IWAKI Toshihiro Toyama University, Dept.of Mechanical System Engineering, Professor, 工学部, 教授 (90019191)
SUZUKI Toshio University of Tokyo, Dept.of Metallurgy Professor, 大学院・工学系研究科, 教授 (70115111)
FUKUSAKO Shoichiro Hokkaido University, Div.Mechanical Science Professor, 工学部, 教授 (00001785)
In general, solidification process consists of a sequential series of the following elementary phenomena : nucleation, crystal growth, morphological boundary and structure formation. This study aimed at the development of ultra-rapid solidification process which brings about substantial modifications in the sequential series and a variety of attractions compared with conventional solidification of relatively heavy sections.
For the sequential series of elementary phenomena, to predict the ultra-rapid solidification process and contribute to material design, we examined the nucleation condition, modeling of the process from nucleation to structure formation, and application of numerical simulation based on molecular dynamics. Experimental data were accumulated for the effects of melt volume and cooling rate on the supercooling at nucleation. It was also shown experimentally that the quenching and nucleation process becomes uniform by selecting the condition of chill surface. A measuring
method of nucleation site distribution was developed, and it was found that smaller values of initial superheating of the melt result in more regular distribution of nucleation sites. Based on these results, a model describing the process from nucleation to the structure formation such as dendrites. As for molecular dynamics simulation, it became possible to pursue the formation of thermal stress and dislocations in ultra-rapid solidification.
Rapid solidification is most readily achieved by concurrently imposing high cooling rates and this requires sufficiently good contact of a sufficiently thin section of melt with a metal chill surface to achieve ultra-rapid solidification. In this study, possible mechanisms which obstacle such good contact were investigated and the following results were obtained. The stability of contact line relates closely to the formation of geometrical surface defects resulting in contact resistance. The contact line formed on a moving chill surface is unstable at low and high velocities but it is stable at moderate velocities. The destabilizing velocity, which is the upper critical velocity of the stable contact line, results from the dynamic non-wetting condition of the contact line. The stabilizing velocity, which is the lower critical velocity of the stable contact line, decreases with decreasing thermal conductivity of the chill surface. To stabilize the contact line and suppress the formation of contact resistance, it is effective to locate the solidification front below the contact line. Less