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Viscous flow of amorphous alloys under compressive and tensile stresses
Materials Science and Engineering, 97 (1988) 483~,85
483
Viscous Flow of Amorphous Alloys Under Compressive and Tensile Stresses* WANG JINGTANG, DING BINGZHE, PANG DEXING, LI SHULING and LI GUSUNG Institute ~?fMetal Research, Academia Sinica, Shenyang (China)
2. Experimental details Abstract In this paper the "region boundary" model is suggested on the basis of the study of the viscous flow o f Ni P, Fe S i - B amorphous alloys under compression and tension. The flow coefficient determined from the experiments is about 0.3-0.5 which is almost consistent with the value estimated from the model. In order to obtain the ideal glass transformation temperature T~, a temperature Tvs is defined at which stable flow starts and extrapolating the Tvs- ~ curve to --, Ocorresponds to T~. The tests were performed with T M S - 2 and D S C - 2 thermal analysers. The obtained viscosity r1 may be extended to the liquid state according to the formula r/= ~/oA exp
B + bT ~1 l/r) T - T~
1. Introduction The viscous flow of amorphous alloys is one of the important characteristics that distinguishes them from the crystalline metal. The flow behaviour at temperatures near the glass transition temperature Tg has been investigated to some extent by the creep method [1 3], however, a more detailed study is desirable for a better understanding of the flow mechanism in amorphous metals. In this paper, research has been carried out using two methods, tension-expansion and compression-expansion tests, and the relationship between the two has been established. It is important to understand the formation ability of amorphous metal and to explore the internal relation between the liquid and amorphous metals. In particular, the viscous flow behaviour under compressive stress may provide the possibility of consolidating layers of amorphous ribbon to produce a thick strip for use as a large stacking transformer of amorphous metals.
*Paper presented at the Sixth International Conference on Rapidly Quenched Metals, Montr6al, August 3-7, 1987. 0025-5416/88/$3.50
Amorphous Fe7sSigB13 (Metglass 2605S2) and Ni89.3Plo7 alloys were prepared by single-roller melt spinning. Samples about 2 mm wide, 30/~m thick with a gauge length of 12 mm were taken from the ribbon for use in the tension creep tests. For the compression-expansion tests, the samples were about 3 mm wide, 30/~m thick and I0 mm in length, and X-ray diffraction confirmed that both samples were amorphous. Considering transients in the isothermal experiments, one hour was taken for pre-annealing the samples at a temperature lower than Tg. The viscous flow tests for Fe-Si-B and Ni-P glasses were performed with a Perkin-Elmer TMS-2 thermomechanical analyser equipped with a quartz tube assembly especially designed for the tests [4]. The measuring precision can be up to 0.01/~m. The external stress was applied to the sample by adding a weight to a loading adaptor. The friction between the specimen and the quartz tubes may be neglected because there is appropriate clearance between them. The crystallization temperature of the amorphous material was measured by a DSC-2 differential scanning calorimeter. All tests were conducted in an argon atmosphere. The observation of the fracture surface of the amorphous samples after measuring viscous flow was conducted in JSM-T200 scanning electron microscope.
3. Results and discussions The amorphous Fe-Si-B alloy was heated at a constant rate of 20 K min 1 under tension (A) and compression (B) in the TMS-2 instrument, the experimental curves obtained are shown in Fig. 1. The temperature T~v corresponds to the intersecting point of the extrapolated lines for the thermal expansion at low temperature and the viscous flow at high temperature and defines the temperature for the initiation of viscous flow. The temperature for the initiation of steady state flow is defined as steady-state viscous flow temperature Tvs. T~ shows the crystallization temperature.
,~? Elsevier Sequoia/Printed in The Netherlands
484 90 8o 7o 6o
0,~ A .J
.~ 0,3
50 ~o .q
30
0,2
2o
I
1o
TIK)
l"sv
I Tvs Tc
o,~
Fig. l. The expansion and viscous flow curves: A, under tension; B, under compression.
~o
7~0
7~
~o
~o
T(K) Fig. 3. The relative deformation curves of amorphous Fe-Si-B alloy: A, isothermal tension; B, isothermal compression.
