Formation Manner of Monotectic Structure in Al–In Alloys Solidified Unidirectionally

Abstract

Monotectic Al-17.5 mass%In alloys were solidified unidirectionally at various growth rates and temperature gradients, and were quenched during growth to reveal the solid-liquid interface morphology. The alloys having fibrous composite structures were held near the monotectic temperature for various time durations to examine the thermal stability of L2 fibers. When fibrous composite structures are forming, the planar solid-liquid interface proceeds with some projections of L2 phase toward L1. Periodical fluctuation of the growth front and depressions form at about 109 K·s/m2 of G⁄R, and funnel-like L2 droplets are incorporated into the aluminum matrix intermittently. These L2 droplets spheroidize during cooling to form regular arrays of spherical L2 droplets. The planar solid-liquid interface no longer lies at G⁄R below 109 K·s/m2, and regular monotectic composite structures can not be formed. Droplet composite structures are also formed by necking down, pinching off and spheroidizing of L2 fibers during cooling at high temperatures below the monotectic. The structural transition during cooling is enhanced, as the diameter of L2 fibers decreases, i.e., as growth rates increase.