Y 

I 

NASA 

Technical 

Memorandum 

NASA TM -86573 

( &AS A - T M- 8 6 5 7 3 1 

THE EFFECTS OF GRAVITY LEVEL DURING 

DIRECTIONAL SOLIDIFICATION ON THE 

MICROSTRUCTURE OF HYPERMONOTECTIC 

AI-In-Sn ALLOYS 

Center Director's Discretionary Fund Final Report 

By P. A. Curreri and W. F. Kaukler 

Space Science Laboratory 

Science and Engineering Directorate 

November 1986 

3hE EEEkCIS C€ GEAVITY 

LEVBL DURING CIFECTICNAL SCLICIFICASION ON 

AI-In-Sn ALLOYS (tiASA) 41 F CSCL 

TEE flICECS'LRUCTUBE OE HYPEbBC hClJiCTIEC 

National Aeronautics and 

Space Admlmstration 

George C. Marshall Space Flight Center 

MSFC - Form 3190 (Rev. May 1983) 
TECHNICAL REPORT STANDARD TITLE PAGE 

1. REPORT NO. 12. GOVERNMNT ACCESSION NO. 13. RECIPIENT’S CATALOG NO. 

NASA TM-86573 I 

4. TITLE AND SUBTITLE 

The Effects of Gravity Level During Directional Solidificatiorr,. 

on the Microstructure of Hypermonotectic AI-In- Sn Alloys 

P. A. Curreri and W. F, Kaukler* 

7. AUTHOR(S) . 

9. PERFORMING ORGANIZATION NAME AND ADDRESS 

George C. Marshall Space Flight Center 

Marshall Space Flight Center, Alabama 35812 

12. SPONSORING AGENCY NAME AN0 ADDRESS 

National Aeronautics and Space Administration 

Washington , D . C . 20546 

15. SUPPLEMENTARY NOTES 

5. REPORT DATE 

November 1986 

PERFORM,NG CC~E 

8. PERFORMING ORGANIZATION REPOR r d 

10. WORK UNIT NO. 

11. CONTRACT OR GRANT NO. 

L 

13. TYPE OF REPORi’ & PERIOD COVEREt 

Technical Memorandum 

1.1. SPONSORING AGENCY CODE 

16. ABSTRACT 

Five hypermonotectic Al-In-Sn compositions were directionally solidified in a Bridgman-type furnace 

at normal gravity and during aircraft low-gravity maneuvers. The. tendency of the AI-301n alloy to form an 

indium-rich band at the start of unidirectional growth (SUG) made it difficult to study the integration of L2 

into the solidification interface. Hypermonotectic compositions closer to monotectic “slightly hypermonotectic” caused only a partial band of L2 to form at SUG and allowed the study of such variables as 

gravity, composition, and monotectic dome height on integration of excess L2 into the solid plus L2 interface. 

It was found that formation of aligned composite structures for the Al-In-Sn system is not only a function of 

G and R but also of the degree to which the composition varies from on monotectic. Most of the aligned 

fibrous structures formed from hypermonotectic Al-In-Sn had spacings that were of the order of irregular 

fibrous structures reported for on monotectic Al-In-Sn. The spacings for the large fibers and aligned globules 

found for ground and low-gravity processed Al- 18In-22Sn, respectively, were significantly larger than the 

others measured and were of the order expected for cell spacings under the growth conditions utilized. It was 

found that the integration into the solidification front of excess L2 in low gravity was a function of the Sn 

composition of the alloy. 

17. KEY WORDS 

Low -Gravity Alloy Solidification , Immiscible Alloys , Al- In- Sn , Hypernonotectic , Directional 

Solidification During KC- 135 

Low-Gravity Maneuvers 

19. SECURITY CLASSIF. (d thls rop&) 20. SECURITY CLA! 

18. DI STR I B UT I ON STATEMENT 

Unclassified - Unlimitea 

21. NO. OF PAGES 22. PRICE it F. (of this pee) 

Unclassified I Unclassified I NTIS 
ACKNOWLEDGMENTS 

The authors wish to thank M. H. McCay whose encouragement and enthusiasm were instrumental to 

beginning this work. We wish to acknowledge the assistance of D. Warden for preparative and photometallography, W. K. Witherow for image analysis, and R. E. Shurney, G. Smith, R. Bond, J. Theiss, and G. Workman for providing the technical expertise needed to integrate and operate the directional solidification furnace in 

the KC-I35 aircraft flight environment. We wish to also thank the Johnson Space Center personnel who operate 

the KC- 135 aircraft. This research was supported by the Marshall Space Flight Center Director’s Discretionary 

Fund and the NASA Microgravity Science and Applications Program. 

ii 
TABLE OF CONTENTS 

I . INTRODUCTION ............................................................................................... 

I1 . EXPERIMENTAL METHOD ............................................................................... 

A . 

C . Compositions Chosen for Study ........................................................................... 

Directional Solidification Furnace System ............................................................... 

B . KC- 135 Aircraft Furnace Integration ..................................................................... 

D . Sample Preparation ......................................................................................... 

E . Quantitative Microscopy ................................................................................... 

I11 . EXPERIMENTAL RESULTS ................................................................................. 

A . Directional Solidification Under Normal Gravity ....................................................... 

B . Directional Solidification During Low-Gravity Maneuvers ........................................... 

IV . DISCUSSION .................................................................................................... 

A . Segregation of Excess L2 Under a Thermal Gradient in a 

Gravitational Field .......................................................................................... 

B . The Effect of Gravity on the Integration of Primary Macrodroplets ................................... 

D . Spacings of Aligned Structures ............................................................................ 

E . Interfacial Stability and Gravity Level .................................................................... 

C . Compositional Dependence for the Formation of Aligned Composite Structure .................... 

V . CONCLUSIONS ............................................................................................ 