Figure 2 shows a relationship between flow temperature Tsv, Tvs and the heating rate ~b. The TS, values obtained from extrapolating the Tvs-~b curve to ~b ~ 0 can be defined as a critical viscous flow temperature, and the point Tv~ obtained from extrapolating the Tw-~b curve to ~b ~ 0 corresponds to the ideal glass transformation temperature Tg. For this it is assumed that the atoms in the amorphous solid are thermally activated over a long period of time, as the sample is heated to TS~ over an infinite time. The relative change in the length during isothermal-tension (A) and isothermal-compression (B) for amorphous Fe--Si-B alloy is shown in Fig. 3. The difference between A and B is caused by the sign of the additional external stress. The viscosity of the amorphous alloy near Tg was investigated by both the tensile stress and the
compressive stress methods. The experimental results under isothermal tension (A) and isothermal compression (B) for various external stresses are given in Fig. 4. It can be seen that tension-expansion is identical with compression-expansion for the flow mechanism of amorphous Fe-Si-B alloy. The viscosity ~/of the amorphous state as well as that of liquid state [5] can be determined from eqn. (1). r/0A exp{B + b T (1 T - T~s
r/=
l/T)}
(1)
If it is assumed that a volume unit in the amorphous alloy is subjected to a force in a stress field, the contraction process which is related to the tension condition shortens the dimension of the sample across the lines of external stress, the flow direction is normal to the external stress, and the deformation takes place in three dimensions. Therefore one can try to correct
r
J
29
/
/
700 -~
28
I I
I
10 20
I
I
40
80
i
160
qb(K/min ) Fig. 2. The relationship between heating rate and viscous flow
temperature.
Fig. 4. The viscosity curves of amorphous Fe-Si-B alloy: A, isothermal tension; B, isothermal compression.
485 for compression stress with some kind of internal frictional coefficient #. Hence an estimated viscosity r/can be expressed as follows [6]: d~/dt = ~r/t
30
(2)
where d~/dt is the tension rate and tr is the external stress. Experimentally determined/~ values are about 0.30.5 for Ni-P and Fe-Si-B alloys in compression, which is almost consistent with the values estimated from the model described below. Because of the distinction between tensile and compressive stress, the dimension of yielding viscous flow is quite different. According to the model of region boundary, ~t should be 1/3 for the compression test and 1 for the tension test. A model called "region boundary" is suggested on the basis of the aforementioned experiments on the viscous flow of Fe-Si-B and Ni P amorphous alloys under the tension and compression. It is assumed that the amorphous alloys obtained from the melt consist of blocks of foundation bodies and mucous layers around them; the foundation bodies are dense, and congealed liquid regions which are kept apart by the less dense and congealed mucous layers, e.g. boundary layers during solidification of the melt. As the boundaries contain more free volume and defects than body regions they possess high mobility, and the mucous layers will be the first to unfreeze and extend as the amorphous alloy is heated, later followed by the activation of the foundation bodies. If there is additional external stress, the boundary layers may provide lubrication, and the amorphous alloys exhibit viscous flow behaviour as all regions and boundaries unfreeze.
Fig. 5. SEM fractography of fracture surface of Ni-P glass. The fracture surface after viscous flow of Ni-P glass is shown in Fig. 5. It can be seen that partial crystallization of the sample began on the shiny side (air), but the amorphous state remained near the dull side (wheel). However, the block foundation bodies and mucous layers would obviously be observed in the zone where crystallization has not taken place. The size of a block body was of estimated order of magnitude of I0-J/~m according to the size of the layers.
References 1 H. S. Chen and D. Turnbull, J. Chem. Phys., 48(1968) 25602571. 2 Hisamich Kimura, Takeo Murata and Tsuyoshi Masumoto, Sei. Rep. Res. Inst. Tohoku Univ. Set. A, 26(4-5) (1977). 3 A. I. Taub, in S. Steeb and H. Warlimont (eds.), Rapidly Quenched Metals, Vol. 2, North-Holland, Amsterdam, 1985, pp. 136.%1368. 4 Wang Jingtang, Ding Bingzhe, Li Shuling and Wei Xueli, in S. Steeb and H. Warlimont (eds.), Rapidly Quenched Metals, Vol. 2, North-Holland, Amsterdam, 1985, pp. 1361 1364. 5 Wang Jingtang et. aL, Proe. 6th Int. Conf on LiquM and Amorphous Metals, in Z. Phys. Chem., 156 (1987) 373-377. 6 O. V. Mazuriu, J, Non-Cryst. Solids, 25(1977) 129.
