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Structural study of a phase transition in a NiP metallic glass
MATERIALS SCIENCE & E1IGIYEERINC
ELSEVIER
Structural
Materials Science and Engineering A226-228
study of a phase transition
M. Calvo-Dahlborg
A
(1997) 197-203
in a NiP metallic glass
a,*, F. Machizaud b, S. Nhien l@, B. Vigneron b, U. Dahlborg 2+
a Depammt
of Neutmn P&ysics, Royal Institute of Technology, b LSGZMUA 159, Ecole des Mines, Nmcy Cedex,
Sioclcholm, Fmnce
Sweden
Abstract The structure of NiP amorphousalloys has beenthe subject of severalcontroversies,especiallyconcerningthe nature of the short- and medium-rangeorders.It hasalready been reported that thesemetallic glasses can exhibit polymorphism,i.e., different types of local order, dependingupon the production technique. In this study, amorphousribbons of Ni,,P,, produced by two different techniques,melt spinning and electrodeposition,have been investigated. The results obtained from various types of structural analysis have been correlated: differential scanning calorimetry (DE) and neutron diffraction during heating at different ratesuntil completecrystallization. A hilly region in the DSC curve as well as some significant changes in the diffraction pattern could be observed prior to crystallization in both the melt-spun and the electrodepositedsamples.The results are
interpreted and discussed in terms of a phase transition in the material. 0 1997 Elsevier Science S.A. KeywoA: AmorphousNi,,P19alloy; Crystallization; Differential scanning calorimetry; Neutron diffraction; Phase transformation
1. Introduction Amorphization and crystallization kinetics of metallic glasses has been extensively studied for many years and it continues to be a hot topic. The reason is mainly the need to, as accurately as possible, determine the lifetime of these materials and to what extent the physical properties change during nucleation and crystalline growth. The present report is the first in a series dealing with phase transitions and crystallization phenomena in glasses of the NiP, FeB and PdSi systems. The crystallization process of melt-spun NIP metallic glasses has been studied in many works ([1,2] and references therein). It has generally been found that crystallization is strongly depending on the phosphorus content and that the thermal stability of the amorphous alloys increases as the P content approaches the eutectic composition. The crystallization process in NiP glasses produced by electrodeposition has also been studied extensively [2-51 and found to follow the same crystallization scheme as melt-spun NiP glasses [2]. In this * Corresponding author. Current address: LSGZM-UA 159, Ecole des Mines, Nancy Cedex, France. ’ Current address: CRISMAT, ISMRa, Caen Cedex, France. ’ Current address: LSG2M-UA 159, Ecole des Mines, Nancy Cedex, France. 0921-5093]97/$17.00 0 1997 Elsevier Science S.A. All rights reserved. - ___^^_ _ _^^_,^ _._^_._ .
study, we report on some new features seen in the amorphization and crystallization process of NiP glasses of eutectic composition produced by two different techniques, melt-spinning and electrodeposition. The experimental methods employed include differential scanning calorimetry (DSC) and real-time neutron diffraction.
2. Experimental
procedure and results
2.1. Neutron difft*action measurements
The intensity IObs(Q) measured in a neutron diffraction experiment on a binary system can in the static approximation formally be written as 1&Q) = A -S,,,(Q) where A is a coupling constant. S,,,(Q) is given by
&o,(Q) = ~,@a)~ + c&J2 + ~t
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atoms of kinds a and ,f?, and Q is the wavevector transfer in the scattering process. For the eutectic composition used here, the following relative Q dependent weighing factors are easily calculated s&Q)
= 0.82OSN+Ji(Q) + O.17osp&Q)
+ 0.001&p(Q) (2)
It is obvious, therefore, that the main features of a measured total structure factor are determined by NiNi correlations and that no information about P-P correlations can be obtained. For that purpose, the isotope substitution technique has to be employed, as extensive measurements on a N&P,,, glass have demonstrated [6]. The neutron diffraction experiments were performed on the LAD diffractometer at the neutron spallation source ISIS at Rutherford Appleton Laboratory, Chilton, UK, while the DSC measurements were carried out at LSG2M in Nancy, France. The metallic glasses studied all had the composition NislP19. The melt-spun ribbons were obtained from Allied Signal while the electrodeposited one was made by F. Machizaud, Nancy [4]. For the neutron diffraction measurements, the ribbons were cut in pieces of length 2-3 mm. The electrodeposited ribbon was taken away from the platinum support in small pieces less than 2 mm long. The pieces were put in a cylindrical vanadium can of an inner diameter of about 6 mm and put inside a furnace. This procedure ensured that all samples can be regarded as isotropic. The samples were heated stepwise in the furnace at somewhat different speeds, as is seen in Fig. 1. For practical purposes, the following notations will be used below to identify the different samples: NIP3A (meit-spun), NIP3B (meltspun), NIPDl (electrodeposited). From Fig. 1, it is seen that the average heating rate is rather low compared with a DSC measurement, of the order of 0.2-0.3 K min- ‘. The time required to obtain 500
i-
--
Time
7
[min]
Fig. 1. Heating rates for the melt-spun samples NIP3A (filled circles), NIP3B (triangles) and for the electrodeposited sample NIPDl (crosses).
