Influence of low-gravity solidification on heterogeneous nucleation in stable iron-carbon alloys
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INTRODUCTION
T H E space environment is characterized by four specific properties: weightlessness/microgravity, high vacuum, high incident solar radiation, and extremely low temperature (on the shadow side). I1-41 Of the four, however, microgravity seems to be the most promising for materials science and technology in space. The uniqueness of low gravity for materials processing derives from the (1) possibility of containerless handling of liquids to suppress unwanted container reaction, (2) elimination or reduction of natural convection flow arising from thermally or compositionally induced density gradients in fluids, (3) reduction of sedimentation and Stokes flow, and (4) elimination or reduction of hydrostatic pressure and its influence on phase equilibria. Solidification of alloys can be rationalized in terms of nucleation and growth kinetics. Nucleation is influenced by the liquid state and by solidification conditions. Under ground conditions, convection in the liquid occurring due to gravity-induced and gravity-independent heat and mass transport processes can contribute to grain multiplication. I5,61A suggested mechanism for grain multiplication in dendritic alloys consists in the fracture of the tips of the growing dendrites by convection currents and trans-
HUAMENG TIAN, Visitng Scholar, permanently with the Department of Metals and Materials Science Technology, Harbin Institute of Technology, People's Republic of China, and DORU M. STEFANESCU, University Research Professor and Director of the Solidification Laboratory, are with the Department of Metallurgical and Materials Engineering, The University of Alabama, Tuscaloosa, AL 35487. PETER A. CURRERI, Materials Scientist, is with NASA/ Marshall Space Flight Center, AL 35812. Manuscript submitted October 21, 1988. METALLURGICAL TRANSACTIONS A
port of these tips away from the melt interface, where they serve as nuclei for new grains. In a low-g environment, however, the intensity of convection during solidification for the same solidification velocity will be much lower than that encountered under 1-g (or high-g) conditions, although some convection may still result from surface tension-driven flow or phase change-induced flow. [7-10] It has been reported that under low-g conditions, the value of liquid diffusivity is greatly reduced, [11'121 which will have an obvious effect on the cooperative growth process of the s o l i d i f y i n g p h a s e s . [7,13,14[
It has been found that in a low-g environment, the grains of a Ni-32.4 pct Mo-6.2 pct A1 eutectic alloy were larger than those of ground specimens, which points to the important role of convection during eutectic solidification, t151 Carlberg [161 has given two explanations for the lower number of grains found in eutectic A1-Cu alloys solidified in space, but he has found no difference in A1-Si samples. The influence of convection on the eutectic morphology has been summarized by Curreri et al. [~7]
For the particular case of stable Fe-C alloys (gray cast iron), which is the material studi
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