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Deformation of amorphous metals
Materials Science and Engineering, 25 (1976) 71 - 75
71
© Elsevier Sequoia S.A., Lausanne -- Printed in the Netherlands
Deformation of Amorphous Metals
T. MASUMOTO and T. MURATA The Research Institute for Iron, Steel and Other Metals, Tohoku University, Sendai (Japan)
I. INTRODUCTION
Recently, amorphous metals (or metallic glasses) have generated great interest as a new class of solids, and the study of their properties has become an independent field in itself in Materials Science [1, 2]. Because of the disordered nature of the atomic arrangements, their properties must be inherently different from those of crystalline metals which are in the long range ordered state of constituent atoms. The mechanical properties of these amorphous metals seem to be most interesting; they exhibit higher strength as well as degrees of ductility and toughness. The discovery of these unexpected characteristics fascinates materials scientists because inorganic glasses such as oxides and chalcogens are, in general, extremely brittle. It has been known, empirically, that some amorphous metals can be deformed severely, w i t h o u t cracking, by cold-rolling. The first direct observation of plastic flow was made independently by Masumoto and Maddin [ 3 ] and by Leamy, Chen and Wang [4]. They observed distinct and relatively sharp shear-step markings on the surfaces of bent specimens. Since then, various studies of the deformation behavior, and several suggestions for the deformation mechanism have been made for amorphous metals. In this paper we discuss "What we do n o t know about deformation of amorphous metals".
2. EXPERIMENTAL EVIDENCE OBTAINED SO FAR AND SOME PROBLEMS TO BE CLARIFIED Before discussing the problem, let us ask ourselves "What do we know about the deformation of amorphous metals?". Fortunately, most of the results obtained so far have been summarized in several reviews [5 - 8]. Accord-
ing to these articles, sufficient knowledge has not been accumulated to understand fully the deformation of these solids in spite of the many efforts. Some reliable phenomena obtained by experiments on this problem are as follows: Elastic and anelastic d e f o r m a t i o n
The shear and Young's moduli are about 30% less than those in the fully crystallized state. By contrast, the bulk modulus changes relatively little by crystallization. Amorphous metals also exhibit a large temperature dependence of the elastic properties as compared with the same crystalline metals. However, the experimental values of these moduli are not well defined for either the amorphous or the crystalline state. Most of the measured values of the elastic moduli seem to involve considerable experimental error because the specimens used are neither large enough, nor of a regular form to obtain accurate values. To measure accurately the absolute values of these moduli in the amorphous state, and also to compare them with those in the crystalline state are important problems to be considered. To study these problems a new experimental m e t h o d should be developed in order to measure these values in amorphous samples having thicknesses less than several microns. The values are considered to be essential not only for the estimation of the theoretical strength, but also for consideration of the anharmonic effect in the amorphous state if the temperature dependence of these moduli and thermal expansion coefficients are known. There are several reports on the anelasticity of amorphous metals. The internal friction is usually larger in the amorphous state than in the crystalline state. It increases abruptly as a function of temperature and exhibits a high value around the crystallization temperature; this behavior seems to be related to a large
72
creep recovery after removal of the applied stress at higher temperatures. Besides the thermoelastic damping, several relaxation peaks of unknown origin are observed.
General characters of plastic deformation At temperatures well below the glass transition temperature, Tg, amorphous metals are ductile on a microscopic scale, while macroscopically they behave in a brittle manner; that is, slip deformation occurs which is limited to a localized shear band in the cases of uniaxial tension and compression. At temperatures around Tg, plastic deformation occurs homogeneously by viscous-like flow. As one important aspect of amorphous metals, it is found in several experiments that strain hardening is negligibly small, even though it shows apparently either softening or hardening. Since an ideal plastic material where no strain hardening occurs would, in principle, become unstable in tension and begin to neck as soon as yielding took place, this fact implies that amorphous metals can be considered as elastic, perfectly plastic solids. In other words, they can be used as materials for examining the mechanics of the ideal solid. Temperature and strain rate effects on plastic deformation Mode of deformation The mode of deformation transfers apparently from the inhomogeneous type to the more homogeneous type around a certain temperature. This critical temperature seems to be lower (about 100 - 150 °C) than Tg depending on the strain rate. In Fig. 1, the temperature- and strain rate-dependence of the strength and mode of deformation is shown [9] (the broken line indicates an approximate boundary between inhomogeneous and homogeneous modes in deformation). In the two regions delineated by this boundary there are distinct differences in the strain rate dependence of strength as well as the mode of deformation. In the homogeneous region, the strength decreases as the strain rate increases, while in the inhomogeneous region its dependence is small or opposite. The temperature dependence of the strength changes at this boundary. The strength is less sensitive to temperature in the inhomogeneous region, whereas a strong temperature dependence exists in the
15C
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o
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•
4--4
200)~
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"
"
0 "' E
80
HOMOGENEOUS
~
/
~ 6o
.
