Fe-based metallic glasses exhibit high fracture toughness in the as-cast condition and an embrittlement after annealing above a critical temperature, which causes problems in handling and application. However, the annealing step is necessary to improve the soft magnetic properties of these materials for industrial applications. For the future development of ductile (partial-) nanocrystalline Fe-based ribbons with excellent soft magnetic properties, it is important to understand the mechanisms behind the annealing embrittlement.
Therefore, tensile tests of single edge notched specimens, as well as bending tests and nanoindentations were performed to determine the embrittlement of 15-20 µm thin, flash annealed Fe85.2B9.5P4Cu0.8Si0.5 ribbons in terms of critical stress intensity factor and bending ductility. In case of SENT specimens, the ribbons were notched and subsequently fatigue pre-cracked or directly pre-cracked during the notching process. Microstructure analysis has been done via X-ray diffraction (XRD) and atom probe tomography (APT).
The investigated material shows a drop in bending fracture strain at annealing temperatures between 330°C and 350°C. The critical stress intensity factor is decreasing from 70.4 MPam0.5 to 3.2 MPam0.5 at annealing temperatures above 350°C. Whereas the fracture morphology is changing from a typical vein-like (ductile) fracture pattern to a periodic striped pattern (Wallner lines, brittle fracture) and finally to an intercrystalline-like fracture behavior with increasing annealing temperature. Furthermore, Cu-clustering was investigated by APT before the onset of primary crystallization, the precipitation of α-Fe, which occurs at approx. 400°C.
Thus, the DBT already takes place in the X-ray amorphous condition. Relaxation processes lead to a decrease in free volume and a reduced shear band activity. Changes in the short range order, as well as precipitation due to annealing, lead to local stress concentrations and subsequently to local fracture events, promoted by a decrease in free volume. In consequence a global embrittlement/failure of the annealed Fe85.2B9.5P4Cu0.8Si0.5 alloy takes place.