Understanding yielding and the unusual ductile-brittle-ductile transition in Fe-based amorphous nanocrystalline alloy: A combined micromechanical and thermodynamic study

Fe-based amorphous nanocrystalline alloys have been attracting tremendous research interest because of their excellent soft magnetic properties. However, their applications are hindered because of their room-temperature brittleness and the underlying mechanisms are yet to be fully understood. In this work, we successfully fabricated a series of Fe-based amorphous nanocrystalline alloys through the controlled nanocrystallization in Fe-based amorphous ribbons, which resulted in rather thin amorphous inter-granular films with an average thickness reducing from 10 nm to less than 0.5 nm as the volume fraction (V_f) of the nanocrystals increased from 16% to 95%. According to the extensive microcompression experiments, we demonstrated that the yielding strain of the amorphous nanocrystalline alloys decreases progressively with the increasing V_f and identified three types of distinctive deformation behaviors, including (I) shear banding for V_f < 70%, (II) shear banding induced cracking for 70% < V_f < 90% and (III) distributed plasticity for V_f > 90%. As combined with the transmission electron microscopy and strain rate sensitivity analyses, we developed a micromechanical and thermodynamic model which quantitatively explains the yielding behavior of the amorphous nanocrystalline alloys and the unusual phenomenon of ductile-brittle-ductile transition, as manifested by the three deformation behaviors. Our current findings provide important insights into the design of strong-yet-ductile soft magnetic amorphous nanocrystalline alloys.