Solidification structures in submicron spheres of iron-nickel: Experimental observations

ha

~I-61~/88

melall. Vol. 36, No. 9, pp. 2523-2536. 1988

$3.00 +o*oo

Copyright 0 1988Pergsmon Press pk

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SOLIDIFICATION STRUCTURES IN SUBMICRON SPHERES OF IRON-NICKEL: EXPERIMENTAL OBSERVATIONS’f YEON-WOOK KIM, HONG-MING LIN and THOMA!3 F. KKLLY Department

of Me~l~~c~

and Mineral Engineering, University of Wisconsin, Madison, WI 53706, U.S.A.

(Received 17 June 1987; in revised~rm

9 December 1987)

Abstract-Alloys of 0, 30, 40, and 5Oat.% nickel in iron have been processed in vacuum by electrohydrodynamic atomization (EHD) to produce submicron droplets. The as-solidified spheres are studied to determine which of several solidification phases has appeared. Field-emission scanning transmission electron microscopy (STEM) is used to determine the microstructure, composition, and crystal structure of the 10-150 run diameter spheres. It is believed that homogeneous nucleation must be the predominant nucleation mechanism in EHD droplets during free flight. The alternative crystallization phase, b.c.c., in the Fe-Ni alloy system, is found in 30 and 40 at.% Ni alloys but not in the 50 at.% Ni alloy. A new hexagonal crystal structure of Fe50 at.% Ni is discovered. Furthermore, the smallest spheres of each alloy (< 50 nm diameter) including pure iron are found to be amorphous. These findings are consistent with &z&ions (detailed in a second paper [Acta met& 36,2537 (1988)), based on classical mrcleation theory which predict the conditions under which alternative crystallization phases may appear. R6smn&Nous avons &bore des alliages de fer ;i 0, 30, 40 et 50% en atomes de fer, par atomisation ~l~rohydr~~~que (EHD) sous vide, ce qui permet d’obtenir des gout& inferieures au pm. Now avons itudit les spheres brutes de solidfication pour determiner quelle est. parmi les nombreuses phases de solidification, celle qui se forme. Nous avons d&ermine, par microscopic ilectronique en transmission, avec balayage et emission de champ, la microstructure, la composition et la structure cristalline des spheres de diametres compris en&e 10 et 150 nm. Nous pensons que la germination homogene est le m&canisme de germination p&dominant dans les gouttes obtenues par EHD pendant leur temps de vol. Nous observons l’autre phase de cristallisation, cubique cent&, du systeme fer-nickel dans les alliages a 30 et a 40% en atomes de nickel, mais pas dans l’alliage a 50% en atomes de nickel ou nous mettons par contn en evidence une nouvelle structure cristalline hexagonale. En outre, nous observons que les plus petites spheres (de diam&re inferieur a 5Onm) de chaque alliage, y compris le fer pur, sont amorphes. Nos observations sont en accord avec les calculs que nous detaillerons dans un second article [,4ctu tnet4jl. 36, 22537 (1988)) et qui sont bases sur la theorie classique de la germination qui predit les conditions vacations d’autres phases de ~stalli~tion. ~~Ei~nie~erungen mit 0,30,40 und 50 At.-% Nickel wurden in Form von Kilgelchen mit einem Durchmesser von 10 bis I50 nm hergestellt, indem entsprechende Triipfchen im Vakuum durch elektrodynamische Atomisierung gebildet wurden. Die erstarrten Kiigelchen wurden dann im RasterDurchstrahhmgselektronenmikroskop untersucht. Die auftretendea Erstarrungsphasen, deren Mikrostruktur, Zusammensetxung und Kristallstruktur wurden bestimmt. Es wird angenommen, da0 die Kiigelchen im Fluge vorwiegend durch homogene Keimbildung erstarren. Die im Eisen-Nickelsystem alternative krx Kristallisationsphase wird in den Legierungen mit 30 und 40 At.-% Ni gefunden, jedoch nicht in der mit 50 At.-%. In dieser Legierung wurde eine neue hexagonale Strukture entdeckt. AuDcrdem sind die kleinsten Ktlgelchen (Durchmesser c SOnm) &mtlicher Legierungen einschlieBlich des reinen Eisens amorph. Diese Ikfunde enstprechen Rerechnungen (behandelt in einer xweiten Arbeit [Acre metoll. 36,2537(1988)], welche auf der klassischen Keimbildungstheorie beruhen und die Redingungen angeben, unter denen alternative K~stalli~tionsphasen auftreten kiinnen.

