TRANSPORT PHENOMENA IN MATERIALS PROCESSING

Only in the last decade or so has it been realized that transport phenomena in the fluid phase during the processing of materials have a profound influence on the structure and quality of the solid phase. During this period there has been an ever-increasing effort to gain some understanding of these phenomena so that commercial products could be improved. A great deal of this activity has been directed to the growth of single crystals and to the related field of solidification. For the most part, the earlier work in this regard was rather rudimentary and empirical and consisted essentially of qualitative descriptions or explanations of specific aspects that are observed in current processing techniques. Crystal growth and solidification of alloys are inherently mass transfer processes and the parent phase is a fluid in almost all the commonly used techniques. The process of forming a crystal is called vapor growth when there is a solid-vapor-solid transformation. The process is called melt or solution growth when the crystal is obtained from a liquid. Roughly speaking, melt growth is a higher temperature process than solution growth. As in most nonequilibrium processes involving a fluid, convection (macroscopic fluid motion) plays a dominant role, in that it affects the fluid-phase composition and temperature a t the phase interface. The fluid motion, in turn, is determined by the interplay of externally imposed motion (forced convection) and locally generated motion (natural convection). The latter flows are due either to the effects of temperature and concentration density gradients in a body force field or to the effects of surface tension gradients. Thus, in general, crystal growth and solidification are coupled processes that include heat and mass transfer, fluid flow, phase transformations, and chemical reactions. Most of the earlier work, particularly in regard to crystal growth, was based on the premise that convection is deleterious. Thus, it was thought, with the advent of space flight, that crystal growth would be enhanced by a low-gravity environment that could eliminate or greatly reduce the natural convection. However, crystal growth experiments aboard Skylab did not produce such definitive results. Perhaps the greatest contribution to date of the materials processing in space program is that it brought about closer interactions among materials scientists and fluid dynamicists. However, this by itself did not bring about an immediate improvement in understanding because many of the relevant transport problems were beyond the frontiers of current knowledge in the related fields. What has occurred is a more intensive and basic research effort that is beginning to have an impact on the problems. It is the purpose of this paper to identify the problenis of importance and to indicate some of the results obtained by my group to date. This will set the stage for the other papers of this session, which will present other significant results. 256