Dendrite growth velocity in the undercooled melt of glass forming Ni₅₀Zr₅₀ compound

Glass-forming Ni₅₀Zr₅₀ intermetallic compound is containerless undercooled and solidified using electrostatic levitation. Large undercoolings up to ∆T = 300 K are achieved. The dendrite growth velocity of the congruently melting alloy is measured as a function of undercooling using a high-speed camera technique. The experimental data is analysed within a sharp interface theory. It is found that the driving force of crystallisation is controlling the growth kinetics at ∆T < 250 K but at larger undercoolings the growth kinetics is progressively controlled by atomic diffusion. This leads to a slowing down of the growth velocity. The maximum velocity and the temperature at which the maximum occurs (T_max) are inferred from the dendrite growth velocity–undercooling relation. The relation of the temperature T_max and the glass temperature fits into a general classification scheme for glass-forming systems. The kinetic and thermal undercooling terms are calculated within dendrite growth theory as a function of the total undercooling. At ∆T > 126 K, the kinetic undercooling dominates and increases rapidly with the undercooling ∆T. The maximum prefactor of the kinetic undercooling is plotted vs. the reciprocal temperature. Its temperature dependence is discussed.