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a diffraction pattern with sufficient statistical accuracy from an amorphous sample on LAD was about 1 h and this measurement time was used for the NIP33 and NIPDl samples. The heat treatment and the measuring time was different for the NIP3A sample: fewer steps of 3 h each. It should, though, be noted that even if the ISIS spallation source was very stable during most of the experiments, some measurement times were longer than others because of accelerator problems. However, the main difference between NIP3B and NIPDl has only been a longer stage at 90°C for NIPDl (5 h instead of 1 h), which results in a shift of the two curves in Fig. 1. The heating procedure of these two samples can thus be considered to be very close. The main consequence of these differences can only be a slightly earlier occurrence of the transformations in NIPDI relative to NIP3B as compared with the DSC results. The heating procedure for NIP3A in longer steps was intended to stabilize the structural transformations occurring in the material. The evolvement of S,,,(Q), measured at 90” scattering angle, with temperature for the NIP3A, NIP3B and NIPDI samples are shown in Figs. 2-4, respectively. The differences between the three samples will be discussed below over the whole range of temperature, The shape of the diffraction pattern at this large angle detector and, moreover, the complete absence of any sign of crystallization at the lowest temperatures (20°C in Fig. 2; 40°C in Figs. 3 and 4) clearly indicate that all samples were amorphous. The first sign of crystallization can be observed for the electrodeposited sample NIPDI in the diffraction patterns recorded at 300°C (Fig. 4), at 350°C for the melt-spun NIP3B (Fig. 3), and at 300°C for the meltspun NIP3A (Fig. 2). 2.2. DSC measurements In order to characterize the samples, measurements were performed on a differential scanning calorimeter (SETARAM DSClll). The heating rate was 5 K min-‘. The results shown in Fig. 5 clearly reveal a difference between the melt-spun and the electrodeposited samples. The melt-spun sample exhibits only one peak (starting at about 300°C and located at 338°C) originating from the crystallization process, while the electrodeposited one shows two transitions (starting at about 261 and 372°C and located at 282 and 415’C, respectively). It has to be concluded, therefore, that the crystallization process in the two samples are taking place along different routes. Furthermore, as will be shown in more detail in the discussion below, the two-step crystallization of the electrodeposited sample is not to be attributed to an excess or lack of phosphorus in the composition of the alloy similar to what is reported in Refs. [1,2,7]. The first part of the curve
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Fig. 2. Measured static structure factors S,,,(Q) for the melt-spun NIP3A sample at temperatures (“C) indicated.
prior to crystallization is similar for the two samples: a first hilly region, until 155°C for the electrodeposited sample and 180°C for the melt-spun one, and a flat region afterwards up to the first crystallization peak.
3. Discussion 3.1. The nmorpholrs-to-crystal tsnnsition
From the results reported above and from the careful examination of Figs. 2-5, several comments can be made: (1) The final stage of the crystallization is the same for all the samples. The diffraction patterns at 500°C for NIPDl and NIP3B, and at 400°C for NIP3A are identical. The peak in the DSC curve of the melt-spun samples consequently corresponds to the second peak of the DSC curve of the electrodeposited one (see Fig. 5). (2) Differences between the temperature corresponding to the tist sign of crystallization can be observed for the three samples in the DSC and the neutron diffraction measurements: 308°C in DSC and 250300°C in neutron diffraction for NIP3A, 308°C and 300-350°C for NIP3B, 261°C and 270-300°C for NIPDl, respectively. This can be explained as follows. (i) The temperature intervals in Fig. 2 (250-3OO”C), Fig. 3 (300-350°C) and Fig. 4 (270-300°C) do not allow a precise determination of the temperature of first crystallization. In a preliminary set of measurements performed on the SLAD diffractometer at Studsvik, Sweden, on another melt-spun sample taken from the same ribbon, heated at a rate of 0.07 K min-’ in
shorter temperature intervals, the first crystallization could be detected between 293 and 305°C [S]. (ii) The heating procedure used for the neutron diffraction experiment has been carried out in long steps (1 h for NIP3B and NIPDl and 3 h for NIP3A), while the DSC heating is continuous. This has resulted in a better stabilization of the structural transformations in the neutron diffraction experiments, thus allowing an earlier initiation of crystallization. This is particularly obvious for NIP3A. Furthermore, the crystallization temperatures reported in Refs. [1,6] for Ni,,P,, alloys are higher than those in Fig. 5 for the simple reason that the DSC measurements in these cases were carried out at much higher heating rates, namely 20 and 40 K min - ‘, respectively. (3) From a comparison of Figs. 2-4, it is obvious that the crystallization process is proceeding differently in the samples produced by melt spinning than in the one produced by electrodeposition, in disagreement with earlier results obtained, however, at another heating rate [2]. From the three comments above and from the results reported in Refs. [1,6] on the influence of the phosphorus content on the crystallization process of electrodeposited and melt-spun Ni-P alloys, it is obvious that the two-stage crystallization observed in NIPDl is not to be attributed to a non-eutectic composition of the sample, although the accuracy in the composition of the alloys is about 1% in both the melt-spun and the electrodeposited samples. (4) After the first step of crystallization, the structure does not apparently change after further heating in the melt-spun samples (see Figs. 2 and 3). The NIPDl sample exhibits a similar behavior for the second steps
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Q [A-II Fig. 3. Measured static structure factors S,,,(Q) for the melt-spun NIP3B sample at temperatures (“C) indicated.
