Early Stages of Crystallization in Phase-Separated Amorphous Copper-Niobium Alloy Thin Films

The binary Cu-Nb system is rather interesting, because Cu and Nb are virtually immiscible in the solid state, exhibit a relatively strong clustering (phase separating) tendency in the liquid state, and therefore can only be alloyed in a highly metastable form, for example, by vapor quenching techniques such as sputtering. In this article, metastable amorphous Cu-Nb alloy thin films with nominal compositions of Cu-45 at. pct Nb have been deposited by magnetron cosputtering. The amorphous phase in these thin films exhibits nanoscale phase separation in the as-deposited condition. The early stages of devitrification of this amorphous phase have been investigated using three-dimensional atom probe tomography coupled with transmission electron microscopy. The results indicate that devitrification of this phase-separated amorphous phase is initiated by the formation of nanoscale fcc Cu-rich grains uniformly distributed in the amorphous matrix. These nanoscale fcc Cu-rich grains are often faulted and appear to retain some Nb supersaturation. The role of the nanoscale phase separation in the as-deposited amorphous film on the subsequent devitrification is discussed in the context of free energy–composition plots for this system.