In-situ observation of unidirectional solidification using transparent substance has been performed to investigate the growth direction of solid phase. Cell and/or dendrite, the preferred growth directions of which are not parallel to the heat flow direction, were observed with various solidification conditions. Dimensionless growth direction (π = (angle between heat flow direction and growth direction)/(angle between heat flow direction and preferred growth direction)) changes from zero to unity with increasing growth velocity at a constant temperature gradient. Introducing the normalized growth velocity (V /V c , where V c is the critical growth velocity for breaking down a planar interface), the relation between π and growth condition could be correlated and π could be expressed by a unique line with respect to the normalized growth velocity. Furthermore, the growth directions of cells or dendrites under the condition of unidirectional solidification have been analyzed by the phasefield model. The calculated results agree with the experimental results and the functional relationship between growth velocity and growth direction is qualitatively explained.

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