NANOCRYSTAL DEVELOPMENT DURING PRIMARY CRYSTALLIZATION OF AMORPHOUS-ALLOYS

During primary crystallization of multicomponent amorphous alloys a hi gh density of nanocrystals can develop at levels up to 10(23) m(-3) an d volume fractions > 0.30. For Al-base amorphous alloys at high Al nan ocrystal densities, diffusion field impingement develops quickly above the glass transition and provides for a kinetic stabilization. A kine tics analysis has been developed to account for nanocrystal growth wit h diffusion field impingement and unequal component diffusivities. The analysis together with a thermodynamic model of the f.c.c.-liquid pha se equilibria for Al-Y-Fe was applied to model DSC exotherms correspon ding to primary f.c.c. nanocrystal formation. From the kinetics analys is an estimate of the diffusion coefficient of yttrium in the liquid i s obtained above the glass transition as 1.4 x 10(-17) m(2)/s. New all oying strategies are discussed based upon the implications of the kine tics analysis. (C) 1998 Acta Metallurgica Inc.