Page 

1 

2 

2 

2 

3 

3 

3 

3 

3 

5 

6 

6 

6 

7 

7 

8 

8 

REFERENCES .................................................................................................................... 9 

... 

111 
LIST OF ILLUSTRATIONS 

Figure Title Page 

1 . 

2 . 

3 . 

4 . 

5 . 

6 . 

7 . 

8 . 

9 . 

IO . 

II . 

I2 . 

13 . 

14 . 

15 . 

16 . 

Al-x.In-y.Sn immiscibility dome after Reference 23 ....................................................... 

A1-30111 sample showing the disruption of the solidification process by an 

indium-rich band near the start of unidirectional growth ................................................... 

Hypermonotectic Al-x.In-y.Sn alloys directionally solidified under 

normal gravity (ground) ........................................................................................ 14 

Transverse sections for Al- 18.6In ground sample 15 

Quantitative microscopy for Al- 18.61n ground sample ..................................................... 16 

Transverse sections for Al-18.5In-6.6Sn ground sample .................................................. 17 

Quantitative microscopy for Al- 18.5In-6.6Sn ground sample ............................................ 18 

i2 

13 

I 

a ........................................................... 

Transverse sections for Al- 18.9In- 14.6Sn ground sample ................................................. 19 

Quantitative microscopy for Al- 18.9111- 14.6Sn ground sample ........................................... 20 

Transverse sections for Al- 18.1 In-22Sn ground sample ................................................... 21 

Quantitative microscopy for Al- 18.1 In-22Sn ground sample ............................................. 22 

Hypermonotectic Al-x.In alloys directionally solidified during 

KC- 135 maneuvers (flight) .................................................................................... 23 

Quantitative microscopy for AI-301n flight sample ......................................................... 24 

Hypermonotectic Al- 18.5111-6.6Sn and Al- 18.9111- l4.6Sn flight samples ............................... 25 

Quantitative microscopy for Al- 18.51n-6.6Sn flight sample .............................................. 26 

Quantitative microscopy for Al- 18.9111- 14.6Sn flight sample ............................................. 27 d 

I7 . Microstructural transition between low and high gravity during 

the fourth parabolic maneuver for Al- I8 . I In-22Sn ......................................................... 28 
LIST OF ILLUSTRATIONS (Concluded) 

Figure Title Page 

18. Microstructure around the fourth low-gravity zone relative to a corresponding 

section of the control sample for AI- 18.1 In-22Sn ........................................................... 29 

19. Quantitative microscopy for AI- 18.1 In-22Sn flight sample around 

the fourth low-gravity zone .................................................................................... 30 

20. Transverse sections for AI-18. IIn-22Sn flight sample through the fourth 

low- and high-gravity zones ................................................................................... 3 1 

2 1 . Liquid composition gradient resulting from a hypermonotectic 

sample under gravity in a temperature gradient.. ............................................................ 32 

22. Schematic summary of the macrostructures of AI-x.In-y .Sn hypermonotectic 

alloys directionally solidified upwards in one gravity and directionally 

solidified during KC- 135 low-gravity maneuvers.. ......................................................... 33 

23. Transverse section through the fourth low-gravity zone for AI- 18.1 In-22Sn 

flight sample showing the relative size of the spacing of the aligned particles ........................... 34 

V 
TECHNICAL MEMORANDUM 

THE EFFECTS OF GRAVITY LEVEL DURING DIRECTIONAL 

SOLIDIFICATION ON THE MICROSTRUCTURE OF 

HYPERMONOTECTIC AI-In-Sn ALLOYS 

1. INTRODUCTION 

Many potentially important alloy systems have a monotectic dome (a region in their phase diagram where 

I two immiscible liquid phases simultaneously exist). During solidification through the monotectic dome the alloy 

is especially susceptible to buoyancy-driven segregation. For this reason the low gravity of space is particularly 

attractive for the study of the solidification mechanisms of monotectic alloys, and for the processing of 

monotectic alloys with microstructures and properties that are not possible in the presence of gravitationallydriven segregation. This paper reports on results of experiments on directional solidification of immiscible AlIn-Sn alloys during aircraft multiple low-gravity maneuvers. 

Like eutectic alloys, monotectic alloys can be made to grow with both regular and irregular aligned 

composite structures depending on growth rate, R, thermal gradient, G, and composition [I ,2]. For monotectic 

alloys the interfacial energies between the two liquid phases (L, and L2) and the wetting of the liquid phases to 

the solid (SI) are believed to play a critical role in determining the solidified microstructure [3]. If L2 west SI 

steady state growth and regular composite structures can be obtained if G/R is sufficiently large. However, since 

L, is chemically closer to SI it is likely that LI will be the wetting liquid in most monotectics. Theory predicts 

[3] that even if L2 is not the wetting phase, composite growth can occur if the growth velocity is large enough 

to overcome the disjoining pressure. Further, it is expected that L, will perfectly wet SI unless the monotectic 

temperature is far below the critical temperature. To test these theories it was suggested that ternary monotectic 

systems be studied where the third component would change the monotectic dome height and provide a range 

disjoining pressures. Experimental studies of the solidification of AI-In-Sn [4,5] and the Al-Cu-Pb [4,5] have 

verified that as the concentration of the third component increases, lowering the monotectic dome height, there 

is a transition from steady state composite-type growth to irregular composite-type growth. 

The influence of gravity on monotectic solidification has been studied to a limited extent by furnace 

inversion techniques. By changing the orientation of the solidification direction relative to the gravitational vector, the relative sensitivity of the monotectic microstructure to the presence of gravity can be determined [5,7,8]. 

Furnace inversion has limited use for studying hypermonotectic compositions, since the magnitude of the gravitational field remains constant and only the direction of the segregation relative to the solidification interface is 

changed [SI. 

