Influence of Diffusion and Convective Transport on Dendritic Growth in Dilute Alloys
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INFLUENCE OF DIFFUSION AND CONVECTIVE TRANSPORT ON DENDRITIC GROWTH IN DILUTE ALLOYS
M. E. GLICKSMAN, NARSINGH BAHADUR SINGH AND M. CHOPRA Materials Engineering Department, Rensselaer Polytechnic Institute, Troy, New York, USA
ABSTRACT Extensive experimentation has been carried out in which the kinetics and morphology of dendritic growth were measured as a function of thermal supercooling, solute concentration, and spatial orientation of the dendritic growth axis. The crystal growth system studied is succinonitrile [NC(CI! 2 )2CN] with additions of argon (up to 0.1 mole %). This system is especially useful as a model for alloy studies because kinetic data are available for high purity (7-9's) succinonitrile. The addition of argon provides a simple, controllable dilute solute that now permits the first comparably detailed dendritic growth studies on binary alloys. One dramatic influence of the solute, at fixed thermal supercooling, is to increase the growth velocity (to a maximum) and correspondingly decrease intrinsic crystal dimensions (tip radius). Morphological measurements will be described in detail relating tip size, perturbation wavelength, supercooling, and solute concentration. The analysis of these effects based on morphological stability theory will also be discussed. Finally, experiments permitting the separation of convective and diffusive heat transport during crystal growth of succinonitrile will be described briefly. These studies clearly underscore
the importance of gravitationally-induced
buoyancy
effects on crystal growth kinetics and morphology.
INTRODUCTION The quantitative measurement of the kinetics of alloy solidification is an area in materials science in which major experimental and theoretical developments have emerged recently [1,2]. Glicksman et al. [3,4] have intensively investigated solidification behavior in pure melts. Prior to their work, it was believed that the operative growth state of a dendrite could be predicted from the so-called "maximum velocity principle", but recent ex.periments uncovered a 6.5-fold error in the growth-rate prediction. Theorists [5-7] then suggested that the dendrite must instead achieve a stable tip configuration in order to establish steady-state growth, and the so-called "stability criterion" has replaced the "maximum velocity principle". The role of stability in dendritic growth in pure melts has been examined carefully since its introduction a few years ago, and morphological stability theory has been shown quantitatively to select the correct growth state. Specifically, the theories of Mullins and Sekerka [5] and Langer and MUller-Krumbhaar [7] which introduced time-dependent stability considerations were found to be reasonably accurate, based on the kinetic and morphological studies reported by Glicksman [1] and by Langer, Sekerka and Fujioka [8]. The present study is the first attempt to extend quantitative kinetic and
462 morphological measurements to dendritic growth in alloy systems. This paper is concerned with the solidification
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