Development of grain structures in highly undercooled germanium and copper

The microstructures of bulk Ge and Cu samples solidified from undercooled liquid were investigated. High levels of undercooling were attained by containerless electromagnetic processing and melt fluxing. For both faceted material Ge and non-faceted Cu, a twin microstructure was formed in a certain range of undercooling, whereas the twins were completely replaced by fine equiaxed grains at larger undercooling. The structural transformation in the two materials was analyzed with respect to the relation between undercooling and the velocity of free crystal growth, twinning mechanisms, and a dendrite break-up model.