~RODU~ION Background

From the vestigations [2-S], small isolation of

very beginnings of experimental ininto liquid-to-crystal nucleation in melts droplet processing has been exploited for potential heterogeneous nucleation cata-

YI’his work was sponsored by the Department of Energy, OiIice of Rasic Energy Sciences, under Contract No. DE-FGO2-85ER45215.

lysts. By this approach, major fractions of a liquid sample have been supercooled a large amount prior to nucleation of the crystal. The results of small droplet processing experiments have generally been derived from observation of physical parameters that are sensitive to crystallization. In the pioneering work of Tumbull et al. [3,4] visual observations of droplets on a vitreous silica substrate were employed to monitor nucleation events. High melting-temperature metals were studied in this way. In droplet-emulsion techniques, small

2525

2526

KIM et al.:

SOLIDIFICATION STRUCTURES IN SUBMICRON SPHERES: EXPERIMENTAL

liquid metal droplets have been suspended in carrier fluids which were chosen to minimize their effect on the nucleation process. Using the procedure, Vonnegut [2] and others [5-7] used dilatometric measurements of specific volume changes during crystallization as a gauge of nucleation events. Rasmussen and Loper [S] used a differential thermal analyzer to directly measure the evolution of the heat of fusion from small-droplet emulsions as crystallization occurred. The latter technique has been used extensively by Perepezko ef al. [9-121 to study the nucleation behavior of low-melting-temperature metals and alloys. Flemings et al. [13,14] have performed differential thermal analysis studies on higher-melting-temperature, iron- and nickel-base alloys. A major advantage of the above techniques is the ability to monitor the nucleation temperature directly. However, each of these techniques also suffers from the fact that the liquid is in direct contact with a substance that may influence (for example, change the composition of the liquid) the nucleation process. As an alternative to directly studying the nucleation process as it occurs, the products of the nucleation process may be studied after nucleation occurs. Information about the nucleation process can be inferred, however, only if there is more than one possible solidification product. In the present study, alloys that can crystallize as alternative phases are investigated. The key to this approach for studying liquid-to-crystal nucleation is that there is an identifiable event-the appearance or absence of a given crystal phase-that can be correlated with theoretical predictions. This analysis would require that the phase products do not change after nucleation. Experimentally, submicron droplets which are produced by electrohydrodynamic atomization [15-l 7), cool very rapidly ( lo6 K/s) and solidify in free flight through a vacuum. Because of the high cooling rate, phases that nucleate and grow at high temperatures are retained at ambient. Furthermore, the surfaces of the droplets are atomically clean for the duration of the solidification process. Though the temperature of nucleation is not measured, alternative crystallization phases are produced. Kelly and Vander Sande [l&19] have described an analytical treatment of the nucleation of alternative crystallization phases in the supercooled liquid. Analytical modelling is used in support of this approach to studying liquid-to-crystal nucleation. Detailed calculations of nucleation probabilities of primary and alternative crystallization phases are made for comparison with experimental results. In this fashion, nucleation theory is exercised quantitatively and its successes and failures are noted. The iron-nickel

system

The iron-nickel alloy system is of interest for this study because it has an f.c.c. and a b.c.c. phase that

FS

20

40

60

60

Ni

Composition (at.%)

Fig. 1. Fe-Ni phase diagram per Chuang, Hsieh and Chang [21,22], which shows calculated phase boundaries for both liquid-to-f.c.c. and liquid-to-b.c.c..

change in relative stability with composition and temperature (Fig. 1). Alternative crystallization phases are expected from an alloy with these properties. In fact, Cech [20] reported finding what appears to have been an alternative crystallization phase, b.c.c., in the smaller droplets of an Fe-29.5 wt% Ni alloy. The Fe-Ni system was recently modeled thermodynamically by Chuang et al. [21,22]. In conjunction with an analytical model of the nucleation process, this information is used to predict nucleation probabilities for alternative crystallization phase prod-

1600

;

1600

a

% s!

1400

s ‘Z 4 2

1200

“E

3

3 1000 600

1

FS

20

40

Composition

60

60

Ni

(at.%)

Fig. 2. Calculated volume nucleation temperature for f.c.c. and b.c.c. phases in 60nm diameter spheres of Fe-Ni.

