SOLIDIFICATION OF GERMANIUM AT HIGH UNDERCOOLINGS - MORPHOLOGICAL STABILITY AND THE DEVELOPMENT OF GRAIN-STRUCTURE

We present results on germanium solidified at high undercoolings using three techniques: melt fluxing, containerless solidification in a drop-tube and pulsed laser surface quenching. In combination with microstructural observations of Devaud and Turnbull, we find evidence that at large interfacial undercoolings, germanium can grow in a continuous normal manner. We employ a numerical model for spherical growth to estimate the critical interface temperature for the transition from edgewise to normal growth as ≈1057 K, and extend the model to treat growth in a dilute Ge-0.39 at.% Sn alloy, successfully predicting the bulk undercooling required in the alloy for the occurrence of dendritic growth. At deeper undercoolings, we also find evidence for a grain refinement effect, associated with a critical dendrite velocity. Using the model of Trivedi, Lipton and Kurz, we estimate this velocity as ≈17 m s-, and correctly predict the ≈35 K reduction in the undercooling for grain refinement brought about by the addition of 0.39 at.% Sn. © 1990.