Rod of technical grade titanium for biomedicine and method for producing same

Abstract

A rod of technical grade titanium for biomedicine has a structure consisting of ultra-fine grains of alpha phase with a hexagonal close-packed lattice, a fibrous deformation texture and a grain-subgrain structure type, wherein the volume fraction of elongated grains in a size range of 0.1-3 µm with a grain shape factor greater than 2 in mutually perpendicular planes is not less than 80%. Subgrains formed inside the elongated grains have low-angle boundaries that are misoriented from adjacent subgrains by an angle of up to 15°. A method for producing a rod of technical grade titanium for biomedicine includes intense plastic deformation of a billet of technical grade titanium at a temperature of not more than 450°С by rotary swaging at strain rates in a range of 10-2 to 10-4 s-1 and with a total true strain value of e≤2.2. As a result of the formation of an ultra-fine structure, a rod of technical grade titanium produced using the claimed method has high mechanical characteristics.

Description

A rod of technically pure titanium for biomedicine and a method for its production

[0001] The invention relates to the field of medical metallic materials with an ultrafine-grained (UFG) structure with enhanced mechanical and biomedical properties, in particular titanium and its alloys, which can be used for the manufacture of medical implants used in surgery, orthopedics, traumatology and dentistry, as well as to the technology of processing said materials to form structures that provide certain mechanical and biomedical properties.

[0002] It is known that the mechanical and operational properties of metallic materials, including those used in the production of medical implants, depend on the chemical composition and type of the formed structure, which includes a wide range of its parameters, such as phase composition, size and shape of grains, misorientation of their boundaries, dislocation density, microdeformation and other defects of the crystal lattice. [1. Gulyaev A.P. Metal Science: textbook. Moscow: Metallurgy, 1986, 544 p., 2. Shtremel M.A. Strength of alloys. Moscow: Metallurgy, 1982. Part 1: Lattice defects. 280 p.; 3. Shtremel M.A. Strength of alloys. 4.2. Deformation. Moscow, MISiS, 1997, 527 p.]. 3.

[0003] Commercially pure titanium is widely used for the manufacture of implants in dentistry and traumatology due to its high biocompatibility. [D. M. Runette, P. Tengvall, M. Textor, P. Thomsen, "Titanium in medicine", Springer, (2001) p. 1019]. In general, titanium has a fairly high specific strength; however, the creation of minimally invasive and mini implants often requires stronger materials that will also ensure higher performance properties of products made from this titanium. Alloying titanium with aluminum, vanadium, tin, and chromium (medical alloys Ti6A14V, Ti5Al12.5Sn, Ti3Al13V11Cr) allows for a strength higher than that of pure titanium, but also increases the toxicity to the body. [DMBrunette, P.Tengvall, M.Textor, P.Thomsen, “Titanium in medicine”, Springer, (2001) p.1019].

[0004] A titanium alloy alloyed with zirconium (15%) with a strength of 1050 MPa is well-known. It was developed by the Swiss company Straumann for highly biocompatible dental implants under the Roxolid brand (https: / / dzen.ru / a / YgEHcd0hXgccy2DR). The average grain size of this material is 5-15 µm. However, it does not provide the high strength of 1250-1260 MPa required for creating mini implants and traditional implants with high performance properties. Moreover, the alloying process and the use of zirconium, an expensive component, significantly increase the cost of the initial rod.

[0005] A range of medical-grade titanium rods with an average grain size of 3-7 µm, branded ERGITAN®, is also available, developed by the German company BBS i Halmstad AB (https: / / bbshalmstad.se / en / ex-stock / titanium / ergitan-37165 …). The strength range of these rods ranges from 750 to 1100 MPa. Strength is increased through deformation processing. A drawback is the insufficient strength of these rods, which precludes the creation of mini-implants and traditional implants with high performance properties.

[0006] It is known that the formation of ultrafine structural states, including nanostructured grain or grain-subgrain type, makes it possible to achieve a unique combination of strength, ductility, and fatigue life in metals and alloys [1. Raab G.I. Valiev R.Z. Production of nanocrystalline titanium / Materials Science and Heat Treatment of Metals. 2000. No. 9. pp. 27–31; 2. R.Z. Valiev, I.V. Alexandrov. Bulk nanostructured metallic materials. Moscow: ITC “Akademkniga”, 2007. — 398 p.]. In this regard, a patent providing for the formation of an UFG structure in titanium was taken as a prototype [Patent No. 2383654.Nanostructured commercially pure titanium for biomedicine and a method for producing a rod therefrom] characterized by a structure with a volume fraction of grains with a size of 0.1...0.5 μm and a grain aspect ratio of no more than 2 in mutually perpendicular planes of at least 90%, with more than 60% of the grains having high-angle boundaries misoriented relative to adjacent grains at angles from 15 to 90°. This technical solution has been adopted as a prototype. The formation of such a structure requires combined thermomechanical processing with an accumulated deformation level greater than e = 4, requiring controlled precision machining with a gradual decrease in temperature. This approach ensures a high strength of up to 1330 MPa, but is associated with high costs for the formation of the structure and production of rods, sinceThe method for producing such titanium includes intensive plastic deformation of the workpiece by equal-channel angular pressing at a temperature not exceeding 450°C with a total true accumulated deformation of e≥4 and subsequent thermomechanical treatment with a degree of deformation from 40 to 80%, with a gradual decrease in the deformation temperature in the range of T=450...350°C and a deformation rate of 10. -2 … 10 -4 With -1To accumulate strain e≥4 at [units / units / units], at least four equal-channel angular pressing cycles with intermediate heating, application, and consumption of a lubricant coating are required. Subsequent thermomechanical processing with temperature reduction also requires controlled heating, lubrication, and the use of new equipment, significantly complicating and increasing the cost of the end-to-end process of producing nanostructured rods. Another disadvantage of this method is the use of low deformation rates, which increases time costs, and low metal utilization, which increases material and financial costs in producing the finished product.

