Progress in

Crystal Growth

and Characterization

PERGAMON Progress in Crystal Growth and Characterization

of Materials (2002) 163-167 of Materials

http:l/www.elsevier.comllocate/pcrysgrow

Morphology of Cells at the Grain Boundaries of an Anisotropic Material

Om Prakash Singh Chemistry Department, K. N. Post Graduate College, Gyanpur, Ravidas Nagar (UP) India

ABSTRACT

We have studied the formation of ridges on the grain boundaries of an anisotropic crystal known as pivalic acid. These results are compared with very well studied another isotropic material succinonitrile to understand the formation of defects in anisotropic crystals. Preliminary results show that along with anisotropy, orientation has a large influence on the morphology of solid-liquid interface formed after planar interface becomes unstable. The formation of ridges and hence the defects at the interfaces are more favoured in highly anisotropic materials.

Key Words: Morphology, Dendrite, Grain, Solidification, Anisotropy and Pivalic acid

1. INTRODUCTION

Since it was discovered that the solid-liquid interface could become morphologically unstable due to constitutional supercooling, a huge amount of data is published on the experimental and theoretical modelling of morphological instability. Most of the data are on the isotropic materials with very little anisotropy on solid-liquid interface energy. The succinonitrile was extensively studied by Glicksman, Singh and Chopra [l]. For the first time Singh and Glicksman [2-41 discovered materials with extremely high anisotropy and studied effect of anisotropy on the dendritic morphology. These experiments involved detailed studies on measurements of solid-liquid interface energy, growth velocity as a function of undercooling and tip radius of the dendrites. Two important materials under studies were pivalic acid and cyclohexanol. Pivalic acid was very extensively studied and became the subject of space experiments. A comparison was made with succinonitrile a less anisotropic material, for the stability constant and to predict role of convection and diffusion during dendritic growth. Schaefer [5] studied the interface breakdown on the ridges in succinonitrile. This study is lacking in anisotropic material. The objective of present study is to understand the defect formations at the ridges in the conditions where solid-liquid interface is very close to planar conditions.

2. EXPERIMENT AND RESULTS

The experimental observations were carried out in a Pyrex cylindrical chamber designed by Singh and Glicksman [2,3]. Similar chamber was used to measure thermal conductivities and interface energy of pivalic acid. We used a tube of 15x17 mm

0960-8974/02/$ - see front matter 0 2002 Published by Elsevier Science Ltd. PII: SO960-8974(02)00013-X

164 0.P. Singh / Prog. Crystal Growth and Charact. 44 (2002) 163-167

diameter size. As-supplied pivalic acid was listed for 99.9% purity level. It was distilled three timed under reduced pressure to further purify. The purified material was filled in the growth chamber in vacuum and sealed. The observation chamber was placed at 35.6 C approximately 0.1 K below 35.76, the melting point of pivalic acid.

Figure 1 Succinonitrile and pivalic acid showing anisotropy in solid-liquid interface energy ]2,31

Figure 2 shows formation of two grains [Fig.31 when a very small (less than 0.1 K undercooling was used and interface was kept for a period of 24 hours. We did not observe any interface breaking. As the time of the observation was increased we observed that many more grains were formed. We did not observe any interface breaking. However, the grains formed at boundaries started tilting. If we compare this result with the observations of Schaefer [5] taken for succinonitrile, we observed that grains of pivalic started tilting and had asymmetry due to anisotropy in solid-liquid interface. The number of grains increased as a function of time. The tilt in the angle of boundaries was much largere than succinonitrile due to anisotropy. A sequential observation is taken on the developments of grains and results are shown in Figure 4. We decreased the supercooling to 0.2K and decrease the power of the central heater (wire) to half of the original power. It is very clear from the Figure 4 that interface started breaking and cellular growth occurred at the boundaries. This behavior of interface breakdown was less on the grains away from the boundaries. Since the pivalic acid has a less tendency to undercool, 0.2K was large enough for the growth and coarsening.

0.P. Singh /Prog. Crystal Growth and Charact. 44 (2002) 163-167 165

Figure 2 Solid-liquid interface with two large grains of pivalic acid (x 25)

Figure 3 Sequential breakdown of the solid-liquid interface into cells (x2.5)

166 0.P. Singh / Prog. Crystal Growth and Charact. 44 (2002) 163-167

W (4

Figure 4 A sequential breakdown of the solid-liquid interface at the grain boundariesThe diameter of the wire (dark filled cylinder in the middle) was 0.5 mm.. (a) sharp solid- liquid interface with no breakdown of interface, (b) Formation of grain boundaries, (c) starting of ridges (d) well developed grains and (e) cells on boundaries.

The solid liquid interface is shown in Figure 4(a) and grain boundary and node formations are shown in Figure 4 (b-c). Figures 4(d-e) indicate fast growth, node formation at the boundaries and collapse of boundaries by coarsening. It is clear from the

0.R Singh / Prog. Crystal Growth and Charact. 44 (2002) 163-167 167

Figure 4 that in addition to the boundaries nodes are also tilted. This is tilt was not observed by Schafer [5] in succinonitrile which has smaller anisotropy than pivalic acid. The nodes always formed in a regularly spaced arrays, and were slightly tilted.

3. SUMMARY We observed that pivalic acid which has 10 times higher anisotropy than succinonitrile has larger tendency to develop larger numbers of grains at the grain boundaries. It has greater tendency to break at the interface and hence larger number of defects formation. The regularly spaced nodes formed at the boundaries are also tilted.

4. REFERENCES 1. M. Chopra, M. E. Glicksman and N.B.Singh, Met. Trans. A, 19A (1987) 3087 2. N. B. Singh and M. E. Glicksman, J. Crystal Growth, 98 (1989) 277. 3. N. B. Singh and M. E. Glicksman, J. Crystal Growth, 98, (1989) 534. 4. M. E. Glickaman, N. B. Singh and M. Chopra, CSME proceedings ed. by R.

Currie 1990. 5. N. B. Sir@, Northrop Grumman Corporation, Linthicum, MD (Private

Communication)