influence of quenching rate on the curie temperature, resistivity, internal friction and structure...

Volume 7, number 7,8 MATERIALS LETTERS December I988

INFLUENCE OF QUENCHING BATE ON THE CURIE TEMPERATURE, RESISTMTY, INTERNAL FRICTION AND STRUCTURE OF Fe-BASED AMORPHOUS ALLOYS

D.X. PANG, J.T. WANG, A.Q. HE and B.Z. DING Institute ofMetal Research, Academia Sinica. Shenyang 110015, China

Received 22 August 1988

The effects of the quenching rate on the Curie temperature, resistivity, internal friction and structure of Fe-based amorphous alloys were investigated. The results indicate that the quenching rate affects the microstructure (atomic arrangement), internal friction behavior and magnetic properties of Fe-based amorphous alloys, and that the Curie temperature is related to certain relaxation characteristics. It has been found that the internal friction behavior for as-quenched samples produced at different quenching rates below the glass transition temperature TS is very different, but the Q-’ curves approach each other in the region of the amorphous-crystalline transition.

1. Introduction

The factors which determine the structures and properties of as-quenched amorphous alloys are ba- sically the chemical composition (alloying effect), atomic arrangement (short-range order or long-range disorder) and metastable character (structural relaxation). As soon as the chemical composition is fixed, the atomic arrangement and metastable character are mainly controlled by the processing conditions. A large number of studies have been reported on the effects of processing conditions on the structures and properties of amorphous alloys produced by melt spinning techniques [ l-41. We previously [ 51 reported that the quenching rate strongly affects the microstructures (such as chemical short-range order and topological short-range order), mechanical properties (such as microhardness, Young’s modulus, fatigue fracture behavior) and thermal behavior (such as crystallization enthalpy and crystallization temperature). A further research [ 6 ] on the structure and crystallization process of amorphous selenium illustrated that the topological short-range order and the metastructures formed during the crystallization processes are closely related to the quenching rate. Posgay and co-workers [ 71 studied the effect of cooling conditions on the internal fiic- tion of a multicomponent metallic glass and discov-

ered the starting temperature of structure relaxation r, depends on the strip-wheel contact time f,, and above T, the internal friction Q-’ obviously in- creases with the increase in temperature. Grossinger et al. [ 81 researched the influence of cooling rate on the magnetic and elastic properties of Fe-based amorphous alloys. Up to date, it has been progres- sively realized that the processing parameters determine not only the geometrical, mechanical and thermal properties of as-quenched amorphous alloys, but also the magnetic and electrical properties, and among the processing parameters, the quenching rate is of great importance. In the present work, the effects of quenching rate on the Curie temperature, resistivity, internal friction and microstructure of Fe-based amorphous alloys as well as a Co-based one are reported.

2. Experimental procedures

The samples, amorphous Fe79Ni4M05SiZB i. and Co66Fe4V2Si8B20, are both prepared by the melt spinning technique. At linear speeds of the wheel surface from 17.0 to 41.5 m s-‘, the molten alloys are ejected by argon at a pressure of 140 kPa to form ribbons of about 2 mm wide and 20-50 pm thick- ness; the ribbons are determined to be amorphous by

0167-577x/88/% 03.50 0 Elsevier Science Publishers B.V. ( North-Holland Physics Publishing Division )

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Volume 7, number 7,8 MATERIALS LETTERS December 198 8

X-ray diffraction. The jet casting temperature is about 100°C higher than the melt temperature for each of the alloys. The thermal analysis experiments are carried out in a DSC-II differential scanning cal- orimeter. High-resolution electron microscopy is used to study the microstructures of as-quenched alloys produced at different quenching rates. Internal friction Q-’ measurements are performed with an in- verted torsion pendulum, a small tensile stress ( < 1 MPa) is applied to the samples. The measurements are conducted in a vacuum of about 5 x 1 0e5 Torr and at a heating rate of 5 ’ Cfmin. The as-quenched sample is scanned from room temperature to the crystallization temperature, the resistances are measured by the usual four-probe dc technique in a sample-holder at a heating rate of 5’ C/min in a vacuum of about 1 x 10e2 Tot-r. An AGA Thermovision 780 system is used to measure temperatures of the top surface of the melt spun ribbons versus the ribbon- wheel surface contact time. A filtered indium-anti- monide detector cooled by liquid nitrogen is chosen for the peak quantum efficiency and has a scanning rate of twenty-five fields per second. The thermal sensitivity is 0.1 ‘C at 30’ C. The color monitor pro- vides the quantitative images in selectable colors which correspond to the discrete thermal levels, The Curie temperatures are measured on an M300 magnetic balance with a vacuum of 1 x 10d5 Torr.

