In situ observations of solid-liquid interfaces

Ultramicroscopy 39 (19911 110-117 North-Holland

~

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In situ observations of solid-liquid interfaces H. Saka, K. Sasaki, T. O h a s h i , I. O h t s u k a Department qf Materials Science and Engineering, Faculty of Engineering, Nagoya Um~'ersity, Nagoya 464-01, Japan

T. K a m i n o Hitachi Instrument Engineering, Co. Ltd., 882 Ichige, Kutsuta, lbaraki 312, Japan

and M. Tomita Naka Works, Hitachi, Ltd., 882 Ichige, Katsuta, lbaraki 312, Japan Received 26 March 1991

Recent observations by in situ electron microscopy of the melting processes in lnSb, In and Pb have been reviewed with special reference to the morphology and structure of solid-liquid interfaces. It has been shown that, in some cases, the structure of the solid-liquid interface can be observed at a resolution allowing the lattice-fringe image. The results of the observations have been compared with the theoretical model of solid-liquid interfaces by Jackson.

1. I n t r o d u c t i o n

The morphology of the solid-liquid interface plays a very important role in the processes of solidification and melting of crystals [1]. In situ electron microscope observation is a very useful technique to observe directly the solid-liquid interface, and some in situ observations of the solid-liquid interfaces have been carried out on metallic materials {2-4]. Unfortunately, the resolution of the in situ observations carried out so far was not high enough for obtaining information on the solid-liquid interfaces at the nearatomic resolution level. However, recently some attempts have been made by the present authors to observe the solid-liquid interface by in situ

high-resolution electron microscopy, and will be described in this review.

2. S o l i d - l i q u i d

interfaces in I n S b

Solid-liquid interfaces in lnSb were observed in Hitachi HU-1000D and H-1250ST microscopes at an accelerating voltage of 1000 kV [5]. Fig. 1 shows the phase diagram of the I n - S b system [6]. The intermetallic compound InSb occurs at the 1 : 1 stoichiometric composition and two eutectic reactions, namely L ~ In + InSb and L -* InSb + Sb, occur at 155 ° and 500 ° C, respectively. The mutual solubility of the solid phases is very small The intermetallic compound lnSb melts congru-

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H. Saka et al. / In situ obseruations of solid-liquid interfaces

g~ 600

400

200

y

500"C

155"C

i

i

I

I

I

8O In (at% Sb) Fig. 1. Phase diagram of In-Sb (from ref. [6]).

I

100 Sb

111

ently at 530 ° C. H o w e v e r , w h e n foil s p e c i m e n s of I n S b are h e a t e d in the v a c u u m of t h e e l e c t r o n m i c r o s c o p e s , the d e p l e t i o n of Sb a t o m s starts well b e l o w the m e l t i n g p o i n t of InSb owing to the p r e f e r e n t i a l e v a p o r a t i o n of Sb. A s a result, the overall c h e m i c a l c o m p o s i t i o n of the h e a t e d foil s p e c i m e n s deviates from the original s t o i c h i o m e t ric 1 : 1 c o m p o s i t i o n t o w a r d s the In-rich side. This m e a n s that, above the eutectic t e m p e r a t u r e bet w e e n In a n d I n S b (155 o C), the foil s p e c i m e n has an In-rich liquid p h a s e a n d an InSb solid p h a s e , the c o m p o s i t i o n a n d a m o u n t of the liquid p h a s e d e p e n d i n g on t h e t e m p e r a t u r e a n d the d e v i a t i o n from the stoichiometry. Figs. 2a a n d 2b show the

Fig. 2. Structure of a foil specimen of initially stoichiometric InSb heated in an electron microscope: (a) bright field image with the beam direction B = 113; (b) dark-field image using part of the first halo ring of the diffraction pattern from the liquid droplets; (c) diffraction pattern from the regions marked with L (from ref. [5]).

