Properties of Deformable Medium Element as a Factor of Superplasticity of Metallic Materials

Theoretical study of the phenomenon of superplasticity of metallic materials has been carried out. Numerical calculations were performed on the basis of a two-level finite element model of an elastoplastic medium. The plastic properties of the deformable medium element determining the deformation by uniaxial tension in superplasticity mode were found. It was shown that depending on the shape of the stress-strain curve (σ–ε) of the medium element, the various types of plastic flow localizations were observed at the macroscale level. The nonmonotonic dependence σ–ε of the deformable medium element having single maximum was determined as the condition for the appearance of a stable fracture neck. In the case when the dependence σ–ε characterized by the strengthening of the elementary volumes with two maxima, a propagating (running) neck was observed, then a second (counter) neck appeared, while further tension led to the appearance of the third stable neck in which fracture occurred. In the case of more complex oscillating shape of σ–ε curve the multiple running necks were observed. The movement of the multiple necks propagating along the sample leads, ultimately, to a uniform picture of the deformation and made it possible to achieve the strain values observed in superplasticity mode.