3. Results and discussion

The ribbon-wheel surface contact time dependence of the ribbon top-surface temperature for amorphous Fe-Si-B alloy prepared at a wheel surface linear speed of 4 1.7 m s- ‘, and the relationship between quenching rate and wheel surface linear speed are shown in fig. 1. The slope of line C is used to determine the maximum quenching rate related to a certain wheel speed. In the case of the Fe-Si-B alloy, the quenching rates measured by infrared thermal photography are from 1.6~ lo6 to 2.4~ 10’ “C s-’ corresponding to wheel speeds 22.6 to 41.7 m s-l. The quenching rate rises sharply with an increase in the wheel speed (fig. 1) which results in considerable changes in the structures and properties of amorphous alloys.

High-resolution electron microscopy (JEM-

264

N m/set 30

3.0 6.0 9.0 t set, x1Li4

Fig. 1. (a) Ribbon top-surface temperature versus ribbon-wheel surface contact time during the forming process for the Fe-Si-B amorphous alloy prepared at a wheel surface linear speed of 4 1.7 m s-l. (b) Relationship between the quenching rate and the wheel surface linear speed.

200CX) is employed to investigate the effect of quenching rate on the atomic distribution. The mea- surement is performed with a point resolution of 0.25 nm and a coefficient of spheric aberration of 1.2 mm. The HREM images and electron diffraction patterns of as-quenched amorphous Fe-Si-B ribbons are shown in fig. 2. Both electron diffraction patterns of the samples show no obvious evidence of crystallization by the broad halos without sharp reflections. However, the electron diffraction shown in fig. 2d is sharper as compared with that shown in fig. 2b. From micrography (fig. 2a), the bright areas from about 1.0 to 1.6 nm in diameter are seen, which are pre- sumed to arise from ordered regions or so-called small “packets” of lattice fringes in the amorphous samples. These regions with spacing about 0.6 nm con- sist of ordered fringes [ 9 ] which represent the spatial distribution of atomic planes. Around these ordered areas, there exist regions in which the atoms are ar- ranged at random, on the whole. HREM image (fig. 2c) indicates that both the bright and dark regions are “speckled” by bright fringes which are straight and parallel in each direction with spacing about 0.2 nm. In this case, a description in terms of micro- crystallites can therefore be suggested at least to some extent. According to the results, we can conclude that the quenching rate strongly affects the atomic distribution, an increased quenching rate can result in

Volume 7, number 7,8 MATERIALS LETTERS December 1988

Fig. 2. HREM (high-resolution electron microscopy) images and

electron diffraction patterns of as-quenched Fe-Si-B ribbons, re-

corded at an accelerating voltage of 200 kV. (a) HREM image of

as-quenched Fe-Si-B ribbon, the wheel surface linear speed is

37.2 m s- I, (b) Electron diffraction of (a). (c) HREM image of

as-quenched Fe-Si-B ribbon, the wheel surface linear speed is 17.0 m s-‘. (d) Electron diffraction of (c).

an increased disorder of the atom arrangement in

amorphous materials. The internal friction Q-’ is calculated from the

equation

Q-‘=(llnn)ln(M&),

where n is the number of oscillation; A0 and A, are

the amplitudes of the first and the nth oscillations, respectively.

The temperature dependence of the internal friction is presented in fig. 3 for amorphous Fe-Si-B samples quenched at wheel surface linear speeds from 22.6 to 41.7 m s-‘. As is shown in the figure, in the Q- ’ curves, three internal friction peaks are observed: a low-temperature peak at about 360 K, a medium-temperature peak at about 620 K and a high- temperature peak at about 780 K. The minimum in Q - ’ at high temperatures, about 800 K, indicates the

400 600 600 750 800 850 T K

Fig. 3. (A) Temperature dependence of the internal friction Q-’

for amorphous Fe-Si-B alloys. The heating rate is 5°C min-‘.

