Ultrafine grained (UFG) metallic materials obtained by severe plastic deformation (SPD) typically exhibit very high strength properties, whose values are much higher than those predicted by the well-known Hall–Petch relation. Our studies show that the basis for this to occur is that SPD not only forms the UFG structure, but also leads to the formation of other nanostructural features, such as dislocation substructures, nanotwins, and nanosized precipitates of second phases, which additionally contribute to strengthening of materials. At the same time, this analysis of hardening mechanisms indicates that the structure and condition of grain boundaries, namely, their nonequilibrium state and the presence of grain boundary segregations, also substantially contribute to hardening. Taking this into consideration, approaches are discussed to achieve very high strengths in metallic materials by SPD.

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This paper evaluates the fatigue strength of ultrafine-grained (UFG) Grade 4 Ti in the low-cycle fatigue region, as well as the strength of medical implants (plates and screws) made of UFG Ti under various types of loading in comparison with the strength of products made of coarse-grained (CG) Ti. To produce a UFG state, titanium billets after annealing were processed by the ECAP-Conform technique. The fatigue of the prismatic specimens with a thickness of 10 mm from CG and UFG Ti was tested by the three-point bending method using an Instron 8802 facility. The modeling and evaluation of the stress-strain state in the ANSYS software package for finite-element analysis revealed, in particular, the localization of equivalent stresses in the area of hole edges and at fillets during the tension of the plates. The performed research has demonstrated that medical implants (plates and screws) from UFG Grade 4 Ti have a higher strength under different types of loading (tension, fatigue strength, torsion) in comparison with products from CG Ti. This opens up a possibility for the miniaturization of medical products from UFG Ti while preserving their main performance properties at an acceptable level.

The results of studies on the process of precipitation of dispersed second phases in commercially pure titanium Grade 4 and the effect of secondary precipitates on its structure and mechanical properties in two states, coarse-grained and nanostructured ones, are presented. The nanostructured state was obtained by high-pressure torsion (HPT) under a pressure of 6 GPa up to N =10 revolutions at room temperature. A particular consideration is given to the study of changes in the phase composition and microstructure of titanium subjected to deformation processing after annealing at an elevated temperature of 700°C for 30 minutes. In this work, by means of studies in a transmission electron microscope, it was shown that at a temperature of 700°C and higher, in the structure of the samples, nanoparticles of the second phases which differ in size and morphology are precipitated in both states. The nature of the observed particles was studied in SEM, by indexing the electron diffraction patterns taken from the particles, and by carrying out X-ray phase analysis by the "transmission" method. Particles of the second phases are modifications of the high-temperature β-phase of titanium. The HPT treatment of the alloy, according to the XRD data, leads to an increase in the volume fraction of precipitated particles after annealing and, as a result, to an increase in the microhardness of the states under study. The results of microhardness measurements at varying regimes of deformation and annealing are presented. Combination of HPT up to N = 5 revolutions and annealing at 700°C for 30 minutes followed by additional torsion straining also up to N = 5 revolutions provides the highest microhardness values in commercially pure titanium, which reaches 423 HV.

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