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Formation mechanism and structure of spike-like crystals
Scripta METALLURGICA et MATERIALIA
Vol. 30, pp. 373-375, 1994 Printed in the U.S.A.
Pergamon Press Ltd. All rights reserved
FORMATION MECHANISM AND STRUCTURE OF SPIKE-LIKE CRYSTALS Shan Liu
Tao H u a n g
Deyang Lu
Yaohe Zhou
National Lab of Solidification Processing Northwestern Polytechnical University, 7 1 0 0 7 2 , Xi' a n , China
(Received September 17, 1993) (Revised October 27, 1993) 1. INTRODUCTION Fluid flow exists in almost a n y crystal growth process and has a profound influence on the growth behavior and the internal quality of grown crystals. Therefore, a lot of research has been conducted and advances have been m a d e ; although most research concentrates on the case where fluid flows slowly (generally the flow velocity is less than 1 0 e r a / s ) . Only a few researchers point out the potentialities of rapid fluid flow in the preparation of functional crystals. Wilcox E13[-2] once predicted that the crystal quality would be improved if a rather strong regular convection is applied to reduce or eliminate the deleterious effects of irregular natural convection, and that the defects would not form if the flow velocity is high enough to ensure crystal growth to be controlled by interface kinetics. So it is of initial importance to study the crystal growth behavior with rapid fluid flow both experimentally and theoretically. In this paper the effect of rapid melt flow on the constrained dendrite growth was investigated and a new kind of growth morphology n a m e d SPIKE-LIKE CRYSTALS was found. Its formation m e c h a n i s m and structure were explored experimentally.
2. EXPERIMENTAL PROCEDURES The essential part of our experimental setup is sketched in Figure 1. Compared with the currently used directional solidification apparatus, this one has two characteristics : one is the convection generator which drives the propeller in the glass sample ceil with a spacing gap about 3501am in order to make the melt flow )the other is the supplementary heater which is used to ensure that the temperature gradient along the growth direction remains unchanged despite melt flowE3 ~. This is beneficial for analysis. The flow is perpendicular to growth direction. Transparent alloy S C N - 2 . 0 % w t Ace was employed in the experiment. T h e growth process was photographed under a SMI stereo-microscope. . . . . . .
t
. . . . .
7q
-I.
Fig. 1. The central part of our experimental setup 1. solid 2. melt 3. thermocouple 4. propeller 5. convection generator 6. cooler 7. supplementary heater 8. main heater
(6) 3. RESULTSAND DISCUSSION ( 1 ) Microstructure of Spike-like Crystals W h e n melt flows at high velocities (usually greater than 1 0 c m / s ) , there appears a remarkable change in the interface morphology of constrained dendrite growth, and spike-like crystals f o r m ( F i g u r e 2 ) . As can be seen from Figure 2, the s p i k e - - l i k e crystal column has a definite upstream deflection just as primary dendrites deflect to the upstream side when melt flow velocity is slow. But the tip structure is special : it is made up of crystal branches in two directions, one is parallel to the original primary a r m and the other is almost parallel to the original ripened secondary a r m (for convenience, define the former as F I R S T B R A N C H E S , a n d the latter as SECOND B R A N C H E S , as in Figure 3). From experimental observations, the growth process can be depicted in Figure 3. Two kinds of branches appear and grow alternatively at the L / S interface, which results in the formation of spike-like crystals.
373 0956-716X/94 $6.00 + .00 Copyright (c) 1993 Pergamon Press Ltd.
374
S P I K E - L I K E CRYSTALS
Vol.
50,
No.