of the crystallization. It could be conjectured that the crystallization process starting around 300°C in the melt-spun samples is a direct transformation according to amorphous
NiP *cry%
Ni + tryst. N&P
Intermediate metastable phases have, however, been reported in several earlier publications to be formed for NiP glasses of different compositions [1,2,8], but if present in our samples, the temperature step was too large (50 K) to detect them. However, no sign of a metastable phase could be detected in preliminary neutron diffraction measurements carried out with shorter temperature intervals (12 K) [S]. Also, the DSC curve in Fig. 5 does not exhibit a double transition feature. The (111) and the (200) diffraction peaks originating from crystalline f.c.c. Ni are clearly visible at Q = 3.015 and 3.579 A-’ in Figs. 2 and 3, while the other peaks for Q < 4 .&- ’ correspond to N&P. A close investigation of Fig. 4 reveals that the crystallization is proceeding as a two-step process in the electrodeposited sample NIPDl. This is most strikingly seen’ in the changes of the two diffraction peaks between 2.5 and 2.7 A-‘. Thus, NIPDl crystallizes into the same structure as the melt-spun ribbons (the bodycentered tetragonal N&P structure) via a metastable phase which exists together with the N&P structure and the Ni(P) solid solution (with an f.c.c. structure) at temperatures from about 300 to about 450°C (261372°C in the DSC measurements, Fig. 5). The temperature corresponding to the first sign of crystallization is close to the one reported in Ref. [2], taking the difference in heating rates into account. However, the
metastable phase has not been observed in partly crystallized (nanocrystalline) samples [9]. It should in this context be mentioned that in electrodeposited samples of NiT5PZ5, etching patterns have been found [lo] which indicated that Ni,P4 crystals grow in the temperature interval 250-350°C. Whether the metastable phase observed in the NIPDl sample has the same composition will be treated in detail in a separate paper. 3.2. The avlzoiphous-to-nnzorpho~istmnsition
At first glance, no particular changes seem to occur in the amorphous state at increasing temperature until the crystallization takes place. A more detailed investigation of the results presented below shows, however, that this is not the case. One of the main aims of this work was to investigate whether the irregularities in the DSC curves in Fig. 5 around 150°C are real and correspond to a structural transformation from one amorphous phase to another one. Preliminary measurements mentioned above on a melt-spun sample of the same ribbon as NIP3A and NIP3B, heated at a rate of 0.07 K min- l, have also indicated an anomaly in the structural results in this temperature range, although the diffraction pattern recorded was still that of an amorphous alloy [8]. Furthermore, DSC measurements carried out at 5 K min-’ in other preliminary investigations [ll] on another sample from the same ribbon confirm the existence of a bump in the temperature interval lOO-150°C similar to the ones observed in Fig. 5. Fig. 6 presents the variation of the coordination number (i.e., the number of nearest neighbors) calculated from the main
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Q
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201
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;&
Fig. 4. Measured static structure factor S,,,(Q) for the electrodeposited NIPDl
peak of the total pair distribution function prior to crystallization, as taken from Ref. [8]. The minimum around 150°C clearly indicates that some kind of phase transition, maybe of martensitic type, occurs in meltspun N&P,,. The value of the coordination number at 20°C (8.6), shown in Fig. 6, compares well with the one obtained earlier (9.3) from isotope substitution measurements [6]. In the measured total static structure factors shown in Figs. 2-4, there are several characteristic features which change with temperature and which support the idea that a phase transition is taking place in the amorphous phase. Some of these features are shown in Figs. 7-9. The presented results have been averaged over the detectors at scattering angles ranging from 20” to 90”. No error bars are shown in these figures but we
(1997)
sample at temperatures (“C) indicated.
let the spread of the points show the uncertainty. Any present systematic error is expected to influence every point in the same way. The position of the main peak of the total structure factor changes with temperature for all three samples, as seen in Fig. 7. The abrupt change at 270°C for the NIP3A sample shows that the crystallization process has started. Assuming a linear temperature dependence between 0 and 2OO”C, which seems reasonable from the overall spread of the experimental data, the extrapolated value for the position at 0°C is 3.127 A for the melt-spun sample and 3.120 A for the electrodeposited
L
J
F
5
8.6 12 10
fs 2_ e” P P
c rlI 2
8.2t"".'."'.'...'...i 150 -2 ’ 0
100
200
300 Temperature
400
/ 500
[c]
Fig. 5. DSC curves for melt-spun (, . .) and electrodeposited (-) NisP19.
175 200 Temperature [C]
225
Fig. 6. The variation of the coordination number, calculated from the main peak of the total pair distribution function, during the crystallization process of another sample taken from the same ribbon as NIP3A and NIP3B.
