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Serrated plastic flow in metallic glasses
Scripta M E T A L L U R G I C A
Vol. 13, pp. 463-467, 1979 Printed in the U.S.A.
SERRATED
PLASTIC
FLOW IN METALLIC
Pergamon Press Ltd. All rights reserved.
GLASSES*
S. Takayama of M e t a l l u r g y and Materials Science U n i v e r s i t y of Pennsylvania Philadelphia, Pennsylvania 19104
Department
(Received March 15, 1979) (Revised April 27, 1979) Introduction Plastic flow in metallic glasses has been studied m a c r o s c o p i c a l l y (1-9) and m i c r o s c o p i c a l l y (I0). These studies have indicated that metallic glasses show localized (i.e., inhomogeneous) deformation at relatively low temperatures. The deformation becomes more homogeneous at higher temperatures. Observations of rolling and bending deformation (6,11) revealed that pre-existing slip bands can act as obstacles for plastic flow; and that microcracks often nucleate at the intersections of two slip bands. In order to further investigate the m e c h a n i s m of plastic flow in metallic glasses, studies of bending and tensile deformation after drawing PdvT.5Cu6Sil6.5 glass wires were conducted. Experimental
Procedure
. Wires of. Pd 7.5Cu6Si16.5 glass with a diameter of 284 vm were prepared by rapid quenching ~rom the melt. Their glassy structures were confirmed by x-ray diffraction. The glassy wires were drawn through diamond dies in several steps. The samples were e l e c t r o c h e m i c a l l y polished before and after drawing to make reduced gage lengths (~i0 mm long) for tensile testing. This reduced elastic constraint at the grips and surface imperfections. The prepared samples were pulled to 4 failure by an Instron tensile machine with a constant strain rate (~ = 4 x 10sec -1) at room temperature. To study the deformation marking during tensile loading, the load on the specimen was removed temporarily after a desired stress had been reached and surface was examined with an optical microscope. This procedure was continued until the specimen fractured. Bending deformations of drawn and undrawn wires were observed using a microscope and a toolmaker's vise. More detailed observations of slip steps were made with a scanning electron microscope (SEM). Results
and Discussion
After drawing, two families of deformation bands appear on the lateral surfaces of wires as shown in Fig. I. One makes an angle of ~40 ° to the drawing axis, while the other lies nearly p e r p e n d i c u l a r to it (Fig. I). Fheir trajectories are determined by the stress distribution inside the drawing die (12). An analysis of these two families of bands is discussed elsewhere (II). Note that a large number of the slip markings intersect with each other; and some are terminated by others. *This work was done in the Materials Morristown, NJ 07960.
Research
Center,
Allied Chemical
463 0036-9748/79/060463-05502.00/0 Copyright (c) 1979 Pergamon Press Ltd.
Corp.,
464
SERRATED
FLOW IN METALLIC
GLASSES
Vol.
