Rheological behavior of superplastic nanocrystalline and amorphous materials

Rheological behavior of two groups of superplastic materials is analyzed on the base of mass-transfer mechanics considerations. The first group ts distinguished by ultra-fine grains as the basic mass-transfer units and includes FSS alloys, HSRS alloys as well as crystallizing metal melts. The second group unifies materials which exhibit superplastic and superplastic-like flow on the basis of moving atoms or their groups as mass-transfer units. Inorganic glass-forming systems (non-metallic as well as metallic), polymers and materials which display phase-transformation superplasticity may be included into this group. An analysis of apparent sheer viscosity in superplastic flow of this materials as the function of solid-phase volume fracture as well as of strain rate and deformation temperature shows the evidence of rheological similarity among the materials mentioned above. Rheological properties of all these materials vary inside the range between pure viscous and viscoplastic types. Thermodynamical analysis of viscous flow of metallic glasses show that the typical mass-transfer unit in supercooled liquid state is a group of atoms limited by two to three co-ordination spheres. High-strain-rate superplasticity and very low shear viscosity exhibited by bulk amorphous alloys are considered as the promising features for development of new metal forming technologies.