3
melt flow. f first branches second
Fig. 2. Microstructure of spike-like crystals
Fig. 3. Schematic of the spik~like crystals
(2) Formation Process of Spike-like Crystals Figure 4 shows the whole transition process in which constrained dendrites turn into spike-like crystals. Figure 4a is the dendrite growth without melt flow. Dendritic growth is controlled only by the diffusion process. Figure 4b corresponds to the time when melt flows 23 seconds after the convection generator was operated. It is clear in Figure 4b that the dendrite tip deflects markedly to the upstream side and secondary arms on the upstream side grow rapidly but those on the downstream side are retarded completely and the initial secondary side-branches appear just at the dendrite tip. Figure 4c shows that theoriginal primary arms can not grow out of the macroscopic L/S interface, on the contrary, the rapid flow favors the growth of initial secondary arms, resulting in their competitive growth with primary arms. The sidebranches on the upstream side appear at the L/S interface. Figure 4d shows the formation of spike-like crystals, the first branches and second branches grow competitively at the interface. Figure 4e corresponds to the time when melt flows about 796 seconds, which reveals that the spike-like crystals are not a transient morphology but a stable one that can exist indefinitely if the growth condition is ensured (here the condition is the rapid melt flow).
(a) constrained dendrite array without melt flow
(b) 23 seconds after rapid melt flow is started
(c) 50 seconds under melt flow
Fig. 4.
(d) 120 seconds under melt flow
Transition process of constrained dendr~es to spikelike crystals (U.. melt flow velocity ! growth velocity R = 6. 59~m/s)
(e) 796 seconds under melt flow
From the successive photographs in Figure 4, one can inferred that the initial side-branches appear just at the dendrite tip and their growth is accelerated under a fast melt flow, and that the competitive growth of second branches with first branches results in the growth morphology of spike-like crystals. Obviously, all of these take place just at the dendrite tips.
Vol.
30,
No.
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S P I K E - L I K E CRYSTALS
375
(3) Structure of Spike-like Crystals The above description of the formation process of spike-like crystals reveals that their strueture is naturally clear, that is, they are composed of two kinds of branches. This can be verified through the following experiments, as shown in Figure 5. Figures 5a and 5b are similar to Figures 4a and 4d, showing the original constrained dendrites turning into spike-like crystals. Figure 5e indicates the instance when the convection generator is switched off and when the melt flow stops in approximately 6 seconds. It shows that as soon as the flow stops, first branches protrude from the L / S interface at once while second branches cease to grow for a while. Figure 5d shows the interface morphology after the melt stops flowing in about 161 seconds. It is elearer than Fig. 5e in the aspeet that the first branches grow protrusively and the initial side-branches appear behind the tip at a distance of 5-6R, (Rt:tip radius). This is just the same as for the tip shown in Figures 4a and 5a, the growth of whieh is controlled by the diffusion process. So it may be safe to ascribe the formation of spike-like crystals to the rapid melt flow.
(a) constrained dendrite array
m
(b) growth of spike-like crystals
(c) interface morphology for 6 seconds after melt flow is stopped
Fig. 5. Structure of spike-like crystals ( 3 : flow velocity)
(d) interface morphology for 161 seconds after melt flow is mopped
As can be seen from the above photographs, the microstructure of the spike-like crystals is much finer than in the ease where growth is controlled only by the diffusion process. This shows that spike-like crystals have the potential for improvement of engineering materials. 4. CONCLUSIONS (1) (2) (3) tive
Spike-like crystals will appear in the constrained crystal growth process if the melt flows rapidly. The spike-like crystal is a stable morphology. The spike-like crystal is made up of first branches and second branches. Its formation is the result of their competigrowth. REFERENCES
1. 2. 3.
W . R . Wileox, J. Crystal Growth, 124, 65(1983). W . R . Wileox, L.D. Fullmer, J. AppliedPhysies, 2201, 36(1965). Shan Liu, MS Thesis, Effects of Forced Melt Flow on the Constrained Crystal Growth Process (1986, Northwestern Polytechnieal University, Xi I an, China)
ACKNOWLEDGEMENT This research program is supported by the National Natural Science Foundation of China. We give thanks to Prof. N. J. Grant from MIT for his invaluable discussions and opinions about this paper.