[0007] Thus, in the described patents and literary sources, technically pure titanium, which has high biocompatibility, is used to manufacture implants that can remain in the human body for a long time, however, the main disadvantages include low strength and increased costs for its manufacture. Technical challenge

[0008] The objectives of the invention are to develop a rod made of technically pure titanium with improved mechanical properties due to the formation of an ultra-fine structure and to reduce the costs of its production. Solution to the problem

[0009] The stated problem is solved by the fact that a rod made of technically pure titanium for biomedical purposes has a structure of ultra-fine alpha-phase grains with a hexagonal close-packed lattice, is distinguished by a fibrous deformation texture and a grain-subgrain type of structure, while the volume fraction of elongated grains in the size range of 0.1...3 μm and with a grain shape factor of more than 2 in mutually perpendicular planes is at least 80%, and the subgrains formed inside the elongated grains have low-angle boundaries, misoriented relative to neighboring subgrains at angles of up to 15°. Positive effects of the invention

[0010] The creation of this ultrafine structure type, in the range of 0.1 to 3 µm, with a grain cross-section averaging 0.2 µm, ensures high tensile strength, according to the Petch-Hall law, in the longitudinal direction, and bending and impact strength in the cross-section, due to its textural "fibrous" structure. The presence of a substructural component in the structure increases the resource of uniform plasticity, which improves the structural properties of products by eliminating brittle fracture.

[0011] The diagram of the rod section is shown: plane A-A is the cross-section of the workpiece, plane B-B is the longitudinal section. The microstructure of the rod is illustrated by photographs (a) in the cross-section (TEM), (b) (SEM) and (c) (TEM) in the longitudinal section. It is evident that the grain size (1) in the cross-section of the rod averages approximately 200 nm. Figure 4 shows that the longitudinal section of the rod is dominated by grains (4), which have an elongated shape with a length-to-width ratio greater than 2.

[0012] The method for producing a rod of technically pure titanium for biomedicine, having a structure of ultrafine alpha-phase grains with a hexagonal close-packed lattice, includes intense plastic deformation of a workpiece of technically pure titanium at a temperature not exceeding 450°C, while the deformation is carried out by the method of rotational compression at deformation rates in the range of 10 -2 … 10 -4 With -1 with a total value of true deformation e≤2.2, ensuring the formation of a fibrous deformation texture and a grain-subgrain type of structure in the rod, wherein the volume fraction of elongated grains in the size range of 0.1...3 μm and with a grain shape factor of more than 2 in mutually perpendicular planes is at least 80%, and the subgrains formed inside the elongated grains have low-angle boundaries, misoriented relative to adjacent subgrains at angles of up to 15°.

[0013] Rotational compression of the workpiece is carried out by conical dies with frequent changes in the axes of force application, which ensures drawing directed along the longitudinal axis of the original round workpiece and a reduction in its diameter with a degree of deformation of e ~ 0.4-0.5 per pass. (S. Yu. Radyuchenko., Rotational compression. "Machine-building", 1972, 176 p.). Frequent changes in the axis of load application up to 1000 times per minute introduce a factor of non-monotonic deformation, allows for intensive deformation and at the same time ensures a linear speed of rod movement of up to 3 meters per minute, due to the absence of continuous contact of the tool with the workpiece. It is known that plastic deformation, especially under conditions of its non-monotonic nature, undergoes more intensive refinement of the structure. [ 1. Utyashev F.Z., Raab G.I. Scientific foundations of deformation technologies for the formation of ultrafine-grained and nanostructured bulk materials. Infra-Engineering Publishing House. 2021. 160 p. ISBN: 978-5-9729-0632-1.; 2. Alexander P. Zhilyaev, Anatoly I.Pshenichnyuk, Farid Z. Utyashev, Georgy I. Raab. Superplasticity and Grain Boundaries in Ultrafine-Grained Materials / ELSEVIER. 2020. 416 p. ISBN: 978-0-12-819063-0 (print). ISBN: 978-0-12-819077-7 (online) Limiting the accumulated strain to ε = 2...2.2 minimizes processing costs by reducing the number of passes. In addition, one type of universal equipment is used, which is characterized by low energy consumption. At the same time, a high-strength state at the level of 1250-1300 MPa is achieved and the relative plasticity level of ~12-15% is maintained.

