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

摘要

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 Fe_(70)Nb_(10)B_(20) and Fe_(73.5)Nb_3Si_(13.5)Cu_1B_9 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_(71)Cu_(13)Ni_(10)Ti_3Al_3 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 Zr_(71)Cu_(13)Ni_(10)Ti_3Al_3. 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.