KIM et uf.: SOLIDIFICATION STRUCTURES IN SUBMICRON SPHERES EXPERIMENTAL u&on. For example, the nucleation temperature calculated for homogeneous nucleation of f.c.c. and b.c.c. phases in 60 nm diameter droplets is shown in Fig. 2 [I]. This plot indicates that if homogeneous nucleation is achieved, the b.c.c. phase has the higher nucleation temperature and should therefore nucleate in 60nm droplets for compositions less than 47 at.% Ni. Electrohydrodynamic atomization is used to produce quantities of sub-100 nanometer diameter droplets of selected Fe-Ni alloys for this study. The as-solidified droplets have been examined by analytical electron microscopy and the solidification phases are determined. The experimental observations of these spheres are presented in this first paper and the origin of each of the structures is considered. In a second paper 111,these experimental results of droplet processing are compared directly with the analytical model and detailed analysis of calculated nucleation probabilities are made. Furthermore, the ramifications of these results for nucleation theories and theories of liquids are considered. Advantages of the EHD/TEM liquid-to-crystal nucleation

approach for studies of

In experiments of this nature, the advantages of transmission electron microscopy studies of electrohydrod~amically atomized samples are noteworthy. Electrohydrodynamic atomization offers the following advantages for studies of liquid-to-crystal nucleation: Very small droplet size which results in (a) rapid cooling (> IO’ K/s) (b) extrene isolation of heterogeneous nucleants (c) electron transparency without thinning Solidification occurs in free flight Atomically clean droplet surface l(b) plus 2 and 3 above imply that homogeneous nucleation is operative. Furthermore, ~ansmission electron microscopy studies of the as-solidified droplets offers the following advantages: 1. The microstructure can be studied in the image 2. The atomic structure can be dete~ined from the electron diffraction pattern 3. The size of the droplet is determined in the image 4. The composition can be determined from Xray analysis.

EXPERIMENTAL

Pure (0.~~) iron and (0.9~9) nickel stock were alloyed in an arc melter under argon atmosphere. Four alloys, pure iron and Fe-30,40, and 50 at.% Ni, were prepared. Forty gram ingots were melted,

2527

overturned, and remelted several times to ensure homogenization. Wires were prepared by placing the ingots in a copper sheath then swaging and drawing down to OSmm diameter. The copper sheath was removed by thorough etching in a nitric acid solution. The pure iron wire was processed a-received from the supplier, The wires were sprayed by el~trohydrodynamic (EHD) atomization which is a process developed by Phrasor Scientific, Inc. in California [IS-l 7j. With an EHD process, spheres of liquid in the range of 2 nm to 100 pm diameter can be sprayed. For this work, spheres 10 to 150 nm diameter were sprayed. The droplets solidify in free-flight through a vacuum chamber operated at about 10m4Pa and impinge on a collector substrate after solidification is complete. Cooling rates of about 10’ K/s are achieved by radiation in 60 nm spheres at the melting temperature. A potential source of heterogeneous nucleants is oxides on the surface of the droplets. At a pressure of 5 x 10e4 Pa in the EHD chamber, the partial pressure of oxygen will be about 10e4Pa. From the kinetic theory of gasses [23], the flux of oxygen atoms at room temperature impingeing on any surface will be 3 x 1018m-zs-‘. For a 60nm diameter sphere cooling from 1800 to 800 K at an average cooling rate of 3 x IO6K/s, the total number of oxygen molecules hitting the surface during that time is 10. Thus the surfaces of these droplets must be essentially atomically clean for the duration of solidification and oxide formation should not be a significant source of heterogeneous nucleants in the EHD process. Using the vapor pressure of liquid iron or nickel [24], the evaporation rate [25] from the surface of the droplets is calculated to be about lo-’ monolayers during the time that the droplet is molten. Two substrates that have been used are single crystal rock salt and acetate tape. Rock salt is the preferred substrate since it minimizes hydrocarbon contamination in subsequent observation in the TEM. Specimens for TEM were prepared by depositing a very thin film of carbon onto the substrate and cutting this into 3 mm squares. The substrate was then dissolved in an appropriate solvent: distilled water for the rock salt and acetone vapor for the acetate tape. The carbon film with as-solidified spheres was mounted on IO00 mesh TEM grids. By using a fine mesh grid, it was possible to make the support film very thin which aided observation of the diffraction patterns of the spheres. Analytical electron microscopy was used to determine the crystal structure as a function of the composition of the solidified spheres. The crystal structure can be determined directly by electron diffraction techniques as shown below. Diffraction patterns were recorded from spheres in the orientation in which they were found. Most of the diffraction patterns recorded in this fashion could be indexed unambiguously as either f.c.c. or b.c.c. If a pattern could not be indexed, it was not counted. In addition, the