[0014] The invention is implemented as follows.

[0015] A commercially pure titanium rod is used as the starting material. Processing is performed by rotary compression at a temperature no higher than 450°C in 4 or 5 passes until a true cumulative strain of e≤2.2 is achieved. During deformation processing, the structure undergoes a gradual transformation from the initial equiaxed structure with a grain size of 15-25 microns to a banded grain-subgrain structure with a grain aspect ratio greater than 2, grain sizes of 0.1-3 microns, and subgrains within them.

[0016] Thus, as a result of sequential deformation processing in commercially pure titanium, an ultra-fine grain-subgrain structure with a fibrous deformation texture is formed, while the volume fraction of grains in the size range of 0.1...3 µm with a grain shape factor of more than 2 in mutually perpendicular planes is at least 80%, and the subgrains formed inside the elongated grains have low-angle boundaries, misoriented relative to neighboring subgrains at angles of up to 15°.

[0017] An example of a specific implementation of the invention

[0018] A bar of commercially pure CP Grade 4 titanium, 15 mm in diameter and 300 mm in length, was used as the starting material. This bar was heated to 400°C and subjected to four rotary compression passes to a diameter of 6 mm. The total cumulative strain was e = 1.83. This process resulted in a bar with a diameter of 6 mm and a length of approximately 1850 mm.

[0019] Microstructure specimens were prepared from this rod, using scanning microscopy on an EM-30 plus microscope and transmission electron microscopy on a JEM-100 microscope. The specimens were cut using electrical discharge machining (EDM) to form plates in both the rod's cross-section and longitudinal cross-section. Polished plates and thin foils were prepared using standard techniques.

[0020] A 5 mm diameter rod was also produced at a temperature of 400°C in 5 passes. The total cumulative deformation was e = 2.2.

[0021] The diagram of the rod section is shown: plane A-A is the cross-section of the workpiece, plane B-B is the longitudinal section. The microstructure of the rod is illustrated by photographs (a) in the cross-section (TEM), (b) (SEM) and (c) (TEM) in the longitudinal section. It is evident that the grain size (1) in the cross-section of the rod averages approximately 200 nm. Figure 4 shows that the longitudinal section of the rod is dominated by grains (4), which have an elongated shape with a length-to-width ratio greater than 2.

[0022] To study the mechanical properties, samples with a diameter of 3 mm were made from the rod for tensile testing according to GOST 1497-84 (ISO 6892-84).

[0023] The table presents the results of tensile tests at room temperature for samples cut from a rod with a diameter of 6 and 5 mm of commercially pure titanium Grade 4, obtained by the proposed method, and comparative data on the values ​​from the prototype (patent no. 2383654). No. Condition of the rod after deformation Accumulated deformation Tensile strength, MPa Yield strength, MPa Relative elongation, % Relative contraction, % 1 Patent no. 2383654 (Prototype) 4 + 1.4 1330 1280 12562 Proposed invention rod Ф6 mm 1.83 1260 1200 14513 Proposed invention rod Ф5 mm 2.2 1300 1260 1255

[0024] The mechanical properties data for points No. 2 and No. 3 are presented based on the results of three tests.

[0025] The table shows that the mechanical properties of titanium produced by the proposed method are similar to those of ultrafine-grained titanium produced according to the prototype patent No. 2383654. However, machining nanorods made of technical titanium with a strength of 1300-1330 MPa has revealed a significant reduction in cutting tool life. Therefore, a strength of 1250 MPa is considered the most appropriate, minimizing blade machining costs and providing a comprehensive, efficient, and end-to-end technology for creating high-strength mini-implants.

Claims

A rod made of technically pure titanium for biomedicine, having a structure of ultrafine alpha-phase grains with a hexagonal close-packed lattice, characterized in that it has a fibrous deformation texture and a grain-subgrain type of structure, wherein the volume fraction of elongated grains in the size range of 0.1...3 μm and with a grain shape factor of more than 2 in mutually perpendicular planes is at least 80%, and the subgrains formed inside the elongated grains have low-angle boundaries misoriented relative to adjacent subgrains at angles of up to 15°. A method for producing a rod of commercially pure titanium for biomedicine, having a structure of ultrafine alpha-phase grains with a hexagonal close-packed lattice, including intensive plastic deformation of a workpiece of commercially pure titanium at a temperature not exceeding 450°C, characterized in that the deformation is carried out by the method of rotational compression at deformation rates in the range of 10 -2 … 10-4 With -1 . with a total value of true deformation e≤2.2, ensuring the formation of a fibrous deformation texture and a grain-subgrain type of structure in the rod, wherein the volume fraction of elongated grains in the size range of 0.1…3 μm and with a grain shape factor of more than 2 in mutually perpendicular planes is at least 80%, and the subgrains formed inside the elongated grains have low-angle boundaries, misoriented relative to adjacent subgrains at angles of up to 15°.

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