OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 24 No. 4 2022 Introduction The design of metallic materials for medical purposes, combining high mechanical properties and low elastic modulus, as well as mechanical and biological compatibility, is an important challenge today [1]. In this regard, a promising trend in the field of medical materials science is the development of titanium-based alloys doped with bioinert metals that do not have a toxic effect on human body. These are alloys of the following systems: Ti-Nb, Ti-Nb-Ta, Ti-Nb-Zr, Ti-Nb-Sn, Ti-Nb-Ta, Ti-Nb-Hf, Ti-Nb-Zr-Sn, Ti-Nb-Ta-Sn, Ti-Nb-Ta-Zr [1, 2]. The alloying of titanium with stabilizing elements of certain concentrations, such as niobium, zirconium, and tantalum, allows the formation of a b-phase that contributes to a low modulus of elasticity in the alloy. The elastic modulus of such alloys, depending on the elemental composition, can vary in the range of 14–50 GPa, which is comparable with the elastic modulus of bone tissue (10–30 GPa) [2]. The interest in alloys with a low modulus of elasticity is reflected in a number of scientific studies carried out for alloys of ternary systems based on titanium, niobium and zirconium (TNZ): Ti‑13Zr‑13Nb, Ti-19Nb14Zr, Ti-Nb(18-19)-Zr(5-6) [3–8]. The advantage of TNZ alloys is the absence of toxic effects on the body. However, its wide application in medicine is limited by its low strength properties, such as yield strength, ultimate strength, fatigue strength, etc. The formation of an ultrafine-grained (UFG) structure in β-titanium alloys by the severe plastic deformation (SPD) method provides a significant increase in fatigue strength and cyclic durability without alloying with “toxic” elements and increases the strength and yield strength up to the level of coarsegrained (CG) medium-strength “α + b” titanium alloys for medical applications. It was shown in [9] that, depending on the modes of thermomechanical treatments, the elastic modulus of the Ti-13Nb-13Zr alloy ranges from 79–84 GPa. For Ti-Nb-Zr alloys with different concentrations of niobium and zirconium after rolling and heat treatment, the elastic modulus and ultimate strength can vary from 59 to 75 GPa and from 345 to 810 MPa, respectively [9–11]. However, the issues related to achieving the required mechanical properties and the regularities of structure formation during SPD require further development due to the large variety of forming structures and phase transformations for multicomponent systems based on titanium with a stabilized b-phase and low modulus of elasticity. All these factors determine the relevance of the research aimed at designing alloys based on titanium, niobium, zirconium and further solving problems associated with increasing the level of mechanical properties and reducing the value of the elastic modulus. The aim of the work is to reveal the effect of severe plastic deformation on the microstructure and mechanical properties of an alloy of the Ti-Nb-Zr system. Materials and research methodology The alloy of the Ti-Nb-Zr system, Ti-42Nb-7Zr, was used as a research material. The experimental Ti-42Nb-7Zr alloy ingots were fabricated from pure iodide titanium, niobium, and iodide zirconium by arc melting in a shielding argon atmosphere using a non-consumable tungsten electrode in a Buhler furnace [12]. To ensure the homogeneity of the chemical composition, a fivefold remelting was carried out. The ingots were obtained in the form of disks (diameter – 25 mm, height – 8 mm) with a mass of 20 g. According to the X-ray microanalysis data, the ingots had the following composition (wt %): Ti - 50.3; Nb - 42.3; Zr- 7.4.After remelting, the ingots were held at 1,000 °C for 3 hours in an argon atmosphere and subsequently quenched in water. Billets were prepared from the ingots and subjected to heat treatment and SPD according to two schemes to obtain the UFG state. Figure 1 shows the schemes of thermal and deformation treatments of the alloy ingots. According to the first scheme, billets in the form of parallelepipeds with the dimensions of 7×8×15 mm3 were cut from the ingot by an electrospark discharge machine. Then billets were subjected to SPD, which consisted of multi-pass flat rolling. Before rolling, the billets were preheated to 200 °C, and the rolling was carried out in the room temperature rolls to a total logarithmic strain of 2.19. In the second scheme, a combined SPD method was used, consisting of abc-pressing and subsequent multi-pass rolling in grooved and then in flat rolls. A billet with the dimensions of 13×15×18 mm3 was
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