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Solidification characteristics of atomized AlTi powders
S c r i p t a METALLURGICA e t MATERIALIA
V o l . 26, pp. 6 9 7 - 7 0 2 , 1992 Printed in the U.S.A.
Pergamon P r e s s p l c All rights reserved
SOLIDIFICATION CHARACTERISTICS OF ATOMIZED AI-Ti POWDERS M. Gupta, F.A. Mohamed and E.J. Lavernia Materials Science and Engineering Section Department of Mechanical and Aerospace Engineering University of California Irvine, CA 92717 ( R e c e i v e d December 20,
1991)
1. Intro4ucfion The highly non-equilibrium conditions that are present during rapid solidification processing have prompted the development of novel alloy compositions that would otherwise be unattainable by conventional solidification (1, 2). Inspection of the available scientific literature demonstrates that the field of rapid solidification has continued to evolve since this phenomenon was first reported by Duwez et al. (3) in 1960. Rapid solidification may be attained experimentally through a variety of techniques, including: melt spinning, atomization and gun quenching (4, 5). Among these techniques, atomization has received considerable attention since it allows for the synthesis of a broad range of alloy compositions with extensive control over the resulting powder characteristics and properties. An interesting example of the application of rapid solidification processing to the development of novel alloy compositions is provided by the AI-Ti system. Rapidly solidified AI-Ti alloys derive their excellent strength, ductility and creep resistance from their refined microstructure and dispersion of AI3Ti particles, in combination with the low solid solubility [0.8 at.%] and low volume diffusivity [3.86 x 10-15 at.% cm2/sec, at 427 °C] of Ti in AI (6-9). The objective of the present study was to provide insight into the solidification microstructures and segregation patterns that are present in micron-sized AI-Ti powders, with emphasis on the synergism between microstructure and processing. 2. Experimental Procedure ProcessinP
The composition of the AI-Ti alloy used in the present study was obtained by mixing an AI-6.0 wt.%Ti master alloy provided by KBAlloys Inc. [Robards, KY] with commercial purity A1 [99.99%] provided by the Aluminum Company of America [Pittsburgh, PAl. These two master alloys were mixed in order to obtain a target alloy composition of AI-5.1 wt.% Ti. In order to ensure dissolution of the primary AI3Ti intermetallic phase and to promote sufficient mixing during melting, the alloy was slowly heated [under a nitrogen atmosphere] in a zirconia crucible to the desired superheat temperature, as shown in Table 1. The melt was subsequently maintained at the superheat temperature for approximately 10-20 minutes prior to atomization. The chemical composition of the material was verified following the atomization experiment. The powders were synthesized according to the following procedure. The matrix material was superheated to the particular temperature of interest [see Table 1], and disintegrated into a fine dispersion of micron-sized droplets using nitrogen gas at a preselected pressure. Following atomization, the rapidly quenched powders were separated from the atomization gas using a cyclone separator. In order to avoid extensive oxidation of the AI - Ti alloy during processing, the experiment was conducted inside an environmental chamber. The latter was evacuated down to a pressure of 150 microns of Hg and backfilled with inert gas [N2] prior to melting and atomization. The primary experimental parameters used during the experiment are listed in Table 1.
697 0056-9748/92 $5.00 + .00 Copyright (c) 1992 Pergamon Press plc
698
SOLIDIFICATION OF A 1 - T i POWDERS
Vol.
26, No.
