Al-Rare Earth-Transition Metal Alloys: Fragility of Melts and Resistance to Crystallization

Abstract

The stability of M-transition metal (TM)-rare earth (RE) alloys isconsidered with reference to transport and thermodynamic properties ofthe melt. The mobility of species or groups of atoms is described abovethe melting point and in the proximity of the glass transition.Viscosity is analysed at first in the frame of the strong/fragileclassification of liquids. Al-based glass-formers are shown to befragile systems. Evidences that for some molecular and metallicglass-formers the Stokes-Einstein equation does not hold below andabove T, are reviewed. The alloys are, however, resistant tocrystallization on rapid quenching. Some of them display also peculiardevitrification behaviour, often, but not always, with formation ofprimary compact nanocrystals. Analysing the transformation paths it canbe inferred that mobility is belated by composition gradients in theamorphous matrix. The resistance to crystallization may result from theshape and relative position of free energy curves because crystal andliquid phases are such that the driving force for nucleation ofintermetallics does not increase steadily but tends to level off onundercooling.