2528

KIM er af.: SOLIDIFICATION STRUCTURES IN SUBMiCRON SPHERES: EXPERIMENTAL

Specimen OL OA

L r7tl

SADA

c2

c2

Cl

Cl

VOA

VOA

Normal imogc (Higha,Low L)

Low mag image (Lowa, High L)

Fig. 3. Illustration of the electron diffraction modes used in this work.

diameter of each sphere can be determined with great accuracy from the image. The analysis was performed on a Vacuum Generators HB501 field-emission STEM. There are two different operating modes on this ins~ment that have been used for diffraction and imaging work in this investigation as shown in Fig. 3. In normal image mode, all lenses are operational and the scanned beam is focussed to a 0.5 nm spot. Images with about 0.4 nm spot. Images with about 0.5 nm resolution are obtained. The convergence of the beam in this mode, however, is on the order of 5 mrad which is large for electron diffraction work. Because of the large convergence angle of the incident illumination, the diffracted beams appear as discs in the diffraction pattern [26]. Complex diffraction patterns with diffracted beams that are close to each other cannot be sorted out in this mode. It is also possible to operate the instrument with the objective lens turned off to facilitate low magnification imaging. In this latter mode, the scanned beam is focussed to a 1Onm spot with a beam convergence of less than 1 mrad. The low beam convergence of this mode is beneficial for observing tine detail in the diffraction pattern. Howerver, the image resolution of this mode is too poor to resolve microstructural information in the nominally 100 m diameter spheres of this study. Thus, both highresolution imaging and Iow-beam-convergence

diffraction can be obtained but not simultaneously. Furthermore, though it is possible to obtain both the high-resolution image and a low-beam-convergence diffraction pattern from the same sphere, in practice this is not readily a~omplished. The composition of the spheres was studied by energy dispersive X-ray analysis using an ultra-thin paralene/aluminum window. The composition was found to be essentially nominal for each alloy. In order to study the carbon content of the spheres, aluminum was used as a support film for several samples, This made it possible to study the carbon content of the spheres with high precision. The composition of all other elements, including oxygen, could be studied on carbon films. All samples were baked at 60°C for 1 h in order to drive off hydrocarbons and water. For spheres mounted on carbon films, no impu~ty elements were detected above background except for carbon. For spheres mounted on aluminum films, no impurity elements were detected above background except for oxygen and aluminum. It is possible to detect on the order of 0.5 wt.% (or 0.1 at.%) of these elements with this technique. RESULTS Images and diffraction patterns were obtained from spheres in the size range of 10 to 150 nm

KIM ef al.: SOLIDIFICATION STRUCTURES IN SUBMICRON SPHERES: EXPERIMENTAL

2529

Fig. 4. STEM bright-field image of an Fe-50 at.% Ni alloy sphere processed by EHD and a high beam convergence diffraction pattern of the f.c.c. phase.

Fig. 5. STEM bright-field image of an Fe-30 at.% Ni alloy sphere processed by EHD and low beam convergence diffraction pattern of the b.c.c. phase.

diameter. Each sphere was surveyed by moving the beam to all parts of the image while observing the diffraction pattern and it was found that most of the solidified spheres were single crystals independent of whether they are f.c.c. or b.c.c. Representative microstructures and diffraction patterns of solidified droplets are shown in Fig. 4 for f.c.c. and Fig. 5 for b.c.c. A small fraction of the spheres, however, consist of more than two grains. As indicated in Table 1, very fine grained poly crystalline structures were observed in pure iron and in Fe-50 at.% Ni. Diffraction patterns from two of these spheres are shown in Fig. 6. The sphere associated with Fig. 6(a) is b.c.c., pure iron, and is on the order of 35 nm diameter while the sphere associated with Fig. 6(b) is f.c.c., Fe-50 at.% Ni and is on the order of 55 nm diameter. The two polycrystalling patterns observed for pure iron were almost identical. The electron beam is on the order of 10 nm diameter for these low beam-convergence diffraction