TABLE 1. Experimental Parameters. Variable
Description
Units
Matrix alloy Atomization pressure Atomization gas Pouring Temperature Metal flow rate Gas flow rate
A1 - 5.1Ti 1.3 8 N2 1573 0.032 0.011
wt. % MPa I( kg/s kg/s
M icrostructur¢ Microstructural characterization studies were conducted on the atomized powders in order to establish the morphological features and the segregation pattern of Ti at the surface, as compared to that in the center of the powders. Scanning electron microscopy [SEM] studies were conducted on plastic mounted, polished, and gold coated samples using a HITACHI S-500 microscope in order to determine the rnicrostructural characteristics of powders of different sizes. The samples were etched with Keller's reagent [0.5 HF - 1.5 HCI - 2.5 HNO3- 95.5 H2OI prior to examination. Transmission electron microscopy studies were performed using a Philips CM 20 equipped with EDS [Energy Dispersive Spectroscopy] at an operating voltage of 200 keV. The TEM specimens were prepared by dispersing powders on a polymer coated Cu grid. The amount of Ti present at a particular location was then determined from microanalysis studies using EDS with a beam spot size of 40 rim. The objective of these studies was to determine the quantity of Ti present on the surface of the powders, relative to that present in the interior of the powders. A schematic diagram showing the locations on the powder surface used for EDS analysis is shown in Figure 1. ~, Results Microstructure SEM studies revealed that, for the size range of interest [e.g., 3 I.tm - 120 I.tm] the powders generally exhibited a spherical morphology, which is typical of gas atomized powders [see Figures 2-5]. Examination of the solidification morphology revealed that there were three types of microstructures present in the atomized powders. In powders less than 10 I~m in diameter, the microstructure was often featureless, and no dendrites or ceils were evident [see Figure 2a]. In powders with a diameter larger than 10 I.tm, but smaller than 100 I,tm, the microstructure generally exhibited a cellular morphology [see Figure 2b]. Finally, in powders larger than 100 I.tm the microstructure was typically dendritic [see Figure 2c]. In most of the powders studied, the solidification microstructure was indicative of multiple nucleation events. One notable exception is shown in Figure 3, where the microstructural features are suggestive of a single nucleation event. In addition, SEM studies also revealed the presence of star shaped precipitates, presumably of AI3Ti (10) type, on the surface of the powders [see Figure 4]. The spherical morphology of the powders was also evident from the TEM studies, as shown in Figure 5. The presence of satellites [small spherical protuberations] attached to the surface of the powders is also evident in this figure. These nanometer sized powders, formed as a result of the violent interaction of the solidifying metal and the atomization gas, are typical of gas atomized powders. The EDS results conducted on the surface and the center of powders of various sizes are summarized in Table 2 and Figure 6. The results show a high concentration of Ti at the surface and lower concentration in the center, for all the powders sizes studied [ the ratio R (CTi stnraee/C,ri center ) was always greater than 1]. In addition, EDS analyses conducted at different surface location] on a single" powder also yielded a value of the ratio R greater than 1, irrespective of the surface location [see Table 3].
5
Vol.
26,
No.
5
SOLIDIFICATION OF A 1 - T i POWDERS
699
TABLE 2. Results of EDS Analyses Showing Segregation Pattern of Ti in Atomized Powders. Powder size(I.tm)
3.12
4.08
6.28
8.20
8.86
R = CTi' surface/ CTi. center
1.23
1.78
1.05
2.28
4.28
TABLE 3. Results of the EDS Analyses Conducted at Different Locations in a Single Powder. Powder size (I.tm)
*Location
**R = CTi ' surface/CTi ' center
8.40 8.40 8.40 8.40
Surface Surface Surface Surface
1.40 1.84 3.09 3.55
*Each surface location was 90" apart from each other. **The concentration of Ti in the central region of the powder was 1.76 at. % [3.17 wt. %].
4. Discussion The solidification microstructures formed in the droplets depend to a great extent on the thermal history, undercooling and size of the droplets (11-13). The various stages encountered by droplets of small and large diameters during solidification, and their anticipated microstructural features are shown schematically in Figure 7 (14, 15). During atomization, small diameter droplets [e.g., <10 lam] will experience higher cooling rates, with concomitant microstructural refinement and decreased concentration gradients, than those experienced by large diameter droplets [e.g. > 100 I.tm] (5). The present results, summarized in Table 2 and Figure 6, show that the ratio 'R' approaches 1 with decreasing powder size, and hence are consistent with the expected trend. These results are supported by the microstructural studies from Figure 2a, which show that powders smaller than 10 I.tm generally exhibited a solidification microstructure indicative of plane front solidification, whereas powders larger than 10 t.tm generally exhibited a cellular or dendritic rnicrostructure [Figures 2b and 2c]. Regarding the initiation of solidification a few comments are in order. In the peritectic AI-Ti system the first solid to form is rich in Ti [k > 1], relative to the starting liquid composition (16, 17). EDS results conducted on powders in the 3 I.tm to 9 ~m size range [see Table 2, Table 3 and Figure 6] show that the Ti concentration is higher at the surface relative to that present in the center of the powders. These results suggest, first, that the solidification events were initiated on the surface of the powders, and second, that solidification was not partitionless, even in powder sizes as small as 3 I.tm. In addition, careful examination of Figure 2b shows that the microstructural features present on the surface of the powders were more refined relative to those present in the central region. This may be attributed to the higher quench rates experienced by the surface of the powders [due to the efficient heat convection to the atomizing gas] relative to those experienced by the central region [due to the thermal effects associated with recalescence]. In addition, initiation of solidification at the surface will enrich the surface with Ti which in turn will favor the formation of AI3Ti at the surface. This is evident from the AI-Ti phase diagram which shows that the formation of AI3Ti is favored with increasing Ti concentration [from 1.15 wt. % to ~ 37.3 wt. % Ti]. Experimental support to this suggestion is provided by the presence of AI3Ti precipitates on the surface of the powders [see Figure 4]. Regarding the nucleation of solidification in the droplets, the microstructural features of the powders are suggestive of multiple nucleation events on the surface. This was generally noted to be the case for most of the powder sizes studied. One notable exception is shown in Figure 3, where the solidification features are indicative of a single nucleation event in this particular powder.
