 to 0.30~m in diameter.
Again, only NiZr 2 crystals were found, as
confirmed by electron diffraction.
From TEM therefore we can say that there
is a single crystallization event in the sense that only one phase is evolved, and furthermore the total heat evolved was, within error, the same for all ribbons.
This suggests that the difference in DSC must be due to differences
472
Y.L. Walter et al. / The influence o f quench temperature
in the kinetics of crystallization for the two ribbons. From Kissinger's peak-shift method 5, the activation energy for ribbon #1 was found to be 3.1 eV, very close to that found for the polymorphous crystallization of Cu33Zr67 6.
For ribbon #4 the activation energies of the
two peaks were 3.8 eV and 2.4 eV respectively ; in fact for all nine ribbons the activation energy of the first peak lay between 3.1 eV and 4.0 eV whereas the second peak (where observable) always
gave a value of 2.4 eV.
This
indicates that the second peak is associated with growth of the crystals rather than nucleation 2.
Isothermal
crystallization studies in the DSC con-
firm this conclusion. It has been shown that for most metallic glasses the kinetics of isothermal crystallization are reasonably well described by an equation of the JohnsonMehl-Avrami type 6 where the Avrami exponent n is characteristic of the transformation process.
Applying this to samples of ribbon #i and #4 gives two
well defined and distinctly different exponents of 3.44 and 2.92 respectively. As the final product and the total enthalpy evolved is the same for both ribbons, one expects the mean activation energy, Ea, to be the same also. In cases where growth and nucleation processes are separated and well defined,
(a)
(a)
(b) FIGURE 3
(b) FIGURE 4
a) TEM of ribbon #I heated to 675 K
a) TEM of ribbon #4 heated to 692 K
b) Diffraction pattern of highly striated crystal in figure 3a.
b) Diffraction pattern of crystal and amorphous matrix in figure 4a.
J.L. Walter et al. / The influence o f quench temperature
473
E a is given by 7 (nn E n + n g Eg) / (nn + ng) where n and g as subscripts stand for nucleation and growth respectively ; the overall Avrami exponent n = n n + n . For polymorphous transformations one expects growth to be diffusion g controlled 6 for which n = 1.5 8. Since n = ..v92 for ribbon #4, this suggests g that n = 1.42 for this ribbon, which gives a mean activation energy of 3.1 eV, n 8 exactly the same as for ribbon #1. The nucleation rate , I, being proportional to tnn-1 implies, approximately, that for ribbon #1 I ~ t while for ribbon #4, I ~ ~
- i.e. nucleation for ribbons quenched from higher temper-
atures is more difficult, requiring more time and more energy.
This is con-
sistent with the higher temperatures and the few nuclei found in TEM.
We
suggest that ribbon #I, being quenched from a temperature close to the melt, shows more chemical short range order (CSRO) than a ribbon #4 ; there are more local configurations close to crystalline NiZr 2 in ribbon #I so that less diffusion is required to form nuclei,
Such CSRO would be invisible to
electron microscopy.
4. CONCLUSIONS The microstructure of glassy NiZr 2 quenched from the melt shows no evidence for phase separation or demixing of the components down to a scale of 1 nm. The two peaks in DSC correspond to nucleation and growth processes, rather than the evolution of two different crystallization products.
The second
principal diffuse ring reported previously in TEM 1 is, we believe, due to oxide and is unrelated to the DSC results.
Differences in crystallization
characteristics with quench temperature are therefore caused by differences in structure on the atomic scale - probably differences in CSRO which are not mutually accessible by thermal annealing ; it seems one can really obtain different metastable glassy configurations simply" by quenching the liquid from different temperatures.
ACKNOWLEDGEbIENTS The authors thank E.F. Koch for the TEM work.
This research was supported
in part by the Natural Sciences and Engineering Research Council of Canada.
REFERENCES 1) H. van Swijgenhoven, L.M. S t a l s and K.H.J. Buschow, Phys, S t a t . 72 (1982) 153.
