STRUCTURE AND EMBRITTLEMENT OF METALLIC GLASSES

Metallic glasses are of commercial interest for Various applications b ecause 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 cha racteristics from a ductile to a brittle behavior. The mechanical prop erties 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 c haracteristics. Classical tools such as Weibull statistics can be appl ied to check the quality of the produced materials; (ii) on a microsco pic scale, i.e. on the scale of the first shells of neighbors, the var ious existing local orders determine the mechanisms involved in the ma croscopic behavior of the material, e.g. its resistance to fracture or corrosion. Classical mechanisms can also be applied in order to descr ibe this correlation. Three cases of embrittlement of metallic glasses are identified: embrittlement by heat treatment, even below the cryst allization temperature, embrittlement at very low temperatures and emb rittlement at high strain rates. Fracture mechanics can be applied to present a macroscopic characterization of each step of the embrittleme nt. 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 a s the stability, relaxation and phase transformations of the alloys. I t is emphasized that classical mechanisms can be applied in order to i nterpret the behavior of metallic glasses. Some examples are given whe re different production parameters have influenced their structure and mechanical properties on the two scales. (C) 1997 Elsevier Science S. A.