700
SOLIDIFICATION OF A I - T i POWDERS
Vol.
26, No.5
5. Conclu~ign~ The following conclusions may be drawn for the solidification of atomized A1-Ti droplets: 1.
2. 3. 4.
Solidification is not partitionless for droplet sizes as small as 3 l.tm in diameter, and the first solid formed is rich in Ti. The segregation patterns obtained for powders of diameter less than 10 I.tmindicates that solidification during atomization initiated at the surface. In powders less than 10 I.tm, the microstructure is often featureless, and no dendrites or cells are evident. In powders larger than 10 I.tm, but smaller than 100 I~m,the microstructure is generally cellular. Finally, in powders larger than 100 gm, the microstructure is generally dendritic. The solidification microstructures present in the entire range of powder sizes studied are indicative of multiple nucleation events. Acknowledgements
Financial support from the Army Research Office under grant no. grant # DAALO3-89-K-0027 and the National Science Foundation under grant no. MSS-8957449 is gratefully acknowledged. In addition, the authors would like to thank Prof. M. Mecartney and Vikram Joshi for useful discussions. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.
H. Jones, J. Mat. Sci.. 19, 1043 (1984). Wolfgang G. J. Bunk, Mat. Sci. Eng., 134A, 1087 (1991). P. Duwez, R.H. Willens and W. Klement, J. Appl. Phys., 31, 1136 (1960) T.R. Ananthraman, C. Suryanarayana, J. Mat. Sci., 6, 1111 (1971). C.G. Levi and R. Mehrabian, Metall. Trans., 13A, 221 (1982). " M. E. Fine, Dispersion Strengthened Aluminum Alloys, Y. W. Kim, W. M. Griffith, eds., The Metallurgical Society, Warrendale, PA, 1988, p. 103. P.K. Mirchandani and R. C. Benn, Experimental High Modulus Elevated Temperature AI-Ti Based Alloys by Mechanical Alloying, SAMPE, Covina, CA (1988). W.E. Frazier and M. J. Koczak, Elevated Temperature Aluminum-Titanium alloy by Powder Metallurgy Process, United States Patent, No. 4,834,942, May 30, 1989. W.E. Frazier and M. J. Koczak, High Strength Powder Metallurgy Aluminum Alloys II, G. H. Hildeman, M. J. Koczak, eds., The Metallurgical Society, Warrendale, PA, 1985, p.353. M. Gupta, F. A. Mohamed and E. J. Lavernia, Metall. Trans. B, in press, 1991. W.J. Boettinger, L. Bendersky and J. G. Early, Metall. Trans., A17, 781 (1986). W.J. Boettinger, Mat. Sci. and Eng., 98, 123 (1988). R.F. Cochrane, P. V. Evans and A. L. Greer, Mat. Sci. and Eng., 98, 99 (1988). R. Mehrabian, Int. Met. Rev., 27 (4), 186 (1982). M. Cohen and R. Mehrabian, Rapid Solidification Processing Principles and Technologies, III, Proceedings of the Third Conference on Rapid Solidification Processing, held at the National Bureau of Standards, R. Mehrabian, editor, Dec. 6-8, 1982, Gaithersburg, MD., p. 1. H.W. Kerr, J. Cisse and G. F. Boiling, Acta Metall., 22, 677 (1974). G.F. Boiling and W. A. Tiller, J. Appl. Phys., 32, 2587 (1961). 40 nm spot size
center location
~
Lll
. ~ _
s u r f i ~ f i ~ p' o w d e r
FIG. 1. Schematic diagram showing the location of EDS analyses conducted on powders of varying sizes.
