Solidification structure of monotectic alloys

The formation manner of the monotectic structure is shown 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 L2 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 L2 droplets in the aluminium solid matrix &rarr periodic regular array of L2 droplets &rarr fibrous L2 composite &rarr aluminium single-phase region without the L2 phase. In Cu-Pb monotectic alloy the monotectic structure changes with decreasing growth rates as follows: irregularly shaped rod-like L2 composite &rarr coalesced coarsened discontinuous L2 composite &rarr periodic banded structure consisting of L2-rich regions and L2-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 L2 separated through the monotectic reaction and the interfacial energies between the solid and two liquids at the monotectic growth front.