Effect of nanoheterogeneities on the fracture toughness and tensile ductility of CuTa metallic glass thin films

Nanoheterogenenous metallic glasses (MG) can offer improved ductility through a nanoscale modulation in their mechanical properties. However, the relationship between the modulation parameters and the mechanical behavior is not well understood. Physical vapor deposition can directly control the compositional and morphological parameters of nanoheterogeneous MGs and enables the systematic investigation of this problem. This work explores the microstructure and mechanical properties of a range of CuTa-based nanolayered amorphous/amorphous (A/A) and amorphous/semi-crystalline (A/SC) nanoheterogeneous MGs and MG composites. The first step was the identification of three microstructural regimes in CuTa, namely, a fully amorphous form (23-65 at.% Ta), a Ta-rich amorphous-crystalline composite (65-75 at.% Ta), and a Cu-rich amorphous-crystalline composite (16-23 at.% Ta). The hardness of the films increased from 6 GPa to 17 GPa with increasing Ta content. Next, a range of CuxTa1_ x/CuyTa1_y nanolayers composed of A/A and A/SC nanolayers were investigated. The hardness of all nanolayers follows the rule of mixture. A/A structures do not provide a significant increase in fracture toughness and only a minor increase in tensile ductility despite the high amplitude modulation of hardness between layers. A/SC nanolayers' hardness and toughness was higher than A/A nanolayers as well as monolithic amorphous films, but still remained below the monolithic Cu25Ta75 SC film. The results show that the design of nanoheterogeneities in MGs requires careful optimization to achieve a useful improvement in mechanical properties.