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100
200 Tem&-ature
Fig. 7. The position of the main peak of S&Q) Notations as in Fig. 1.
for the three samples.
one, indicating that the nearest-neighbor distance is slightly shorter in the melt-spun ribbons. The thermal expansion coefficient in the same temperature range is found to be 0.36 x 10U4 and 0.48 x 10B4 K-‘, respectively. The value for the electrodeposited sample agrees with an earlier determination [12]. It is to be noted that the three curves shown in Fig. 7 have very similar shape, with a small irregularity at 150°C for NIP3A, 12O’C for NIP3B and 140°C for NIPDl. The height of the main peak of S(Q) is, as seen in Fig. 8, very different for the different samples, although the shape of the three curves is similar. When extrapolated to O”C, the melt-spun samples give the same value, 3.58 + 0.08, while for the electrodeposited sample, one obtains 3.19 & 0.06. The onset of the crystallization is different for the three samples and has been discussed above. It is to be noted here again that a hilly region is observed in the three curves. Also, in the
amorphous phase, the different heating rates seem to induce different behavior. The rapid heating up to 11O’C of NIP3A (see Fig. 1) has obviously delayed the small increase in peak height starting the hilly region, which takes place in NIP3B around lOO”C, to a temperature of around 150°C. As a matter of fact, the time it
2.0'
, 0
Fig.
8. Height
x
'
100
of the main
Notationsasin Fig. 1.
200
peak
Temperature
[C]
of S,,,(Q)
for
' 300
the three
3
a
J 400
samples.
300
[c]
Fig. 9. Extrapolated value of S(Q) for Q = 0 A- 1 for the three samples. The horizontal line indicates the calculated theoretical value for nuclear scattering only, i.e., no magnetic scattering component is included. Notations as in Fig. I.
has taken for these two features to occur is almost the same for the two samples. In NIP3B, the slow heating rate results in another feature, a sudden decrease in the peak height, in the temperature range 170-180°C. This second feature is related to the effect shown in Fig. 6 obtained in a preliminary set of measurements performed at a heating rate of 0.07 K min -I [8]. The same kind of variation in peak height is also seen in the electrodeposited sample NIPDI but in the temperature range 160-170°C. As mentioned above, the DSC curves shown in Fig. 5 also exhibit some irregularities in the same temperature region. All these effects indicate that a phase transition from one amorphous state to another amorphous state is occurring in the Ni81P19 system at a temperature of about 160°C. This transition could be connected to a ductile-to-brittle transition of a
martensitic type as proposed in [13] and further investigations are carried out to check this hypothesis. It is in this context appropriate to mention a determination of the elastic properties of electrodeposited Nis2P,* [14], heated at a rate of 1 K min-‘, which supports this conjecture. It was shown in this study that the temperature variation of the shear modulus in the amorphous phase is composed of two effects: the normal reversible, linear decrease with increasing temperature, and an irreversible increase relative to this decrease starting at about 120°C. Another quantity of the measured static structure factor, namely Jhe value obtained from an extrapolation to Q = 0 A-‘, gives information both about the accuracy of the measurement as well as about the existence of a possible magnetic component in S,,,(Q). If only nuclear scattering is present, the value, calculated from cross-section data is 0.335, which level is shown as a horizontal line in Fig. 9. The lowest measured and reliable Q is about 1.4 A-l and the extrapolation
is therefore difficult to make in an unambiguous
way. The results displayed
in Fig.
9
as well as the
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behavior of S,,,(Q) at large Q (not shown here) show that accuracy in the determination of the absolute value of&,,(Q) for the melt-spun samples is very good, of the order of 0.03. There is no reason to believe that the accuracy is worse for the electrodeposited sample, the measurements and the correction of data having been performed by the same team on the same instrument. It has thus to be inferred that the difference in behavior between the two sets of samples is real and that in amorphous NIPDl at least some of the Ni atoms have a magnetic moment which gives rise to a magnetic scattering component at small Q. This has been indicated earlier by annealing an electrodeposited sample of N&PZ5 in a magnetic field at a temperature of 140°C [lo]. As Ni is a ferromagnetic element and amorphous Ni-P is non-magnetic, it is conceivable that the local order is not exactly the same in a melt-spun and a electrodeposited Ni-P glass. This conclusion is in agreement with the difference in the height of the main peak of S(Q) as discussed above.
4. Conclusion
It has been shown that many properties of metallic glasses must be discussed not within a ‘frozen liquid’ concept but within the concept ‘disordered solid’ built of chemical and/or topological units. Metallic glasses produced under different conditions can thus have different structures leading to different procedures of crystallization. Furthermore, it was shown that a structural transformation occurs prior to crystallization around lOO-150°C which could correspond to the ductile-tobrittle transition for this alloy.
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Acknowledgements M.C.-D., U.D. and S.N. are grateful to the Swedish Natural Science Research Council for tiancial support. Thanks are due to Dr W.S. Howells for his support during the LAD measurements.
References [l] Z.F. Dong, Y.H. Ma and K. Lu, Ser. &fall.
Wzter.,
32 (1994)
81.
[2] E. Wachtel, I. Bakonyi, J. Bahle et al., kfafer. Sci. Eng., Al33 (1991) 196. [3] F. Machizaud, F.A. Kuhnast and .I. Flechon, J. Non-Cryst. Solids, 68 (1984) 271. [41 F. Machizaud, T&e d’Etnt, University of Nancy, France, 1973. [51 D.S. Lashmore, L.H. Bennett, H.E. &hone, P. Gustafson and R.E. Watson, Phys. Rev. Lett., 48 (1982) 1760. PI P. Lamparter and S. Steeb, Proc. 5th Int. Conf. Rapidly Quenched IMetals, 1985, p, 459. Sci. Elzg., Al33 r71 S. Budurov, V. Fotty and S. Toncheva, Mater. (1991)
452.
