- Email: [email protected]
A gas-water atomization process for producing amorphous powders
Scripta
METALLURGICA
Vol. 13, pp. 6 7 3 - 6 7 6 , 1979 P r i n t e d in the U.S.A.
P e r g a m o n Press Ltd. All r i g h t s r e s e r v e d .
A GAS-WATER ATOMIZATION PROCESS FOR PRODUCING AMORPHOUS POWDERS
S.A. Miller and R.J. Murphy Department of Mechanical Engineering Northeastern University Boston, Massachusetts (Received
February
8,
1979)
Introduction The p r o m i s e o f n o v e l a p p l i c a t i o n s of the u n i q u e p r o p e r t i e s o f m e t a l l i c g l a s s e s ( 1 , 2 , 3 ) h a s p r o m p t e d s e r i o u s e f f o r t o v e r t h e p a s t d e c a d e t o p r o d u c e amorphous a l l o y s i n q u a n t i t y . P r e s e n t l y t e c h n i q u e s e x i s t t o p r o d u c e r i b b o n s ( 4 ) , t h i n s h e e t s ( 5 ) , w i r e , and L/D powder ( s h o r t r o d s ) ( 6 ) . However, a t t e m p t s a t t h e p r o d u c t i o n o f t r u e powders w h i c h a r e e n t i r e l y amorphous h a v e met w i t h o n l y l i m i t e d s u c c e s s ( 7 , 8 ) . This paper presents an atomizing system b y wb_~ch h i g h y i e l d s of 37 Um a n d f i n e r amorphous Cu60Zr~0 powder h a v e b e e n p r o d u c e d . Experimental The a t o m l z i n g u n i t ( F i g u r e I ) c o n s i s t s o f t h r e e s e p a r a t e s e c t i o n s ; a r e s i s t a n c e h e a t e d vacuum f u r n a c e , a l a r g e d l a m e t e r a c r y l i c t u b e t o c o n f i n e t h e a t o m i z i n g and q u e n c h i n g p r o c e s s , and a c o n e - s h a p e d c o l l e c t i o n and s e d i m e n t a t i o n s y s t e m . The furnace utilizes primary and secondary heaters for the graphite crucible and melt guide tube, respectively. The charge is bottom poured from the crucible through the melt guide tube and the gas atomization nozzle. The nozzle is mounted on the inside floor of the furnace directly above a port which opens into the acrylic atomlzing-quenching tube by means of a sliding door. At the top of the large acrylic tube is a series of argon ports used to provide a blanket of inert gas in the atomization region. Directly below the ports are four water Jets positioned such that their focal point is on the axis of the melt stream and located approxlmerely 1 cm below the focal point of the atomization nozzle. This tube empties into a collection/sedimenta~ion system located below. Baffled outlets allow the spent gas to escape while separating out the suspended water and fines. In this experiment, the Cu60Zr40 alloy was prepared by vacuum melting reactor grade 21 zirconium sponge with OFHC copper in an induction furnace. In the atomization unit the alloy was remelted and superheated to 1470 K under vacuum. Prior to atomization the furnace was backfilled with argon to a slight positive pressure, the sliding door opened, and the gas atomization nozzle and surrounding water nozzles activated. The stopper rod was then removed, allowing the alloy to flow to the nozzle for atomization. Argon gas at 4.2 MPa was used for atomization and the water nozzles were operated at a pressure of .45 MPa. After sedimentation, the damp powders were vacuum dried, sieved, and analyzed by x-ray diffraction (~R%D) for the presence of equilibrium and non-equilibrlum structures. Those powders which were found to be amorphous were then further evaluated by scanning calorimetry. Results Figure 2 shows the XRD patterns of the Cu60Zr~0 powder atomized with this unit. The patterns show that powders less than 37 microns in diameter are amorphous. They also reveal the gradual transition with increasing particle size from amorphous to equilibrium structure. The results of the powder classification are shown in Table i. All Cus0Zr40 runs were initiated with a crucible charge of 400 g. Complete atomization of the entire charge was
673 0036-9748/79/080673-04502.00/0 C o p y r i g h t (c) 1979 P e r g a m o n Press
Ltd.
