Method for preparing polycrystalline alloy for orthopedic implant, and polycrystalline alloy prepared by the method

Abstract

Provided is a method for preparing a polycrystalline alloy for an orthopedic implant containing titanium, zirconium, and niobium. An alloy containing 46% by weight of Ti, 46% by weight of Zr, and 8% by weight of Nb is heated in a vacuum up to a deformation temperature of 950 °C. The alloy becomes a β phase, and then the heated alloy is subjected to pressure deformation at a deformation rate of 1.0×10 -2 s -1 ~5.0×10 -4 s -1 . Thereby, a polycrystalline alloy having coarse grains with a microstructure oriented in the <001> direction parallel to the compression axis is prepared. The polycrystalline alloy contains 46% by weight of Ti, 46% by weight of Zr, and 8% by weight of Nb, has an elastic modulus of less than 50 GPa, and the microstructure of the alloy includes coarse grains of a microstructure oriented parallel to the <001> direction.

Description

Technical Field

[0001] The present invention relates to the field of biomechanics, and more specifically, to a polycrystalline alloy for orthopedic implants and a method for manufacturing the same.

Background Art

[0002] Currently, most of the materials used in the manufacture of orthopedic implants are made of stainless steel, cobalt-chromium (Co-Cr) alloys, or titanium and titanium alloys. Using titanium in the manufacture of orthopedic implants is advantageous because it has excellent properties such as high strength, low density, high corrosion resistance due to the formation of a thin passive TiO2 oxide layer, high biocompatibility, and the ability of titanium alloys to bond with bone and other tissues. Titanium is a metal formed in two crystalline (allotropic) modifications called α and β. Titanium alloys often use alloying elements such as aluminum, vanadium, tin, or chromium because they can achieve much higher strength characteristics than when using pure metal. The most commonly used titanium alloy is Ti6Al4V.

[0003] It is known that the elastic modulus of commercially available pure titanium is about 100 GPa, while the elastic modulus of conventional biomedical alloys such as stainless steel and Co-Cr alloys is about 200 GPa. Considering that the elastic modulus of cortical bone is about 10 - 30 GPa, which is one order of magnitude lower than that of most metals, stress shielding caused by the incompatible elastic modulus between the implant material and natural bone is a major problem. When a part of the bone is replaced with this orthopedic implant material, the load transfer between the bone and the implant becomes non-uniform. This phenomenon is known as "stress shielding", where the implant with a high elastic modulus receives most of the load and the natural bone is not loaded much, thus inhibiting bone formation or reconstruction. Since most of the load is transferred from the implant, the bone around the implant is not loaded, and the bone becomes significantly weaker. This phenomenon is highly undesirable because it increases the risk of implant failure or damage to surrounding tissues.

[0004] The combination of titanium, zirconium, and niobium is considered to be a suitable alloy with suitable properties. Numerous alloys containing these elements with different properties and amounts are known. Patent Document 1 of Chinese Patent No. 111676407 describes a medical composition containing titanium diboride, titanium, niobium, and zirconium. The elastic modulus of this composition is about 30 GPa. Patent Document 2 of European Patent No. 0601804 describes a medical implant made of a titanium alloy with a low elastic modulus containing titanium, niobium, and zirconium. The elastic modulus of this alloy is a value close to that of bone. In both documents, the amount of niobium in the alloy exceeds 10% by weight.

[0005] Crystal structure is one of the important properties of alloys for biomedical applications, especially orthopedic implants. The description of each crystal is generally related to a reference coordinate axis called a crystal axis or crystal axis intersection. The reference coordinate axis is usually parallel to the edge of an important crystal plane or perpendicular to the symmetry plane of the crystal. In most crystal systems, the axes are called a, b, and c. Miller symbols or Miller indices are used to represent crystal edges. The Miller symbol defines the plane of atoms in a crystal according to the intersection with the crystal axes by finding the intersection of the three basic crystal axes and a plane and symbolizing the intersection as the length of the edge of the unit cell. Crystallographically, the direction of the a-axis is the Miller symbol

[0100] , the direction of the b-axis is the Miller symbol

[0010] , and the direction of the c-axis is the Miller symbol

[0001] . In the structure, nodal lines correspond to these directions, and these directions only differ in their orientation with respect to the coordinate axes, and there is no difference in the occupancy density of the nodes.

[0006] In the Miller symbol system, several types of brackets with specific meanings are used. (hkl) - The symbol indicates a specific plane (region on the crystal). [hkl] - The symbol indicates a specific line (edge on the crystal). {hkl} - The symbol indicates a set of crystallographically equivalent planes belonging to a certain crystal shape. <hkl>The "-" symbol indicates a set of crystallographically equivalent lines.

