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Currently, studies of structural superplasticity (SP) are of great interest, since the use of this mode in metalworking technologies allows for the production of parts of various shapes in one operation (with high repetition accuracy of even very complex shapes), while requiring less energy and material resources (relatively low pressures and tool wear) compared to deformation in the ‘normal plasticity’ mode. Other advantages of using a structural SP are improved physical and mechanical characteristics of the finished product: better surface quality after deformation, high ductility at elevated temperatures, increased strength at temperatures close to room one without reducing ductility (most often there is an increase in ductility), increased cyclic strength, hardness, impact resistance, elevated corrosion-resistance durability, and absence of anisotropy of properties after superplastic deformation.

The morphology of the fibrous structures formed in the working parts of the 01420T alloy samples with the initial bimodal grain structure, deformed to fracture under optimal conditions of superplastic deformation at a temperature Т = 520°С and flow stress σ = 4,5 MPa is investigated. The maximum elongation of specimens deformed to failure δ is 670%. It has been suggested that the specific type of fibrous structures found in the specimens of the investigated alloy 01420T probably depends on the volume of the metastable liquid-solid phase, which was concentrated in the form of inclusions at some grain boundaries and made a viscous flow during superplastic deformation, its shear viscosity , the characteristics of its surface tension, the degree of dynamic oxidation of the melt, and the kinetics of the development of this process. The final view of the fibers and their shape, likely, depends not only on the nature of the viscous flow of the liquid-solid material, but also on the process of its crystallization during the cooling of the specimen in air to room temperature after mechanical tests. It was found that in view, all fibrous structures found in the working parts of the specimens can be conditionally divided into the following: cylindrical fibers; tapered fibers; cylindrical fibers on which there is a thickening or one or more drop-like formations; ribbon-like fibers; fibers that look like stalactites or stalagmites. The reasons for the formation of cracks on ribbon-like fibers are considered. It is assumed that they were formed as a result of relaxation of internal stresses, which were not fully minimized in the course of recrystallization, which was carried out when the sample was cooled. The reasons for the formation of droplets on the fibers are considered. It has been suggested that fibrous structures similar to stalactites and stalagmites were formed from a viscous material, which, in the course of superplastic deformation, as a result of crystallization, occured in local microvolumes of fibers, gradually turned from liquid-solid to solid-liquid. This led to the fact that in the crystallized microvolume of this fiber, the viscous homogeneous flow of the material probably turned into a localized flow, which is characteristic of the plastic flow carried out as a result of displacement of dislocations in the solid phase, and leads to the formation of stalagmitic fibers.

The aim of the study is to determine the correlation between the structure evolution and features of submicrocrystalline VT6 alloy deformation behavior under superplastic deformation. in the alloy was obtained by the multi-directional pressing method changing the deformation axis and gradual decrease of deformation temperature in the interval of 823-1073 К. Deformation behavior of VT6 alloy submicrocrystalline state was studied under tensile tests of 10-2 Pa in vacuum with the initial stretching rate of 6,9 10-3 sec.-1 and in the temperature range of 773-1073 К. The TEM method has been used to investigate the alloy structure in the initial state as well as in the submicrocrystalline state after the elongation of 40, 60 and 200 %. Submicrocrystalline structure formation in the alloy is found to lead to decreasing the temperature interval of superplastic flow beginning from 1173 to 873 K. It was shown that the increase in temperature of superplastic deformation of submicrocrystalline VT6 alloy samples from 873 to 973 K results in appearance of strain hardening stage in the stress-strain curves in the coordinates of the "true stress - true strain". Structure evolution of the submicrocrystalline VT6 alloy during superplastic deformation depends on the test temperature. At 973 K one can observe the change in the character of size distribution histogram of grain-subgrain structure elements from unimodal to bimodal. The strain hardening of the alloy under superplastic flow is caused by bimodal structure formation in it and growth in volume fraction of relatively large grains at deformation.