Sol.(a)
2) M.G. S c o t t , G. Gregan and Y.D. Dong, Proc. o f the 4th I n t ' l Conf. on Rapidly Quenched Metals. T. Masumoto and K. Suzuki, eds. J a p a n , I n s t . M e t a l s , Sendai, 1982, p. 671. 3) Z. A l t o u n i a n , Tu guo-hua and a . o , 3111.
S t r o m - O l s e n , J. Appl. Phys. 54 (1983)
of
474
ZL. Walter et al. / The influence o f quench temperature
4) Z. Altounian, J.O. Strom-Olsen and J.L. Walter, unpublished. 5) H.E. Kissinger, Anal. Chem. 29 (1957) 1702. 6) Z. Altounian, Tu Guo-hua, J.O. Strom-Olsen, and W.8. Muir, PHys. Rev. B24 (1981) 505. 7) S. Ranganathan and M. von Heimendahl, J. Mater. Sci. 16 (1981) 2401. 8) J. Burke, "Kinetics of Phase Transformations in Metals" (Pergamon, Oxford, 1965).
469
THE INFLUENCE OF QUENCH TEMPERATURE ON THE STRUCTURE AND CRYSTALLIZATION OF GLASSY NiZr 2
J.L. WALTER General Electric Research and Development Center, Schenectady,
Z. ALTOUNI~N and J.O.
N.Y. 12301.
STROH-OLSEN
R u t h e r f o r d P h y s i c s B u i l d i n g , McGill U n i v e r s i t y , M o n t r e a l , Quebec, Canada, H3A 2T8.
3600 U n i v e r s i t y S t r e e t ,
The crystallization characteristics of glassy NiZr2 are very sensitive to quench temperature. Samples quenched just above tile melt show a single peak in DSC which splits into two peaks as the quench temperature is raised to several hundred degrees above the melt. In all cases the crystallization product is the same and electron microprobe, transmission electron microscopy and electron diffraction show the same microstructure in the glassy phase down to a scale of 1 nm with no evidence for phase separation. Studies of crystallization kinetics show that the differences in DSC are caused by differences in nucleation and growth rates ; we believe these are the outcome of different degrees of chemical short range order frozen in from the different quench temperatures.
i. INTRODUCTION A recent study I on Ni-Zr glasses in the range 33 to 37 at.% Zr concluded that phase separation occurred in these alloys : two diffuse haloes were seen in transmission electron micrographs
(TEM) and two crystallization peaks in
differential scanning calorimetry (DSC).
These results were in conflict with
those of other studies 2'3 where, at the composition 33 at.% Zr,
a single
diffuse halo was seen in TEM with a polymorphous transformation into crystalline NiZr 2. In an effort to resolve these differences and to answer the important question of whether phase separation occurs we undertook a detailed study of the crystallization of Ni33Zr67 prepared by quenching from different temperatures.
2. EXPERIMENTAL METHODS The glasses were prepared by melt spinning under high purity helium at 50 kPa pressure with all the manufacturing parameters carefully monitored so that the only difference between ribbons was the temperature from which they were quenched.
Nine ribbons were made with the quench temperature covering the
range 1150 K to 1450 K.
Crystallization temperatures,
0022-3093/84/$03.00 ©Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
(Tx) , heats of
J.L. Walter et al. / The influence o f quench temperature
470
crystallization,
activation energies,
(E), and kinetics of crystallization
were all measured by a calibrated Perkin-Elmer DSC-2C under high purity argon. Structural information both before, during and after crystallization was obtained by TEM, electron and X-ray diffraction.
Compositional homogeneity
was examined by electron beam microprobe analysis on the scale of i~.
3, RESULTS AND DISCUSSION Representative DSC scans at a heating rate of 40 K/min. are shown in figure I.
Ribbons quenched from low temperatures show only a single peak which
starts to separate into two peaks as the quench temperature increases. crystallization temperature also increases with quench temperature.
ribbon #I T x = 680 K, while for ribbon #4 Txl = 692 K and Tx2 = 712 K. #i shows the same DSC characteristics same as in reference I.