:.,:',
•
¢t: \\
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. M~,~u. STRESs ........
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i I 0t
,
, i IIIlll
,
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Fig. 1. Effect of strain rate on the tensile strength at various temperatures.
homogeneous region. These experimental facts imply that the flow can be controlled by an athermal mechanism at lower temperatures and by a thermally-activated one at higher temperatures. Such a difference in the mechanism between these two kinds of flow is supported by experimental results on the structural change during plastic deformation; briefly, the inhomogeneous deformation causes a disordering in the amorphous state [5, 10, 11], while the homogeneous deformation promotes atomic ordering [5, 9]. Figure 2 shows a schematic representation of the temperature dependences of the stress in both the homogeneous and the inhomogeneous states (this diagram is constructed from the previous arguments for the plastic deformation of amorphous metals). Here, the stress is a value necessary to induce continuous deformation at a constant strain rate, and Tp is
0
Tp
TEMPERATURE
Fig. 2. Schematic diagram showivg the variation of the mode of deformation with temperature. The vertical axis denotes the stress which is necessary to deform amorphous metals at a constant rate. If the testing temperature is higher than Tp, homogeneous deformation takes place because the stress is smaller than that for inhomogeneous deformation.
73
a critical temperature at which the stresses for the two modes of deformation coincide. This critical temperature, T~, resembles the so-called equicohesion temperature which has been proposed in the phenomenological study of the fracture of crystalline metals [ 1 2 ] . In order to obtain a better understanding of the critical temperature, a comparison of the resistance to inhomogeneous and homogeneous deformation is important, and the variation with strain rate should be clarified.
I PdsoS]20
~
20C
O1130 ...I
0
0
5
I0
ELONGATION (pro)
Inhomogeneous deformation Quantitative data on the flow characteristics of inhomogeneous deformation are meagre because of its occurrence within a limited, localized shear band. There are, however, qualitative data from compression tests carried o u t under more complex conditions of stress [13, 14]. Generally, the measurement of apparent yield- and fracture stresses shows that these values are higher, usually, than those of the major element in the crystalline state. Theoretical considerations of the strengths of amorphous metals has been made based on a dislocation model [15, 16] and the theoretical strength is estimated to be 0.03 - 0.05 of the Young's modulus. However, because of experimental difficulties, there have been no exact values of "critical resolved shear stress" (CRSS) of amorphous metals for comparison with the theoretical values. In the case of the tensile experiment using a conventional tensile testing machine, amorphous metals at temperatures below Tp rupture in a brittle manner inhibiting the observation of the generation of slip lines and the behavior of the displacement along them. Slip of this kind seems to occur instantly because of negligible strain hardening from the slip deformation in the slip band. As the slip is restricted to such a limited region, the apparent fracture strain and deformation energy assume smaller values as compared with those for homogeneous deformation. The energy required for plastic deformation or fracture is much smaller than the elastic energy of the tensile testing machine and the sample. This fact causes the catastrophic fracture. Accordingly, it is essentially important to develop a tensile testing machine of a very hard type for the purpose of clarifying the detailed processes of the slip deformation and the values of CRSS of amorphous metals. Recently,
Fig. 3. Load-elongation curve of amorphous Pd80Si20 alloy obtained under the extension speed of 0.05 pm/s at 20 °C. The gauge length is approximately 0.5 ram.
measurement of CRSS has been made by using a new hard-type tensile testing machine developed for this purpose. Figure 3 shows the load-elongation curve of amorphous Pds0-Si20 alloy at 20 °(3 [17]. This sample yields abruptly at the time when slip takes place and the slip proceeds in an intermittent manner along a particular slip band. This fact shows that slip takes place when the shear stress acting on this slip plane exceeds a certain critical value of the applied stress. By estimating the relationship between a m o u n t of slip and applied stress, we will be able to determine whether the strain hardening is operative or not. Clarification of the nature of the inhomogeneous deformation is a matter of primary importance for a better understanding of the deformation of amorphous metals. In some kinds of amorphous, iron-base alloys it is known that a remarkable decrease of the plastic deformation prior to failure occurs at a certain temperature below room temperature, implying the presence of a ductile-brittle transition similar to that of steels at low temperatures [18].