13, No. 6
Typical stress-strain curves for undrawn and drawn (R = 26%, where R denotes a total reduction in area) wires are shown in Fig. 2. The deviation from elastic behavior (the apparent yield point (5)) is much lower and the fracture stress is slightly higher for the drawn wire than for the undrawn wire. The apparent yield stress and the fracture stress for drawn wires is an average of ~28% lower and ~7% higher than those of the undrawn wires, respectively (II). The decrease of the apparent yield stress most likely results from preferential plastic flow within pre-existing slip bands (i.e., work softening); while the increase (or no considerable decrease) of the fracture stresses may result from the inhibition of slip bands that tend to cut through pre-existing deformed areas (see the terminations of the slip lines in Fig. i). Furthermore, the curve for the drawn wire in Fig. 2 shows distinct serrations (plastic instabilities), whereas the curve for the undrawn wire shows no serrations. These plastic instabilities were further explored by continuous observations of the surfaces during a tensile test. These revealed that in the case of the undrawn wires, visible deformation markings are seldom observed until a specimen fractures. In contrast, for drawn wires many slip lines appear on the surface when the applied load exceeds the apparent yield point and lies 5-10% below the fracture stress. This is clearly shown in Fig. 3 which includes the load vs. extension curve (the thick arrow indicates the apparent yield point) and photographs taken at points a, b, and c on the curve. The deformation lines which make angles of ~Sl ° with the wire axis appeared during the tensile test. Photograph (a) shows the surface within the gauge length immediately after the stress drop at point (a). Photographs (b) and (c) show the additional slip lines produced by strain increments. The appearance of separate additional slip lines may indicate an inhibition of further flow within the initial slip lines. Such inhibition probably results from the blocking effects of intersecting pre-deformed regions. Note that slip lines in Fig. 3(a), (b), and (c) must intersect some of the pre-existing lines shown in Fig. i. On the other hand, slip lines may nucleate relatively easily in the drawn wires. This may account for the susceptibility of plastically predeformed area for further plastic flow. Serrated plastic flow may result from interactions between slip systems in metallic glasses. In order to further investigate this, drawn wires were continuously bent under an optical microscope. It was found that some of the preexisting slip lines deform additionally to accommodate the applied strain, resulting in large slip steps. A closer view of some of the large steps in Fig. 4a is shown in Fig. 4b. It reveals that each large slip step is composed of a number of smaller ones. This is further revealed in Fig. 4c and 4d. The striae have rather regular spacings. The distances between the individual steps in Fig. 4d are roughly 1.3 ~m. The Y-modulatipn mode of the microscope reveals that the striations are small steps with slopes. These striations can be largely suppressed by reducing the amount of mutual intersection between slip systems. For example, Fig. 5 shows large slip steps produced by bending an as-quenched ribbon around the axis of its width direction. For such a specimen, most of the slip lines lie perpendicular to the ribbon axis and parallel to each other. Only a few striations, sometimes none, appear on the slip steps as pointed out by the arrow in the photograph. From the above observation we suggest that serrated flow in metallic glasses results from the interactions between slip systems. Although the micromechanism of these intersections is not clear in glasses, it might be similar to dislocation intersections in crystalline solids. Conclusions Serrated plastic flow in cold-drawn gated with results as summarized below.
metallic
(I) Samples of Pd-Cu-Si glass always curve after cold-drawing. The appearances of specimens reveal a correlation between and bursts of slip lines. (2) A large number of striations was
glass wires has been investi-
show serrations on their stress-strain of deformation bands on the surfaces serrations on the stress-strain curves observed
on the large slip steps of
Vol. 13, No. 6
samples
SERRATED FLOW IN METALLIC GLASSES
that
were drawn and subsequently
(3) The s e r r a t e d plastic flow in metallic interactions between slip systems.
465
bent.
glasses
is
explained
in terms
of
Acknowledgements The a u t h o r g r a t e f u l l y acknowledges fruitful G i l m a n a n d R. H a s e g a w a a n d P r o f . J . C. M. L i . doing
He a l s o a p p r e c i a t e s scanning electron
the assistance micrography.
discussions
with
o f Dr. A. R e i m s c h u e s s e l
Drs.
J.
J.
a n d Ms. B r a d y i n
References 1. 2 3 4 5 6 7 8 9 I0 Ii. 12.
T. Masumoto and R. M a d d i n , A c t a M e t . ; 1 9 , 725 ( 1 9 7 1 ) . H.J. L e a m y , H. S. Chen a n d T. T. Wang, M e t . T r a n s . , 3, 699 ( 1 9 7 2 ) . C . A . Pampillo and H. S. Chen, Mat. Sci. Eng., 13, 181 (1974). F. Spaepen and D. Turnbull, Scripta Met., 8, 563 (1974). S. Takayama and R. Maddin, Scripta Met., 9, 343 (1975). S. Takayama and R. Maddin, Acta Met., 23, 943 (1975). S. Takayama and R. Maddin, Phil. Mag., 32, 457 (1975). L . A . Davis and S. Kavesh, J. Mat. Sci., i0, 453 (1975). S. Takayama and R. Maddin, Met. Trans., 7A, 1065 (1976). S. Takayama and R. Maddin, 2nd International Conf. Proc., Boston, Mass., USA, p. 261, Elsevier Sequoia S.A., Lausanne (1975). S. Takayama, Mat. Sci. Eng., in press (1979). R. Hill, The Mathematical Theory of Plasticity, Chapter VII, Oxford University Press, London (1950).