Experimentation under the reduced gravity conditions of free fall or in space offers an opportunity to 

study hypermonotectic alloy solidification free from buoyancy-driven segregation. Solidification of hypermonotectic alloys in low gravity has sometimes resulted in greatly reduced macrosegregation [ 10- 131, but has 

mere efter! resu!ted in massive segregatinn caused by other nonbuoyancy-driven mechanisms [ 14- 181 that 

normally would have been masked by the effect of gravity. These results clearly show the need for the detailed 

systematic investigations that can separate buoyancy-driven segregation from other mechanisms such as surface 

tension-driven flows and particle pushing by the solidification interface. In this study hypermonotectic AI-In-Sn 
alloys over a composition range in which the solidification mechanism normally ranges from steady state 

composite for the lower Sn compositions to irregular composite growth for the higher Sn compositions are directionally solidified under normal gravity and during KC- 135 low-gravity maneuvers. 

II. EXPERIMENTAL METHOD 

A. Directional Solidification Furnace System 

The directional soldification furnace utilized in this research is the same unit that has been used in previously described studies on superalloys [19,20] and iron-carbon alloys [21,22]. The furnace is of the Bridgman 

type, equipped with a platinum rhodium resistance heating wire double wound around an alumina core below 

which is a water-cooled copper quench block. A sample of about 8 cm in length and 0.4 cm in diameter was 

placed in an alumina crucible 0.5 cm id and 45 cm long, with argon passing through the top. The furnace was 

positioned such that the bottom 1.5 to 2.0 cm of the sample remained unmelted. After about 10 min at temperature, the furnace was then translated upwards at controlled rates, the remainder of the sample being thus solidified. 

. 

b 

The thermal profile of the furnace was such that the maximum temperature was 1040°C and about a 6 cm 

length of the crucible interior was above 900°C. The thermal gradient was determined with samples instnimented with an alumina coated thermocouple. The thermal gradient calculated at 640°C was 219"C/cm. The 

thermal gradients measured in this manner were found to be reproducible within IO percent. Within this experimental error the gradients for flight and ground-based samples were found to remain consistent for solidification 

under all the gravitational conditions studied. 

B. KC-1 35 Aircraft Furnace Integration 

The method used in this research for studying the effects of low-g on solidification was to have the 

directional furnace flown on the NASA KC- 135 aircraft during repetitive low-g maneuvers. Each maneuver 

gives from 20 to 30 sec of low-g ( 

to 1.8 g). Therefore, a sample that is being solidified experiences a repetitive sequence of low-gravity (low-g) 

and high-gravity (high-g) forces parallel to the longitudinal growth axis. Acceleration experienced by the sample 

is monitored by three accelerometers mounted to the furnace assembly on three orthogonal axes. For a typical 

maneuver during low-g, the acceleration on all axes averages below 

acceleration parallel to the longitudinal axis of the sample reaches 1.75 g, while the accelerations on the other 

two axes are less than 0.15 g. 

g, where g is 980 cm/sec2), and up to 1.5 min of pullout and climb (up 

g. During pullout and climb, the high-g 

The furnace translation rate is kept constant regardless of power fluctuations by a servo control circuit 

which maintains constant drive r/min. The furnace translation rate during each experiment is measured directly 

by a calibrated potentiometric translation detector and continuously recorded. The rate for these experiments was 

maintained at 5 % 0.05 mm/min. 

Selection of an initial sample position relative to the furnace such that about 2 cm of sample remains unmelted allows reliable identification of the start of unidirectional growth. The known solidification rate of the 

sample can then be correlated with accelerometer data to determine the gravity level during solidification for any 

location on the sample. For the typical parabola when entering low-g, the acceleration drops from 1 g to g 

in less than 1.5 sec. For convenience in the figures, a low-g zone represents the time difference from when the 

system just went under 1 g going into low gravity to when it just went over 1 g coming out of low gravity. 

2 
C. Compositions Chosen for Study 

The compositions chosen for study are shown in Figure I relative to the AI-In-Sn immiscibility dome 

[23]. Five hypermonotectic compositions were studied with Sn content from 0 to 22 wt.%. Their initial 

“nominal” compositions were (in wt.%) AI- 18.6111, A1-30.OIn, A1-18.5In-6.6Sn, A1-18.9In-14.6Sn, and Al18.1111-22.OSn. In some of the figures and text the compositions are referred to by the integer values of the 

weight percent for convenience. 

D. Sample Preparation 

The samples were prepared by placing the appropriate weights of five nines pure indium and tin, and 

four nines pure aluminum into a 99.8 percent alumina crucible. The sample was then melted under argon using 

an electromagnetic furnace. After about 10 min at about 1OOO”C the sample was quenched by aspirating it into a 

4 mm id quartz tube. 

After quenching, the samples were broken out of the quartz tubes and 1 cm was cut from the top for 

chemical analysis. Atomic adsorption analysis indicated that all the specimens were hypennonotectic and within 

1.3 wt. % of the nominal tin content. 

E. Quantitative Microscopy 

Quantitative microscopy was used to obtain data on the change in volume fraction of the In or In-Sn-rich 

phase with solidification distance. An Omnicon image analysis system and enlarged photomicrographs were 

used for this purpose. This technique was chosen for the initial analysis over wet chemical analysis because the 

nondestructive nature of the former technique allowed subsequent electronic properties measurements which are 

described in another paper [24]. 

Area fraction of the dark In or In-Sn-rich phase was measured. It was assumed that the volume fraction 

was equal to the area fractions measured. Each measurement shown in the figures represents the average volume 

fraction of an area about 3 mm wide (perpendicular to the longitudinal axis) and 0.3 mm long (parallel to the 

longitudinal axis). 

