MODEL STUDY ON THE TRANSITION FROM COLUMNAR CRYSTAL ZONE TO EQUIAXIAL.

Transition from a columnar to an equiaxial crystal, of importance for cast structure formation, consists of two steps: (1) free crystals are formed in liquid zone; (2) the crystals are able to grow and to pile up before the solidification front, interrupting the columnar growth. Two-dimensional solidification of H//2O-NH//4Cl solution has been experimentally observed with emphasis on the second step of the transition process. Close attention has been paid the motion of free crystals in liquid zone caused by gravity and liquid flow and the means by which free crystals pile up before different kinds of solidification fronts. Methods to estimate the length of columnar crystal zone are proposed, and a means to promote the formation of equiaxial crystals is suggested.