Nonequilibrium crystal growth and kinetic roughening in greatly undercooled Ge and Ge-2at%Sn

Containerless electromagnetic (EM) levitation processing was used to undercool pure Ge and Ge-2at%Sn alloy up to 426 and 368 K, respectively. The crystal growth velocities in undercooled melts were directly measured with a photodiode technique. Experiments and calculations with the slightly modified current dendrite growth theory have shown that solidification mode was transformed from lateral growth (LG), to continuous growth (CG), and eventually to rapid growth (RG), as the driving force for crystallization, i.e. undercooling was increased, correspondingly leading to faceted twins, randomly oriented dendrites, and refined equiaxed grains. It was found that a small addition of solute Sn to pure Ge made roughening of solid-liquid (S/L) interface occur at a lower undercooling, and the growth velocities rise markedly.