New Application of Unidirectional Solidification Method for Observation of Interfacial Morphology of Al–Si and Al–Si–Sr Alloys

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Abstract
As a fundamental study to investigate the mechanism of modification of Al–Si alloys, the change in silicon phase morphology at the solid/liquid interface due to the addition of strontium was observed using an improved unidirectional solidification method. A constant composition was required throughout the experiment, especially in the case of the unidirectional solidification method. In our research, a self sealing method was used to prevent the strontium from evaporating and oxidizing. We discovered that the Al–Si alloy was best aligned at a solidification speed of 2 mm/h and that strontium worsened the degree of alignment. As the solidification rate decreased, the degree of the interfacial smoothness also decreased in the Al–Si–Sr alloy, while it increased in the Al–Si alloy. The silicon phase protruded into the liquid phase in the Al–Si–Sr alloy whereas, the silicon phase was covered by α phase in the Al–Si alloy. This phenomenon was caused by the change in the balance between the α phase/liquid interfacial energy and the silicon phase/liquid interfacial energy. This was confirmed by observing the interface in equilibrium condition which was achieved by holding the furnace stationary for 5 h.

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