20
15
~: ,4 x 10"4s¢*c"1 T "R.T.
(a)
(b)
/'~
//[
'// 5
1.0
FIG. 1 Optical micrograph of the deformation bands on the peripheral surface of Pd77 5Cu6Silh 5 metallic glass wire after drawing (area reduction R = 23%).
~'1
STRAIN( Ln I~llox10"2)
FIG. 2 Typical stress-strain curves (a) before and (b) after drawing (R=26%) Pd77Cu6.5Si16.5 metallic glass wires.
466
SERRATED
FLOW IN METALLIC
GLASSES
Vol.
13, No.
70f 60--
50
z 4o
I ~ 3o 20
1C
I
1
I
I
I
1
2
3
,4
5
Z~L
Optical micrographs drawn Pd77 5Cu6Si~6 stress levels durlng of applied load vs. taken at the stages arrows in the curve.
x lo-' (ram)
FIG. 3 of slip lines on the peripheral surface of 5 metallic glass wire (R = 23%) at different a tensile test, and the corresponding curve extension. Photos (a), (b) and (c) were a, b and c respectively, as indicated by
6
Vol. 13, No. 6
SERRATED FLOW IN METALLIC GLASSES
467
FIG. 4 Scanning electron micrographs of the offsets introduced after drawing (R = 53%) and subsequently bending a PdvvCu6.5Si16.5 metallic glass wire: (a) whole view of the bent part of the wire; (b) high magnification in the middle of photo (a); (c) the outlined area of pboto (b) is magnified; (d) the outlined area of photo ~c) is further magnified.
FIG. 5 Scanning electron micrograph of the large offsets introduced after bending a ribbon Ni40Fe40PI4B 6 metallic glass filament.
Vol. 13, pp. 463-467, 1979 Printed in the U.S.A.
SERRATED
PLASTIC
FLOW IN METALLIC
Pergamon Press Ltd. All rights reserved.
GLASSES*
S. Takayama of M e t a l l u r g y and Materials Science U n i v e r s i t y of Pennsylvania Philadelphia, Pennsylvania 19104
Department
(Received March 15, 1979) (Revised April 27, 1979) Introduction Plastic flow in metallic glasses has been studied m a c r o s c o p i c a l l y (1-9) and m i c r o s c o p i c a l l y (I0). These studies have indicated that metallic glasses show localized (i.e., inhomogeneous) deformation at relatively low temperatures. The deformation becomes more homogeneous at higher temperatures. Observations of rolling and bending deformation (6,11) revealed that pre-existing slip bands can act as obstacles for plastic flow; and that microcracks often nucleate at the intersections of two slip bands. In order to further investigate the m e c h a n i s m of plastic flow in metallic glasses, studies of bending and tensile deformation after drawing PdvT.5Cu6Sil6.5 glass wires were conducted. Experimental
Procedure
. Wires of. Pd 7.5Cu6Si16.5 glass with a diameter of 284 vm were prepared by rapid quenching ~rom the melt. Their glassy structures were confirmed by x-ray diffraction. The glassy wires were drawn through diamond dies in several steps. The samples were e l e c t r o c h e m i c a l l y polished before and after drawing to make reduced gage lengths (~i0 mm long) for tensile testing. This reduced elastic constraint at the grips and surface imperfections. The prepared samples were pulled to 4 failure by an Instron tensile machine with a constant strain rate (~ = 4 x 10sec -1) at room temperature. To study the deformation marking during tensile loading, the load on the specimen was removed temporarily after a desired stress had been reached and surface was examined with an optical microscope. This procedure was continued until the specimen fractured. Bending deformations of drawn and undrawn wires were observed using a microscope and a toolmaker's vise. More detailed observations of slip steps were made with a scanning electron microscope (SEM). Results
and Discussion
After drawing, two families of deformation bands appear on the lateral surfaces of wires as shown in Fig. I. One makes an angle of ~40 ° to the drawing axis, while the other lies nearly p e r p e n d i c u l a r to it (Fig. I). Fheir trajectories are determined by the stress distribution inside the drawing die (12). An analysis of these two families of bands is discussed elsewhere (II). Note that a large number of the slip markings intersect with each other; and some are terminated by others. *This work was done in the Materials Morristown, NJ 07960.