[0007] Another important property of alloys is their arrangement in single crystals or polycrystals. A single crystal refers to a crystal (such as NaCl or diamond) in which particles are arranged so that their distribution in space repeats periodically. Due to the regular alignment of the particles, a single crystal has a regular geometric shape. A polycrystal refers to a crystal composed of a large number of small crystals, that is, coarse grains with a particle size of 10 μm to several mm. Although the particles are regularly aligned inside the coarse grains, the positions of the coarse grains are random. Most solids occur in this form.

[0008] Known alloys used in the manufacture of orthopedic implants are in the form of single crystals. However, the production of samples of single crystal orthopedic implants is very complex and time-consuming, making the production of alloys for orthopedic implants very expensive and disadvantageous for industrial applications.

[0009] Therefore, developing a new preparation method for polycrystalline alloys for orthopedic implants containing titanium, zirconium, and niobium is the correct solution to achieve a lower elastic modulus at a cost suitable for practical use. Therefore, the object of the development is not only various changes to the design solutions of the shape of the prosthesis, but also the effort to form a material with a sufficiently low elastic modulus and approach the mechanical parameters of bone, which is the object of the present invention. Therefore, it is necessary to develop a new material for orthopedic implants for biomedical applications, especially those having an elastic modulus close to that of bone and sufficient strength at the same time.

Prior Art Documents

Patent Documents

[0010]

Patent Document 1

Patent Document 2

Non-Patent Documents

[0011]

Non - Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0012] The stated object is solved by a method for preparing a polycrystalline alloy for orthopedic implants containing titanium, zirconium, and niobium according to the present invention.

Means for Solving the Problems

[0013] As the gist of the present invention, an alloy containing 46 wt% Ti, 46 wt% Zr, and 8 wt% Nb is heated in a vacuum up to a deformation temperature of 950 °C, thereby turning the alloy into the β phase. Then, the heated alloy is pressure - deformed at a (od) deformation rate of 1.0×10 -2 s -1 ~5.0×10 -4 s -1 to prepare a polycrystalline alloy having coarse grains with a microstructure oriented in the <001> direction parallel to the compression axis, thereby reducing the elastic modulus while maintaining the strength characteristics. That is, due to this microstructure, the elastic modulus decreased. At the same time, the strength characteristics of the original material were maintained. The combination of high temperature and very slow deformation reliably forms a unique polycrystalline alloy containing coarse grains of a microstructure oriented in the <001> crystal direction parallel to the compression axis. This completely unique process of thermomechanical processing of titanium alloys reliably prepares a new polycrystalline alloy having the above - described characteristics and reduces the elastic modulus by the <001> microstructure while maintaining the strength characteristics. To form the <001> orientation structure of the crystal direction parallel to the compression axis of Ti - 46Zr - 8Nb, high - temperature plastic deformation is utilized. The alloy is heated to a high temperature in the β - phase region and 11.0×10 -2 s -1 ~(do)5.0×10 -4 s -1 By applying compressive deformation at a deformation rate of , grain boundary migration is controlled. As a result, a microstructure oriented in the <001> direction parallel to the compression axis is obtained. The driving force for this process is the deformation energy.

[0014] The present invention also relates to a polycrystalline alloy for orthopedic implants prepared by the above method according to the present invention. The gist of the present invention is that the alloy contains 46% by weight of Ti, 46% by weight of Zr, and 8% by weight of Nb, which is also referred to as Ti-46Zr-8Nb. The polycrystalline alloy has an elastic modulus of 50 GPa or less, and its microstructure includes coarse grains oriented parallel to the crystal direction <001>.

[0015] The advantages of the polycrystalline alloy for orthopedic implants are, in particular, that the elastic modulus is close to that of bone, and at the same time the alloy has sufficient strength, and the cost of its preparation is suitable for actual use. The present invention will be described in more detail with reference to the following drawings.

Brief Description of the Drawings

[0016]

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Mode for Carrying Out the Invention

[0017] A polycrystalline alloy for orthopedic implants was produced by melting and solidifying at 950 °C, and then rolled at a high temperature of 950 °C to obtain a plurality of square bars with a diameter of 25 mm and a length of 300 mm. Rolling was omitted for the small ingots for uniaxial compression tests. That is, after melting and solidifying, some samples were subjected to a homogenization treatment, and other samples were not heat-treated. The samples for uniaxial compression are small samples without rolling. Also, some samples were used as castings that were not heat-treated immediately after casting. Then, samples with a diameter of 8 mm and a length of 12 mm were prepared. At a high temperature of 950 °C, at deformation rates of 1.0×10 -2 s -1 、1.0×10 -3 s -1 、and (a) 5.0×10 -4 s -1 the samples were compressed and deformed to half of their original height, specifically, from 6 mm to 12 mm of the original sample height. After this deformation, the microstructure of the polycrystalline alloy appeared in the deformed samples. Since there were no major problems with the heating rate, this heating rate was used as long as the equipment, i.e., the furnace and the press, allowed. The shorter the heating time, the lower the possibility of sample oxidation.