Homogeneous deformation The flow character at temperatures above Tp is more quantitatively investigated by creep testing. Creep deformation of the amorphous metals occurs homogeneously at temperatures above Tp, and especially near Tg. It is known that a transient creep is induced and that the deformation is of viscoelastic origin. There is a distinct difference between the creep curves of amorphous metals and those of crystalline
74
metals which also exhibit viscoelastic flow (diffusion creep) at just below the melting point. By contrast with the case of diffusion creep, the creep rate of amorphous metals decreases as the creep strain increases. This phenomenon shows that there should be some strengthening effect, such as an increase in creep activation energy, an increase in internal stress, and a decrease in uncertain elements which participate in the deformation. The structural change to a more stable state is also considered to be responsible for this effect when the testing temperature is much higher than Tp. In order to distinguish the most effective factor, it will be important to clarify not only the activation energy for transient creep, but also the internal stress, by using the latest techniques [19, 20] developed for obtaining these values for crystalline metals. Estimation of the internal or effective stress acting in the rate-controlling process is an essential problem to be solved before a detailed knowledge of the viscoelastic properties and, hence, a better understanding of the homogeneous deformation of amorphous metals can be obtained. Further, diffusion measurements in amorphous metals would be desirable to substantiate the conclusions from the creep experiments.
Other important effects on plastic deformation Structural effect Since the amorphous phase is thermodynamically unstable, the transformation to the crystalline phase occurs during cooling from a molten state with a relatively slow rate, or heating and aging at elevated temperatures. In general, the ductility of amorphous metals disappears gradually as crystallization proceeds. Recently, it has been found that the loss of ductility is induced by heating or aging, even at temperatures appreciably lower than the crystallization temperature (determined by means of X-ray and electron analyses [11, 21] ). This phenomenon is seen in iron-base amorphous alloys. The development of embrittlement by aging at the incipient stage of crystallization may be caused by a change in the bonding nature between constituent atoms, e.g., an increase of the covalent bonds due to ordering of atoms in the short range. In addi-
tion, there is other empirical evidence which appears to be related to the structure dependence of flow; for example, amorphous metals such as C o - P and Ni-P, synthesized by electroor chemical deposition, are entirely brittle, in spite of the fact that the same alloys produced from the liquid state are ductile. As mentioned above, the ductility of some of the amorphous metals is sensitive to a slight change in the structure which is not easily detected by the conventional methods used for structural analysis. Accordingly, more modern experimental procedures will be needed to provide more detailed information about the atomic configuration in the short range, and the bonding nature between atoms relative to the amorphous structure and their flow mechanisms.
Compositional effect In amorphous metals which contain, almost invariably, metalloid atoms as one of the constituents, the strength is greatly affected by the composition of the alloys. In a recent study, the compositional effects on the hardness were examined using amorphous ironbase alloys with various alloying elements [22] Figure 4 shows the experimental results, which m a y be explained in terms of the outer electron concentration of constituent metals; it is implied that the major role of the outer electrons of transition elements in amorphous metals is n o t to increase the cohesive energy of solids, as in the case of crystalline solids, but to weaken the bond strength associated with metalloid atoms, which seems to play the dominant role in determining the flow property of amorphous metals. At present there is only scanty information on this problem, in spite of its being one of the most im• A • • o
FIPTi-P- C Fe-V -P-C Fe--Cr-P-C Fm-P-C Fe- Co-P~C
~4
~ 500 9 AVERAGE ELECTRON CONCENTRATION re/o)
Fig. 4. Microhardness plotted against the average outer electron (s, d) c o n c e n t r a t i o n of metallic atoms (e/a) for amorphous iron alloys.
75 p o r t a n t subjects f r o m b o t h a f u n d a m e n t a l and a practical p o i n t o f view.