Research
Center,
Allied Chemical
463 0036-9748/79/060463-05502.00/0 Copyright (c) 1979 Pergamon Press Ltd.
Corp.,
464
SERRATED
FLOW IN METALLIC
GLASSES
Vol.
13, No. 6
Typical stress-strain curves for undrawn and drawn (R = 26%, where R denotes a total reduction in area) wires are shown in Fig. 2. The deviation from elastic behavior (the apparent yield point (5)) is much lower and the fracture stress is slightly higher for the drawn wire than for the undrawn wire. The apparent yield stress and the fracture stress for drawn wires is an average of ~28% lower and ~7% higher than those of the undrawn wires, respectively (II). The decrease of the apparent yield stress most likely results from preferential plastic flow within pre-existing slip bands (i.e., work softening); while the increase (or no considerable decrease) of the fracture stresses may result from the inhibition of slip bands that tend to cut through pre-existing deformed areas (see the terminations of the slip lines in Fig. i). Furthermore, the curve for the drawn wire in Fig. 2 shows distinct serrations (plastic instabilities), whereas the curve for the undrawn wire shows no serrations. These plastic instabilities were further explored by continuous observations of the surfaces during a tensile test. These revealed that in the case of the undrawn wires, visible deformation markings are seldom observed until a specimen fractures. In contrast, for drawn wires many slip lines appear on the surface when the applied load exceeds the apparent yield point and lies 5-10% below the fracture stress. This is clearly shown in Fig. 3 which includes the load vs. extension curve (the thick arrow indicates the apparent yield point) and photographs taken at points a, b, and c on the curve. The deformation lines which make angles of ~Sl ° with the wire axis appeared during the tensile test. Photograph (a) shows the surface within the gauge length immediately after the stress drop at point (a). Photographs (b) and (c) show the additional slip lines produced by strain increments. The appearance of separate additional slip lines may indicate an inhibition of further flow within the initial slip lines. Such inhibition probably results from the blocking effects of intersecting pre-deformed regions. Note that slip lines in Fig. 3(a), (b), and (c) must intersect some of the pre-existing lines shown in Fig. i. On the other hand, slip lines may nucleate relatively easily in the drawn wires. This may account for the susceptibility of plastically predeformed area for further plastic flow. Serrated plastic flow may result from interactions between slip systems in metallic glasses. In order to further investigate this, drawn wires were continuously bent under an optical microscope. It was found that some of the preexisting slip lines deform additionally to accommodate the applied strain, resulting in large slip steps. A closer view of some of the large steps in Fig. 4a is shown in Fig. 4b. It reveals that each large slip step is composed of a number of smaller ones. This is further revealed in Fig. 4c and 4d. The striae have rather regular spacings. The distances between the individual steps in Fig. 4d are roughly 1.3 ~m. The Y-modulatipn mode of the microscope reveals that the striations are small steps with slopes. These striations can be largely suppressed by reducing the amount of mutual intersection between slip systems. For example, Fig. 5 shows large slip steps produced by bending an as-quenched ribbon around the axis of its width direction. For such a specimen, most of the slip lines lie perpendicular to the ribbon axis and parallel to each other. Only a few striations, sometimes none, appear on the slip steps as pointed out by the arrow in the photograph. From the above observation we suggest that serrated flow in metallic glasses results from the interactions between slip systems. Although the micromechanism of these intersections is not clear in glasses, it might be similar to dislocation intersections in crystalline solids. Conclusions Serrated plastic flow in cold-drawn gated with results as summarized below.
metallic
(I) Samples of Pd-Cu-Si glass always curve after cold-drawing. The appearances of specimens reveal a correlation between and bursts of slip lines. (2) A large number of striations was
glass wires has been investi-
show serrations on their stress-strain of deformation bands on the surfaces serrations on the stress-strain curves observed
on the large slip steps of
Vol. 13, No. 6
samples
SERRATED FLOW IN METALLIC GLASSES
that
were drawn and subsequently
(3) The s e r r a t e d plastic flow in metallic interactions between slip systems.
