Crystallization embrittlement of Ni-Ti-B glasses

While metallic glasses have excellent toughness and ductility in the as-cast and unrelaxed state, the process of crystallization leads to a nearly continuous and significant embrittlement. This change is examined on three Ni-Ti-B glasses and related to the morphology and distribution of the crystals obtained. For the partially crystallized materials, failure still occurs after intense shear on one well-defined shear plane, and it is shown how the crystals act as stress and strain incompatibilities causing localized crack or cavity formation. The relationships between sample mechanical properties, fracture surface characteristics and crystal distributions allow an analysis of cavity nucleation and growth rates, and thereby make it possible to suggest microstructures which may maintain reasonable toughness.