111. EXPERIMENTAL RESULTS 

A. Directional Solidification Under Normal Gravity 

The directionally solidified AI-30111 samples often exhibited gross segregation of the In-rich-phase near 

the beginning of unidirectional growth. Figure 2 shows a typical sample in which the first millimeter after the 

start of unidirectional growth (SUG) consisted of an indium-rich band. Indium-rich metal also coated portions of 

the outside of the unmelted region of the sample. This run down of indium-rich liquid could result in only a 

broken band of indium-rich metal at the start of unidirectional growth (as occurred in the AI-301n flight sample 

shown later). The remainder of the AI-30111 sample in Figure 2 consists of irregular shaped indium-rich particles 

dispersed in an aluminum-rich matrix. This latter mic:n,st:ucture is essentia!!y that expected [4] for the 

monotectic composition solidified at the G and R used in this study. 

3 
Figure 3 shows photomicrographs of longitudinal sections of the four other compositions after directional 

solidification. The AI-18In sample in contrast to the AI-301n sample did not form a complete band of indiumrich metal at SUG. Instead, the first few millimeters of unidirectional solidified sample consists of indium-rich 

fibers aligned to the growth direction. This fibrous section consists of a few fibers of about 0.1 mm thickness 

and numerous finer fibers. The fibrous growth ends abruptly after about 2 mm; the microstructure then consists 

of irregular indium-rich particles dispersed in an aluminum-rich matrix with some intermittent aligned fibers 

occurring after about 4 mm of unidirectional growth. Figure 4 shows photomicrographs of transverse sections 

across the aligned fibers at 1.5 mm from SUG and the irregular indium-rich particles at 6.5 mm from SUG. The 

transverse section (Fig. 4A) reveals that the finer fibers are grouped in bands of about 50 microns thick and 100 

microns spacing, while the larger fibers do not appear to have a regular spacing. The volume fraction 

indium-rich phase versus distance from SUG obtained from quantitative microscopy is given in Figure 5 for the 

AI- 18In composition. It is clear that the composition of the fibrous portion of the sample is indium-rich relative 

to the rest of the sample. The indium content of the first fibrous band increases with solidification distance 

reaching a maximum of about 30 volume percent and then abruptly decreasing below that of the nominal 

composition at the end of the first fibrous region. The volume fraction In increases again at a position 

corresponding to regions of intermittent fibers. 

The longitudinal section for the A1-18In-6Sn sample (Fig. 3) shows In-Sn-rich phase near SUG in the 

form of irregular macrodroplets somewhat oriented to the growth direction. At 3 mm from SUG there appears a 

partial band of lens-shaped droplets. After 3 mm irregular In-Sn-rich particles gradually are replaced by aligned 

spheres and then by aligned fibers at about 8.5 mm from SUG. The aligned fibers end abruptly and the remaining sample consists of irregular In-Sn-rich particles in an AI-rich matrix. Figure 6A shows a transverse section 

of the aligned fibers at 8.5 mm from SUG contrasted with (Fig. 6B) the irregular structure at 11 mm from SUG. 

In place of the fine irregular structures in Figure 6B are groupings of regularly spaced fibers in Figure 6A 

surrounded by a coarser microstructure that is common to both sections. Figure 7 gives the volume fraction 

change with distance from SUG for the A1-18In-6Sn sample. The volume fraction In-Sn-rich phase peaks well 

above the nominal composition in the area of the particle band of lens-shaped droplets (3 mm from SUG). 

Another increase occurs as the fraction of the sample consisting of aligned spheres increases. A sharp drop in 

measured volume fraction precedes the transformatin to aligned fibers. 

The longitudinal section for AI-1 8In-14Sn (Fig. 3) shows large irregular-shaped macrodroplets in the first 

few millimeters after SUG. The microstructure then consists of irregular In-Sn-rich particles in an Al-rich 

matrix until at about 6 mm from SUG a concentrated band of irregular macrodroplets and fibers is evident 

ending at 8 mm from SUG in a band of lens-shaped droplets. The remainder of the sample consists of irregular 

In-Sn particles. Figure 8A shows a transverse section of the band of droplets at 8 mm from SUG. The 

macrodroplets are seen to form a somewhat regular pattern. The coarser connected microstructure of the 

transverse section at 10 mm from SUG (Fig. 8B) is common to both sections. The quantitative microscopy for 

the 14Sn sample, Figure 9, shows that the two regions dominated by macrodroplets correspond to dramatic 

increases in volume fraction of In-Sn-rich phase. 

The AI-18In-22Sn sample (Fig. 3) did not exhibit the macrodroplets that were prominent in the 6Sn and 

14Sn samples. Instead, intermittent large (0.05 mm) fibers begin at about 2 mm from SUG and continue to 

about 5 mm from SUG. Figure 10 shows a transverse section of the fibrous region at 4.5 mm from SUG contrasted with the irregular microstructure at 7 mm from SUG. It is apparent that the fibers are regularly spaced 

except for fiberless gaps. The quantitative microscopy (Fig. 11) shows an increase in volume fraction In-Sn-rich 

phase with the start of fibrous growth but a decrease is not evident in volume fraction after fibrous growth 

ceases at 55 mm from SUG. 

4 
B. Directional Solidification During Low-Gravity Maneuvers 

Figure 12 shows longitudinal sections of AI-18In and AI-301n samples that were solidified during KC135 low-gravity maneuvers. The AI-I8In sample has a 1 mm band of aligned fibers that begins at about the 

middle of the first low-g zone and ends abruptly slightly before the end of the first low-g zone. The remainder 

of the sample consists of irregular particles of In-rich phase in AI-rich matrix with no obvious microstructural 

correlation to the gravity level at solidification. The microstructure of the AI-30In sample is similar to that of 

the AI- 18In flight sample except that aligned fibers appear immediately after SUG and are followed by 

macrodroplets. Like the AI-1 8In flight sample the remaining sample is dominated by irregular In-rich particles 

that start before the end of the first low-g zone. Figure 13 gives the volume fraction versus distance from SUG 

for the AI-301n sample. It is evident that the volume fraction In-rich phase near SUG is greatly enriched in 

indium over the nominal composition but then drops quickly with distance from SUG. 

