ON THE ROLE OF SMALL ADDITIVES FOR NANOCRYSTALLINE FORMATION FROM AMORPHOUS MATRIX

The roles of dilute additives on grain refinement by devitrification a re studied by means of XAFS. Local structure changes during nanocrysta lline formation are observed for Cu in Fe73.5Cu1Nb3Si13.5B9 and for Ce in Al87Ni10Ce3. Although both Cu and Ce promote precipitation of alph a-Fe and alpha-Al respectively, the manner in which they do so are qui te different. The Cu inclines to form fee Cu clusters prior to the ons et of alpha-Fe precipitation. The local structure of Ce does not show any definite changes through alpha-Al precipitation indicating that Ce does not directly correlate with alpha-Al precipitation. The Ce addit ion extends amorphous formability of this alloy towards the pure Al re gion resulting in the formation of quenched-in nuclei for an as-quench ed state. These pre-existing alpha-Al nuclei induce alpha-Al precipita tion at an early stage of crystallization.