patterns and there are on the order of 50 spots in the f 10 ring of this b.c.c. pattern. Assuming that there are 15 grains in the interaction voIume with the electron beam, this indicates that the average grain size of the sphere of Fig. 6(a) is 6 nm diameter. Figure 6(b) shows some strong spots that correspond to a (112) zone axis pattern in f.c.c. in addition to the polycrystalline pattern. There may be one large grain present in addition to the very small grains. The observations for pure iron and Fe 30, 40, 50 at.% Ni alloys are summarized in Table 1 according to the crystal structure observed. There is a clear trend for the predominance of b.c.c. in pure iron white f.c.c. predominates in Fe-50 at.% Ni. In order to study the dist~bution of crystal phases as a function of droplet size, a size distribution histogram for f.c.c. and b.c.c. phases was determined for F&at.% Ni, Fig. 7. There does not appear to be a distinct transition point between f.c.c. and b.c.c. with droplet size in the range of this histogram.

Table 1. Summary of microstructures Alloy Pure iron Fe-30 at.% Ni Fe40 at.% Ni Fe-S&at.% Ni

observed according to crystal structure

f.c.c.

br.c.

f.c.c. + b.c.c.

8 :d

41 52 25 0

0 2 0 0

61

Polycrystal

Hexagonal

2 b.c.c. 0 0

0 0 0

I ccc.

5

2530

KIM et 41.: SOLIDIFICATION

STRUCTURES

IN SUBMICRON

SPHERES: EXPERIMENTAL Fe- 40 at.% Ni

0

5

10

15 20

25

90

35

40

45

50

5560

65

TO

Diameter (nm) Fig. 7. Size distribution histogram according to crystal structure of F&O at.% Ni;;bnicron spheres processed by

Fig. 6. Diffraction patterns from very fine grain poly crystalline spheres of (a) pure iron and (b) Fe-50 at.% Ni.

The microstructure are usually relatively free from features and do not show evidence of a large point or line defect content (Figs 4 and 5). A few spheres contain microstructural features that have the appearance of twins. An example of this type of microstructure is shown in Fig. 8. The diffraction patterns from two spheres similar to that of Fig. 8 are shown in Fig. 9. The high-beam-convergence diffraction pattern shown in Fig. 9(A) is taken from a twinned area of a sample and exhibits a f.c.c. 111 I} habit pfane. A (I IO) zone axis diffraction pattern of the matrix is shown in gig. 9(B). Furthermore, it is possible to identify the twinned crystal more clearly with the low-beam-convergence diffraction pattern in Fig. 9(C) which is schematically detailed in Fig. 9(D). In addition to the appearance of extra spots due to twinning, a diffraction pattern, Fig. 10, is afso obtained from an Fe-30 at.% Ni sphere in which two phases with a particular mutual orientation relationship are present. Indexing of the pattern indicates the co-existence of f.c.c. and b.c.c. phases where the relative orientation is consistent with the Nishiyama-Wasserman orientation relationship.

Thus this suggests that a partial martensitic transformation of f.c.c. to b.c.c. has taken place. Several diffraction patterns were recorded for Fe-50at.% Ni that could not be indexed as either f.c.c. or b.c.c. Formation of an ordered cry&I phase might be possible at the equiatomic com~si~on. However, the structure factor for a superlattice reflection in the FeNi system will be small because the difference between the atomic scattering factors of Fe and Ni is less than 2. Thus even if the ordered phase exists for this alloy composition, the intensity of a superlattice reflection is too weak to be identified in a diffraction pattern. In order to obtain systematic info~ation about the unit cell of this unknown structure, diffraction patterns were obtained from a single sphere by tilting the sphere on the order of 20deg at a time about one axis and recording the diffraction pattern. When a major crystallographic pole was encountered, the sphere was tilted about a perpendicular axis. This series of diffraction patterns is shown with a unit stereographic triangle in Fig. 11. One of spot patterns, (co001 > zone axis), shown that this crystal contains a 3-fold rotation axis that can only be found in the cubic and hexagonal crystal systems (and also in an icosohedral phase).

Fig. 8. STEM bright-field image of an f.c.c. sphere processed by EHD with twinning in the structure.

