On Dendrite Growth in Directional Solidification.

The structure and properties of cast alloys are determined to a large extent by dendritic growth. Due to the complications which arise in treating this phenomenon theoretically, only partial solutions are known. One fundamental problem is the growth of the tip of the primary crystals. The tip can take the form of a needle (metallic-dendrite) or a plate (C in Fe, Si in Al, Ni//3Nb in Ni). An approximate solution for needle and plate growth in directional solidification (temperature gradient greater than 0) is presented. Using this model, the primary crystallization of Fe and C in an eutectic Fe-C alloy has been calculated. The results show that plate growth requires a higher undercooling than needle growth. Therefore, the plate form will appear only when growth kinetics prevent the development of the optimum morphology.