Dendritic crystallization from the undercooled melts: effect of tiny amount of impurity

Abstract

The kinetics of dendrite crystal growth in pure Ni and diluted Ni–B melts is analyzed. Using a model for anisotropic dendrites rapidly growing from undercooled melts, numerical solutions for dendrite tip velocity V and tip radius R are presented. The influence of boron content on tip velocity V and tip radius R of the dendrite growing in Ni-B alloys is shown in comparison with the growth kinetics of pure Ni–dendrites. Particularly, we show that there is a well-known regime of growth in which the impurity (boron) decreases the velocity of the alloying Ni–B–dendrite regarding the nickel dendrite velocity. In addition to this well-established regime, we also show quantitatively that there is a tiny amount of boron which enhances the dendrite velocity in comparison with the velocity of pure nickel dendrite. Explanations of this non-linearity are given using the results of calculations for dendrite tip radii and dendrite growth velocity for alloying Ni–B and pure Ni systems.