The
For Ribbon
as in references 2 and 3 ; ribbon #4 the
These two ribbons therefore were selected for ex-
haustive analysis by TEM and crystallization kinetics.
Through measurements
of superconducting transition temperatures of all nine ribbons 4 in the as-made and thermally relaxed state we have eliminated the possibility that different quenching temperatures merely introduce different degrees of thermal relaxation.
'692
r~2
w I
1
I L 660
I
I
I
I I I I 700 T(K)
I 740
FIGURE 1 DSC thermograms of some representative ribbons heated at 40 K/min.
FIGURE 2 TEH of ribbon #i with electron diffraction pattern inset.
ZL. Walter et al. / The influence o f quench temperature
471
Electron beam microprobe measurements made on transverse and longitudinal sections of both ribbons, showed no difference in composition and no variations in composition from surface to surface or from surface to centre of each ribbon.
As made the samples had an oxide layer of perhaps I0 nm thick-
ness which was removed by abrasion. crystallization characteristics.
The abraded samples showed no change in
Samples were thinned for TEM in an electro-
lyte consisting of perchloric acid in methanol.
The ribbons contained some
deep pits where oxide films could be seen in the thinned TEM samples.
Figure
2 shows a TEM and electron diffraction pattern of the as made ribbon #i. The speckles (about 1 nm diameter) are probably a surface artifact of the electropolishing of this reactive alloy and are not a reflection of phase separation or demixing since the electron diffraction patterns are the same for all parts of the surface and show only a single principal halo at 0.238 nm, and a secondary ring at 0.143 nm.
Ribbon #4 shows an essentially identical
microstructure with the same speckles and the same electron diffraction pattern though the d-values corresponding to the principal and secondary ring are slightly larger : 0.242 and 0.149 nm, respectively. no evidence for phase separation.
Once again there is
We are able, however, to reproduce the
results of reference 1 by deliberately incorporating one of the abovementioned pits into the area of diffraction. a d-value of 0.249 nm was observed.
In this case an inner ring with
We conclude that there is no phase
separation in glassy NiZr 2 and that, down to the scale of 1 nm, the microstructure is independent of quench temperature. To study the differences in crystallization, samples of ribbon #I and #4 were heated in the DSC at 40 K/min. to 675 K and 692 K respectively, where 25% crystallization has occurred, then rapidly cooled (see figure I).
Figure
3, a TEM and electron diffraction pattern for ribbon #1 shows an area of highly striated, elongated NiZr 2 crystals averaging 0.2 to 0.3~m in length. Figure 4 shows an area of ribbon #4 which indicates, in spite of the position in the DSC thermogram, that the extent of crystallization is less than in ribbon #i and that the NiZr 2 crystals are smaller also, averaging about 0.1~m in diameter.
Another sample of ribbon #4 was heated to 712 K (see figure l)
corresponding to about 75% crystallization.
TEM analysis confirms that the
sample is indeed about three quarters crystallized with large crystals about 0.25 to 0.30~m in diameter.
Again, only NiZr 2 crystals were found, as
confirmed by electron diffraction.
From TEM therefore we can say that there
is a single crystallization event in the sense that only one phase is evolved, and furthermore the total heat evolved was, within error, the same for all ribbons.
This suggests that the difference in DSC must be due to differences
472
Y.L. Walter et al. / The influence o f quench temperature
in the kinetics of crystallization for the two ribbons. From Kissinger's peak-shift method 5, the activation energy for ribbon #1 was found to be 3.1 eV, very close to that found for the polymorphous crystallization of Cu33Zr67 6.
For ribbon #4 the activation energies of the
two peaks were 3.8 eV and 2.4 eV respectively ; in fact for all nine ribbons the activation energy of the first peak lay between 3.1 eV and 4.0 eV whereas the second peak (where observable) always
gave a value of 2.4 eV.
This
indicates that the second peak is associated with growth of the crystals rather than nucleation 2.
