Transition towards ultrastable metallic glasses in Zr-based thin films

The underlying structural changes that provide the perceived unique properties of ultrastable metallic glasses (SMGs) are presently not well understood. Herein, Zr50Cu44.5Al5.5 and Zr50Cu41.5Al5.5Mo3 thin film metallic glasses (TFMGs) fabricated by direct current magnetron sputtering deposition at room temperature were investigated systematically. By tuning the deposition rate from similar to 250 nm/min to similar to 5 nm/min, the thermophysical and mechanical properties, as well as the corresponding structural evolution of TFMGs was examined. A clear transition from bulk-like to ultrastable-like behaviour was observed, whereby reducing the deposition rate results in a gradual enhancement in thermal stability and mechanical properties. A distinct structural difference was observed between conventional metallic glasses (MGs) and SMGs, with the latter yielding a more homogeneous and looser-packed structure under greater induced geometric frustration. SMGs show a greater resistance to crystallization as seen by a change in crystallization pathway. The results also show that MGs with more pronounced slow beta relaxation have a greater potential to form SMGs with more significant variations in key properties. This work provides new insights into the structural evolution of SMGs with varying deposition rate and has implications in the design and fabrication of SMGs by considering their relaxation dynamics.