Review of experimental studies on structural superplasticity: internal structure evolution of material and deformation mechanisms

A wide variety of metals and metal alloys can be transferred to a specific state in which materials are capable of extremely large (hundreds and thousands of percents) strains without fracture at relatively low (compared with normal plasticity) stresses. For that materials should be fine-grained (with an average grain size less than some critical size for a given class of alloys, usually less than 10 microns) and subject to deformation in a certain range of temperatures and strain rates. The property of materials to test anomalously large deformations under the specified conditions is classified as structural superplasticity and is widely used in manufacturing processes by formation methods various (in the first place - large-sized) products in many industries (aerospace, automotive and etc.). Mathematical modelling is the most effective “tool” in developing rational regimes of technological processes, which, in turn, requires the creation of constitutive models (constitutive relations) allowing to adequately describe the physics and mechanics of superplastic deformation processes. To date many dozens of constitutive relations of various classes (macrophenomenological, structural-mechanical, thermodynamic, physical) have been created. Identification and verification of such models is carried out on the basis of the experimental data which are obtained on macro-samples, as a rule. The extensive experimental material about features of structural superplasticity has been currently accumulated for various materials. In most cases the experiments are carried out on cylindrical specimens by using uniaxial tension on kinematic type machines. The proposed review pays special attention to the issue of the staged nature of “longitudinal stress - strain” dependence which is observed in experimental tests with a transition to the superplasticity regime. This review attempts to systematize data from experimental studies which can be useful to form a more complete picture of physical nature of structural superplasticity phenomenon in various materials and necessary changes in material structure for transition to this deformation regime. It considers the effect of initial temperature-velocity conditions, meso- and microstructure of materials on the form of curves. The analysis of the considered experimental data confirms that the grain-boundary sliding is a predominant mechanism of superplastic deformation, but other mechanisms and processes play an important role: intragranular dislocation sliding, grain-boundary diffusion and rotation of crystallite lattice, along with the dynamic recrystallization. The paper performs and analyzes factographic data and their descriptions about a possible action of the above mechanisms and processes, their influence on each deformation stage and effect on the change in stress-strain state and material structure.