Vol.
26, No.
5
S O L I D I F I C A T I O N OF AI-Ti
,,~,., ~
.
j.
~:'-._'%--
(a)
(b)
701
.-,~.
! t
POWDERS
'
'~''
,
(c)
FIG. 2.
SEM micrographs showing: a) featureless microstracture for powders less than 10 p.m, b) cellular solidification morphology for powders in the 10-100 lam range, and c) dendritic microstructure tbr powders greater than 100 gm in diameter.
FIG. 3.
SEM micrograph of a powder suggestive of a single nucleation event.
FIG. 4. SEM micrograph of a powder showing the presence of star-shape precipitates [AI3Ti] on the surface of the powders.
702
SOLIDIFICATION OF A 1 - T i POWDERS
Vol.
26, No. 5
o
....~
ta
2 .,..,"
Powder Diameter [I.tm] FIG. 6. Segregation behavior of Ti for powders of varying sizes.
FIG. 5. TEM micrograph showing morphological features of AI-Ti atomized powders. Small Droplets
]
Large Droplets
Steps to complete solidification J
]
I Liquid phase High undercooling
Undercooling
v
Low undercooling v
Rapid Recalescence Slow Recalescence Shorter time interval
Mushy stage cooling
gelatively longertime interv~ v
~,elatively longertime interval
Shorter time interval Solid state cooling
Isenthalpic+Isothermal
Close to isenthalpic
v
Solidification type
Anticipated microstructure
I
Amorphous/ Crystalline
I
Metastable Phases
Less/No segregation
I
Crystalline
I
I
Less/No Metastable Finite phases present segregation
FIG. 7. Schematic diagram showing the solidification stages for small and large droplets (14, 15).
V o l . 26, pp. 6 9 7 - 7 0 2 , 1992 Printed in the U.S.A.
Pergamon P r e s s p l c All rights reserved
SOLIDIFICATION CHARACTERISTICS OF ATOMIZED AI-Ti POWDERS M. Gupta, F.A. Mohamed and E.J. Lavernia Materials Science and Engineering Section Department of Mechanical and Aerospace Engineering University of California Irvine, CA 92717 ( R e c e i v e d December 20,
1991)
1. Intro4ucfion The highly non-equilibrium conditions that are present during rapid solidification processing have prompted the development of novel alloy compositions that would otherwise be unattainable by conventional solidification (1, 2). Inspection of the available scientific literature demonstrates that the field of rapid solidification has continued to evolve since this phenomenon was first reported by Duwez et al. (3) in 1960. Rapid solidification may be attained experimentally through a variety of techniques, including: melt spinning, atomization and gun quenching (4, 5). Among these techniques, atomization has received considerable attention since it allows for the synthesis of a broad range of alloy compositions with extensive control over the resulting powder characteristics and properties. An interesting example of the application of rapid solidification processing to the development of novel alloy compositions is provided by the AI-Ti system. Rapidly solidified AI-Ti alloys derive their excellent strength, ductility and creep resistance from their refined microstructure and dispersion of AI3Ti particles, in combination with the low solid solubility [0.8 at.%] and low volume diffusivity [3.86 x 10-15 at.% cm2/sec, at 427 °C] of Ti in AI (6-9). The objective of the present study was to provide insight into the solidification microstructures and segregation patterns that are present in micron-sized AI-Ti powders, with emphasis on the synergism between microstructure and processing. 2. Experimental Procedure ProcessinP
The composition of the AI-Ti alloy used in the present study was obtained by mixing an AI-6.0 wt.%Ti master alloy provided by KBAlloys Inc. [Robards, KY] with commercial purity A1 [99.99%] provided by the Aluminum Company of America [Pittsburgh, PAl. These two master alloys were mixed in order to obtain a target alloy composition of AI-5.1 wt.% Ti. In order to ensure dissolution of the primary AI3Ti intermetallic phase and to promote sufficient mixing during melting, the alloy was slowly heated [under a nitrogen atmosphere] in a zirconia crucible to the desired superheat temperature, as shown in Table 1. The melt was subsequently maintained at the superheat temperature for approximately 10-20 minutes prior to atomization. The chemical composition of the material was verified following the atomization experiment. The powders were synthesized according to the following procedure. The matrix material was superheated to the particular temperature of interest [see Table 1], and disintegrated into a fine dispersion of micron-sized droplets using nitrogen gas at a preselected pressure. Following atomization, the rapidly quenched powders were separated from the atomization gas using a cyclone separator. In order to avoid extensive oxidation of the AI - Ti alloy during processing, the experiment was conducted inside an environmental chamber. The latter was evacuated down to a pressure of 150 microns of Hg and backfilled with inert gas [N2] prior to melting and atomization. The primary experimental parameters used during the experiment are listed in Table 1.
