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

Abstract

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 (Vf) 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 Vf and identified three types of distinctive deformation behaviors, including (I) shear banding for Vf < 70%, (II) shear banding induced cracking for 70% < Vf < 90% and (III) distributed plasticity for Vf > 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.