3 

A longitudinal section of AI-18In-6Sn flight sample is given in Figure 14. There is an interesting contrast 

in the segregation behavior of the 6Sn flight sample with that of the OSn flight samples. Whereas for OSn the 

excess In-rich phase appears in the first low-g zone, the 6Sn sample does not have macrodroplets until the 

beginning of the first high-g period. Quantitative microscopy (Fig. 15) shows that the volume fraction In-Sn-rich 

phase remains essentially the same as it was before SUG throughout the first low-g zone, then increases rapidly 

with the onset of high-g peaking at over 60 volume percent and then returning to the level before SUG by the 

middle of the first high-g zone to remain at that level for the rest of the sample. 

I 

A1-18In-14Sn samples like the 6Sn samples also did not have macrodroplets in the microstructure until 

the first high-g zone. For some samples for each Sn composition, furnace translation was stopped after 10 sec or 

less of the first high-g zone and then solidification was continued only in low-g for the next four maneuvers 

before returning to continuous solidification in low- and high-g. No new information was apparent for the OSn 

and 22Sn compositions from using this procedure, however, some interesting results were obtained for the 6Sn 

and 14Sn compositions. For the 6Sn and 14Sn compositions, when this procedure was followed, it was found 

that the excess L2 could be made to incorporate more gradually in smaller macrodroplets by shortening the 

solidification time in high-g. For the 6Sn sample some of the smaller macrodroplets incorporated in low-g 

zones. 

When the 14Sn alloy was processed using this procedure the excess In-Sn-rich phase did not appear to 

incorporate except at or immediately following high gravity as is evident in the irregular macrodroplets in the 

14Sn sample in Figure 14. Unlike the lower Sn flight samples, the 14Sn sample also exhibited a second band of 

macrodroplets on the transition from the second high-g zone to the third low-g zone. This band of macrodroplets 

is similar to that in the ground 14Sn sample at 8 mm from SUG that abruptly terminated the macrodroplet 

microstructure. A comparison of Figure 9 with Figure 16 shows that the volume fraction In-Sn-rich phase of the 

two bands of droplets is about 40 volume percent. 

The A1-18In-22Sn flight sample like the 22Sn ground sample did not exhibit macrodroplets. The microstructure of the 22Sn flight samples remained irregular In-Sn-rich particles dispersed in an AI-rich matrix 

throughout solidification through the first several parabolas. Beginning at about the fourth maneuver, an abrupt 

difference in microstructure between the material solidified in low-g and high-g becomes evident. Figure 17 

shows the microstructure at the transition between the fourth low- and high-g zones. By inspection of Figure 17 

it is evident that the In-Sn particles in the low-g zone are finer and tend to be more aligned. Figure 18 shows a 

longitudinal section spanning the area from the third to the fourth high-g zones of the flight sample and the 

corresponding section of a control sample solidified under the same conditions except for gravity. It is evident 

from such comparisons that the change in microstructure is a function of gravity level during solidification and 

distance from SUG. A comparison of the transverse microstructure for high-g and low-g is given in Figure 19. 

5 
A finer microstructure and greater degree of alignment in the low-g microstructure is also evident but to a lesser 

degree in the transverse sections. Figure 20 gives the volume fraction versus distance from SUG corresponding 

to the section of the flight sample shown in Figure 19. An abrupt decrease in volume fraction In-Sn-rich phase 

is evident at the beginning of low-g coinciding with the aligned irregular microstructure. The volume fraction 

then increases steadily through the low-g zone falling sharply again at the low-g to high-g transition. 

IV. DISCUSSION 

A. Segregation of Excess L2 Under a Thermal Gradient in a 

Gravitational Field t 

To interpret the results of these experiments it is critical first to understand the segregation that takes 

place in a hypermonotectic melt standing in a thermal gradient relative to a gravitational field [25]. Figure 21 is 

an illustration of this situation with L2 more dense than L, as is the case for the systems in this study. Since the 

system (Fig. 21A) is hypermonotectic, the temperature gradient results in a band of liquid that exists in the 

temperature range that falls in the immiscibility dome (Fig. 21B). Stokes settling of the L2 droplets to the solid 

+ L2 interface occurs due to gravity. The composition of the liquid above the L2-rich band at the interface is 

continually homogenized, due to convective flows, resulting in nucleation of L2 droplets in the temperature 

gradient zone. The process continues until the composition of the liquid above the L2-rich region at the solid + 

L2 interface reaches the monotectic composition (Fig. 21B). If the system is rich enough in component B then 

the droplets of L2 at the interface merge to form a continuous band at the solid + L2 interface (Fig. 21C). 

When the sample is translated relative to the thermal gradient, to begin growth, unless R is slow relative to diffusion of solute through the L2 band, growth will be interrupted and begin after the L2 band is passed. The Al301n microstructure given in Figure 2 is an example of this phenomenon. Other examples are in the literature for 

Al-In [4] and for Cu-Pb [7]. Since growth is interrupted at practical rates, experiments at these compositions 

yield little information about the behavior of excess L2 at the growing interface. The “slightly-hypermonotectic” 

compositions Al- 1 gin, AI- 18In-6Sn, Al- I8In- 14Sn, and Al-I 8In-22Sn were selected for study because the concentration of B apparently was not great enough to cause droplets of L2 at the solid + L2 interface to merge 

into a band, Figure 21C. In this way the effect of variables such as disjoining pressure and gravity on the incorporation of excess L2 in the growth front could be studied. 