Environmental effects S o m e a m o r p h o u s iron-base alloys are k n o w n t o be a f f e c t e d s t r o n g l y b y e n v i r o n m e n t a l aspects such as h y d r o g e n e m b r i t t l e m e n t [23, 24] a n d stress c o r r o s i o n [7, 2 4 ] ; their plasticity is c o m p l e t e l y lost b y these effects. F u r t h e r m o r e , it has been o b s e r v e d t h a t a del a y e d f r a c t u r e takes place in s o m e o f t h e amorp h o u s iron-base alloys e x p o s e d u n d e r stresses a b o v e the critical level, in air, at r o o m t e m p e r a t u r e [ 2 5 ] . Such a r e m a r k a b l e decrease in plastic flow seems to be due, m a i n l y , to the harmful effect of hydrogen atoms diffused into the i n h o m o g e n e o u s shear slip bands. H o w e v e r , f u r t h e r investigations seem to be n e e d e d in o r d e r to u n d e r s t a n d the m o r e definite roles o f t h e h y d r o g e n a t o m s o n the b o n d ing n a t u r e b e t w e e n a t o m s , as well as the diffusibility o f h y d r o g e n a t o m s in the a m o r p h o u s structure. 3. FINAL REMARKS In c o n c l u s i o n , s y s t e m a t i c studies o f t h e f u n d a m e n t a l and practical aspects o f the def o r m a t i o n b e h a v i o r o f a m o r p h o u s metals have o n l y just begun. Hence, o n e needs m o r e experimental work to develop a better understanding o f their various behaviors a n d t o obtain a unified view o f the physical m e c h a n i c s o f a m o r p h o u s solids. ACKNOWLEDGEMENTS T h a n k s are d u e t o Prof. R. M a d d i n for m a n y fruitful discussions o n this subject. REFERENCES 1 J. J. Gilman, Phys. Today, 28 (1975} 46. 2 T. Masumoto, K. Hashimoto and H. Fujimori, Sci.
Rep. RITU, A-25 (1975) 232. 3 T. Masumoto and R. Maddin, Acta Metall., 19 (1971) 725. 4 H.J. Leamy, H. S. Chen and T. T. Wang, Metall. Trans., 3 (1972) 699. 5 T. Masumoto and R. Maddin, Mater. Sci. Eng., 19 (1975) 1. 6 J. J. Gilman, J. Appl. Phys., 46 (1975) 1625. 7 C. A. Pampillo, J. Mater. Sci., 10 (1975) 1194. 8 L. A. Davis, in N. J. Grant and B. C. Giessen (eds.), Proc. Second Intern. Conf. on Rapidly Quenched Metals, M.I.T., 1975, Section I, MIT Press, Cambriclge, Mass., 1976. 9 R. Maddin and T. Masumoto, Mater. Sci. Eng., 9 (1972) 153. 10 H. S. Chen, Scr. Metall., 9 {1975) 411. 11 T. Masumoto, Y. Waseda, H. M. Kimura and A. Inoue, Sci. Rep. RITU, A-26 (1976) 21. 12 C. Crussard and R. Tamhankar, Trans. Metall. Soc. AIME, 212 (1958} 718. 13 H. S. Chen, Scr. Metall., 7 (1973) 931. 14 C. A. Pampillo and H. S. Chen, Mater. Sci. Eng., 13 (1974) 181. 15 J. J. Gilman, J. App|. Phys., 44 (1973) 675. 16 J. C. M. Li, in Distinguished Lectures in Materials Science, Marcel Dekker, New York, 1974. 17 T. Murata and T. Masumoto, Scr. Metall., 1976, to be published. 18 C. A. Pampillo and D. E. Polk, Acta Metall., 22 (1974) 741. 19 K. Toma, H. Yoshinaga and S. Morozumi, Trans. Jpn. Inst. Met., 17 (1976) 102. 20 T. Murata and Y. Imai, Trans. Jpn. Inst. Met., 17 (1976)35. 21 T. Egami, P. J. Flanders and C. D. Graham, Jr., AIP Conf. Proc., 24 (1975) 697. 22 M. Naka, S. Tomizawa, T. Watanabe and T. Masumoto, in N. J. Grant and B. C. Giessen (eds.), Proc. Second Intern. Conf. on Rapidly Quenched Metals, M.I.T., 1975, Section I, MIT Press, Cambridge, Mass., 1976. 23 M. Nagumo and T. Takahashi, in N. J. Grant and B. C. Giessen (eds.), Proc. Second Intern. Conf. on Rapidly Quenched Metals, M.I.T., 1975, Section I, MIT Press, Cambridge, Mass., 1976. 24 A. Kawashima, K. Hashimoto and T. Masumoto, in N. J. Grant and B. C. Giessen (eds.), Proc. Second Intern. Conf. on Rapidly Quenched Metals, M.I.T., 1975, Section I, MIT Press, Cambridge, Mass., 1976; Corrosion Sci., 1976, in press. 25 T. Masumoto and H. M. Kimura, unpublished data.