(B) DSC traces of amorphous Fe-Si-B samples. The DSC scan-

ning rate is 10°C min-‘. Curves (a), (b), (c) and (d) corre-

spond to wheel surface linear speeds of 22.6. 26.2, 3 1.7 and 4 1.7

m s-l, respectively.

onset of crystallization. The internal friction behavior for the samples below the glass transition tem-

perature Tg (about 770 K) is very different, but the Q-’ curves approach each other in the region of the amorphous-crystalline transition.

Internal friction has been used to obtain infor- mation related to lattice defects and their motion in crystalline materials; however, in the case of amorphous materials, an internal friction peak appears in a plot of internal friction versus temperature where

a relaxation process is concentrated [ lo]. From fig. 3, we can see that there exists only one crystallization peak at about 8 10 K in each of the DSC traces

for the samples, therefore the crystallization process for the Fe-Si-B amorphous alloys are responsible for the high-temperature peak in the internal friction. As

is known, above the glass transition temperature T,, the glass is in an internal equilibrium state, and relaxation is associated with a cooperative diffusive re-

laxation process following the low-temperature relaxation process below T,. Although the differ- ences in the internal friction behavior of the samples below T, are pronounced, above T, the Q- ’ curves run together due to the relaxation process below Tg which may reduce the difference in the structure (such as the stress field, defects, short-range order, atomic disposition, etc. ).

265


In each of the Q - ’ curves, there is a small peak at about 360 K, the peaks are weakened gradually with increasing quenching rate (fig. 3). This fact indicates the relative stability of the structural state for the high quenching rate. Accordingly, it is assumed that defects [ 111 and the internal stress [ 121 intro- duced by quenching during the preparation of amorphous alloys affect the internal friction behavior at low temperature. The difference in the internal fiic- tion behavior at low temperature among the differ- ently quenched samples does not allow us to conclude that the atomic environment is drastically changed; it is reasonably proposed that the low-temperature behavior is mainly due to the internal stress relief and the development or polymerization of the defects [ 13,141.

The plots of AR/R versus temperature Tare presented in fig. 4, and agree with the low-temperature Q-i curve.

The medium-temperature internal friction peaks related to sub-T, relaxation processes occur at about 620 K. It is clear that the quenching rate has an obvious effect on this relaxation process, the related internal friction behavior being strongly influenced by the quenching condition. Based on the extended free- volume model, Chen and Morito [ 15 ] gave an account of the observed sub-T, relaxation: ( 1) internal friction arises from relaxation within a localized structural entity associated with the local free volume; (2) there exists in a glass a fluctuation in the local free volume v with the most probable distribution [ 161,

370 lr70

T K

Fig. 4. Plots of (R-Ro) /R versus temperature T, R. is the resistivity at 25”C, the heating rate is 5°C mitt-‘. Curves (a), (b), (c), (d) and (e) correspond to wheel surface linear speeds of 41.7,26.2, 31.7,22.6and37.2mse’,respectively.

where vf is the average free volume. In accordance with this, the sub-T, internal friction results from a redistribution in the local free volume which is quenched in during the forming process of amorphous alloys.

On the other hand, let us consider the internal stress distribution theory proposed by Smith and co- workers. This theory predicts that internal friction is caused by the irreversible jumps of 90” domain walls opposed by the local internal barriers. It has been used for the description of internal friction in Co- based amorphous alloys. Based on this model, the interaction of local stresses with magnetic domains can be predicted, which should affect not only the internal friction behavior but also the magnetic properties.

As pointed out by Gubanov [ 17 1, the effect of structural disorder on the Curie temperature can be attributed to two basic ingredients, first the spatial dependence of the exchange integral between magnetic atoms, second the pair radial distribution func- tion about a given atom, which means that the Curie temperature depends on the disposition of the atom structure and the exchange interaction for the most part between neighbouring atoms. The Curie temperatures for both Fe-.%-B and Co-Fe-Si-B amorphous alloys versus wheel surface linear speeds are shown in fig. 5. It can be seen that the T, is less sen- sitive to structural disorder in Co-based than in Fe- based amorphous alloys which can be interpreted in terms of electronic band structure. It is interesting to note that the Curie temperature T, is related to the sub-T, internal friction behavior for the Fe-based amorphous alloy, the onset of the medium-temper-

615- Y

,’

= 595 -

a-___ ---__y

<

575

t 9__~___A--‘~

I I

20 30 40

N mlsec

Fig. 5. Relationship between the Curie temperature Tc and the wheel surface linear speed N. The heating rate is 5°C min-‘. (a) Amorphous Fe-%-B alloy, (b) amorphous Co-Fe-Si-B alloy.