KIM et al.: SOLIDIFICATION

STRUCTURES

IN SUBMICRON

SPHERES: EXPERIMENTAL

2531

Fig. 9. High beam convergence diffraction patterns from (A) a twinned area and (B) the matrix for a (110) tc.c. zone axis of the same sphere. (C) Low beam convergence diffraction Pattern from Fe-50 at.% Ni sphere processed by EHD showing twin reflections. (D) Plotting the twin spots (opencircle) of (C) shows a (1 IO) f.c.c. zone axis pattern with a {11 I) twin axis.

This structure indexes as a hexagonal crystal with a = 0.252 nm and c = 0.621 nm or a c/a ratio of 2.41.

If this phase is assumed to have the density of pure iron or nickel, then it is found that there must be 3 atoms in this unit cell. Three atoms per unit cell is disturbing in light of the fact that this is an equiatomic composition. The atomic positions have not been determined but a logical first guess is 000, 3 4 $, and f : j. This proposed atomic arrangement is similar to that obtained if a hexagonal cell is used to describe an f.c.c. structure, The f.c.c. structure in hexagonal format has a c/a ratio of 2.45. Thus this hexagonal crystal is very similar to f.c.c., but, the sequence of closest-packed planes is different from f.c.c. There has been no report in the literature of this structure in any Fe-Ni alloy previously. In a fraction of the smallest spheres, an amorphous structure is observed, Fig. 12. Even in pure iron, the amorpous structure was found in half of the spheres that are less than 30 nm diameter. These histograms show that a transition size between amorphous spheres and crystal spheres exists. These transitions occur at about 35, SO,45 and 40 nm diameter for the pure iron, and Fe-30, 40, and SOat.% Ni compositions, respectively. This transition size is lowest for pure iron and is decreasing with increasing nickel content for the alloys. An example of the amorphous

microstructure and a diffraction pattern is shown in Fig. 13. Note the salt-and-pepper appearance of the image contrast which is characteristic of an amorphous phase. Additionally, one of the more interesting findings is that some spheres were observed to be part crystalline and part amorphous. AtI of these mixed spheres were found in the size range near the transition between the crystalline and amorphous structures for pure iron and each composition of the alloys. Figure 14 shows an example of a pure iron sphere which consists of a b.c.c. phase growing into an amorphous phase. DiSCUSSION The fact that Fe-SOat.% Ni is exclusively f.c.c., that the diffraction patterns are clean and show no signs of strain, and that the phase diagram indicates that f.c.c. should be stable to low temperatures implies that the f.c.c. phase solidified from the liquid. For the same latter two reasons, the f.c.c. phase in Fe-30 and 40 at.% Ni alioys appears to be due to solidification from the liquid. The b.c.c. phase in the Fe-30 and 40 at,% Ni alloys also appears to have solidified from the liquid. Indeed, Fig. 14 shows b.c.c. growing from the liquid. The diffraction pattern shows no signs of strain in the lattice. Furthermore,

2532

KIM et al.: SOLIDIFICATION STRUCWRES IN SUBMICRON SPHERES: EXPERIMENTAL

. l

a

.

0

.

e

1

0

*

l

*

.

.

* l

0

. 0

.

.

“*

l

.

0 l

.

. . .* l . . .

10 ” a-

f 0 L IL

0

ot. X Ni

Oiamcter fnm)

Fe - 40

Oiameter (nm)

_I 65

(b)

14

16

16

20

(d)

1 6E

20

m

&twrphour

50

at.%

Ni

Diameter (nm)

Fe-

Diameter tnml

m

Fe-30at.XNi CrYLt0UiM)

Fig. 12. Size distribution histograms of EHD spheres according to solidification structure as either crystalline or amorphous. 20 spheres were counted in each 10 nm diameter incremental range of (a) pure iron, (b) iron-30 at.% Ni, (c) Fe-40 at.% Ni, and (d)Fe-SO at.% Ni.

2-

4-

6-

$2-

G E

14 -

16 -

16 -

(c)

0

(a)

20

nm

Fig. 13. STEM bright-field image of amorphous sphere processed by EHD and low beam convergence diffraction pattern.

