Unraveling Contributions to Thermal Stability in Nanocrystalline Alloys Using Nanometallic Multilayers

Despite their intrinsic instabilities, nanocrystalline metals have demonstrated improved properties relative to their coarse-grained counterparts. Targeted doping of grain boundaries to offset their energetic penalty has been pursed as a thermodynamic pathway for increasing thermal and mechanical stability. However, as recently demonstrated in several systems, dopant species thought to promote thermodynamically preferred nanocrystalline states instead contain features such as small nanoprecipitates that act to inhibit grain boundary migration – rather than stabilize against its driving force. In this study, we use thermodynamic nanostructure stability models to select a model system for exploring the interplay between thermodynamic and kinetic contributions to stability in nanocrystalline alloys. A unique starting microstructure containing nano-metallic multilayers in the Mo-Au binary system was employed and thermally aged through in-situ electron microscopy experiments to access evolving segregation states. A combination of imaging and analytical microscopy was used to map solute segregation at different microstructural features and corresponding phase transitions as a function of temperature. Our results demonstrate the microstructure initially evolved through a phase transformation at lower homologous temperatures (< 600 °C) where Au solute atoms cluster and segregate to the underlying grain structure, consistent with predictions from thermodynamic stability models. With increasing temperature, grain boundaries are shown to migrate and produce an increase in grain size, which was accompanied by intermittent pinning events from solute-rich regions. This regime transpired at temperatures involving phase separation, thus demonstrating kinetic contributions to stability at higher homologous temperatures.