Dynamic effects arising from high-speed solidification

Abstract

Convective mass transfer occurs during solidification because of the difference in mass density between the participant solid and liquid phases. The relationship between interphase mass transfer and the mechanical behavior of a bounded system undergoing rapid solidification is developed from an analysis of the kinematics and dynamics of dendritic freezing. The kinematic analysis yields theoretical expressions for the displacement, velocity, and acceleration imparted to the center of mass of a constrained melt by the solidification-induced mass transfer, whereas the dynamical analysis relates the changes in external force and internal pressure with the accelerations of the solidifying body's mass center. These analyses reveal that a critical degree of supercooling exists, above which the dendrite velocity can increase without a concomitant increase in the acceleration of the center of mass, and that this condition coincides with the onset of cavitation in the specimen. Some new experiments, which provide quantitative information on the mechanical behavior of rapidly freezing bismuth melts, are then discussed. The results obtained from these experiments justify the major assumptions employed in the theory, and substantiate several of the theoretical predictions. Finally, a unified approach to solidification dynamics is presented in a discussion of the following effects: 1. (1) the emission of acoustical disturbances during solidification; 2. (2) the occurrence of an anomalous refinement in the as-cast grain size of nickel and cobalt specimens frozen from highly supercooled melts; 3. (3) the development of cavitation pits on the constrained surfaces of rapidly frozen transition metal specimens; 4. (4) the presence of pressure pulses both during and after rapid dendritic growth.