Nanoscale phase separation in metallic glasses studied by advanced electron microscopy techniques

To understand atomic structures of metallic glasses, advanced electron microscopy was employed to analyze local structures of some Fe-based and Zr-based metallic glasses. In the as-formed Fe₇₀Nb₁₀B ₂₀ and Fe_73.5Nb₃Si_13.5Cu ₁B₉ glasses, crystalline nano-clusters as small as 1 nm were observed using HREM (high resolution electron microscopy) under optimized defocus conditions. Nano-diffraction with a probe size of 1 nm revealed diffraction patterns of bcc-Fe and Fe-compound, suggesting the presence of nanoscale phase separation in these as-formed specimens. Atomic pair distribution function was calculated from analyzing electron diffraction intensity obtained from a large area in the samples using energy-filtering. For the Fe-Nb-B glass, an averaged atomic structural model with 2500 atoms was constructed using reverse Monte-Carlo calculation followed by Voronoi polyhedral analysis. The analysis revealed the presence of bcc-Fe-like, B-centered trigonal prism and highly coordinated Nb-Fe polyhedral structures, which was consistent with experimental results of HREM and nano-diffraction. A Zr ₇₁Cu₁₃Ni₁₀Ti₃Al₃ bulk metallic glass was also studied using HREM and high angle annular dark-field (HAADF) techniques. Crystalline clusters with sizes of about 1-2 nm were detected in the as-cast Zr₇₁Cu₁₃Ni₁₀Ti ₃Al₃. In the HAADF image, bright contrasts extending as small as 2 nm were found to fluctuate locally, indicating a local compositional fluctuation caused by nanoscale phase separation.