697 0056-9748/92 $5.00 + .00 Copyright (c) 1992 Pergamon Press plc
698
SOLIDIFICATION OF A 1 - T i POWDERS
Vol.
26, No.
TABLE 1. Experimental Parameters. Variable
Description
Units
Matrix alloy Atomization pressure Atomization gas Pouring Temperature Metal flow rate Gas flow rate
A1 - 5.1Ti 1.3 8 N2 1573 0.032 0.011
wt. % MPa I( kg/s kg/s
M icrostructur¢ Microstructural characterization studies were conducted on the atomized powders in order to establish the morphological features and the segregation pattern of Ti at the surface, as compared to that in the center of the powders. Scanning electron microscopy [SEM] studies were conducted on plastic mounted, polished, and gold coated samples using a HITACHI S-500 microscope in order to determine the rnicrostructural characteristics of powders of different sizes. The samples were etched with Keller's reagent [0.5 HF - 1.5 HCI - 2.5 HNO3- 95.5 H2OI prior to examination. Transmission electron microscopy studies were performed using a Philips CM 20 equipped with EDS [Energy Dispersive Spectroscopy] at an operating voltage of 200 keV. The TEM specimens were prepared by dispersing powders on a polymer coated Cu grid. The amount of Ti present at a particular location was then determined from microanalysis studies using EDS with a beam spot size of 40 rim. The objective of these studies was to determine the quantity of Ti present on the surface of the powders, relative to that present in the interior of the powders. A schematic diagram showing the locations on the powder surface used for EDS analysis is shown in Figure 1. ~, Results Microstructure SEM studies revealed that, for the size range of interest [e.g., 3 I.tm - 120 I.tm] the powders generally exhibited a spherical morphology, which is typical of gas atomized powders [see Figures 2-5]. Examination of the solidification morphology revealed that there were three types of microstructures present in the atomized powders. In powders less than 10 I~m in diameter, the microstructure was often featureless, and no dendrites or ceils were evident [see Figure 2a]. In powders with a diameter larger than 10 I.tm, but smaller than 100 I,tm, the microstructure generally exhibited a cellular morphology [see Figure 2b]. Finally, in powders larger than 100 I.tm the microstructure was typically dendritic [see Figure 2c]. In most of the powders studied, the solidification microstructure was indicative of multiple nucleation events. One notable exception is shown in Figure 3, where the microstructural features are suggestive of a single nucleation event. In addition, SEM studies also revealed the presence of star shaped precipitates, presumably of AI3Ti (10) type, on the surface of the powders [see Figure 4]. The spherical morphology of the powders was also evident from the TEM studies, as shown in Figure 5. The presence of satellites [small spherical protuberations] attached to the surface of the powders is also evident in this figure. These nanometer sized powders, formed as a result of the violent interaction of the solidifying metal and the atomization gas, are typical of gas atomized powders. The EDS results conducted on the surface and the center of powders of various sizes are summarized in Table 2 and Figure 6. The results show a high concentration of Ti at the surface and lower concentration in the center, for all the powders sizes studied [ the ratio R (CTi stnraee/C,ri center ) was always greater than 1]. In addition, EDS analyses conducted at different surface location] on a single" powder also yielded a value of the ratio R greater than 1, irrespective of the surface location [see Table 3].