266


ature internal friction peak corresponds to the amorphous Curie temperature. A high Curie temperature corresponds to an intense internal friction process. The T, has a maximum when the wheel speed is 37.2 m s-‘, this indicates that the relaxation process at about 620 K enhances the fe~omagnetic exchange between iron atoms which may be caused by the an- nihilation of the local free volume and the irreversible jumps of 90” domain walls mentioned before. It may be suggested that the effects of the quenching rate on the chemical and topological short-range or- ders result in a specific disposition of the atoms and exchange energy between nei~~u~ng atoms, which correspond to certain relaxation characteristic, internal friction behavior and magnetic property.

The explanation of the Tc curve (fig. 5a) is not well understood so far, but it may be attributed to the effect of the relaxation process. An increase in wheel speed from 30 to 37.2 m s-’ results in an increase in “local free volume” and the quenching in of more “local free volume” should make the sub-T, relaxation process more intensive. A further increase in the quenching rate can strongly stabilize the structure of as-quenched amorphous material and weaken the sub-T, relaxation process on account of the sat- uration of the “local free volume” quenched in and the variation of the internal stress. As a result, the Curie temperature T, decreases. Below a wheel speed of 30 m s- ‘, a decrease in quenching rate results in a decrease in the topological disorder and an increase in pre-existing nuclei or even the occurrence of microcrystallinity, which is possibly responsible for the increase in the value of T,.

Acknowledgement

The authors would like to thank Professor G.P.

Shui for his help during the internal friction experiments.

References

[I J H.H. Liebermann and CD. Graham, IEEE Trans. Magn. MAG 12 (1976) 921.

[2] S. Takayama and T. Oi, J. Appl. Phys. 50 ( 1979) 1595. [ 31 A.L. Greer, Acta Metall. 30 ( 1982 ) 17 1, [4] U. Koster, Scripta Metall. 17 (1983) 867. [ 5 ] J.T. Wang, D.X. Pang and B.Z. Ding, in: Proceedings of the

6th International Conference on Rapidly Quenched Metals, Montreal, Canada ( 1987), to be published.

[ 6 ] D.X. Pang, J.T. Wang and B.Z. Ding, J. Phys. F, submitted for publication.

[7] G. Posgay, F.J. Kedves, B. Albert, S. Kiss and I.Z. Harangozo, in: Proceedings of the 5th International Conference on Rapidly Quenched Metals, Vol. 1, eds. S. Steeb and H. Warlimont (Wurzburg, Germany, 1984) p. 723.

[ 8) R. Grossinger, H. Sassik, Ch. Schotzko and A. Lovas, in: Proceedings of the 5th Intemational Conference on Rapidly Quenched Metals, Vol. 2, eds. S. Steeb and H. Warlimont (Wurzburg, Germany, 1984) p. 1255.

[ 91 P.H. Gaskell, L.A. Freeman and D.J. Smith, in: Proceedings of the 4th International Conference on Rapidly Quenched Metals ( 198 1) p. 439.

[IO] S. Suzuki, M. Takahashi and H. Sakamura, J. Non-Cry& Solids 85 ( 1986) 375.

[ 1 I ] D. Srolovitz, K. Maeda, V. Vitek and T. Egami, Phil Mag. 44(1981)847.

[ 121 G.S. Smithand J.R. Birchak, J. Appt. Phys. 40 (1969) 5174. [ 131 Ii. Kronmuller, J. Appl. Phys. 52 (1981) 1859. [ 141 C.H. Bennett, P. Chaudhari, V. Moruzzi and P. Steinhardt,

Phil. Mag. A 40 (1977) 48.5. [ 151 H.S. Chen and N. Morito, in: Proceedings of the 5th

International Conference on Rapidly Quenched Metals, Vol. 1, eds. S. Steeb and H. Warlimont (Wurzburg, Germany, 1984) p. 715.

[ 161 M.H. Cohen and D. Turnbull, J. Chem. Phys. 3 I (1959) 1164.

]I 7 J A.1. Gubanov, Soviet Phys. Solid State 2 ( 1960) 468.

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influence of quenching rate on the curie temperature, resistivity, internal friction and structure...

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