[81 S. Nhien,
Dip&e
France, 1993. 191M. Yamamoto, SOL,
138 (1991)
d’Etudes
Approfondies,
Universiiy of Caen,
K. Shirai and N. Watanabe, J. Electrochem. 2052.
[lOI M. Yamamoto, N. Watanabe, Y. Zhao and B. Ge, Appl.
Phys.,
1111B. Vigneron,
32 (1993) 3586. Dipl&ne d’Etudes
Jpn.
J.
Approfondies, University of Nancy I, France, 1993. WI G. Dietz and R. Sonnberger, Z. Phys. 3, 46 (1982) 213. 1131 M. Calve, Int. J. Rapid Solidzx, 5 (1990) 105. [I41 G. Dietz and W. Heinen, Proc. 5th Int. Con$ Rapidly Quenched Metals,
1985, p. 127.
ELSEVIER
Structural
Materials Science and Engineering A226-228
study of a phase transition
M. Calvo-Dahlborg
A
(1997) 197-203
in a NiP metallic glass
a,*, F. Machizaud b, S. Nhien l@, B. Vigneron b, U. Dahlborg 2+
a Depammt
of Neutmn P&ysics, Royal Institute of Technology, b LSGZMUA 159, Ecole des Mines, Nmcy Cedex,
Sioclcholm, Fmnce
Sweden
Abstract The structure of NiP amorphousalloys has beenthe subject of severalcontroversies,especiallyconcerningthe nature of the short- and medium-rangeorders.It hasalready been reported that thesemetallic glasses can exhibit polymorphism,i.e., different types of local order, dependingupon the production technique. In this study, amorphousribbons of Ni,,P,, produced by two different techniques,melt spinning and electrodeposition,have been investigated. The results obtained from various types of structural analysis have been correlated: differential scanning calorimetry (DE) and neutron diffraction during heating at different ratesuntil completecrystallization. A hilly region in the DSC curve as well as some significant changes in the diffraction pattern could be observed prior to crystallization in both the melt-spun and the electrodepositedsamples.The results are
interpreted and discussed in terms of a phase transition in the material. 0 1997 Elsevier Science S.A. KeywoA: AmorphousNi,,P19alloy; Crystallization; Differential scanning calorimetry; Neutron diffraction; Phase transformation
1. Introduction Amorphization and crystallization kinetics of metallic glasses has been extensively studied for many years and it continues to be a hot topic. The reason is mainly the need to, as accurately as possible, determine the lifetime of these materials and to what extent the physical properties change during nucleation and crystalline growth. The present report is the first in a series dealing with phase transitions and crystallization phenomena in glasses of the NiP, FeB and PdSi systems. The crystallization process of melt-spun NIP metallic glasses has been studied in many works ([1,2] and references therein). It has generally been found that crystallization is strongly depending on the phosphorus content and that the thermal stability of the amorphous alloys increases as the P content approaches the eutectic composition. The crystallization process in NiP glasses produced by electrodeposition has also been studied extensively [2-51 and found to follow the same crystallization scheme as melt-spun NiP glasses [2]. In this * Corresponding author. Current address: LSGZM-UA 159, Ecole des Mines, Nancy Cedex, France. ’ Current address: CRISMAT, ISMRa, Caen Cedex, France. ’ Current address: LSG2M-UA 159, Ecole des Mines, Nancy Cedex, France. 0921-5093]97/$17.00 0 1997 Elsevier Science S.A. All rights reserved. - ___^^_ _ _^^_,^ _._^_._ .
study, we report on some new features seen in the amorphization and crystallization process of NiP glasses of eutectic composition produced by two different techniques, melt-spinning and electrodeposition. The experimental methods employed include differential scanning calorimetry (DSC) and real-time neutron diffraction.
2. Experimental
procedure and results
2.1. Neutron difft*action measurements
The intensity IObs(Q) measured in a neutron diffraction experiment on a binary system can in the static approximation formally be written as 1&Q) = A -S,,,(Q) where A is a coupling constant. S,,,(Q) is given by
&o,(Q) = ~,@a)~ + c&J2 + ~t
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et al. /Materials
Science
atoms of kinds a and ,f?, and Q is the wavevector transfer in the scattering process. For the eutectic composition used here, the following relative Q dependent weighing factors are easily calculated s&Q)
= 0.82OSN+Ji(Q) + O.17osp&Q)
+ 0.001&p(Q) (2)
It is obvious, therefore, that the main features of a measured total structure factor are determined by NiNi correlations and that no information about P-P correlations can be obtained. For that purpose, the isotope substitution technique has to be employed, as extensive measurements on a N&P,,, glass have demonstrated [6]. The neutron diffraction experiments were performed on the LAD diffractometer at the neutron spallation source ISIS at Rutherford Appleton Laboratory, Chilton, UK, while the DSC measurements were carried out at LSG2M in Nancy, France. The metallic glasses studied all had the composition NislP19. The melt-spun ribbons were obtained from Allied Signal while the electrodeposited one was made by F. Machizaud, Nancy [4]. For the neutron diffraction measurements, the ribbons were cut in pieces of length 2-3 mm. The electrodeposited ribbon was taken away from the platinum support in small pieces less than 2 mm long. The pieces were put in a cylindrical vanadium can of an inner diameter of about 6 mm and put inside a furnace. This procedure ensured that all samples can be regarded as isotropic. The samples were heated stepwise in the furnace at somewhat different speeds, as is seen in Fig. 1. For practical purposes, the following notations will be used below to identify the different samples: NIP3A (meit-spun), NIP3B (meltspun), NIPDl (electrodeposited). From Fig. 1, it is seen that the average heating rate is rather low compared with a DSC measurement, of the order of 0.2-0.3 K min- ‘. The time required to obtain 500
i-
--
Time
7
[min]
Fig. 1. Heating rates for the melt-spun samples NIP3A (filled circles), NIP3B (triangles) and for the electrodeposited sample NIPDl (crosses).