I-

I-a

t;l z

!iz

VI

2534

KIM et ai.: SOLIDIFICATION STRUCTURES IN SUBMICRON SPHERES: EXPERIMENTAL

Fig. 14.STEM bright-field image of pure iron sphere and the corresponding diffraction pattms showing part b.c.c., part amorphous. the fact that b.c.c. is seen with the amorphous phase implies that it must have been retained to ambient temperatures without solid state transformation. Based on what is known about martensitic transformations in Fe-Ni alloys, the b.c.c. phase in the 4Oat.% Ni alloy is not expected to be martensitic. The MS temperature of Fe-30 at.% Ni is below room temperature though surface martensite can form at room temperature. Still, only one potential example structure was found in of a marten&c Fe-30 at.% Ni (Fig. 10). No martensitic structures were found in the Fe-40 and 50 at.% Ni alloys. Experimentally, the cross-over composition between f.c.c. and b.c.c. nucleation is here found to be between 40 and 50 at.% Ni. The trends in the experimental results shown in Table 1 agree qualitatively with the calculations for homogeneous nucleation shown in Fig. 2. There is not a sharp cross-over, however, between f.c.c. and b.c.c. nucleation with either composition or droplet size. This suggests that the nucleation rates of the two phases are nearly equal and that statistically, the resutt is a mixture of the phases. Alternatively, the presence of the f.c.c. phase in the pure iron, Fe-30 and 40 at.% Ni might be attributed to heterogeneous nucleation. It was shown above that the possibility of formation of a surface oxide is negligible in EHD processing. In most droplet processing, droplet sizes below about 10 pm diameter are

needed to obtain large liquid supercooling. If it is assumed that there is on the order of one heterogeneous nucleant per droplet at lOpm, diameter, then at 1OOnm diameter there is only one nucleant per 1O6droplets. Furthermore, recent work by L&era 1271 shows that as a heterogeneous nucleant’s size decreases below about 100 nm, is potency decreases rapidly independent of its species. Also, the impurity content of the pure iron wire is very low and second phases nucleating from the liquid is not likely. For these reasons, it appears that heterogeneous nucleation is not likely and homogeneous nucleation is dominant. The hexagonal phase of Fe-SO at.% Ni must represent an alternative crystallization phase that has not been observed in any manifestation previously. It is not so surprising that it is found in the equiatomic composition because there may be some tendency to order which influences the kinetics sufficiently to affect nucleation. Most of the submicron droplets are found to solidify as single crystals. This indicates that: (1) there is only one nucleation event per droplet and (2) crystal growth rates are high enough to cause the droplet to fully crystallize before a second nucleation event occurs. However, the growth rate decreases rapidly as the temperature approaches the glass transition temperature. The mixed crystalline and amorphous sphere in Fig. 14 shows that the crystal nucle-

KIM et al.:

SOLIDIFICATION

STRUCTURES

ated but that growth was slowed ,,down and ‘halted; presumably as the temperature of the sphere approached T,. This indicates that the liquid never reached a point where copious nucleation could occur. It is apparent though, that very large liquid supercoohngs have been achieved in this process since the growth was halted. A small fraction of the spheres contains two or more grains which is consistent with the statistical nature of nucleation. In pure iron and the Fe-50 at.% Ni alloy, however, randomly oriented fine-grain polycrystalline structures are observed in spheres that have a diameter just greater than the maximum diameter of the amorphous spheres. This fine grained structure must be due to copious nucleation of the crystal phase from the liquid. Again, though it is possible to envision scenarios based on heterogeneous nucleation that could produce the fine-grain structure, it appears to be more realistic to ascribe the structure to homogeneous nucleation. In classical nucleation theory, the occurrence of copious nucleation requires more liquid supercooling than is expected for a single nucleation event. The amorphous phase is found in smaller spheres, on average, in the pure iron and in the highest nickel content alloys. Since the radiative cooling rate varies as inverse diameter, this indicates that pure iron and the higher nickel content alloys require a greater cooling rate in order to avoid nucleation. The reduced glass transition temperature must therefore be decreasing with increasing nickel content for the alloys. It is thus interesting to note that the polycrystalline structures are observed in the materials with the lowest apparent glass transition temperature. The amorphous phase in the Fe-Ni alloy spheres is found exclusively at the smaller diameters, Fig. 12. It appears that the spheres of less than 30nm diameter predominantly amorphous and the maximum diameter of the amorphous spheres decreases with nickel content from 55 nm for 30 at.% Ni to 40 nm

