On the glass transition in metallic melts

Experimental determinations of the glass transition and related properties in alloys are reviewed. Several recent measurements of viscosity, specific volume and heat capacity in the undercooling regime have become possible after the discovery of bulk glass forming alloys. The data allow verification of models on the kinetics and thermodynamics of such melts. The notion of melt fragility, borrowed from the description of inorganic and molecular glasses, has become popular also for metallic alloys: the kinetic fragility is described by the viscous behavior of the liquid and the thermodynamic fragility by the entropy loss on undercooling. The experimental viscosity data for metallic glass formers presently available can be reasonably framed within the strong–fragile classification. On the other hand, there are major discrepancies for the thermodynamic fragility of metallic glass formers with respect to conventional ones. The reason for such disagreement is twofold: on the experimental ground there are difficulties in obtaining fully reliable thermodynamic data in sufficiently wide temperature ranges; on the conceptual side the definition of the configurational entropy of the liquid with a suitable reference state is controversial. Among fragility indexes, the reduced width of the transition range proves most useful. The trend of the glass transition as a function of composition is also discussed in relation to the progressive ordering of the liquid on undercooling.