Solidification structure of monotectic alloys

Abstract

The formation manner of the monotectic structure is shown from the observation of the evolution of crystallization and growth of CuPb and some aluminium-based monotectic alloys solidified in a free direction and unidirectionally. The morphological change of the monotectic composite structure is discussed in relation to the solid-liquid interfacial morphology at the monotectic growth front as well as the interfacial energy balance between the solid and two liquids in the monotectic reaction. In free-directional solidification, a characteristic monotectic cell with a spherical shape is formed which is unlike dendritic and lamella morphologies. The monotectic cell consists of a spherical solid and a separated L 2 liquid lying along the radii of the solid sphere. In unidirectional solidification, the monotectic structure of AlPb, AlBi and AlIn alloys changes in the following sequence with decreasing growth rate under a constant temperature gradient; random dispersion of L 2 droplets in the aluminium solid matrix → periodic regular array of L 2 droplets → fibrous L 2 composite → aluminium single-phase region without the L 2 phase. In CuPb monotectic alloy the monotectic structure changes with decreasing growth rates as follows: irregularly shaped rod-like L 2 composite → coalesced coarsened discontinuous L 2 composite → periodic banded structure consisting of L 2-rich regions and L 2-poor regions. These morphological transformations of monotectic structure are strongly affected by the ratio of the temperature gradient to the growth rate, the volume fraction of liquid L 2 separated through the monotectic reaction and the interfacial energies between the solid and two liquids at the monotectic growth front.