71
© Elsevier Sequoia S.A., Lausanne -- Printed in the Netherlands
Deformation of Amorphous Metals
T. MASUMOTO and T. MURATA The Research Institute for Iron, Steel and Other Metals, Tohoku University, Sendai (Japan)
I. INTRODUCTION
Recently, amorphous metals (or metallic glasses) have generated great interest as a new class of solids, and the study of their properties has become an independent field in itself in Materials Science [1, 2]. Because of the disordered nature of the atomic arrangements, their properties must be inherently different from those of crystalline metals which are in the long range ordered state of constituent atoms. The mechanical properties of these amorphous metals seem to be most interesting; they exhibit higher strength as well as degrees of ductility and toughness. The discovery of these unexpected characteristics fascinates materials scientists because inorganic glasses such as oxides and chalcogens are, in general, extremely brittle. It has been known, empirically, that some amorphous metals can be deformed severely, w i t h o u t cracking, by cold-rolling. The first direct observation of plastic flow was made independently by Masumoto and Maddin [ 3 ] and by Leamy, Chen and Wang [4]. They observed distinct and relatively sharp shear-step markings on the surfaces of bent specimens. Since then, various studies of the deformation behavior, and several suggestions for the deformation mechanism have been made for amorphous metals. In this paper we discuss "What we do n o t know about deformation of amorphous metals".
2. EXPERIMENTAL EVIDENCE OBTAINED SO FAR AND SOME PROBLEMS TO BE CLARIFIED Before discussing the problem, let us ask ourselves "What do we know about the deformation of amorphous metals?". Fortunately, most of the results obtained so far have been summarized in several reviews [5 - 8]. Accord-
ing to these articles, sufficient knowledge has not been accumulated to understand fully the deformation of these solids in spite of the many efforts. Some reliable phenomena obtained by experiments on this problem are as follows: Elastic and anelastic d e f o r m a t i o n
The shear and Young's moduli are about 30% less than those in the fully crystallized state. By contrast, the bulk modulus changes relatively little by crystallization. Amorphous metals also exhibit a large temperature dependence of the elastic properties as compared with the same crystalline metals. However, the experimental values of these moduli are not well defined for either the amorphous or the crystalline state. Most of the measured values of the elastic moduli seem to involve considerable experimental error because the specimens used are neither large enough, nor of a regular form to obtain accurate values. To measure accurately the absolute values of these moduli in the amorphous state, and also to compare them with those in the crystalline state are important problems to be considered. To study these problems a new experimental m e t h o d should be developed in order to measure these values in amorphous samples having thicknesses less than several microns. The values are considered to be essential not only for the estimation of the theoretical strength, but also for consideration of the anharmonic effect in the amorphous state if the temperature dependence of these moduli and thermal expansion coefficients are known. There are several reports on the anelasticity of amorphous metals. The internal friction is usually larger in the amorphous state than in the crystalline state. It increases abruptly as a function of temperature and exhibits a high value around the crystallization temperature; this behavior seems to be related to a large
72
creep recovery after removal of the applied stress at higher temperatures. Besides the thermoelastic damping, several relaxation peaks of unknown origin are observed.
General characters of plastic deformation At temperatures well below the glass transition temperature, Tg, amorphous metals are ductile on a microscopic scale, while macroscopically they behave in a brittle manner; that is, slip deformation occurs which is limited to a localized shear band in the cases of uniaxial tension and compression. At temperatures around Tg, plastic deformation occurs homogeneously by viscous-like flow. As one important aspect of amorphous metals, it is found in several experiments that strain hardening is negligibly small, even though it shows apparently either softening or hardening. Since an ideal plastic material where no strain hardening occurs would, in principle, become unstable in tension and begin to neck as soon as yielding took place, this fact implies that amorphous metals can be considered as elastic, perfectly plastic solids. In other words, they can be used as materials for examining the mechanics of the ideal solid. Temperature and strain rate effects on plastic deformation Mode of deformation The mode of deformation transfers apparently from the inhomogeneous type to the more homogeneous type around a certain temperature. This critical temperature seems to be lower (about 100 - 150 °C) than Tg depending on the strain rate. In Fig. 1, the temperature- and strain rate-dependence of the strength and mode of deformation is shown [9] (the broken line indicates an approximate boundary between inhomogeneous and homogeneous modes in deformation). In the two regions delineated by this boundary there are distinct differences in the strain rate dependence of strength as well as the mode of deformation. In the homogeneous region, the strength decreases as the strain rate increases, while in the inhomogeneous region its dependence is small or opposite. The temperature dependence of the strength changes at this boundary. The strength is less sensitive to temperature in the inhomogeneous region, whereas a strong temperature dependence exists in the
15C
0"c
"~E 14C tj) ~J n,, ira
o
130 12C iio
•
4--4
200)~
INHOMOGENEOUS
"
"
0 "' E
80
HOMOGENEOUS
~
/
~ 6o
.