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a diffraction pattern with sufficient statistical accuracy from an amorphous sample on LAD was about 1 h and this measurement time was used for the NIP33 and NIPDl samples. The heat treatment and the measuring time was different for the NIP3A sample: fewer steps of 3 h each. It should, though, be noted that even if the ISIS spallation source was very stable during most of the experiments, some measurement times were longer than others because of accelerator problems. However, the main difference between NIP3B and NIPDl has only been a longer stage at 90°C for NIPDl (5 h instead of 1 h), which results in a shift of the two curves in Fig. 1. The heating procedure of these two samples can thus be considered to be very close. The main consequence of these differences can only be a slightly earlier occurrence of the transformations in NIPDI relative to NIP3B as compared with the DSC results. The heating procedure for NIP3A in longer steps was intended to stabilize the structural transformations occurring in the material. The evolvement of S,,,(Q), measured at 90” scattering angle, with temperature for the NIP3A, NIP3B and NIPDI samples are shown in Figs. 2-4, respectively. The differences between the three samples will be discussed below over the whole range of temperature, The shape of the diffraction pattern at this large angle detector and, moreover, the complete absence of any sign of crystallization at the lowest temperatures (20°C in Fig. 2; 40°C in Figs. 3 and 4) clearly indicate that all samples were amorphous. The first sign of crystallization can be observed for the electrodeposited sample NIPDI in the diffraction patterns recorded at 300°C (Fig. 4), at 350°C for the melt-spun NIP3B (Fig. 3), and at 300°C for the meltspun NIP3A (Fig. 2). 2.2. DSC measurements In order to characterize the samples, measurements were performed on a differential scanning calorimeter (SETARAM DSClll). The heating rate was 5 K min-‘. The results shown in Fig. 5 clearly reveal a difference between the melt-spun and the electrodeposited samples. The melt-spun sample exhibits only one peak (starting at about 300°C and located at 338°C) originating from the crystallization process, while the electrodeposited one shows two transitions (starting at about 261 and 372°C and located at 282 and 415’C, respectively). It has to be concluded, therefore, that the crystallization process in the two samples are taking place along different routes. Furthermore, as will be shown in more detail in the discussion below, the two-step crystallization of the electrodeposited sample is not to be attributed to an excess or lack of phosphorus in the composition of the alloy similar to what is reported in Refs. [1,2,7]. The first part of the curve
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Fig. 2. Measured static structure factors S,,,(Q) for the melt-spun NIP3A sample at temperatures (“C) indicated.
prior to crystallization is similar for the two samples: a first hilly region, until 155°C for the electrodeposited sample and 180°C for the melt-spun one, and a flat region afterwards up to the first crystallization peak.
3. Discussion 3.1. The nmorpholrs-to-crystal tsnnsition
From the results reported above and from the careful examination of Figs. 2-5, several comments can be made: (1) The final stage of the crystallization is the same for all the samples. The diffraction patterns at 500°C for NIPDl and NIP3B, and at 400°C for NIP3A are identical. The peak in the DSC curve of the melt-spun samples consequently corresponds to the second peak of the DSC curve of the electrodeposited one (see Fig. 5). (2) Differences between the temperature corresponding to the tist sign of crystallization can be observed for the three samples in the DSC and the neutron diffraction measurements: 308°C in DSC and 250300°C in neutron diffraction for NIP3A, 308°C and 300-350°C for NIP3B, 261°C and 270-300°C for NIPDl, respectively. This can be explained as follows. (i) The temperature intervals in Fig. 2 (250-3OO”C), Fig. 3 (300-350°C) and Fig. 4 (270-300°C) do not allow a precise determination of the temperature of first crystallization. In a preliminary set of measurements performed on the SLAD diffractometer at Studsvik, Sweden, on another melt-spun sample taken from the same ribbon, heated at a rate of 0.07 K min-’ in
shorter temperature intervals, the first crystallization could be detected between 293 and 305°C [S]. (ii) The heating procedure used for the neutron diffraction experiment has been carried out in long steps (1 h for NIP3B and NIPDl and 3 h for NIP3A), while the DSC heating is continuous. This has resulted in a better stabilization of the structural transformations in the neutron diffraction experiments, thus allowing an earlier initiation of crystallization. This is particularly obvious for NIP3A. Furthermore, the crystallization temperatures reported in Refs. [1,6] for Ni,,P,, alloys are higher than those in Fig. 5 for the simple reason that the DSC measurements in these cases were carried out at much higher heating rates, namely 20 and 40 K min - ‘, respectively. (3) From a comparison of Figs. 2-4, it is obvious that the crystallization process is proceeding differently in the samples produced by melt spinning than in the one produced by electrodeposition, in disagreement with earlier results obtained, however, at another heating rate [2]. From the three comments above and from the results reported in Refs. [1,6] on the influence of the phosphorus content on the crystallization process of electrodeposited and melt-spun Ni-P alloys, it is obvious that the two-stage crystallization observed in NIPDl is not to be attributed to a non-eutectic composition of the sample, although the accuracy in the composition of the alloys is about 1% in both the melt-spun and the electrodeposited samples. (4) After the first step of crystallization, the structure does not apparently change after further heating in the melt-spun samples (see Figs. 2 and 3). The NIPDl sample exhibits a similar behavior for the second steps
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Q [A-II Fig. 3. Measured static structure factors S,,,(Q) for the melt-spun NIP3B sample at temperatures (“C) indicated.
