Nanocrystallization of complex Fe23B6-type structure in glassy Fe-Co-B-Si-Nb alloy

Local structural development in the bulk metallic glass (Fe0.5Co0.5)72B20Si4Nb4 on annealing has been investigated using transmission electron microscopy. Nanoscale crystalline grains with the Fe23B6-type structure were densely formed during annealing around the first crystallization temperature. In the state prior to the crystallization (below Tg), we found extended medium range ordered (MRO) regions as small as 2 nm with an extremely high density in the glass matrix. The nanocrystallized microstructure is presumably ascribed to the presence of the dense MRO regions. On the basis of nanobeam diffraction analysis, however, it was found that most of the MRO regions have no clear order of the Fe23B6-type structure. The clear Fe23B6 order was found in regions extending as large as 5 nm at a higher temperature (around Tg). A relationship between the structural change and the glass stability is discussed.