5
Vol.
26,
No.
5
SOLIDIFICATION OF A 1 - T i POWDERS
699
TABLE 2. Results of EDS Analyses Showing Segregation Pattern of Ti in Atomized Powders. Powder size(I.tm)
3.12
4.08
6.28
8.20
8.86
R = CTi' surface/ CTi. center
1.23
1.78
1.05
2.28
4.28
TABLE 3. Results of the EDS Analyses Conducted at Different Locations in a Single Powder. Powder size (I.tm)
*Location
**R = CTi ' surface/CTi ' center
8.40 8.40 8.40 8.40
Surface Surface Surface Surface
1.40 1.84 3.09 3.55
*Each surface location was 90" apart from each other. **The concentration of Ti in the central region of the powder was 1.76 at. % [3.17 wt. %].
4. Discussion The solidification microstructures formed in the droplets depend to a great extent on the thermal history, undercooling and size of the droplets (11-13). The various stages encountered by droplets of small and large diameters during solidification, and their anticipated microstructural features are shown schematically in Figure 7 (14, 15). During atomization, small diameter droplets [e.g., <10 lam] will experience higher cooling rates, with concomitant microstructural refinement and decreased concentration gradients, than those experienced by large diameter droplets [e.g. > 100 I.tm] (5). The present results, summarized in Table 2 and Figure 6, show that the ratio 'R' approaches 1 with decreasing powder size, and hence are consistent with the expected trend. These results are supported by the microstructural studies from Figure 2a, which show that powders smaller than 10 I.tm generally exhibited a solidification microstructure indicative of plane front solidification, whereas powders larger than 10 t.tm generally exhibited a cellular or dendritic rnicrostructure [Figures 2b and 2c]. Regarding the initiation of solidification a few comments are in order. In the peritectic AI-Ti system the first solid to form is rich in Ti [k > 1], relative to the starting liquid composition (16, 17). EDS results conducted on powders in the 3 I.tm to 9 ~m size range [see Table 2, Table 3 and Figure 6] show that the Ti concentration is higher at the surface relative to that present in the center of the powders. These results suggest, first, that the solidification events were initiated on the surface of the powders, and second, that solidification was not partitionless, even in powder sizes as small as 3 I.tm. In addition, careful examination of Figure 2b shows that the microstructural features present on the surface of the powders were more refined relative to those present in the central region. This may be attributed to the higher quench rates experienced by the surface of the powders [due to the efficient heat convection to the atomizing gas] relative to those experienced by the central region [due to the thermal effects associated with recalescence]. In addition, initiation of solidification at the surface will enrich the surface with Ti which in turn will favor the formation of AI3Ti at the surface. This is evident from the AI-Ti phase diagram which shows that the formation of AI3Ti is favored with increasing Ti concentration [from 1.15 wt. % to ~ 37.3 wt. % Ti]. Experimental support to this suggestion is provided by the presence of AI3Ti precipitates on the surface of the powders [see Figure 4]. Regarding the nucleation of solidification in the droplets, the microstructural features of the powders are suggestive of multiple nucleation events on the surface. This was generally noted to be the case for most of the powder sizes studied. One notable exception is shown in Figure 3, where the solidification features are indicative of a single nucleation event in this particular powder.
700
SOLIDIFICATION OF A I - T i POWDERS
Vol.
26, No.5
5. Conclu~ign~ The following conclusions may be drawn for the solidification of atomized A1-Ti droplets: 1.
2. 3. 4.
Solidification is not partitionless for droplet sizes as small as 3 l.tm in diameter, and the first solid formed is rich in Ti. The segregation patterns obtained for powders of diameter less than 10 I.tmindicates that solidification during atomization initiated at the surface. In powders less than 10 I.tm, the microstructure is often featureless, and no dendrites or cells are evident. In powders larger than 10 I.tm, but smaller than 100 I~m,the microstructure is generally cellular. Finally, in powders larger than 100 gm, the microstructure is generally dendritic. The solidification microstructures present in the entire range of powder sizes studied are indicative of multiple nucleation events. Acknowledgements
Financial support from the Army Research Office under grant no. grant # DAALO3-89-K-0027 and the National Science Foundation under grant no. MSS-8957449 is gratefully acknowledged. In addition, the authors would like to thank Prof. M. Mecartney and Vikram Joshi for useful discussions. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.