674
.ATOMIZATION
OF A M O R P H O U S
POWDERS
Vol.
13, No.
TABLE 1 Powder Classification and Yields Resulting from Crucible Charges of 400 Grams Ren#
Powder Yield ( ~ 0-595 ~m
1
2*
3*
4
322.6
28.8
72.3
287.8
.17
.05
Fraction of Yield by Particle Size 297-595 ~m
.19
105-297 tea
.34
.33
.17
.37
63-105 ~m~
.16
.06
.6
.19
44-63 tim
.i0
.10
.12
.15
37-44 ~tm
.04
.11
.8
.10
0-37 ~m
.17
.40
.40
.14
m
l,
*gas atomization nozzle shut off b e c a u s e of melt flow stoppage
never achieved because of crucible wetting and process instabilities which at times necessitated shutting off the gas nozzle and allowing the melt to pour directly into the collection unit. Thereforep the yields shown are the total amount of powders produced that are less than 595 ~m in diameter. This criteria results in low ylelds for runs 2 and 3 since melt flow stoppage occurred and the atomization process was terminated to prevent damage to the nozzle. Amorphous powder fractional yields varied from a low of .14 to a high of .40. These measured fractions are probably lower than the actual fractions because of the large range of particle sizes included in the standard sieve size increments and the next larger particle size range (Table 1) includes significant evidence of the diffuse peak (Figure 2) characteristic of amorphous materials. The morphology of the glassy metal powders is shown in Figure 3. The shape varies from spherlcal (for the smaller particles) to highly irregular (for the larger particles). Work is now in progress to correlate the particle shape dlstributlon to particle history during the atomlzation-quenchlng process. An AI95Cu05 alloy was used to quantify the cooling rate obtained with this process. By means of reported dendrite arm spacing correlations (9,10), the cooling rate was deter~,4-ed to be in the order of 105 to 106 K/s. The differential sc--ning calorimeter results (Figure 4) show that the alloy exhibits a glass transition temperature ( T ~ . However~ the glass-undercooled liquid transformation is interrupted by the onset of crystallization. Since it is possible that the full change in the specific heat is not observed, the actual Tg cannot be determined using the midpoint method. Therefore, Tg was evaluated by the less common method of using the intersection of the extrapolated baseline with the steepest tangent associated with the rise of the endothermlc transformatlon. For these powders, the measured T g = 742 K was approximately 20 K higher than that reported for the splat cooled alloy (ii). This difference was initially believed to be the result of a hlgh oxygen content (12), however, chemlcal analysis has r e v e a l e d a content of lass than 15 PPM oxygen. Work is in progress to detezmlne the source of this ATg.
8
Vol.
13, No.
8
A T O M I Z A T I O N OF A M O R P H O U S POWDERS
- -
voc~
675
Che~ber
CruCible
R n ~ l t ~ ¢ ¢ H*oters
g
~
~ ~
Ar~lon 8 . . . . --
Portl
Cleor ~ r y l i ¢ Tube
Mesh Bofflir~ 0
5*ttlinQ TQnk
z~ z
m
FIG. 3 SEM photomicrograph of powders less than 37 ~m in diameter. lqG. 1 Schematic of powder atomizationquenching unit.
' ' I ' ' '
63_,o5. I
i
5O
I
.
,
I i 500
,
' I ' ' ' ' I
,
i
I
,
,
i
,
I 1 1 1 1 1 1 , ,
l , 600
,
,
,I
,
TEMPERATURE,
i
4O
5O
20 FIG. 2 XRD p a t t e r n s of d i f f e r e n t powder f r a c t i o n s ( C u ~ r a d i a t i o n ) .
i
I , , I , I
,
,
i i 700
,
,
t
i,
i
,
QK
FIG. 4 Thermog~ams of amorphous and c r y s t a l l i n e powder heated a t 80 K/rain.
,i
,
,
J
I 800
676
ATOMIZATION
OF A M O R P H O U S
POWDERS
Vol.
15, No.