of the crystallization. It could be conjectured that the crystallization process starting around 300°C in the melt-spun samples is a direct transformation according to amorphous
NiP *cry%
Ni + tryst. N&P
Intermediate metastable phases have, however, been reported in several earlier publications to be formed for NiP glasses of different compositions [1,2,8], but if present in our samples, the temperature step was too large (50 K) to detect them. However, no sign of a metastable phase could be detected in preliminary neutron diffraction measurements carried out with shorter temperature intervals (12 K) [S]. Also, the DSC curve in Fig. 5 does not exhibit a double transition feature. The (111) and the (200) diffraction peaks originating from crystalline f.c.c. Ni are clearly visible at Q = 3.015 and 3.579 A-’ in Figs. 2 and 3, while the other peaks for Q < 4 .&- ’ correspond to N&P. A close investigation of Fig. 4 reveals that the crystallization is proceeding as a two-step process in the electrodeposited sample NIPDl. This is most strikingly seen’ in the changes of the two diffraction peaks between 2.5 and 2.7 A-‘. Thus, NIPDl crystallizes into the same structure as the melt-spun ribbons (the bodycentered tetragonal N&P structure) via a metastable phase which exists together with the N&P structure and the Ni(P) solid solution (with an f.c.c. structure) at temperatures from about 300 to about 450°C (261372°C in the DSC measurements, Fig. 5). The temperature corresponding to the first sign of crystallization is close to the one reported in Ref. [2], taking the difference in heating rates into account. However, the
metastable phase has not been observed in partly crystallized (nanocrystalline) samples [9]. It should in this context be mentioned that in electrodeposited samples of NiT5PZ5, etching patterns have been found [lo] which indicated that Ni,P4 crystals grow in the temperature interval 250-350°C. Whether the metastable phase observed in the NIPDl sample has the same composition will be treated in detail in a separate paper. 3.2. The avlzoiphous-to-nnzorpho~istmnsition
At first glance, no particular changes seem to occur in the amorphous state at increasing temperature until the crystallization takes place. A more detailed investigation of the results presented below shows, however, that this is not the case. One of the main aims of this work was to investigate whether the irregularities in the DSC curves in Fig. 5 around 150°C are real and correspond to a structural transformation from one amorphous phase to another one. Preliminary measurements mentioned above on a melt-spun sample of the same ribbon as NIP3A and NIP3B, heated at a rate of 0.07 K min- l, have also indicated an anomaly in the structural results in this temperature range, although the diffraction pattern recorded was still that of an amorphous alloy [8]. Furthermore, DSC measurements carried out at 5 K min-’ in other preliminary investigations [ll] on another sample from the same ribbon confirm the existence of a bump in the temperature interval lOO-150°C similar to the ones observed in Fig. 5. Fig. 6 presents the variation of the coordination number (i.e., the number of nearest neighbors) calculated from the main
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Fig. 4. Measured static structure factor S,,,(Q) for the electrodeposited NIPDl
peak of the total pair distribution function prior to crystallization, as taken from Ref. [8]. The minimum around 150°C clearly indicates that some kind of phase transition, maybe of martensitic type, occurs in meltspun N&P,,. The value of the coordination number at 20°C (8.6), shown in Fig. 6, compares well with the one obtained earlier (9.3) from isotope substitution measurements [6]. In the measured total static structure factors shown in Figs. 2-4, there are several characteristic features which change with temperature and which support the idea that a phase transition is taking place in the amorphous phase. Some of these features are shown in Figs. 7-9. The presented results have been averaged over the detectors at scattering angles ranging from 20” to 90”. No error bars are shown in these figures but we
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sample at temperatures (“C) indicated.
let the spread of the points show the uncertainty. Any present systematic error is expected to influence every point in the same way. The position of the main peak of the total structure factor changes with temperature for all three samples, as seen in Fig. 7. The abrupt change at 270°C for the NIP3A sample shows that the crystallization process has started. Assuming a linear temperature dependence between 0 and 2OO”C, which seems reasonable from the overall spread of the experimental data, the extrapolated value for the position at 0°C is 3.127 A for the melt-spun sample and 3.120 A for the electrodeposited
L
J
F
5
8.6 12 10
fs 2_ e” P P
c rlI 2
8.2t"".'."'.'...'...i 150 -2 ’ 0
100
200
300 Temperature
400
/ 500
[c]
Fig. 5. DSC curves for melt-spun (, . .) and electrodeposited (-) NisP19.