[0018] Optical microscope observation Figure 1 shows the optical microscope observation results of the central parts of the materials (a) during casting, (b) during rolling, and (c) during homogenization. Figure 2 shows the optical microscope observation results of the lower parts of the materials (a) during casting, (b) during rolling, and (c) during homogenization. A columnar crystal structure was observed at the lower end (cooling end) of the casting material, and an equiaxed crystal structure was observed in the central part. Also, no second phase was confirmed in this observation. In the rolling material, a second phase considered to be stress-induced martensite was confirmed, which is consistent with the report by Hisada et al. [Non-Patent Document 1]. In the homogenization material, an equiaxed crystal structure was observed, and it was confirmed that the crystal grains were coarser compared to the casting material and the rolling material. [Non-Patent Document 1] Y. hisada, E. kobayashi, and T. Sato, "Materials Transactions", 2015, Vol. 56, No. 9, p. 1553-1557

[0019] Micro-Vickers hardness measurement Figure 3 shows the hardness test results of the casting material, the rolling material, and the homogenization material. In the rolling material, due to the strain introduced by cold working, the maximum hardness was 301.1 HV, and the second phase was presumed to be stress-induced martensite. On the other hand, in the homogenization material, the strain during melting was eliminated and the crystal grains were coarsened, so the hardness reached the lowest value of 242.3 HV.

[0020] Phase identification by XRD Figure 4 shows the XRD results of the casting material, the rolling material, and the homogenization material. Based on the results of the microstructure observation, the peaks of the β phase were identified in the casting material and the homogenization material, and the peaks of the β phase and the α' phase were identified in the rolling material, and each peak was used as an index.

[0021] Electron backscatter diffraction (EBSD) analysis Figure 5 shows the IPF maps of the as-cast material and the homogenized material. No anisotropy was confirmed in the as-cast material, and there was also variation in the grain size. The average grain size was 0.37 mm. It was confirmed that the homogenized material had a non-anisotropic equiaxed crystal structure and an average grain size of 1.1 mm. This is considered to be because the recrystallized coarse grains grew after recrystallization occurred using the strain introduced by cold rolling as the driving force.

[0022] High-temperature uniaxial compression test True stress-strain curve For the as-cast material and the homogenized material, under the conditions of total strain -1.0 and temperature 1223 K, the strain rates were 1.0×10 -2 s -1 、1.0×10 -3 s -1 、and 5.0×10 -4 s -1 . Figure 6 shows the true stress-true strain diagrams of the high-temperature uniaxial compression tests with the strain rates changed to 1.0×10 -2 s -1 、1.0×10 -3 s -1 、and 5.0×10 -4 s -1 . For both the as-cast material and the homogenized material, the flow stress tended to decrease with an increase in the strain rate. An obvious yield phenomenon was confirmed in the as-cast_r material, the as-cast_m material, and the homogenized_s material. Yield during high-temperature deformation suggests dynamic recrystallization. A steady state was confirmed for all samples with a strain of -0.1 to -0.8. After an obvious steady state with a strain of -0.8 or more, the flow stress increased. This is because the sample was barrel-shaped and the sample surface came into contact with the platen and rubbed. The compression speeds were changed to 1.0×10 -2 s -1 、1.0×10 -3 s -1 、and 5.0×10 -4 s -1 . These speeds correspond to "r", "m", and "s" respectively, representing "high-speed compression", "medium compression", and "low-speed compression".

[0023] Figure 7 shows a log-log plot of both the true stress and the true strain rate at a strain of -0.5, where a state close to the steady state is obtained in this stress-strain curve. From Figure 7, it was found that there is a linear relationship and the stress exponent n obtained from the slope of the straight line is 2.65. This suggests that the deformation is dominated by the resistance movement of the solute atmosphere.

[0024] EBSD-based microstructure analysis (a) IPF map and grain boundary map Figure 8 shows the IPF map of a sample subjected to high-temperature uniaxial compression under the conditions of a total strain of -1.0, a temperature of 1223 K, and a strain rate of 1.0×10 -2 s -1 ~5.0×10 -4 s -1 . Figure 9 shows the grain boundaries in the field of view of.4.8. In Figure 8, the large-angle grain boundaries with an orientation difference of 15° or more are shown by solid black lines. In the as-cast material, the growth of crystal coarse grains with an orientation close to <001> and large bulging grain boundaries were observed at all strain rates. This suggests that the grain boundaries moved due to the bulging mechanism. In the homogenized material, small crystal coarse grains were seen around the large crystal coarse grains. Expansion was confirmed at the grain boundaries at both strain rates.

