OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 24 No. 4 2022 are low intensity reflexes from the second phase, a″-martensite, which was formed during rapid cooling in the areas depleted of alloying elements [15]. At the same time, in the state after the combined SPD method, the a″-phase reflexes are not detected on the X-ray diffraction patterns, there are reflexes belonging only to the β-phase (Fig. 5, d). In this case, a noticeable increase in the width of X-ray lines is observed after the deformation of the alloy, which indicates the formation of the developed dislocation substructure. In the recrystallized CG state, the reflexes from the β-phase are observed in the X-ray diffractogram (Fig. 5, e). It should be pointed out that the ω-phase could not be identified by the X-ray diffraction analysis, probably due to its small volume fraction. Thus, according to the given results, as a result of rolling of the alloy samples, a stripe UFG microstructure is formed. It is represented by b-subgrains, dispersion strengthened by the nanosized ω-phase, and a small amount of a″-martensite. The use of multi-pass rolling after abc-pressing leads to the formation of a more dispersed UFG structure, represented by b-subgrains, dispersion strengthened by ω-phase nanoparticles. Figure 6 shows the engineering curves of the Ti-42Nb-7Zr alloy samples during static tensile tests for different states. It should be noted that, due to the small size of the samples in the initial cast state and in the state after quenching, it was not possible to evaluate its tensile properties. Therefore, the mechanical properties of the UFG alloy samples were compared with those in the CG (recrystallized) state. The tensile tests have shown that after multi-pass rolling of the hardened state samples the following mechanical properties are achieved: offset yield strength (σ0.2) is 390 MPa, the ultimate strength (σu) is 710 MPa and the fracture strain (εf) is 5.7 %. It can be seen that for the UFG alloy formed as a result of multi-pass rolling, the ultimate strength is 1.3 times higher compared to the CG state. It is worth noting that in this case the value of the offset yield strength for the UFG alloy does not differ from that of the CG alloy, which is associated with its dispersion strengthening by the w-phase particles. After the combined SPD the alloy samples have the maximum mechanical characteristics, namely: the offset yield strength (σ0.2) is 480 MPa and the ultimate strength (σu) is 1,100 MPa at the fracture strain (εf) equaled to 4.6 %. Grain refinement as a result of the two-stage SPD leads to an increase in the yield strength by more Fig. 5. X-ray diffraction pattern of Ti-42Nb-7Zr alloy in different states: cast (a); quenched (b); UFG, rolling (c); UFG, abc-pressing with rolling (d); CG (e) Fig 6. Engineering curves for Ti-42Nb-7Zr alloy samples in different states: 1 – CG; 2 – UFG (rolling); 3 – UFG (abc-pressing with rolling)
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