Acknowledgements We a r e g r a t e f u l to P r o f e s s o r N . J . Grant f o r the use of the n o z z l e , Kevin C. Hughes f o r h i s l a b o r a t o r y a s s i s t a n c e , and Dr. D.E. Polk f o r the c a l o r i m e t r y work. This r e s e a r c h was funded by a g ran t from The G i l l e t t e Company. References 1. 2. 3. 4. 5.
L.A. Davis, M e t a l l i c Glasses, p. 190, ASM, Metals Park, Ohio (1978). H . J . GUntherodt and H.U. KUnzu, M e t a l l i c C l a s s e s , p. 247, ASM, Metals Park, Ohio (1978). E.M. CY0rgY, M e t a l l i c C l a s s e s , p. 275, ASM, Metals Park, Ohio (1978). R. Pond and R. Raddin, Trans. TMS-AL~E 245, 2475 (1969). A.E.E. S i n g e r , Rapid S o l i d i f i c a t i o n P r o c e s s i n g , p. 154, Massachusetts I n s t i t u t e of Technology P r e s s , Cambridge, Massachusetts (1978). 6. E.E. R a r l n g e r and C.E. Nobley, Rapid S o l i d i f i c a t i o n P r o c e s s i n g , p. 208, Massachusetts I n s t i t u t e of Technology P r e s s , Cambridge, Massachusetts (1978). 7. T. Yamaguchi and K. N a r l t a , Appl. Phys. L e t t . 33, 468 (1978). 8. E. Smith, M.S. T h e s i s , N o r t h e a s t e r n U n i v e r s i t y , Boston, Massachusetts (1976). 9. H. Hatya, B.C. ~ e s s e n , and N.J. Grant, J . I n s . Metals 96, 30 (1968). 10. H. J o n e s , J . Hats. S c i . 7, 349 (1972). J1. E.V. Raman, Ph.D. T h e s i s , N o r t h e a s t e r n U n i v e r s i t y , Boston, Massachusetts (1977). 12. D.E. Polk, C.E. Dube, and B.C. Giessen, Rapidly Quenched Metals I I I , 1, p. 220, The Metals S o c i e t y , London (1978)
8
METALLURGICA
Vol. 13, pp. 6 7 3 - 6 7 6 , 1979 P r i n t e d in the U.S.A.
P e r g a m o n Press Ltd. All r i g h t s r e s e r v e d .
A GAS-WATER ATOMIZATION PROCESS FOR PRODUCING AMORPHOUS POWDERS
S.A. Miller and R.J. Murphy Department of Mechanical Engineering Northeastern University Boston, Massachusetts (Received
February
8,
1979)
Introduction The p r o m i s e o f n o v e l a p p l i c a t i o n s of the u n i q u e p r o p e r t i e s o f m e t a l l i c g l a s s e s ( 1 , 2 , 3 ) h a s p r o m p t e d s e r i o u s e f f o r t o v e r t h e p a s t d e c a d e t o p r o d u c e amorphous a l l o y s i n q u a n t i t y . P r e s e n t l y t e c h n i q u e s e x i s t t o p r o d u c e r i b b o n s ( 4 ) , t h i n s h e e t s ( 5 ) , w i r e , and L/D powder ( s h o r t r o d s ) ( 6 ) . However, a t t e m p t s a t t h e p r o d u c t i o n o f t r u e powders w h i c h a r e e n t i r e l y amorphous h a v e met w i t h o n l y l i m i t e d s u c c e s s ( 7 , 8 ) . This paper presents an atomizing system b y wb_~ch h i g h y i e l d s of 37 Um a n d f i n e r amorphous Cu60Zr~0 powder h a v e b e e n p r o d u c e d . Experimental The a t o m l z i n g u n i t ( F i g u r e I ) c o n s i s t s o f t h r e e s e p a r a t e s e c t i o n s ; a r e s i s t a n c e h e a t e d vacuum f u r n a c e , a l a r g e d l a m e t e r a c r y l i c t u b e t o c o n f i n e t h e a t o m i z i n g and q u e n c h i n g p r o c e s s , and a c o n e - s h a p e d c o l l e c t i o n and s e d i m e n t a t i o n s y s t e m . The furnace utilizes primary and secondary heaters for the graphite crucible and melt guide tube, respectively. The charge is bottom poured from the crucible through the melt guide tube and the gas atomization nozzle. The nozzle is mounted on the inside floor of the furnace directly above a port which opens into the acrylic atomlzing-quenching tube by means of a sliding door. At the top of the large acrylic tube is a series of argon ports used to provide a blanket of inert gas in the atomization region. Directly below the ports are four water Jets positioned such that their focal point is on the axis of the melt stream and located approxlmerely 1 cm below the focal point of the atomization nozzle. This tube empties into a collection/sedimenta~ion system located below. Baffled outlets allow the spent gas to escape while separating out the suspended water and fines. In this experiment, the Cu60Zr40 alloy was prepared by vacuum melting reactor grade 