Iron, nickel, and cobalt-base alloy melts can undercool by reproducible amounts, up to approximately 300 °C, prior to nucleation of the solid, provided that high-purity materials are used, and metling and solidification are conducted in vacuum or under reduced inert atmosphere, using crucibles made of amorphous materials (fused silica) or crystalline crucibles (alumina) coated with a fluid glass slag. Highly undercooled alloys exhibit microstructural uniformity and compositional homogeneity. With increasing amount of undercooling, the dendrite morphology, and hence the microsegregation pattern, change drastically. Beyond a critical undercooling of about 170 °C for all the alloys investigated, the grains become very fine and the dendritic growth is replaced by a spherical growth accompanied by a substantial reduction in microsegregation. Mechanical-property evaluation conducted on as-cast Ni-30% Cu and Fe-25% Ni and fully heat-treated AISI 4330 low-alloy steel and Ni-20% Cr indicated a substantial improvement of room-temperature ductility with undercooling. Typically, percent reduction in area of fully heat-treated AISI 4330 was tripled and fracture toughness was increased by about 40% by a 200 °C undercooling.