H. Jones, J. Mat. Sci.. 19, 1043 (1984). Wolfgang G. J. Bunk, Mat. Sci. Eng., 134A, 1087 (1991). P. Duwez, R.H. Willens and W. Klement, J. Appl. Phys., 31, 1136 (1960) T.R. Ananthraman, C. Suryanarayana, J. Mat. Sci., 6, 1111 (1971). C.G. Levi and R. Mehrabian, Metall. Trans., 13A, 221 (1982). " M. E. Fine, Dispersion Strengthened Aluminum Alloys, Y. W. Kim, W. M. Griffith, eds., The Metallurgical Society, Warrendale, PA, 1988, p. 103. P.K. Mirchandani and R. C. Benn, Experimental High Modulus Elevated Temperature AI-Ti Based Alloys by Mechanical Alloying, SAMPE, Covina, CA (1988). W.E. Frazier and M. J. Koczak, Elevated Temperature Aluminum-Titanium alloy by Powder Metallurgy Process, United States Patent, No. 4,834,942, May 30, 1989. W.E. Frazier and M. J. Koczak, High Strength Powder Metallurgy Aluminum Alloys II, G. H. Hildeman, M. J. Koczak, eds., The Metallurgical Society, Warrendale, PA, 1985, p.353. M. Gupta, F. A. Mohamed and E. J. Lavernia, Metall. Trans. B, in press, 1991. W.J. Boettinger, L. Bendersky and J. G. Early, Metall. Trans., A17, 781 (1986). W.J. Boettinger, Mat. Sci. and Eng., 98, 123 (1988). R.F. Cochrane, P. V. Evans and A. L. Greer, Mat. Sci. and Eng., 98, 99 (1988). R. Mehrabian, Int. Met. Rev., 27 (4), 186 (1982). M. Cohen and R. Mehrabian, Rapid Solidification Processing Principles and Technologies, III, Proceedings of the Third Conference on Rapid Solidification Processing, held at the National Bureau of Standards, R. Mehrabian, editor, Dec. 6-8, 1982, Gaithersburg, MD., p. 1. H.W. Kerr, J. Cisse and G. F. Boiling, Acta Metall., 22, 677 (1974). G.F. Boiling and W. A. Tiller, J. Appl. Phys., 32, 2587 (1961). 40 nm spot size
center location
~
Lll
. ~ _
s u r f i ~ f i ~ p' o w d e r
FIG. 1. Schematic diagram showing the location of EDS analyses conducted on powders of varying sizes.
Vol.
26, No.
5
S O L I D I F I C A T I O N OF AI-Ti
,,~,., ~
.
j.
~:'-._'%--
(a)
(b)
701
.-,~.
! t
POWDERS
'
'~''
,
(c)
FIG. 2.
SEM micrographs showing: a) featureless microstracture for powders less than 10 p.m, b) cellular solidification morphology for powders in the 10-100 lam range, and c) dendritic microstructure tbr powders greater than 100 gm in diameter.
FIG. 3.
SEM micrograph of a powder suggestive of a single nucleation event.
FIG. 4. SEM micrograph of a powder showing the presence of star-shape precipitates [AI3Ti] on the surface of the powders.
702
SOLIDIFICATION OF A 1 - T i POWDERS
Vol.
26, No. 5
o
....~
ta
2 .,..,"
Powder Diameter [I.tm] FIG. 6. Segregation behavior of Ti for powders of varying sizes.
FIG. 5. TEM micrograph showing morphological features of AI-Ti atomized powders. Small Droplets
]
Large Droplets
Steps to complete solidification J
]
I Liquid phase High undercooling
Undercooling
v
Low undercooling v
Rapid Recalescence Slow Recalescence Shorter time interval
Mushy stage cooling
gelatively longertime interv~ v
~,elatively longertime interval
Shorter time interval Solid state cooling
Isenthalpic+Isothermal
Close to isenthalpic
v
Solidification type
Anticipated microstructure
I
Amorphous/ Crystalline
I
Metastable Phases
Less/No segregation
I
Crystalline
I
I
Less/No Metastable Finite phases present segregation
FIG. 7. Schematic diagram showing the solidification stages for small and large droplets (14, 15).