:.,:',
•
¢t: \\
.,,,,i
........
J I 0 "4
I 0 -s
. M~,~u. STRESs ........
STRAIN RATE
i I 0t
,
, i IIIlll
,
I I
10 ''=
(sac"]
Fig. 1. Effect of strain rate on the tensile strength at various temperatures.
homogeneous region. These experimental facts imply that the flow can be controlled by an athermal mechanism at lower temperatures and by a thermally-activated one at higher temperatures. Such a difference in the mechanism between these two kinds of flow is supported by experimental results on the structural change during plastic deformation; briefly, the inhomogeneous deformation causes a disordering in the amorphous state [5, 10, 11], while the homogeneous deformation promotes atomic ordering [5, 9]. Figure 2 shows a schematic representation of the temperature dependences of the stress in both the homogeneous and the inhomogeneous states (this diagram is constructed from the previous arguments for the plastic deformation of amorphous metals). Here, the stress is a value necessary to induce continuous deformation at a constant strain rate, and Tp is
0
Tp
TEMPERATURE
Fig. 2. Schematic diagram showivg the variation of the mode of deformation with temperature. The vertical axis denotes the stress which is necessary to deform amorphous metals at a constant rate. If the testing temperature is higher than Tp, homogeneous deformation takes place because the stress is smaller than that for inhomogeneous deformation.
73
a critical temperature at which the stresses for the two modes of deformation coincide. This critical temperature, T~, resembles the so-called equicohesion temperature which has been proposed in the phenomenological study of the fracture of crystalline metals [ 1 2 ] . In order to obtain a better understanding of the critical temperature, a comparison of the resistance to inhomogeneous and homogeneous deformation is important, and the variation with strain rate should be clarified.
I PdsoS]20
~
20C
O1130 ...I
0
0
5
I0
ELONGATION (pro)
Inhomogeneous deformation Quantitative data on the flow characteristics of inhomogeneous deformation are meagre because of its occurrence within a limited, localized shear band. There are, however, qualitative data from compression tests carried o u t under more complex conditions of stress [13, 14]. Generally, the measurement of apparent yield- and fracture stresses shows that these values are higher, usually, than those of the major element in the crystalline state. Theoretical considerations of the strengths of amorphous metals has been made based on a dislocation model [15, 16] and the theoretical strength is estimated to be 0.03 - 0.05 of the Young's modulus. However, because of experimental difficulties, there have been no exact values of "critical resolved shear stress" (CRSS) of amorphous metals for comparison with the theoretical values. In the case of the tensile experiment using a conventional tensile testing machine, amorphous metals at temperatures below Tp rupture in a brittle manner inhibiting the observation of the generation of slip lines and the behavior of the displacement along them. Slip of this kind seems to occur instantly because of negligible strain hardening from the slip deformation in the slip band. As the slip is restricted to such a limited region, the apparent fracture strain and deformation energy assume smaller values as compared with those for homogeneous deformation. The energy required for plastic deformation or fracture is much smaller than the elastic energy of the tensile testing machine and the sample. This fact causes the catastrophic fracture. Accordingly, it is essentially important to develop a tensile testing machine of a very hard type for the purpose of clarifying the detailed processes of the slip deformation and the values of CRSS of amorphous metals. Recently,
Fig. 3. Load-elongation curve of amorphous Pd80Si20 alloy obtained under the extension speed of 0.05 pm/s at 20 °C. The gauge length is approximately 0.5 ram.
measurement of CRSS has been made by using a new hard-type tensile testing machine developed for this purpose. Figure 3 shows the load-elongation curve of amorphous Pds0-Si20 alloy at 20 °(3 [17]. This sample yields abruptly at the time when slip takes place and the slip proceeds in an intermittent manner along a particular slip band. This fact shows that slip takes place when the shear stress acting on this slip plane exceeds a certain critical value of the applied stress. By estimating the relationship between a m o u n t of slip and applied stress, we will be able to determine whether the strain hardening is operative or not. Clarification of the nature of the inhomogeneous deformation is a matter of primary importance for a better understanding of the deformation of amorphous metals. In some kinds of amorphous, iron-base alloys it is known that a remarkable decrease of the plastic deformation prior to failure occurs at a certain temperature below room temperature, implying the presence of a ductile-brittle transition similar to that of steels at low temperatures [18].
