Is There a Link between Melt Fragility and Elastic Properties of Metallic Glasses?

The concepts of melt strength or 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 behaviour of the liquid and the thermodynamic fragility by the entropy loss on undercooling. Using the data presently available on viscosity and introducing a new index, the reduced span of the glass transition range, it is shown that kinetic data comply reasonably well to the general trend of the strong-fragile classification. On the other hand, there are discrepancies for the thermodynamic fragility of metallic glass formers with respect to conventional ones. A recent report in Nature (Novikov and Sokolov, October 2004) has suggested a correlation between melt strength/fragility and the elastic moduli of the glassy material (namely the ratio of the bulk to shear modulus). This is checked for metallic glasses which were not discussed by the Authors of the Nature paper, next to inorganic and organic glasses. It is shown that considering a larger material basis and extending the number of property values in the data base, the correlation becomes rather poor. There is, however, the possibility of distinguishing among each class of glasses, e.g. inorganic, organic, metal-metal and metal-metalloid.