Vol. 30, pp. 373-375, 1994 Printed in the U.S.A.
Pergamon Press Ltd. All rights reserved
FORMATION MECHANISM AND STRUCTURE OF SPIKE-LIKE CRYSTALS Shan Liu
Tao H u a n g
Deyang Lu
Yaohe Zhou
National Lab of Solidification Processing Northwestern Polytechnical University, 7 1 0 0 7 2 , Xi' a n , China
(Received September 17, 1993) (Revised October 27, 1993) 1. INTRODUCTION Fluid flow exists in almost a n y crystal growth process and has a profound influence on the growth behavior and the internal quality of grown crystals. Therefore, a lot of research has been conducted and advances have been m a d e ; although most research concentrates on the case where fluid flows slowly (generally the flow velocity is less than 1 0 e r a / s ) . Only a few researchers point out the potentialities of rapid fluid flow in the preparation of functional crystals. Wilcox E13[-2] once predicted that the crystal quality would be improved if a rather strong regular convection is applied to reduce or eliminate the deleterious effects of irregular natural convection, and that the defects would not form if the flow velocity is high enough to ensure crystal growth to be controlled by interface kinetics. So it is of initial importance to study the crystal growth behavior with rapid fluid flow both experimentally and theoretically. In this paper the effect of rapid melt flow on the constrained dendrite growth was investigated and a new kind of growth morphology n a m e d SPIKE-LIKE CRYSTALS was found. Its formation m e c h a n i s m and structure were explored experimentally.
2. EXPERIMENTAL PROCEDURES The essential part of our experimental setup is sketched in Figure 1. Compared with the currently used directional solidification apparatus, this one has two characteristics : one is the convection generator which drives the propeller in the glass sample ceil with a spacing gap about 3501am in order to make the melt flow )the other is the supplementary heater which is used to ensure that the temperature gradient along the growth direction remains unchanged despite melt flowE3 ~. This is beneficial for analysis. The flow is perpendicular to growth direction. Transparent alloy S C N - 2 . 0 % w t Ace was employed in the experiment. T h e growth process was photographed under a SMI stereo-microscope. . . . . . .
t
. . . . .
7q
-I.
Fig. 1. The central part of our experimental setup 1. solid 2. melt 3. thermocouple 4. propeller 5. convection generator 6. cooler 7. supplementary heater 8. main heater
(6) 3. RESULTSAND DISCUSSION ( 1 ) Microstructure of Spike-like Crystals W h e n melt flows at high velocities (usually greater than 1 0 c m / s ) , there appears a remarkable change in the interface morphology of constrained dendrite growth, and spike-like crystals f o r m ( F i g u r e 2 ) . As can be seen from Figure 2, the s p i k e - - l i k e crystal column has a definite upstream deflection just as primary dendrites deflect to the upstream side when melt flow velocity is slow. But the tip structure is special : it is made up of crystal branches in two directions, one is parallel to the original primary a r m and the other is almost parallel to the original ripened secondary a r m (for convenience, define the former as F I R S T B R A N C H E S , a n d the latter as SECOND B R A N C H E S , as in Figure 3). From experimental observations, the growth process can be depicted in Figure 3. Two kinds of branches appear and grow alternatively at the L / S interface, which results in the formation of spike-like crystals.
373 0956-716X/94 $6.00 + .00 Copyright (c) 1993 Pergamon Press Ltd.
374
S P I K E - L I K E CRYSTALS
Vol.
50,
No.
