Numerical simulaton of cellular/dendritic transition and its growth during unidirectional solidificationin of Ti–Al alloy

The CA method based on solute diffusion controlled model was adopted to simulate cell to dendrite transition of Ti44Al alloys at the initial stage of directional solidification with different velocities.The simulation results show that during cell/dendrite transition the mixed structure is composed of cells and dendrites,in which the secondary dendrites are absent at facing surface of some parallel closely spaced dendrites.The dendrite spacing is larger than cellular spacing at a given rate. The columnar grain spacing sharply increases to a maximum as solidification advanced to coexistence zone.In addition,the simulation also revealed that the increase in the number of the seeds decreased the non-uniform degree of the columnar grain spacing.The main influence factor affecting cell/dendrite transition is slight fluctuations of liquid composition at growth front.The simulation results also are in reasonable agreement with experiment observation at low cooling rate.