B. The Effect of Gravity on the Integration of 

Primary Macrodroplets 

The mechanisms of integration of L2 into the solidifying interface for Al-In alloys has been the subject of 

some controversy in the literature. Grugel and Hellawell [4] found that monotectic AI-In alloys solidified in 

regular aligned composite structures at high G/R. They assumed based on the arguments of Cahn [3] that L2 wet 

the solid. Derby and Favier and Derby [6,26] tested this assumption by interfacial energy calculations and concluded that L, preferentially wets the solid; thus, this assumption is invalid. They suggested that dynamic wetting theory of Chernov et a]. [27] be applied. Potard [8] compared the results of Bridgman and capillary tube 

quench solidification experiments with hypermonotectic Al-In and concluded that under conditions that particle 

inertia becomes negligible, only the weight of the primary globules enables them to overcome the interfacial 

repellent force and integrate into the solid. Potard [ 121 also solidified hypermonotectic AI-In in low gravity in a 

sounding rocket experiment and observed that primary particles well above the size that Chernov’s theory 

predicts appeared to experience repulsion by the solidification interface during solidification in low gravity. 

6 
Figure 22 is a schematic of the macrostructure found in the flight and ground samples for the AI-In-Sn 

compositions studied. The results for the 6Sn and 14Sn compositions support Potard’s finding from sounding 

rocket results that large primary droplets can be repelled by the solidification front when gravity is absent. We 

did not, however, observe a notable difference as a function of gravity in the primary phase integration for Al18In or AI-301n compositions. We instead saw an increased sensitivity in the influence of gravity on primary 

phase integration with increasing Sn content presumably due to increasing disjoining pressure at higher Sn content. The difference in observations for AI-In between the two low-gravity experiments might be due to the 

lower gravity (0.OOOl g) for the sounding rocket experiment relative to that (0.01 g) in the KC-135 maneuver. 

The size of the primary particles apparently repulsed by the interface in both the KC-135 and sounding rocket 

low-gravity experiments is larger by at least an order of magnitude than would be expected from Chernov’s 

dynamic wetting theory [6]. 

C. Compositional Dependence for the Formation of 

Aligned Composite Structure 

Figure 3 shows that at normal gravity all the slightly hypermonotectic compositions studied exhibited 

aligned composite growth within the first centimeter from SUG. At first glance this seems to be in contradiction 

to the study of Grugel and Hellawell, since for monotectic compositions they observed only irregular In or InSn-rich particles in an aluminum-rich matrix at the G and R conditions of our study. Examination of the quantitative microscopy data (Figs. 5, 7, 9, and 11) reveals that the aligned structures are correlated with high volume 

fractions of In or In-Sn-rich phase - with hypermonotectic compositions. The surrounding microstructure which 

is close to monotectic composition is consistent with what would be expected at our growth conditions from 

Grugel and Hellawell’s results. It is interesting to note that the banding of the aligned fibers in the AI-18In 

transverse section (Fig. 4A) is similar to that reported by Grugel and Hellawell for on monotectic AI-In at 

higher G/R. Livingston and Cline [I] found an analogous dependency of alignment of CuPb, for fixed G and R, 

with Pb content. Our study is to our knowledge the first to demonstrate that the tendency to form aligned 

composite microstructure for the Al-In-Sn system is a function of composition. 

D. Spacings of Aligned Structures 

Derby and Favier [6] extended the Jackson-Hunt diffusion theory to monotectic systems. Grugel and 

Hellawell [4] suggest that the A2R is a constant for monotectics where A is the aligned structure spacing but that 

the constant depends on the height of the miscibility gap. Table 1 lists the spacings of aligned structures found 

in this study relative to values measured by Grugel and Hellawell for monotectic compositions of AI-In (regular 

fibrous) and Al-In-Sn (irregular fibrous). Most of the fibrous structures observed in this study are on the order 

of the irregular fibrous spacing found by Grugel and Hellawell. This suggests that the hypermonotectic aligned 

structures solidify by a mechanism similar to that of on monotectic irregular fibrous-type growth. Only the 

spacings measured for aligned spheres in Al- I8In approach that of Grugel and Hellawell measurements for 

uniform fibrous AI-In. The quantitative microscopy data in Figure 7 show. however, that this microstructure 

was accompanied by a lowering of the indium volume fraction to near monotectic levels. 

It is interesting to note that the spacings (Table I) of the Al-In-22Sn aligned globules found in lowgravity solidification were essentially the same as the spacing of the large fibers found in the sample solidified 

at normal gravity. Also it should be noted that the spacings of these two aligned structures are substantially 

greater than any of the other structures listed in Table 1. The transverse sections for the ground (Fig. lU) and 

flight (Fig. 19) sample show evidence for cellular solidification. Figure 23 shows the spacing between aligned 
globules found in low-gravity-solidified Al- 18In-22Sn relative to the transverse section macrostructure. Systematic interfacial stability studies for the Al-In-Sn system have not been done but an extrapolation of data on 

the cell size versus l/RG for AI-2Cu [28] to our growth conditions is consistent with the spacing values 

obtained for the Al- I8In-22Sn aligned structures. 

E. Interfacial Stability and Gravity Level 

A deep etch of the band between the second high and third low-gravity zone in the 14Sn sample reveals 

that it is essentially different from the macrodroplets seen at the first high-g zone in the 14Sn sample and also in 

the 6Sn sample. The deep etch of the latter band in the 14Sn sample reveals a network of aligned fibers that 

suggests a change in solidification mechanism. In addition the position of the latter band in the 14Sn sample - 

the transition from the second high-g zone to the third low-g zone - makes it unlikely that the latter band in 

the 14Sn sample resulted from the same mechanism as the macrodroplets found in the first high-g zones. A 

more likely cause of this microstructure as well as the aligned globules found in low-g zones of the 22Sn 

sample is that these structures result form the decreased interfacial stability caused by low gravity as predicted 

by Lemaignan and Malmejac [29]. It would be expected that the higher Sn compositions would be more sensitive to low-gravity-induced interfacial instability, since as the Sn concentration is increased the resulting change 

in the phase diagram is expected to make the Al-In-Sn alloys more sensitive to constitutional supercooling [29]. 

