OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 24 No. 4 2022 than 1.3 times and in the ultimate strength by 2 times in comparison with the CG state. Significant strengthening of the samples after the combined SPD in comparison with the multi-pass rolling is associated with additional refinement of structural elements. The mechanical properties and the structural-phase characteristics of the Ti-42Nb-7Zr alloy in different states are shown in Table. Mechanical and structural phase characteristics of the alloy in different states State Average size of main b-phase elements, µm Phase composition s0.2, MPa su, MPa ef, % Hm, MPa Cast – dendrites up to 500 µm long; – equiaxed grains 350 ± 100 b – – – 1,900 ± 200 Quenched equiaxed grains 100 ± 30 b + a″ – – – 1,540 ± 100 CG equiaxed grains 20 ± 5 (b + w) 350 ± 20 550 ± 30 8.7 ± 0,2 1,700 ± 100 UFG (rolling) bands: – length (0.2–0.8) – width (0.2–0.7) (b + w) + a″ 390 ± 30 710 ± 50 5.7 ± 0,3 2,570 ± 100 UFG (аbc-pressing+ +rolling) non-equiaxed grains 0.3 ± 0.2 (b + w) 480 ± 30 1,100 ± 50 4.6 ± 0,3 2,800 ± 100 For comparison, Table shows the microhardness value for the CG state of the alloy. The formation of the UFG structure in the alloy as a result of multi-pass rolling and the combined SPD method leads to an increase in the microhardness level to 2,570 and 2,800 MPa, which is 1.6 and 1.8 times higher, respectively, compared to the CG state (1,700 MPa). The value of the elastic modulus of the UFG alloy formed by the combined SPD method is 36 GPa, and for the CG state it is 42 GPa, which is significantly lower than for medium-strength titanium alloys, Ti-6Al4V ELI, Ti-6Al-4V, and the pure titanium (100–110 GPa), that are widely used in medicine [21]. Thus, the UFG (β + w) structure obtained by the combined SPD method with an average size of structural elements equaled to 0.3 μm, makes it possible to achieve a higher level of mechanical properties in the Ti-42Nb-7Zr alloy as compared to the structure after rolling. A significant increase in the offset yield strength and ultimate strength, as well as microhardness after deformation in the UFG alloy is associated with substructural and dispersion strengthening. Conclusions The multi-pass cold rolling of the Ti-42Nb-7Zr alloy in the quenched state leads to the formation of an UFG structure that has a stripe character, in which the main phase is the β-phase dispersion strengthened by ω-particles, and there is also a small amount of precipitates of the a″-martensite phase. The combined SPD method of the Ti-42Nb-7Zr alloy promotes more efficient grain refinement and the formation of a more dispersed UFG structure with an average size of structural elements of 0.3 μm, represented by b-subgrains that are dispersion strengthened by ω-phase nanoparticles. The martensitic a″-phase transforms into the b-phase by the α″→a→β mechanism and is not observed after the combined SPD. The formed UFG state with (β + w)-structure in the Ti-42Nb-7Zr alloy as a result of the combined SPD method provides a significant increase in mechanical properties: offset yield strength is 480 MPa, ultimate strength is 1,100 MPa, microhardness is 2,800 MPa at the fracture strain of 4.6 %, which is associated with substructural and dispersion strengthening.
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