Liquid Metals: Crystal and Glass Formation in Supercooled Liquids Close to Revolution?

Dear Colleagues,

Liquid transformation is based, up to now, on the classical equation of nucleation. Tiny crystals, acting as growth nuclei, do not exist in melts above the melting temperature T_m. Crystal nuclei do not have the time to condense below T_m by homogeneous nucleation in quenched liquids. The vitreous state is frozen at T_g and there is no expected Gibbs free energy change, leading to a first or second-order thermodynamic transition.
Many observations are in contradiction with these views. Magnetic texturing experiments show that intrinsic nuclei govern the crystallization of liquid elements and compounds. These growth nuclei disappear after a strong superheating. Experiments and calculations show that liquids undergo a thermodynamic transition at T_g leading to percolating or interpenetrating icosahedral-like tiny superclusters. The new ultra-stable glass state is obtained at the thin film deposition temperature after recovery of a liquid enthalpy fraction. Liquid He-4 confined in nanoporous media under pressure undergoes a first-order glass transition. The new ultrastable glass state would correspond to an equilibrium phase between those of liquid and crystal. Numerous hyperquenched or vapor-deposited substances, such as pure metals, are amorphous. Their glass transitions are masked by the occurrence of crystallization. This temperature of homogeneous nucleation could be equal to T_g.

All these findings could be explained by the formation of icosahedral-like superclusters, submitted to complementary Laplace pressures, depending on their formation temperature, giving rise to glasses by quenching melts below T_g and to crystals by heating the glass above T_g. The condensation of superclusters of radius R and their growth critical radius would depend on a critical enthalpy saving \( \epsilon_{ls}\times\Delta H_{m} \) Its introduction in the classical nucleation equation as shown below is now in agreement with Lindemann rule at T_m:

A fraction of such nuclei could survive above Tm and limit the supercooling rate of melts. This Special Issue of Metals is devoted to the following fields, in order to clarify whether the critical radius of nuclei is finite at T_m instead of being infinite, and whether the glass transition occurs at the homogeneous nucleation temperature of crystals in the glass state:

1. Existence of intrinsic growth nuclei in liquid metals:
   • Texturing of metals and alloys (mainly with a congruent melting) in a high magnetic field and dependence of the magnetic alignment with the liquid superheating temperature above T_m.
   • Grain size of hyperquenched crystallized metals as a function on the superheating rate applied to the melt.
   • Dependence of the supercooling rate with the duration and the temperature of isothermal overheating.
2. Formation temperature of metallic glasses by hyperquenching liquids or thin film deposition at temperatures below T_m/2 down to low temperatures (T_g < T_m/2).
3. Devitrification temperature of amorphous metals by spontaneous crystallization.
4. Nanostructured and Nanocrystallized metallic glasses having T_g < T_m/2.
5. Formation of superclusters in supercooled metallic liquids.

Dr. Robert F. Tournier
Guest Editor

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