483
Viscous Flow of Amorphous Alloys Under Compressive and Tensile Stresses* WANG JINGTANG, DING BINGZHE, PANG DEXING, LI SHULING and LI GUSUNG Institute ~?fMetal Research, Academia Sinica, Shenyang (China)
2. Experimental details Abstract In this paper the "region boundary" model is suggested on the basis of the study of the viscous flow o f Ni P, Fe S i - B amorphous alloys under compression and tension. The flow coefficient determined from the experiments is about 0.3-0.5 which is almost consistent with the value estimated from the model. In order to obtain the ideal glass transformation temperature T~, a temperature Tvs is defined at which stable flow starts and extrapolating the Tvs- ~ curve to --, Ocorresponds to T~. The tests were performed with T M S - 2 and D S C - 2 thermal analysers. The obtained viscosity r1 may be extended to the liquid state according to the formula r/= ~/oA exp
B + bT ~1 l/r) T - T~
1. Introduction The viscous flow of amorphous alloys is one of the important characteristics that distinguishes them from the crystalline metal. The flow behaviour at temperatures near the glass transition temperature Tg has been investigated to some extent by the creep method [1 3], however, a more detailed study is desirable for a better understanding of the flow mechanism in amorphous metals. In this paper, research has been carried out using two methods, tension-expansion and compression-expansion tests, and the relationship between the two has been established. It is important to understand the formation ability of amorphous metal and to explore the internal relation between the liquid and amorphous metals. In particular, the viscous flow behaviour under compressive stress may provide the possibility of consolidating layers of amorphous ribbon to produce a thick strip for use as a large stacking transformer of amorphous metals.
*Paper presented at the Sixth International Conference on Rapidly Quenched Metals, Montr6al, August 3-7, 1987. 0025-5416/88/$3.50
Amorphous Fe7sSigB13 (Metglass 2605S2) and Ni89.3Plo7 alloys were prepared by single-roller melt spinning. Samples about 2 mm wide, 30/~m thick with a gauge length of 12 mm were taken from the ribbon for use in the tension creep tests. For the compression-expansion tests, the samples were about 3 mm wide, 30/~m thick and I0 mm in length, and X-ray diffraction confirmed that both samples were amorphous. Considering transients in the isothermal experiments, one hour was taken for pre-annealing the samples at a temperature lower than Tg. The viscous flow tests for Fe-Si-B and Ni-P glasses were performed with a Perkin-Elmer TMS-2 thermomechanical analyser equipped with a quartz tube assembly especially designed for the tests [4]. The measuring precision can be up to 0.01/~m. The external stress was applied to the sample by adding a weight to a loading adaptor. The friction between the specimen and the quartz tubes may be neglected because there is appropriate clearance between them. The crystallization temperature of the amorphous material was measured by a DSC-2 differential scanning calorimeter. All tests were conducted in an argon atmosphere. The observation of the fracture surface of the amorphous samples after measuring viscous flow was conducted in JSM-T200 scanning electron microscope.
3. Results and discussions The amorphous Fe-Si-B alloy was heated at a constant rate of 20 K min 1 under tension (A) and compression (B) in the TMS-2 instrument, the experimental curves obtained are shown in Fig. 1. The temperature T~v corresponds to the intersecting point of the extrapolated lines for the thermal expansion at low temperature and the viscous flow at high temperature and defines the temperature for the initiation of viscous flow. The temperature for the initiation of steady state flow is defined as steady-state viscous flow temperature Tvs. T~ shows the crystallization temperature.
,~? Elsevier Sequoia/Printed in The Netherlands
484 90 8o 7o 6o
0,~ A .J
.~ 0,3
50 ~o .q
30
0,2
2o
I
1o
TIK)
l"sv
I Tvs Tc
o,~
Fig. l. The expansion and viscous flow curves: A, under tension; B, under compression.
~o
7~0
7~
~o
~o
T(K) Fig. 3. The relative deformation curves of amorphous Fe-Si-B alloy: A, isothermal tension; B, isothermal compression.