The growth of intercellular or intergranular fibers in Al-In alloys has been noted previously [6]. The destabilization of the solidification interface by low-g to a more cellular form has been reported in a study of Fe-C-P 

alloys E221 directionally solidified in low-gravity maneuvers. 

V. CONCLUSIONS 

1. By selection of hypermonotectic composition such that only a partial band of L2 forms at the solid + 

L2 interface, Bridgman directional solidification can be used to study the integration of excess L2 into the solidification interface. 

2. The formation of aligned composite structures in the Al-In-Sn system is not only a function of G and 

R but also of the degree of departure from the monotectic composition. 

3. The spacings of the aligned structures formed at hypermonotectic compositions were for the Al-181n7 

AI-18In-6Sn, and A1-18In-14Sn alloys on the order of that reported for irregular fibrous Al-In-Sn on monotectic. 

The spacings for the aligned structure for the ground and low-gravity-solidified 22Sn alloy were significantly 

larger and are on the order of that expected for cellular spacing for the growth conditions studied. 

4. The influence of gravity on the integration of excess L2 for hypermonotectic Al-In-Sn alloys increases 

as the monotectic dome height decreases. For AI-In alloys, gravity (above about 0.01 g) was not necessary for 

the integration of excess L2 while for A1-18In-6Sn and A1-18In-14Sn alloys, L2 did not integrate until high 

gravity was present. 

5. The higher Sn (lower monotectic dome height) alloys, Al-l8In- 14Sn and Al- 1 8In-22Sn7 exhibited 

aligned structure that was associated with low gravity or the transition from high to low gravity, and with the 

position on the sample relative to the start of unidirectional growth. It is believed that these microstructures are 

the result of the tendency for low gravity to destabilize the solidification interface. 
REFERENCES 

1. Livingston, J. D., and Cline, H. E.: Trans. AIME, Vol. 245, 1969, pp. 351-357. 

2. Knight, R. J. , Li, C. Y., and Spencer, C. W.: Trans. AIME, Vol. 227, 1963, pp. 18-22. 

3. Cahn, J. W.: Metall. Trans., Vol. 10A, 1979, pp. 119-121. 

4. Grugel, R. N., and Hellawell, A.: Metall. Trans., Vol. 12A, 1981, pp. 669-68 

5. Derby, B., Camel, D., and Favier, J. J.: J. Crystal Growth, Vol. 65, 1983, pp 

6. Derby, B., and Favier, J. J.: Acta. Metall., Vol. 3 I, 1983, pp. 1123-1 130. 

280-285. 

7. Bergman, A., Carlberg, T., Fredriksson, H., and Stjerndahl, J.: Materials Processing in Reduced Gravity 

Environment of Space. Proceedings of the Materials Research Society Annual Meeting, G. E. Rindone, ed., 

North-Holland, 1981 , pp. 579-592. 

8. Potard, C.: AIAA Paper No. 79-0173, New Orleans, LA, 1979. 

9. Delves, R. T.: Brit. J. Appl. Phys., Vol. 16, 1965, pp. 343-351. 

10. Lacy, L. L., and Otto, G. H.: AIAA Journal, Vol. 13, 1975, pp. 219-220. 

11. Hodes, E., and Steeg, M.: Z. Flugwiss, Weltraumforsch, Vol. 2, 1978, pp. 337-341 

12. Potard, C.: Materials Processing in Reduced Gravity Environment of Space. Proceedings of the Materials 

Research Society Annual Meeting, G. E. Rindone, ed., North-Holland, 1981, pp. 543-552. 

13. Kneissl, A., and Feshmeister, H.: Proc. Fifth European Symposium on Materials Sciences Under Microgravity. Results of Spacelab-1 , Schloss Elmau, FRG, ESA SP-222, 1984, pp. 63-69. 

14. Ang, C. Y., and Lacy, L. L.: ASTP Experiment MA-044, NASA TM X-64956, Marshall Space Flight 

Center, AL, 1975. 

15. Gelles, S. H., and Markworth, A. J.: AIAA Journal, Vol. 16, 1978, pp. 431-438. 

16. Carlberg, T., and Fredriksson, H.: Metall. Trans., Vol. 11A, 1980, pp. 1665-1676. 

17. Ahloborn, H., and Lohberg, K.: Proc. Fifth European Symposium on Materials Sciences Under Microgravity. Results of Spacelab-1, Schloss Elmau, FRG, ESA SP-222, 1984, pp. 55-62. 

18. Gelles, S. H.: Liquid Phase Miscibility Gap Alloys. MEA AI Experiments, Final Post Flight Report, 

NAS8-32952, NASA, MSFC, 1984. 

19. Johnston, M. H., Curreri, P. A., Parr, R. A., and Alter, W. S.: Metall. Trans. , Vol. 16A, 1985, pp. 1683- 

1687. 

20. Lee, J. E., McCay, M. H., and Curreri, P. A.: Metall. Trans. , 1986, in press. 

9 
21. Hendrix, J. C., Curreri, P. A., and Stefanescu, D. M.: AFS Trans., Vol. 92, 1984, pp. 435-458. 

22. Stefanescu, D. M., Curreri, P. A., and Fiske, M. R.: Metall. Trans., Vol. 17A, 1986, pp. 1 121-1 130. 

23. Campbell, A. N., Buchanan, L. B., Kuzmak, J. M., and Tuxworth, R. H.: J. Am. Chem. SOC., Vol. 74, 

1952, pp. 1962-1966. 

