Undercooling and solidification of Ni2B under different conditions of convection

Kurzfassung

We investigate the kinetics of crystal growth by measurements of the dendrite growth velocity as a function of undercooling during non-equilibrium solidification. Measurements are conducted under different convective flow conditions. The liquid samples are levitated and undercooled in strong alternating electromagnetic fields leading to forced convection. Inductive stirring is avoided by applying electrostatic levitation and by processing the samples in a liquid or glassy slag where only natural convection is present. Forced convection and natural convection can be reduced by performing undercooling experiments in reduced gravity. The experimental results are compared to each other in order to investigate the influence of convective flow on the growth dynamics of dendrites and the microstructure evolution in undercooled melts. The congruently melting compound Ni2B is chosen as sample system. It forms a chemically ordered intermetallic phase with growth velocities that are comparable to the fluid flow velocities in electromagnetically levitated melts. The results are analyzed within dendrite growth models and reveal that the growth velocity is essentially influenced by forced convection in strong electromagnetic.