Formation of boron-rich zones and embrittlement of Fe–B-type metallic glasses

Two currently proposed structural explanations for thermal embrittlement of Fe-based metallic glasses are discussed: embrittlement due to loss of excess free volume by annealing and embrittlement due to the formation of chemical heterogeneities due to phase separation in the amorphous alloy. It is shown that recent small-angle x-ray scattering results that were thought to be in disaccord with field-ion microscopy (FIM) atom-probe analyses indicating occurrence of phase separation are in fact in agreement with the latter. Then a new mechanism is presented, based on structural models and density measurements in Fe–B-type amorphous alloys, which results in loss of free volume during the occurrence of phase separation, thus integrating an important effect of phase separation in the free-volume models for plastic flow in amorphous alloys.