24. Wu, M. K., Ashbum, J. R., Curreri, P. A., and Kaukler, W. F.: Metall. Trans., 1986, submitted. 

25. Grugel, R. N., Lagrasso, T. A., and Hellawell, A.: Materials Processing in Reduced Gravity Environment 

of Space. Proceedings of the Materials Research Society Annual Meeting, G. E. Rindone, ed., NorthHolland, 1981, pp. 553-561. 

26. Derby, B.: Proc. Fourth European Symposium on Materials Sciences Under Microgravity, Madrid, 5-8 

April 1983, ESA SP-191, pp. 277-280. 

27. Chernov, A. A., Temkin, D. E., and Mel’nikova, A. M.: Sov. Phys. Crystallogr., Vol. 21, 1976, p. 369. 

28. Sharp, R. M., and Hellawell, A.: J. Crystal Growth, Vol. 6, 1970, p. 335. 

29. Lemaignan, C., and Malmejac, Y.: Materials Sciences in Space With Application to Space Processing, L. 

Steg, ed., American Institute of Aeronautics and Astronautics, NY, 1977, pp. 447-454. 

30. Delves, R. T.: Brit. J. Appl. Phys., Vol. 16, 1965, pp. 343-351 
TABLE 1. SPACING OF ALIGNED COMPOSITE STRUCTURE 

System Gravity Level Structure A* v(M3/S X w4) 

Al- I8In 

Al- 18In 

AI-30In 

Al- 18.5111-6.6Sn 

A1-18.9In-14.6Sn 

Al- 18.1 In-22Sn 

Al- 1 8. I In-22Sn 

Al- I8In 

Normal 

Low 

Low 

Normal 

Normal 

Normal 

Low 

Normal 

Fibrous Band 

Fibrous Band 

Fibrous Band 

Fibrous Band 

Fibroud Band 

Fibrous Band 

Aligned Globules 

Aligned Spheres 

2.05 

4.45 

3.71 

I .92 

1.05 

30.5 

33.7 

0.21 

Al-In-Sn (Grugel and Normal 

Hellawell 

AI-In (Grugel and Normal 

Hellawell) 

Irregular Fibrous 

Uniform Fibrous 

5.30 

0.04 

11 
Sn 

AI 

WEIGHT PERCENT 

Figure 1. Al-x.In-y.Sn immiscibility dome after Reference 23. The circled points 

denote the compositions studied. 

C 

In 

12 
Figure 2. AI-301n sample showing the disruption of the solidification process by an 

indium-rich band near the start of unidirectional growth. 

13 
AI-18.6 In 

I 

: 

I 

I 

I A 1-18.5 In-6.6 Sn t 

i 

I 

D A 1-18.9 In-14.6 Sn 

l-4 

UNIDIRECTIONAL lmm 

START 

GROWTH 

Figure 3. Hypermonotectic AI-x.In-y .Sn alloys directionally solidified upwards 

under normal gravity (ground). G = 250"Ckm; R = 0.5 cdmin. 
a 

15 
2 

0 

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I I I I 1 1 I I 

0 

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0 Ln 0 Ln 0 

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16 

1 

e 

1 

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(D 

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M 

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3 

r 

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n 

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0 

c3 

d 

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0 

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w 

K 

2 

3 

u. 

0 

c 

a 

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0 

a: 

L 

W 

0 

2 

c 

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a 

- 

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- n 

a 

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rc 

0 

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5 Y 

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17 
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F 0 Ln 0 m 0 

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3SVHd H31H NS-NI N0113VUJ 3kUn'lOA 

A 

E 

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I 

Is 

0 

U 

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A 

2 

0 

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CT 

2 

3 

LL 

0 

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0 

a 

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0 

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6 

22 
AI-I8 In 

I 

I 

A 1-30 In 

START 

UNIDIRECTIONAL 

GROWTH 

M 

Imm 

Figure 12. Hypermonotectic AI-x.In alloys directionally solidified during KC- 135 

maneuvers (flight). G = 250"Ckm; R = 0.5 cm/min. 

23 
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.* .I 

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31 
A B 

> 

c 

> 

a 

a 

u 

___ t:- - 

a 

A C, CHL BCOMPOSITION 

C 

HYPERMONOTECTIC SLIGHTLY -HYPERMONOTECTIC 

0 

0 

SOLID ’ +L2 0 

00 0 

0 0 OrJ 

SOLID 1 +L2 

Io 

-LARGE 

L2 DROPLETS 

Figure 21. Liquid composition gradient resulting from a hypermonotectic sample under gravity in a 

temperature gradient. (A) Temperature gradient and resulting separation (density of L2 is 

greater than L,). (B) Resulting change in composition of the homogeneous 

liquid. (C) Resulting liquid composition profile for hypermonotectic 

and slightly hypermonotectic starting compositions. 

32 
J 

v, n 

33 
H 

X=60pn 

Figure 23. Transverse section through the fourth low-gravity zone for Al- 18.1 In-22Sn flight sample 

showing the relative size of the spacing of the aligned particles. 

c 
APPROVAL 

THE EFFECTS OF GRAVITY LEVEL DURING DIRECTIONAL SOLIDIFICATION ON 

THE MICROSTRUCTURE OF HYPERMONOTECTIC AI-In-Sn ALLOYS 

Center Director’s Discretionary Fund Final Report 

By P. A. Curreri and W. F. Kaukler 

The information in this report has been reviewed for technical content. Review of any information 

concerning Department of Defense or nuclear energy activities or programs has been made by the MSFC 

Security Classification Officer. This report, in its entirety, has been determined to be unclassified. 

E. A. TANDBERG-HANSSEN 

Deputy Director, Space Science Laboratory 

J 

*US. GOVERNMENT PRINTING OFFICE 1987 730-067/40060 

~-~ ~ ~ -~ ~~ 

35