175 200 Temperature [C]
225
Fig. 6. The variation of the coordination number, calculated from the main peak of the total pair distribution function, during the crystallization process of another sample taken from the same ribbon as NIP3A and NIP3B.
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100
200 Tem&-ature
Fig. 7. The position of the main peak of S&Q) Notations as in Fig. 1.
for the three samples.
one, indicating that the nearest-neighbor distance is slightly shorter in the melt-spun ribbons. The thermal expansion coefficient in the same temperature range is found to be 0.36 x 10U4 and 0.48 x 10B4 K-‘, respectively. The value for the electrodeposited sample agrees with an earlier determination [12]. It is to be noted that the three curves shown in Fig. 7 have very similar shape, with a small irregularity at 150°C for NIP3A, 12O’C for NIP3B and 140°C for NIPDl. The height of the main peak of S(Q) is, as seen in Fig. 8, very different for the different samples, although the shape of the three curves is similar. When extrapolated to O”C, the melt-spun samples give the same value, 3.58 + 0.08, while for the electrodeposited sample, one obtains 3.19 & 0.06. The onset of the crystallization is different for the three samples and has been discussed above. It is to be noted here again that a hilly region is observed in the three curves. Also, in the
amorphous phase, the different heating rates seem to induce different behavior. The rapid heating up to 11O’C of NIP3A (see Fig. 1) has obviously delayed the small increase in peak height starting the hilly region, which takes place in NIP3B around lOO”C, to a temperature of around 150°C. As a matter of fact, the time it
2.0'
, 0
Fig.
8. Height
x
'
100
of the main
Notationsasin Fig. 1.
200
peak
Temperature
[C]
of S,,,(Q)
for
' 300
the three
3
a
J 400
samples.
300
[c]
Fig. 9. Extrapolated value of S(Q) for Q = 0 A- 1 for the three samples. The horizontal line indicates the calculated theoretical value for nuclear scattering only, i.e., no magnetic scattering component is included. Notations as in Fig. I.
has taken for these two features to occur is almost the same for the two samples. In NIP3B, the slow heating rate results in another feature, a sudden decrease in the peak height, in the temperature range 170-180°C. This second feature is related to the effect shown in Fig. 6 obtained in a preliminary set of measurements performed at a heating rate of 0.07 K min -I [8]. The same kind of variation in peak height is also seen in the electrodeposited sample NIPDI but in the temperature range 160-170°C. As mentioned above, the DSC curves shown in Fig. 5 also exhibit some irregularities in the same temperature region. All these effects indicate that a phase transition from one amorphous state to another amorphous state is occurring in the Ni81P19 system at a temperature of about 160°C. This transition could be connected to a ductile-to-brittle transition of a
martensitic type as proposed in [13] and further investigations are carried out to check this hypothesis. It is in this context appropriate to mention a determination of the elastic properties of electrodeposited Nis2P,* [14], heated at a rate of 1 K min-‘, which supports this conjecture. It was shown in this study that the temperature variation of the shear modulus in the amorphous phase is composed of two effects: the normal reversible, linear decrease with increasing temperature, and an irreversible increase relative to this decrease starting at about 120°C. Another quantity of the measured static structure factor, namely Jhe value obtained from an extrapolation to Q = 0 A-‘, gives information both about the accuracy of the measurement as well as about the existence of a possible magnetic component in S,,,(Q). If only nuclear scattering is present, the value, calculated from cross-section data is 0.335, which level is shown as a horizontal line in Fig. 9. The lowest measured and reliable Q is about 1.4 A-l and the extrapolation
is therefore difficult to make in an unambiguous
way. The results displayed
in Fig.
9
as well as the
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behavior of S,,,(Q) at large Q (not shown here) show that accuracy in the determination of the absolute value of&,,(Q) for the melt-spun samples is very good, of the order of 0.03. There is no reason to believe that the accuracy is worse for the electrodeposited sample, the measurements and the correction of data having been performed by the same team on the same instrument. It has thus to be inferred that the difference in behavior between the two sets of samples is real and that in amorphous NIPDl at least some of the Ni atoms have a magnetic moment which gives rise to a magnetic scattering component at small Q. This has been indicated earlier by annealing an electrodeposited sample of N&PZ5 in a magnetic field at a temperature of 140°C [lo]. As Ni is a ferromagnetic element and amorphous Ni-P is non-magnetic, it is conceivable that the local order is not exactly the same in a melt-spun and a electrodeposited Ni-P glass. This conclusion is in agreement with the difference in the height of the main peak of S(Q) as discussed above.
4. Conclusion
It has been shown that many properties of metallic glasses must be discussed not within a ‘frozen liquid’ concept but within the concept ‘disordered solid’ built of chemical and/or topological units. Metallic glasses produced under different conditions can thus have different structures leading to different procedures of crystallization. Furthermore, it was shown that a structural transformation occurs prior to crystallization around lOO-150°C which could correspond to the ductile-tobrittle transition for this alloy.
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Acknowledgements M.C.-D., U.D. and S.N. are grateful to the Swedish Natural Science Research Council for tiancial support. Thanks are due to Dr W.S. Howells for his support during the LAD measurements.
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