112

H. Saka et al. / In situ obsercations of solid-liquid interfaces

typical structure of the mixture of the In-rich liquid phase and InSb matrix. Fig. 2c shows the diffraction pattern from the liquid droplets; halo rings characteristic of non-crystalline (liquid) structure are clearly observed. The liquid droplets are elongated along [110] (intersection of the (111) plane with the surface plane (113)), indicating that the interfacial energy between the liquid and the solid InSb is anisotropic. Fig. 3 shows a high-resolution electron micrograph of the solid-liquid interface of InSb. Within the region of crystalline InSb the lattice-fringe images of the (111) planes were observed clearly, while in the liquid droplet no lattice-fringe images were seen. The interface along the (111) planes is very straight, while the curved interface shows no evidence of facetting. Fig. 4 shows an example of the interaction of pre-existing dislocations with the migrating solid-liquid interface. As the liquid droplet grew, the dislocation moved in such a way that it intersected the solid-liquid interface at a

right angle in order to reduce the line energy. However, neither preferential melting nor preferential solidification was observed at the intersection of the dislocation with the solid-liquid interface. It is thought that in situ H R E M observation is necessary to detect such an interaction.

3. Solid-liquid interfaces in In The melting process of In particles embedded in an A1 matrix has been observed continuously using an in situ heating experiment in a H-1250ST microscope at an accelerating voltage of 1000 kV [7,8]. Fig. 5 shows the phase diagram of the A l - I n system [6]. Again, the mutual solubility between AI and In is very small. A l - I n alloy was splatquenched from the liquid state. Fig. 6 shows a typical structure of A l - I n alloy in the as-splatquenched condition. It is clear that In particles were dispersed uniformly in a matrix of AI. The

Fig. 3. High-resolution electron micrograph of the solid-liquid interface of lnSb. The lattice fringes correspond to the ( 111) plane.

H. Saka et al. / In situ obsert,ations of solid-liquid interfaces

crystalline In particles are cuboctahedral in shape, and b o u n d e d by eight {lll}Al,l n and six {100}Al,ln facets. Melting started at one of the {100} facets and p r o c e e d e d into the interior of the In particle. Fig. 7 shows an example of the solid-liquid interface in an In particle. The solid-liquid interface is not sharp in this case, in contrast to the case of InSb.

113

4. Solid-liquid interface in Pb It is also possible to observe the solid-liquid interface in fine particles. A n example is shown for the case of a Pb particle. The sample studied was small crystals of Pb p r e p a r e d by a conventional evaporation technique. They were deposited onto a m o r p h o u s ho[ey carbon films in a

a

\

iOL

Fig. 4. Interaction of the solid-liquid interface in InSb with a pre-existing dislocation. In-rich liquid droplet grew as the temperature was increased slightly.

114

ft. S'aka et al. / In situ obsen'ations qf solid-liquid inte@a'es

ICCO,

'f i

800F

L

537°C °o

\

600L-

\

8 8 & ~.oo E 200

~55"C

L

at

~

i

I

I

L

L

5O In ( at% )

,

~oo

In

v a c u u m of 3 X 10 6 Torr. After transfer into air, the samples were observed with a Hitachi H9 0 0 0 N A R h i g h - r e s o l u t i o n / a n a l y t i c a l electron microscope at an accelerating voltage of 300 kV. Typical magnifications used were 400000 × a n d 500 000 x with an i n c i d e n t electron b e a m c u r r e n t density b e t w e e n 100 a n d 200 A / c m 2. The base v a c u u m of the microscope in the vicinity of the sample was 8 × 10 s Torr. A fibre-optically coupled T V system, G a t a n Model 622, was used for the video-recording. The recorded video images were analyzed frame by frame using a m o n i t o r system with a freeze-frame capability. No processing was applied to the video images. Fig. 8 shows a s e q u e n c e of b e h a v i o u r of a Pb

Fig. 5. Phase diagram of Al-ln (from ref. [6]).

-(11 O}

{100)

Fig. 6. Electron micrograph of splat-quenched Al-ln alloy taken at room temperature. Inset shows the diffraction pattern: it is clear that there is an orientation relationship between In particles and an AI matrix (from ref. [8]).