Homogeneous deformation The flow character at temperatures above Tp is more quantitatively investigated by creep testing. Creep deformation of the amorphous metals occurs homogeneously at temperatures above Tp, and especially near Tg. It is known that a transient creep is induced and that the deformation is of viscoelastic origin. There is a distinct difference between the creep curves of amorphous metals and those of crystalline
74
metals which also exhibit viscoelastic flow (diffusion creep) at just below the melting point. By contrast with the case of diffusion creep, the creep rate of amorphous metals decreases as the creep strain increases. This phenomenon shows that there should be some strengthening effect, such as an increase in creep activation energy, an increase in internal stress, and a decrease in uncertain elements which participate in the deformation. The structural change to a more stable state is also considered to be responsible for this effect when the testing temperature is much higher than Tp. In order to distinguish the most effective factor, it will be important to clarify not only the activation energy for transient creep, but also the internal stress, by using the latest techniques [19, 20] developed for obtaining these values for crystalline metals. Estimation of the internal or effective stress acting in the rate-controlling process is an essential problem to be solved before a detailed knowledge of the viscoelastic properties and, hence, a better understanding of the homogeneous deformation of amorphous metals can be obtained. Further, diffusion measurements in amorphous metals would be desirable to substantiate the conclusions from the creep experiments.
Other important effects on plastic deformation Structural effect Since the amorphous phase is thermodynamically unstable, the transformation to the crystalline phase occurs during cooling from a molten state with a relatively slow rate, or heating and aging at elevated temperatures. In general, the ductility of amorphous metals disappears gradually as crystallization proceeds. Recently, it has been found that the loss of ductility is induced by heating or aging, even at temperatures appreciably lower than the crystallization temperature (determined by means of X-ray and electron analyses [11, 21] ). This phenomenon is seen in iron-base amorphous alloys. The development of embrittlement by aging at the incipient stage of crystallization may be caused by a change in the bonding nature between constituent atoms, e.g., an increase of the covalent bonds due to ordering of atoms in the short range. In addi-
tion, there is other empirical evidence which appears to be related to the structure dependence of flow; for example, amorphous metals such as C o - P and Ni-P, synthesized by electroor chemical deposition, are entirely brittle, in spite of the fact that the same alloys produced from the liquid state are ductile. As mentioned above, the ductility of some of the amorphous metals is sensitive to a slight change in the structure which is not easily detected by the conventional methods used for structural analysis. Accordingly, more modern experimental procedures will be needed to provide more detailed information about the atomic configuration in the short range, and the bonding nature between atoms relative to the amorphous structure and their flow mechanisms.
Compositional effect In amorphous metals which contain, almost invariably, metalloid atoms as one of the constituents, the strength is greatly affected by the composition of the alloys. In a recent study, the compositional effects on the hardness were examined using amorphous ironbase alloys with various alloying elements [22] Figure 4 shows the experimental results, which m a y be explained in terms of the outer electron concentration of constituent metals; it is implied that the major role of the outer electrons of transition elements in amorphous metals is n o t to increase the cohesive energy of solids, as in the case of crystalline solids, but to weaken the bond strength associated with metalloid atoms, which seems to play the dominant role in determining the flow property of amorphous metals. At present there is only scanty information on this problem, in spite of its being one of the most im• A • • o
FIPTi-P- C Fe-V -P-C Fe--Cr-P-C Fm-P-C Fe- Co-P~C
~4
~ 500 9 AVERAGE ELECTRON CONCENTRATION re/o)
Fig. 4. Microhardness plotted against the average outer electron (s, d) c o n c e n t r a t i o n of metallic atoms (e/a) for amorphous iron alloys.
75 p o r t a n t subjects f r o m b o t h a f u n d a m e n t a l and a practical p o i n t o f view.
