抄録
Al-17.5 mass%In alloys of monotectic composition were solidified unidirectionally at various growth rates and temperature gradients, and were quenched during growth to observe the solid-liquid interface morphology. The morphology of Al solid phase and the dispersion behavior of L2 phase at monotectic growth front were examined, and the formation manner of regular monotectic composite and its structural transition were discussed. Fibrous composite structures were held in the neighborhood of monotectic temperature for various time durations, and the thermal stability of L2 fibers was examined.
When fibrous composite structures were obtained, the solid-liquid interface was generally planar and some projection of L2 phase toward L1 phase was observed. At about 109 K·s/m2 of G⁄R, periodical perturbation of the growth front and liquid depressions formed, and conical glass-like L2 droplets were incorporated into an aluminum matrix intermittently. These conical glass-like L2 droplets were spheroidized during the cooling process to form regular arrays of spherical L2 droplets. Below 109 K·s/m2 of G⁄R, the solid-liquid interface was no longer planar, and regular monotectic composite structures could not be formed. Droplet composite structures were also formed by necking down, pinching off and spheroiffizing of L2 fibers during the cooling process at relatively high temperature below the monotectic temperature. As the diameter of L2 fibers decreased, i.e., as growth rates increased, this structural transition during cooling was enhanced.
