Jet turbine engines have benefited from decades of development of nickel-based superalloys, which have allowed a steady increase in engine operating temperatures and led to improved performance and efficiency. However, operating temperatures are now reaching limits posed by the melting temperatures (T_m) of these materials. New materials, including alloys based on metals with higher melting points, such as molybdenum (Mo) and niobium (Nb) alloyed with silicon (Si), are now being seriously examined as alternatives by academic and industrial groups.

Significance: A shear band is a region of highly localized plastic flow that develops during loading in both crystalline and amorphous materials. Shear bands directly determine the ductility of an amorphous phase, but relatively little is known about their nucleation. We use nanoindentation to probe shear band nucleation by measuring the first pop-in event during loading which is associated with shear band formation. We analyze a large number of independent measurements on four different metallic glasses and reveal a bimodal distribution of nucleation events that operate at different stress levels. The discovery of multiple shear band nucleation sites challenges the current view of a single type of site and offers opportunities for controlling the ductility of amorphous alloys.