Papers by Keyword: Crystal Nucleation

Abstract: An undercooled melt possesses an enhanced free enthalpy that enables to crystallize metastable solids in competition with their stable counterparts. Crystal nucleation selects the crystallographic phase whereas the growth dynamics controls microstructure evolution. We apply containerless processing such as electromagnetic and electrostatic levitation to containerlesss undercool and solidify metallic melts. Heterogeneous nucleation on container-walls is completely avoided leading to large undercooling with the extra benefit that the freely suspended drop is direct accessible for in situ observation of crystallization far away from equilibrium. Results of investigations of maximum undercoolability on pure zirconium are presented showing the limit of maximum undercoolability set by the onset of homogeneous nucleation. Rapid dendrite growth is measured as a function of undercooling by a high-speed camera and analysed within extended theories of non-equilibrium solidification. In such both supersaturated solid solutions and disordered superlattice structure of intermetallics are formed at high growth velocities. A sharp interface theory of dendrite growth is capable to describe the non-equilibrium solidification phenomena during rapid crystallization of deeply undercooled melts.

Abstract: Magnetic field texturing of superconducting oxides has shown the possible existence of intrinsic solid nuclei surviving above the melting temperature Tm and governing the solidification. Tiny crystals could survive above Tm and act as growth nuclei with undercooling ratios θ= (T-Tm)/Tm larger than the theoretical value −2/3 if a negative supplementary volume energy −ε v is added in the Gibbs free energy change associated to the formation of a critical cluster. A double layer of opposite charges could create the solid-liquid interface electrostatic -εv. The observed maximum values θ1 and the dimensionless surface energies α1ls calculated for 38 elements assuming that their melts homogeneous,
 used to determine εv(θ). The εv values at T=Tm were equal to 21.7% of the fusion heat per volume unit. The quantity α2ls 3× Sm was nearly the same for all elements, α2ls being the dimensionless surface energy and Sm the fusion entropy. After melting these tiny crystals around Tm2=1.20Tm, all the undercooling ratios could tend to -2/3. The bidimensional texture of Bi2212, Bi2223 tapes can be induced by these nuclei during crystal growth when the prereacted compounds in the sheath are melted and annealed at a weak overheating temperature smaller than a critical value.