Local structure and dynamics of a Zr-based bulk metallic glass during ex situ and in situ annealing

Advanced transmission electron microscopy (TEM) techniques are utilized to investigate the structural evolution and underlying mechanisms driving relaxation in metallic glasses. We quantify changes in medium-range order (MRO) using fluctuation electron microscopy (FEM), while electron correlation microscopy (ECM) combined with in situ heating allows us to directly probe local dynamics with unprecedented spatial resolution. FEM analysis provides a semi-quantitative estimate of the MRO volume fraction, based on either the peak height or the peak integral of the variance. This measure was found to increase in structurally relaxed samples compared to the as-cast condition, observed across both predefined annealed states and during in situ heating. Complementary in situ analyses uncover a temperature-dependent reduction in atomic mobility. This decrease in dynamics—which aligns with the phenomenon of irreversible structural relaxation, often termed ”aging”—is examined with high spatial resolution for the first time. The findings on structural modifications and local relaxation behavior during heating are discussed in detail and compared with results obtained via other characterization methods and across different material systems, providing a comprehensive understanding of relaxation processes in metallic glasses.