21 zirconium sponge with OFHC copper in an induction furnace. In the atomization unit the alloy was remelted and superheated to 1470 K under vacuum. Prior to atomization the furnace was backfilled with argon to a slight positive pressure, the sliding door opened, and the gas atomization nozzle and surrounding water nozzles activated. The stopper rod was then removed, allowing the alloy to flow to the nozzle for atomization. Argon gas at 4.2 MPa was used for atomization and the water nozzles were operated at a pressure of .45 MPa. After sedimentation, the damp powders were vacuum dried, sieved, and analyzed by x-ray diffraction (~R%D) for the presence of equilibrium and non-equilibrlum structures. Those powders which were found to be amorphous were then further evaluated by scanning calorimetry. Results Figure 2 shows the XRD patterns of the Cu60Zr~0 powder atomized with this unit. The patterns show that powders less than 37 microns in diameter are amorphous. They also reveal the gradual transition with increasing particle size from amorphous to equilibrium structure. The results of the powder classification are shown in Table i. All Cus0Zr40 runs were initiated with a crucible charge of 400 g. Complete atomization of the entire charge was
673 0036-9748/79/080673-04502.00/0 C o p y r i g h t (c) 1979 P e r g a m o n Press
Ltd.
674
.ATOMIZATION
OF A M O R P H O U S
POWDERS
Vol.
13, No.
TABLE 1 Powder Classification and Yields Resulting from Crucible Charges of 400 Grams Ren#
Powder Yield ( ~ 0-595 ~m
1
2*
3*
4
322.6
28.8
72.3
287.8
.17
.05
Fraction of Yield by Particle Size 297-595 ~m
.19
105-297 tea
.34
.33
.17
.37
63-105 ~m~
.16
.06
.6
.19
44-63 tim
.i0
.10
.12
.15
37-44 ~tm
.04
.11
.8
.10
0-37 ~m
.17
.40
.40
.14
m
l,
*gas atomization nozzle shut off b e c a u s e of melt flow stoppage
never achieved because of crucible wetting and process instabilities which at times necessitated shutting off the gas nozzle and allowing the melt to pour directly into the collection unit. Thereforep the yields shown are the total amount of powders produced that are less than 595 ~m in diameter. This criteria results in low ylelds for runs 2 and 3 since melt flow stoppage occurred and the atomization process was terminated to prevent damage to the nozzle. Amorphous powder fractional yields varied from a low of .14 to a high of .40. These measured fractions are probably lower than the actual fractions because of the large range of particle sizes included in the standard sieve size increments and the next larger particle size range (Table 1) includes significant evidence of the diffuse peak (Figure 2) characteristic of amorphous materials. The morphology of the glassy metal powders is shown in Figure 3. The shape varies from spherlcal (for the smaller particles) to highly irregular (for the larger particles). Work is now in progress to correlate the particle shape dlstributlon to particle history during the atomlzation-quenchlng process. An AI95Cu05 alloy was used to quantify the cooling rate obtained with this process. By means of reported dendrite arm spacing correlations (9,10), the cooling rate was deter~,4-ed to be in the order of 105 to 106 K/s. The differential sc--ning calorimeter results (Figure 4) show that the alloy exhibits a glass transition temperature ( T ~ . However~ the glass-undercooled liquid transformation is interrupted by the onset of crystallization. Since it is possible that the full change in the specific heat is not observed, the actual Tg cannot be determined using the midpoint method. Therefore, Tg was evaluated by the less common method of using the intersection of the extrapolated baseline with the steepest tangent associated with the rise of the endothermlc transformatlon. For these powders, the measured T g = 742 K was approximately 20 K higher than that reported for the splat cooled alloy (ii). This difference was initially believed to be the result of a hlgh oxygen content (12), however, chemlcal analysis has r e v e a l e d a content of lass than 15 PPM oxygen. Work is in progress to detezmlne the source of this ATg.