465
bent.
glasses
is
explained
in terms
of
Acknowledgements The a u t h o r g r a t e f u l l y acknowledges fruitful G i l m a n a n d R. H a s e g a w a a n d P r o f . J . C. M. L i . doing
He a l s o a p p r e c i a t e s scanning electron
the assistance micrography.
discussions
with
o f Dr. A. R e i m s c h u e s s e l
Drs.
J.
J.
a n d Ms. B r a d y i n
References 1. 2 3 4 5 6 7 8 9 I0 Ii. 12.
T. Masumoto and R. M a d d i n , A c t a M e t . ; 1 9 , 725 ( 1 9 7 1 ) . H.J. L e a m y , H. S. Chen a n d T. T. Wang, M e t . T r a n s . , 3, 699 ( 1 9 7 2 ) . C . A . Pampillo and H. S. Chen, Mat. Sci. Eng., 13, 181 (1974). F. Spaepen and D. Turnbull, Scripta Met., 8, 563 (1974). S. Takayama and R. Maddin, Scripta Met., 9, 343 (1975). S. Takayama and R. Maddin, Acta Met., 23, 943 (1975). S. Takayama and R. Maddin, Phil. Mag., 32, 457 (1975). L . A . Davis and S. Kavesh, J. Mat. Sci., i0, 453 (1975). S. Takayama and R. Maddin, Met. Trans., 7A, 1065 (1976). S. Takayama and R. Maddin, 2nd International Conf. Proc., Boston, Mass., USA, p. 261, Elsevier Sequoia S.A., Lausanne (1975). S. Takayama, Mat. Sci. Eng., in press (1979). R. Hill, The Mathematical Theory of Plasticity, Chapter VII, Oxford University Press, London (1950).
20
15
~: ,4 x 10"4s¢*c"1 T "R.T.
(a)
(b)
/'~
//[
'// 5
1.0
FIG. 1 Optical micrograph of the deformation bands on the peripheral surface of Pd77 5Cu6Silh 5 metallic glass wire after drawing (area reduction R = 23%).
~'1
STRAIN( Ln I~llox10"2)
FIG. 2 Typical stress-strain curves (a) before and (b) after drawing (R=26%) Pd77Cu6.5Si16.5 metallic glass wires.
466
SERRATED
FLOW IN METALLIC
GLASSES
Vol.
13, No.
70f 60--
50
z 4o
I ~ 3o 20
1C
I
1
I
I
I
1
2
3
,4
5
Z~L
Optical micrographs drawn Pd77 5Cu6Si~6 stress levels durlng of applied load vs. taken at the stages arrows in the curve.
x lo-' (ram)
FIG. 3 of slip lines on the peripheral surface of 5 metallic glass wire (R = 23%) at different a tensile test, and the corresponding curve extension. Photos (a), (b) and (c) were a, b and c respectively, as indicated by
6
Vol. 13, No. 6
SERRATED FLOW IN METALLIC GLASSES
467
FIG. 4 Scanning electron micrographs of the offsets introduced after drawing (R = 53%) and subsequently bending a PdvvCu6.5Si16.5 metallic glass wire: (a) whole view of the bent part of the wire; (b) high magnification in the middle of photo (a); (c) the outlined area of pboto (b) is magnified; (d) the outlined area of photo ~c) is further magnified.
FIG. 5 Scanning electron micrograph of the large offsets introduced after bending a ribbon Ni40Fe40PI4B 6 metallic glass filament.