On the role of small additives for nanocrystalline formation from amorphous matrix

The roles of dilute additives on grain refinement by devitrification are studied by means of XAFS. Local structure changes during nanocrystalline formation are observed for Cu in Fe73.5Cu1Nb3Si13.5B9 and for Ce in Al7Ni10Ce3. Although both Cu and Ce promote precipitation of α-Fe and α-Al respectively, the manner in which they do so are quite different. The Cu inclines to form fcc Cu clusters prior to the onset of α-Fe precipitation. The local structure of Ce does not show any definite changes through α-Al precipitation indicating that Ce does not directly correlate with α-Al precipitation. The Ce addition extends amorphous formability of this alloy towards the pure Al region resulting in the formation of quenched-in nuclei for an as-quenched state. These pre-existing α-Al nuclei induce α-Al precipitation at an early stage of crystallization.