Undercooling, crystal growth and grain structure of levitation melted pure Ge and Ge-Sn alloys

Using containerless electromagnetic (EM) levitation, pure Ge, Ge-0.39 at%Sn and Ge-10 at% Sn alloys were undercooled by up to 426, 404 and 334 K, respectively. The crystal growth velocities in undercooled melts were measured with a photodiode technique. Crystal growth behavior was found to fall into three categories of lateral growth (LG) at low, continuous growth (CG) at moderate, and rapid growth (RG) at high undercoolings, leading to various microstructures. The measured growth velocities were analyzed in terms of slightly modified, current dendrite growth theory. The presence of solute, Sn, was found to have a strong influence not only on the magnitude of the growth velocity, but also on the growth kinetics. The latter effect indicates that the critical undercoolings corresponding to the transitions between solidification mechanisms monotonically decrease as the Sn content increases.