Isothermal
crystallization studies in the DSC con-
firm this conclusion. It has been shown that for most metallic glasses the kinetics of isothermal crystallization are reasonably well described by an equation of the JohnsonMehl-Avrami type 6 where the Avrami exponent n is characteristic of the transformation process.
Applying this to samples of ribbon #i and #4 gives two
well defined and distinctly different exponents of 3.44 and 2.92 respectively. As the final product and the total enthalpy evolved is the same for both ribbons, one expects the mean activation energy, Ea, to be the same also. In cases where growth and nucleation processes are separated and well defined,
(a)
(a)
(b) FIGURE 3
(b) FIGURE 4
a) TEM of ribbon #I heated to 675 K
a) TEM of ribbon #4 heated to 692 K
b) Diffraction pattern of highly striated crystal in figure 3a.
b) Diffraction pattern of crystal and amorphous matrix in figure 4a.
J.L. Walter et al. / The influence o f quench temperature
473
E a is given by 7 (nn E n + n g Eg) / (nn + ng) where n and g as subscripts stand for nucleation and growth respectively ; the overall Avrami exponent n = n n + n . For polymorphous transformations one expects growth to be diffusion g controlled 6 for which n = 1.5 8. Since n = ..v92 for ribbon #4, this suggests g that n = 1.42 for this ribbon, which gives a mean activation energy of 3.1 eV, n 8 exactly the same as for ribbon #1. The nucleation rate , I, being proportional to tnn-1 implies, approximately, that for ribbon #1 I ~ t while for ribbon #4, I ~ ~
- i.e. nucleation for ribbons quenched from higher temper-
atures is more difficult, requiring more time and more energy.
This is con-
sistent with the higher temperatures and the few nuclei found in TEM.
We
suggest that ribbon #I, being quenched from a temperature close to the melt, shows more chemical short range order (CSRO) than a ribbon #4 ; there are more local configurations close to crystalline NiZr 2 in ribbon #I so that less diffusion is required to form nuclei,
Such CSRO would be invisible to
electron microscopy.
4. CONCLUSIONS The microstructure of glassy NiZr 2 quenched from the melt shows no evidence for phase separation or demixing of the components down to a scale of 1 nm. The two peaks in DSC correspond to nucleation and growth processes, rather than the evolution of two different crystallization products.
The second
principal diffuse ring reported previously in TEM 1 is, we believe, due to oxide and is unrelated to the DSC results.
Differences in crystallization
characteristics with quench temperature are therefore caused by differences in structure on the atomic scale - probably differences in CSRO which are not mutually accessible by thermal annealing ; it seems one can really obtain different metastable glassy configurations simply" by quenching the liquid from different temperatures.
ACKNOWLEDGEbIENTS The authors thank E.F. Koch for the TEM work.
This research was supported
in part by the Natural Sciences and Engineering Research Council of Canada.
REFERENCES 1) H. van Swijgenhoven, L.M. S t a l s and K.H.J. Buschow, Phys, S t a t . 72 (1982) 153.
Sol.(a)
2) M.G. S c o t t , G. Gregan and Y.D. Dong, Proc. o f the 4th I n t ' l Conf. on Rapidly Quenched Metals. T. Masumoto and K. Suzuki, eds. J a p a n , I n s t . M e t a l s , Sendai, 1982, p. 671. 3) Z. A l t o u n i a n , Tu guo-hua and a . o , 3111.
S t r o m - O l s e n , J. Appl. Phys. 54 (1983)
of
474
ZL. Walter et al. / The influence o f quench temperature
4) Z. Altounian, J.O. Strom-Olsen and J.L. Walter, unpublished. 5) H.E. Kissinger, Anal. Chem. 29 (1957) 1702. 6) Z. Altounian, Tu Guo-hua, J.O. Strom-Olsen, and W.8. Muir, PHys. Rev. B24 (1981) 505. 7) S. Ranganathan and M. von Heimendahl, J. Mater. Sci. 16 (1981) 2401. 8) J. Burke, "Kinetics of Phase Transformations in Metals" (Pergamon, Oxford, 1965).