Figure 2 shows a relationship between flow temperature Tsv, Tvs and the heating rate ~b. The TS, values obtained from extrapolating the Tvs-~b curve to ~b ~ 0 can be defined as a critical viscous flow temperature, and the point Tv~ obtained from extrapolating the Tw-~b curve to ~b ~ 0 corresponds to the ideal glass transformation temperature Tg. For this it is assumed that the atoms in the amorphous solid are thermally activated over a long period of time, as the sample is heated to TS~ over an infinite time. The relative change in the length during isothermal-tension (A) and isothermal-compression (B) for amorphous Fe--Si-B alloy is shown in Fig. 3. The difference between A and B is caused by the sign of the additional external stress. The viscosity of the amorphous alloy near Tg was investigated by both the tensile stress and the
compressive stress methods. The experimental results under isothermal tension (A) and isothermal compression (B) for various external stresses are given in Fig. 4. It can be seen that tension-expansion is identical with compression-expansion for the flow mechanism of amorphous Fe-Si-B alloy. The viscosity ~/of the amorphous state as well as that of liquid state [5] can be determined from eqn. (1). r/0A exp{B + b T (1 T - T~s
r/=
l/T)}
(1)
If it is assumed that a volume unit in the amorphous alloy is subjected to a force in a stress field, the contraction process which is related to the tension condition shortens the dimension of the sample across the lines of external stress, the flow direction is normal to the external stress, and the deformation takes place in three dimensions. Therefore one can try to correct
r
J
29
/
/
700 -~
28
I I
I
10 20
I
I
40
80
i
160
qb(K/min ) Fig. 2. The relationship between heating rate and viscous flow
temperature.
Fig. 4. The viscosity curves of amorphous Fe-Si-B alloy: A, isothermal tension; B, isothermal compression.
485 for compression stress with some kind of internal frictional coefficient #. Hence an estimated viscosity r/can be expressed as follows [6]: d~/dt = ~r/t
30
(2)
where d~/dt is the tension rate and tr is the external stress. Experimentally determined/~ values are about 0.30.5 for Ni-P and Fe-Si-B alloys in compression, which is almost consistent with the values estimated from the model described below. Because of the distinction between tensile and compressive stress, the dimension of yielding viscous flow is quite different. According to the model of region boundary, ~t should be 1/3 for the compression test and 1 for the tension test. A model called "region boundary" is suggested on the basis of the aforementioned experiments on the viscous flow of Fe-Si-B and Ni P amorphous alloys under the tension and compression. It is assumed that the amorphous alloys obtained from the melt consist of blocks of foundation bodies and mucous layers around them; the foundation bodies are dense, and congealed liquid regions which are kept apart by the less dense and congealed mucous layers, e.g. boundary layers during solidification of the melt. As the boundaries contain more free volume and defects than body regions they possess high mobility, and the mucous layers will be the first to unfreeze and extend as the amorphous alloy is heated, later followed by the activation of the foundation bodies. If there is additional external stress, the boundary layers may provide lubrication, and the amorphous alloys exhibit viscous flow behaviour as all regions and boundaries unfreeze.
Fig. 5. SEM fractography of fracture surface of Ni-P glass. The fracture surface after viscous flow of Ni-P glass is shown in Fig. 5. It can be seen that partial crystallization of the sample began on the shiny side (air), but the amorphous state remained near the dull side (wheel). However, the block foundation bodies and mucous layers would obviously be observed in the zone where crystallization has not taken place. The size of a block body was of estimated order of magnitude of I0-J/~m according to the size of the layers.
References 1 H. S. Chen and D. Turnbull, J. Chem. Phys., 48(1968) 25602571. 2 Hisamich Kimura, Takeo Murata and Tsuyoshi Masumoto, Sei. Rep. Res. Inst. Tohoku Univ. Set. A, 26(4-5) (1977). 3 A. I. Taub, in S. Steeb and H. Warlimont (eds.), Rapidly Quenched Metals, Vol. 2, North-Holland, Amsterdam, 1985, pp. 136.%1368. 4 Wang Jingtang, Ding Bingzhe, Li Shuling and Wei Xueli, in S. Steeb and H. Warlimont (eds.), Rapidly Quenched Metals, Vol. 2, North-Holland, Amsterdam, 1985, pp. 1361 1364. 5 Wang Jingtang et. aL, Proe. 6th Int. Conf on LiquM and Amorphous Metals, in Z. Phys. Chem., 156 (1987) 373-377. 6 O. V. Mazuriu, J, Non-Cryst. Solids, 25(1977) 129.