1. Single

crystal

IN SUBMICRON

produced if the nucleation enough that growth is rapid.

temperature is high point 1 in Fig. 15. As

the droplet diameter decreases. the liquid will tend to supercool more and the sphere will cool more rapidly. In the temperature range between points I and 3, the nucleation rate is increasing and the growth rate is decreasing. At point 2 then, a high nucleation rate is coupled with a modest growth rate and a polycrystalline structure is expected. At point 3, the nucleation rate has begun to decrease but it is still high enough to result in a nucleation event. Growth at point 3, however. is depressed and crystallization is not completed. At the glass transition temperature. point 4, both nucleation and growth of the crystal are suppressed. These arguments are qualitative but are consistet with the observations of the structures as a function of droplet size. The differences in the amount of supercooling expected in each droplet is also consistent with the probabilistic nature of nucleation. Several other observations are suggestive of the probabilistic nature of nucleation phenomena. For example, the distribution of the f.c.c. and b.c.c. phases with sphere size in the Fe-40 at.% Ni alloy

+ crystal

4. Amorphous

1

KJ3

1030

432

3535

for 50 fit:% Ni. Clearly, the liquid was supercooled to the point where atomic mobility was very low and neither nucleation nor growth could occur rapidly. Because of the very small size of these droplets, the total nucleation probability is low. Furthermore, because the surface area-to-volume ratio increases as the diameter of a sphere increases. the cooling rate is highest in the smallest droplets (on the order of 10’ K/s for a 30 nm droplet at 1700 K). Thus, if an amorphous structure was to be produced in any droplet, it is most likely that it should be found in the smallest sphere. The observations of each microstructural type can be explained by the competition between nucleation and growth over a range of temperature. as shown schematically in Fig. 15. A single crystal would be

3. Amorphous

2. Polycryrtal

SPHERES: EXPERIMENTAL

1

Temperature

(K)

Fig. 15. Schematic explanation of observed microstructures according to competition between nucleation and growth. The nucleation rate curve is calculated for f.c.c. Fe-30 at.% Ni and the growth rate is similar to the calculation of Kaufman and Fraser 1281.

2536

KIM

et al.:

SOLIDIFICATION

S~U~URES

does not show a sharp transition from one phase to the other. Presumably, the nucleation rate of each phase is comparable in magnitude and the phase with the higher nucleation probability is found in relatively greater abundance. Similarly, there is a range of sphere size in which the amorphous phase is found and some crystallization is found even in the smallest spheres studied. Again, there is not sharp transition between amorphous and crystalline structures with sphere size. Finally, the hexagonal phase has nucleated even though it is found in less than IO% of the Fe-50 at.% Ni spheres.

CONCLUSIONS

An alternative crystallization phase, b.c.c., nucleates from supercooled liquid droplets of Fe-30 and 40 at.% Ni, but is not observed in Fe-50 at.% Ni. F.c.c. nucleates in pure iron as alternative crystallization phase. According to these experimental findings, the cross-over composition between f.c.c. and b.c.c. nucleation must be between 40 and 50at,% Ni. This latter result is in good ag~ment with a calculated prediction [l]. A hexagonal phase with a = 0.252nm and c = 0.621 nm is observed in less than 10% of the iron-50 at.% nickel alloy. When submicron spheres of iron-nickel are processed by electrohydrodynamic atomization, homogeneous nucleation is the predominant mode of nucleation in the droplets. Fine-grain polycrystalline spheres are observed in pure iron and Fe-SO at.% Ni. This polycrystalline structure suggests that copious nucleation can occur in iron and Fe-Ni alloys and the finding may be taken as evidence of homogeneous nucleation. An amorphous structure is found in about one fourth of the spheres of pure iron, and Fe-30,40 and 50 at.% Ni that are smaller than 50 nm diameter. By considering the size-dependence of the amorphous spheres, the glass transition temperature must be greatest in pure iron and Fe-50 at.% Ni and decreases with decreasing nickel content for the three alloys. Rcknowledgentenrs-This work was supported by grant No. DE-FGO2-XSER45215 from the Department of Energy, Office of Basic Energy*Sciences. The el~trohydr~ynamic atomization of the materials of this study was performed by Phrasor Scientific. Inc., Duarte. Calif. and is gratefully acknowledged.

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