Mechanical properties of metallic glasses

Metallic glasses began to be produced using melt spinning methods in the 1960s, and it wasn't until the early 1980s that they were produced in massive form, i.e. with a smaller dimension than a millimeter, using cold crucible techniques. These alloys are defined above all by their lack of long-range order, which gives them profoundly different mechanical properties from their crystalline counterparts. In the case of ordinary metals, these properties are essentially determined by crystal-scale defects (dislocations, grain boundaries, vacancies, etc.). In the case of glasses, in the absence of crystal lattice and translational symmetry, these defects cannot be defined. In particular, this results in a significant rise in the elastic limit at room temperature, close to the theoretical limit. Moreover, while the structure of metallic glasses distances them from ordinary metals, it also brings them closer to other classes of materials: amorphous materials, such as polymers or oxide glasses. In fact, metallic glasses will exhibit behaviors analogous to these classes of materials. Firstly, two essential characteristic temperatures can be defined: the glass transition temperature (T _g ) and the crystallization temperature (T _x ). In terms of mechanical properties, at high temperatures – close to the glass transition –, they exhibit a very high capacity for plastic deformation, which is also found in the case of polymers and oxide glasses. At room temperature, on the other hand, their fracture occurs under macroscopically brittle conditions (as in the case of ordinary glasses or cold polymers), even though it is preceded by intense, highly localized plastic activity in shear bands. What's more, in the vicinity of the glass transition, metallic glasses exhibit anelastic behavior similar, if not identical, to that of polymers and oxide glasses. These brief observations show that the mechanical properties of metallic glasses are largely determined by the amorphous nature of their structure.

This article first introduces the essential data on the structure of metallic glasses and briefly presents the most widely studied plasticity models. The second part focuses on the homogeneous deformation of metallic glasses, which occurs at temperatures close to the glass transition, and will be of particular interest in the forming stages. The third part focuses on the so-called heterogeneous mode – encountered at room temperature –, which determines the properties of metallic glasses in use. Finally, the last part introduces the more applicative – or "engineering" – aspects, such as specific properties and technologies, for which these materials are already used, as well as prospects for more or less long-term applications....