Structure and embrittlement of metallic glasses

Metallic glasses are of commercial interest for various applications because of their magnetic, anticorrosive and mechanical properties. The major limitation of their commercial use is their embrittlement under certain conditions, i.e. the drastic decrease of their mechanical characteristics from a ductile to a brittle behavior. The mechanical properties depend on production conditions in two different ways: (i) on a macroscopic scale, i.e. on the scale of a ribbon, the quality of the material influences the reproducibility of the different macroscopic characteristics. Classical tools such as Weibull statistics can be applied to check the quality of the produced materials; (ii) on a microscopic scale, i.e. on the scale of the first shells of neighbors, the various existing local orders determine the mechanisms involved in the macroscopic behavior of the material, e.g. its resistance to fracture or corrosion. Classical mechanisms can also be applied in order to describe this correlation. Three cases of embrittlement of metallic glasses are identified: embrittlement by heat treatment, even below the crystallization temperature, embrittlement at very low temperatures and embrittlement at high strain rates. Fracture mechanics can be applied to present a macroscopic characterization of each step of the embrittlement. This approach reveals similarities in mechanical behavior between different metallic glasses and classical crystalline metallic alloys. Mechanical properties and embrittlement in the different length scales are discussed in correlation with the production conditions as well as the stability, relaxation and phase transformations of the alloys. It is emphasized that classical mechanisms can be applied in order to interpret the behavior of metallic glasses. Some examples are given where different production parameters have influenced their structure and mechanical properties on the two scales.