Environmental effects S o m e a m o r p h o u s iron-base alloys are k n o w n t o be a f f e c t e d s t r o n g l y b y e n v i r o n m e n t a l aspects such as h y d r o g e n e m b r i t t l e m e n t [23, 24] a n d stress c o r r o s i o n [7, 2 4 ] ; their plasticity is c o m p l e t e l y lost b y these effects. F u r t h e r m o r e , it has been o b s e r v e d t h a t a del a y e d f r a c t u r e takes place in s o m e o f t h e amorp h o u s iron-base alloys e x p o s e d u n d e r stresses a b o v e the critical level, in air, at r o o m t e m p e r a t u r e [ 2 5 ] . Such a r e m a r k a b l e decrease in plastic flow seems to be due, m a i n l y , to the harmful effect of hydrogen atoms diffused into the i n h o m o g e n e o u s shear slip bands. H o w e v e r , f u r t h e r investigations seem to be n e e d e d in o r d e r to u n d e r s t a n d the m o r e definite roles o f t h e h y d r o g e n a t o m s o n the b o n d ing n a t u r e b e t w e e n a t o m s , as well as the diffusibility o f h y d r o g e n a t o m s in the a m o r p h o u s structure. 3. FINAL REMARKS In c o n c l u s i o n , s y s t e m a t i c studies o f t h e f u n d a m e n t a l and practical aspects o f the def o r m a t i o n b e h a v i o r o f a m o r p h o u s metals have o n l y just begun. Hence, o n e needs m o r e experimental work to develop a better understanding o f their various behaviors a n d t o obtain a unified view o f the physical m e c h a n i c s o f a m o r p h o u s solids. ACKNOWLEDGEMENTS T h a n k s are d u e t o Prof. R. M a d d i n for m a n y fruitful discussions o n this subject. REFERENCES 1 J. J. Gilman, Phys. Today, 28 (1975} 46. 2 T. Masumoto, K. Hashimoto and H. Fujimori, Sci.
Rep. RITU, A-25 (1975) 232. 3 T. Masumoto and R. Maddin, Acta Metall., 19 (1971) 725. 4 H.J. Leamy, H. S. Chen and T. T. Wang, Metall. Trans., 3 (1972) 699. 5 T. Masumoto and R. Maddin, Mater. Sci. Eng., 19 (1975) 1. 6 J. J. Gilman, J. Appl. Phys., 46 (1975) 1625. 7 C. A. Pampillo, J. Mater. Sci., 10 (1975) 1194. 8 L. A. Davis, in N. J. Grant and B. C. Giessen (eds.), Proc. Second Intern. Conf. on Rapidly Quenched Metals, M.I.T., 1975, Section I, MIT Press, Cambriclge, Mass., 1976. 9 R. Maddin and T. Masumoto, Mater. Sci. Eng., 9 (1972) 153. 10 H. S. Chen, Scr. Metall., 9 {1975) 411. 11 T. Masumoto, Y. Waseda, H. M. Kimura and A. Inoue, Sci. Rep. RITU, A-26 (1976) 21. 12 C. Crussard and R. Tamhankar, Trans. Metall. Soc. AIME, 212 (1958} 718. 13 H. S. Chen, Scr. Metall., 7 (1973) 931. 14 C. A. Pampillo and H. S. Chen, Mater. Sci. Eng., 13 (1974) 181. 15 J. J. Gilman, J. App|. Phys., 44 (1973) 675. 16 J. C. M. Li, in Distinguished Lectures in Materials Science, Marcel Dekker, New York, 1974. 17 T. Murata and T. Masumoto, Scr. Metall., 1976, to be published. 18 C. A. Pampillo and D. E. Polk, Acta Metall., 22 (1974) 741. 19 K. Toma, H. Yoshinaga and S. Morozumi, Trans. Jpn. Inst. Met., 17 (1976) 102. 20 T. Murata and Y. Imai, Trans. Jpn. Inst. Met., 17 (1976)35. 21 T. Egami, P. J. Flanders and C. D. Graham, Jr., AIP Conf. Proc., 24 (1975) 697. 22 M. Naka, S. Tomizawa, T. Watanabe and T. Masumoto, in N. J. Grant and B. C. Giessen (eds.), Proc. Second Intern. Conf. on Rapidly Quenched Metals, M.I.T., 1975, Section I, MIT Press, Cambridge, Mass., 1976. 23 M. Nagumo and T. Takahashi, in N. J. Grant and B. C. Giessen (eds.), Proc. Second Intern. Conf. on Rapidly Quenched Metals, M.I.T., 1975, Section I, MIT Press, Cambridge, Mass., 1976. 24 A. Kawashima, K. Hashimoto and T. Masumoto, in N. J. Grant and B. C. Giessen (eds.), Proc. Second Intern. Conf. on Rapidly Quenched Metals, M.I.T., 1975, Section I, MIT Press, Cambridge, Mass., 1976; Corrosion Sci., 1976, in press. 25 T. Masumoto and H. M. Kimura, unpublished data.