Crystal growth limitation as a critical factor for formation of Fe-based bulk metallic glasses

In the present work we study the formation mechanism and crystallization behavior of Fe-based bulk metallic glasses influenced by the addition of rare-earth elements. Samples are characterized by conventional X-ray diffractometry, optical microscopy, and high-resolution field-emission-gun scanning and transmission electron microscopy. In the rare-earth-containing alloys, bulk metallic glasses are formed with populations of quenched-in nuclei. Consequently, crystallization on isothermal annealing occurs without any incubation period. This behavior, not typical of bulk metallic glasses in general, implies that in the rare-earth-containing alloys glass formation is possible only because of restricted crystal growth: both on cooling from the molten and on heating from the glassy state, growth of the primary χ-Fe36Cr12Mo10 crystals is too slow to be significant on a reasonable timescale. The low growth rate is connected with large inhomogeneous strain in the growing nanoparticles, while nucleation of eutectic colonies is hampered by slow diffusion of a rare-earth alloying element.