GLASS-FORMATION IN METALLIC SYSTEMS

The aim of the present paper is to analyse the formation and stability of metallic glasses. Attention is focused to the history of the progr ess on new glassy alloys with specific applications: from soft magneti c Fe-based ribbons obtained by melt spinning or planar flow casting, t hrough metallic glasses acting as precursors of nanocrystalline precip itates embedded in an amorphous matrix, towards he finding of new mult icomponent metallic alloys with large glass-forming ability that open a new area of development and applications of bulk metallic glasses. T he glass formation process is discussed in terms of the structure of t he resultant material and in terms of the thermodynamics and kinetics of the process. The development of structural criteria for glass forma tion is based upon the notion that the free energy of particularly sta ble glassy structures will be low enough to make the driving force for crystallization negligible. Both structural and energetic criteria po int to an enhanced glass-forming ability in multicomponent alloys. Kin etic criteria allow to differentiate between the ease of glass formati on from an undercooled melt (or from a mixture of crystalline grains, as in solid state amorphization) and the stability of the glassy state versus devitrification or crystallization. Key kinetic parameters are viscosity and, more specifically in multicomponent systems, diffusivi ties of the different elements in the crystalline, glassy and metastab le liquid state, respectively. The formation of nanostructured materia ls from controlled thermal treatment and their temperature stability i s discussed in terms of the concentration gradients developing at the interface between the nanocrystals and the amorphous matrix. The quali tative influence of several parameters in the glass-forming ability an d glass-stability as well as their Link to the empirical criteria that have been successfully employed to predict the glass-forming ability are presented.