Annealing embrittlement of Fe₇₈Si₉B₁₃ (METGLAS-2605S2)

The ductile to brittle transition that occurs in amorphous Fe₇₈Si₉B₁₃ (METGLAS-2605S2) has been investigated using mechanical measurements over the temperature range 250-370 °C. The fracture toughness values, K _Ic, have been determined for a range of annealing times (5-30 min) and cooling rates of 15-45 °C/min. A pronounced ductile to brittle transition is observed around 310(10) °C although no obvious structural changes are evident as indicated by x-ray diffraction. Comparison of transmission and back-scattered conversion electron ⁵⁷Fe Mössbauer spectra for the bulk as-received ribbon in the ductile state (K_Ic=52~MPa ̇ √m) and the ribbon annealed to the brittle state (K_Ic~10 MPa ̇ √m) indicates magnetic texture effects in both the bulk and on the surface of these amorphous ribbons, related to the magnetostriction resulting from the quenched-in stress during the ribbon production process, and the ensuing stress-relief upon annealing.