3
melt flow. f first branches second
Fig. 2. Microstructure of spike-like crystals
Fig. 3. Schematic of the spik~like crystals
(2) Formation Process of Spike-like Crystals Figure 4 shows the whole transition process in which constrained dendrites turn into spike-like crystals. Figure 4a is the dendrite growth without melt flow. Dendritic growth is controlled only by the diffusion process. Figure 4b corresponds to the time when melt flows 23 seconds after the convection generator was operated. It is clear in Figure 4b that the dendrite tip deflects markedly to the upstream side and secondary arms on the upstream side grow rapidly but those on the downstream side are retarded completely and the initial secondary side-branches appear just at the dendrite tip. Figure 4c shows that theoriginal primary arms can not grow out of the macroscopic L/S interface, on the contrary, the rapid flow favors the growth of initial secondary arms, resulting in their competitive growth with primary arms. The sidebranches on the upstream side appear at the L/S interface. Figure 4d shows the formation of spike-like crystals, the first branches and second branches grow competitively at the interface. Figure 4e corresponds to the time when melt flows about 796 seconds, which reveals that the spike-like crystals are not a transient morphology but a stable one that can exist indefinitely if the growth condition is ensured (here the condition is the rapid melt flow).
(a) constrained dendrite array without melt flow
(b) 23 seconds after rapid melt flow is started
(c) 50 seconds under melt flow
Fig. 4.
(d) 120 seconds under melt flow
Transition process of constrained dendr~es to spikelike crystals (U.. melt flow velocity ! growth velocity R = 6. 59~m/s)
(e) 796 seconds under melt flow
From the successive photographs in Figure 4, one can inferred that the initial side-branches appear just at the dendrite tip and their growth is accelerated under a fast melt flow, and that the competitive growth of second branches with first branches results in the growth morphology of spike-like crystals. Obviously, all of these take place just at the dendrite tips.
Vol.
30,
No.
3
S P I K E - L I K E CRYSTALS
375
(3) Structure of Spike-like Crystals The above description of the formation process of spike-like crystals reveals that their strueture is naturally clear, that is, they are composed of two kinds of branches. This can be verified through the following experiments, as shown in Figure 5. Figures 5a and 5b are similar to Figures 4a and 4d, showing the original constrained dendrites turning into spike-like crystals. Figure 5e indicates the instance when the convection generator is switched off and when the melt flow stops in approximately 6 seconds. It shows that as soon as the flow stops, first branches protrude from the L / S interface at once while second branches cease to grow for a while. Figure 5d shows the interface morphology after the melt stops flowing in about 161 seconds. It is elearer than Fig. 5e in the aspeet that the first branches grow protrusively and the initial side-branches appear behind the tip at a distance of 5-6R, (Rt:tip radius). This is just the same as for the tip shown in Figures 4a and 5a, the growth of whieh is controlled by the diffusion process. So it may be safe to ascribe the formation of spike-like crystals to the rapid melt flow.
(a) constrained dendrite array
m
(b) growth of spike-like crystals
(c) interface morphology for 6 seconds after melt flow is stopped
Fig. 5. Structure of spike-like crystals ( 3 : flow velocity)
(d) interface morphology for 161 seconds after melt flow is mopped
As can be seen from the above photographs, the microstructure of the spike-like crystals is much finer than in the ease where growth is controlled only by the diffusion process. This shows that spike-like crystals have the potential for improvement of engineering materials. 4. CONCLUSIONS (1) (2) (3) tive
Spike-like crystals will appear in the constrained crystal growth process if the melt flows rapidly. The spike-like crystal is a stable morphology. The spike-like crystal is made up of first branches and second branches. Its formation is the result of their competigrowth. REFERENCES
1. 2. 3.
W . R . Wileox, J. Crystal Growth, 124, 65(1983). W . R . Wileox, L.D. Fullmer, J. AppliedPhysies, 2201, 36(1965). Shan Liu, MS Thesis, Effects of Forced Melt Flow on the Constrained Crystal Growth Process (1986, Northwestern Polytechnieal University, Xi I an, China)
ACKNOWLEDGEMENT This research program is supported by the National Natural Science Foundation of China. We give thanks to Prof. N. J. Grant from MIT for his invaluable discussions and opinions about this paper.