ILL Saka et al. / In situ obserl'ations o f solid-liquid interfaces

115

was round corresponds to the liquid state. In fig. 8e, the right-hand side of the particle lost the lattice-fringe image and had a round shape, while the left-hand side had the lattice-fringe image and facetting. Thus, the former was in the liquid state, while the latter was in the solid state; in between lies the solid-liquid interface (indicated by an arrow).

5. Discussion

Fig. 7. High-resolution electron micrograph of a solid-liquid interface in an In particle (from ref. [8]).

particle. The particle reduced its size while being observed, presumably due to the evaporation caused by beam heating. At the beginning, the particle changed its configuration very frequently. During the change of configuration, the latticefringe image was observed clearly all the time, and the particle was kept facetted, figs 8a-8c. This is exactly what has been observed on various metal particles [9]. However, as the particle reduced in size further, the lattice-fringe image disappeared completely, and the outer shape of the particle became round, figs. 8d and 8f. The lattice-fringe image reappeared, and, at the same time, facetting recovered quickly, fig. 8g. We believe that the state in which the lattice-fringe image disappeared and the outer shape of the particle

Jackson [1] showed, from thermodynamical reasoning, that solid-liquid interfaces can be classified into two types, i.e., atomically smooth and atomically rough, depending on whether or not a parameter a is larger than 2. a = ((L/RTM), where L is the latent heat of melting, T M the melting temperature in kelvin, R the gas constant, and ( a crystallographic factor which is less than, but almost equal to, unity. For InSb, L = 11.8 kcal mo1-1 [10], and hence a = 7 . 4 much larger than 2. For In, L = 0.78 kcal tool [10], and hence a = 0.9 - less than 2. For Pb, L = 1.15 kcal tool- J, and hence a = 0.96 - again less than 2. From these thermodynamical data, Jackson's theory predicts that InSb should have an atomically smooth solid-liquid interface, while In and Pb should have atomically rough solid-liquid interfaces. Comparison of fig. 3 with fig. 6 indicates that this prediction is valid for InSb and In. For Pb, it is difficult to judge from fig. 8e whether the solid-liquid interface is atomically smooth or rough. In addition, Jackson's theory should be applied only to the solid-liquid interfaces in thermodynamical equilibrium, while the solid-liquid interface shown in fig. 8e is certainly not in the equilibrium state. It can be said that, although the in situ H R E M observations of the solid-liquid interfaces carried out so far do suggest the validity of Jackson's theory, more comprehensive in situ H R E M observations should be carried out for a definitive conclusion to be drawn on the validity of Jackson's theory.

116

H. Saka et al. / In situ obser~,ations ~f solid-liquid interfaces

Fig. 8. Sequence of high-resolution electron micrographs showing the melting and freezing processes of a fine particle of Pb. A solid-liquid interface can be observed inside the particle in (e), indicated by an arrow. Numbers at the bottoms refer to time (first digit: minute; next two digits: second: last two digits: frame number).

H. Saka et al. / In situ obsen,ations of solid-liquid interfaces

Acknowledgement This work was partly supported Foundation.

by the Mazda

References [1] K.A. Jackson, Liquid Metals and Solidification (ASM, Cleveland, 1975)p. 174. [2] M.E.Glickman and C.L. Void, Acta Met. 15 (1967) 1409. [3] C. Lemaignan, D. Camel and J. Pelissier, J. Cryst. Growth 52 (1981) 67.

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[4] R. Hamar and C. Lemaignan, J. Cryst. Growth 53 (1981) 586. [5] H. Saka, A, Sakai, T. Kamino and T. Imura, Phil. Mag. 52 (1985) L29. [6] M. Hansen, Constitution of Binary Alloys, 2nd ed. (McGraw-Hill, New York, 1958). [7] H. Saka, Y. Nishikawa and T. Imura, Phil. Mag. A57 (1988) 895. [8] K. Sasaki and H. Saka, Phil. Mag. A 63 (1991) 1207. [9] S. Iijima and T. Takahashi, Phys. Rev. 56 (1986) 616. [10] O. Kubaschewski and C.B. Alcock, Metallurgical Thermochemistry, 5th ed. (Pergamon Press, Oxford, 1979).