8
Vol.
13, No.
8
A T O M I Z A T I O N OF A M O R P H O U S POWDERS
- -
voc~
675
Che~ber
CruCible
R n ~ l t ~ ¢ ¢ H*oters
g
~
~ ~
Ar~lon 8 . . . . --
Portl
Cleor ~ r y l i ¢ Tube
Mesh Bofflir~ 0
5*ttlinQ TQnk
z~ z
m
FIG. 3 SEM photomicrograph of powders less than 37 ~m in diameter. lqG. 1 Schematic of powder atomizationquenching unit.
' ' I ' ' '
63_,o5. I
i
5O
I
.
,
I i 500
,
' I ' ' ' ' I
,
i
I
,
,
i
,
I 1 1 1 1 1 1 , ,
l , 600
,
,
,I
,
TEMPERATURE,
i
4O
5O
20 FIG. 2 XRD p a t t e r n s of d i f f e r e n t powder f r a c t i o n s ( C u ~ r a d i a t i o n ) .
i
I , , I , I
,
,
i i 700
,
,
t
i,
i
,
QK
FIG. 4 Thermog~ams of amorphous and c r y s t a l l i n e powder heated a t 80 K/rain.
,i
,
,
J
I 800
676
ATOMIZATION
OF A M O R P H O U S
POWDERS
Vol.
15, No.
Acknowledgements We a r e g r a t e f u l to P r o f e s s o r N . J . Grant f o r the use of the n o z z l e , Kevin C. Hughes f o r h i s l a b o r a t o r y a s s i s t a n c e , and Dr. D.E. Polk f o r the c a l o r i m e t r y work. This r e s e a r c h was funded by a g ran t from The G i l l e t t e Company. References 1. 2. 3. 4. 5.
L.A. Davis, M e t a l l i c Glasses, p. 190, ASM, Metals Park, Ohio (1978). H . J . GUntherodt and H.U. KUnzu, M e t a l l i c C l a s s e s , p. 247, ASM, Metals Park, Ohio (1978). E.M. CY0rgY, M e t a l l i c C l a s s e s , p. 275, ASM, Metals Park, Ohio (1978). R. Pond and R. Raddin, Trans. TMS-AL~E 245, 2475 (1969). A.E.E. S i n g e r , Rapid S o l i d i f i c a t i o n P r o c e s s i n g , p. 154, Massachusetts I n s t i t u t e of Technology P r e s s , Cambridge, Massachusetts (1978). 6. E.E. R a r l n g e r and C.E. Nobley, Rapid S o l i d i f i c a t i o n P r o c e s s i n g , p. 208, Massachusetts I n s t i t u t e of Technology P r e s s , Cambridge, Massachusetts (1978). 7. T. Yamaguchi and K. N a r l t a , Appl. Phys. L e t t . 33, 468 (1978). 8. E. Smith, M.S. T h e s i s , N o r t h e a s t e r n U n i v e r s i t y , Boston, Massachusetts (1976). 9. H. Hatya, B.C. ~ e s s e n , and N.J. Grant, J . I n s . Metals 96, 30 (1968). 10. H. J o n e s , J . Hats. S c i . 7, 349 (1972). J1. E.V. Raman, Ph.D. T h e s i s , N o r t h e a s t e r n U n i v e r s i t y , Boston, Massachusetts (1977). 12. D.E. Polk, C.E. Dube, and B.C. Giessen, Rapidly Quenched Metals I I I , 1, p. 220, The Metals S o c i e t y , London (1978)
8