[0025] Table 1 in Figure 10 shows the ratio of large-angle grain boundaries to small-angle grain boundaries and the ratio of small-angle grain boundaries at each strain rate. In the as-cast material, the ratio of small-angle grain boundaries was minimized at a strain rate of 1.0×10 -3 s -1 . In the homogenized material, the proportion of small grain boundaries decreased significantly when the strain rate was 5.0×10 -4 s -1 .

[0026] (b)

[0001] inverse pole figure Total strain -1.0, temperature 1223 K, and strain rate 1.0×10 -2 s -1 ~5.0×10 -4 s -1 The inverse pole figure of the sample subjected to high-temperature uniaxial compression under the conditions is shown in Fig. 11. In the as-cast material, the pole density of <001> increased at all strain rates. When the strain rate was 1.0×10 -3 s -1 , the pole density exceeded 21.1 times that of random orientation. Even in the homogenized material, the pole density of <001> increased in both the homogenized _m material and the homogenized _s material. In the homogenized _s material with a low strain rate, the pole density was more than 6.3 times that of random orientation. The pole density of <001> was confirmed. When the strain rate was 1.0×10 -3 s -1 , an increase in the pole density of <111> was also confirmed.

[0027] Effect of deformation mechanism and initial microstructure In the as-cast material, <001> oriented coarse grains coarsened and the pole density increased at any strain rate. The pole density of <001> also increased in the homogenized material, but was lower than that of the as-cast material. This is considered to be due to the difference in the initial crystal grain size.

[0028] During the high-temperature uniaxial test at 1223 K, the stress exponent n was 2.65. This suggests that the deformation is dominated by the movement of dislocations that resist the solute atmosphere. In this case, the stored energy in the crystal coarse grains is determined by the Taylor factor, and the stored energy of the <001> oriented coarse grains with a small Taylor factor decreases due to dynamic recovery during deformation. Therefore, there is a difference in the stored energy between the <001> oriented coarse grains and the surrounding crystal coarse grains.

[0029] In the bulging mechanism, the grain boundaries of the crystal coarse grains with few dislocations and low stored energy protrude toward the crystal coarse grains with many dislocations, and the grain boundaries move. Therefore, when grain boundary migration occurs due to the bulging mechanism, the small-angle grain boundaries decrease. In fact, in Table 1 of Fig. 10, it was confirmed that the lower the strain rate, the lower the ratio of small-angle grain boundaries. In Fig. 8, the grain boundary swelling of the <001> oriented coarse grains suggesting grain boundary migration was confirmed at any initial structure and strain rate.

[0030] Therefore, it is considered that the {001} texture formation is formed by grain boundary migration caused by a bulging mechanism driven by the difference in stored energy. On the other hand, in Fig. 6, a decrease in yield with a peak stress was confirmed, and it is considered that dynamic recovery and dynamic recrystallization were competing with each other. As described in the previous chapter, the strength of the texture weakens due to recrystallized coarse grains. From Fig. 8, it is considered that the small crystal coarse grains existing at the grain boundaries of the small crystal coarse grains with random orientations and the large crystal coarse grains of the _m material during homogenization, which are seen in the material during casting, were generated by recrystallization. However, in the material during casting, the number of <001> oriented coarse grains increased significantly, and it is considered that dynamic recovery was dominant at 1223K.

[0031] The initial crystal grain size of the material during casting was 0.37 mm, while the grain size of the material during homogenization was as large as 1.1 mm. As described above, in high-temperature uniaxial compression at 1223K, it is considered that dynamic recovery and dynamic recrystallization compete with each other. In the {001} texture formation by high-temperature uniaxial compression, the <001> oriented coarse grains of the initial structure and the <001> oriented coarse grains formed by recrystallization are stable against slip deformation and maintain their orientation during deformation. This is because the <001> oriented coarse grains grew by the bulging mechanism. When the initial grain size is small, the length of the grain boundary per unit area becomes large. Also, when the viscous motion of dislocations that resists the solute atom atmosphere dominates the deformation, as seen in Fig. 8(d) for the whole sample, in the coarse grains, dynamic recrystallization is less likely to occur at once. Dynamic recrystallized coarse grains are formed in that region. Therefore, since the growth of <001> oriented coarse grains and dynamic recrystallization are promoted by the bulging mechanism, when the initial crystal grain size is small, the pole density of <001> increases. Therefore, it is considered that the {001} texture is more developed in the material during casting compared to the material during homogenization.

[0032] Summary In this chapter, in the material during casting and the material during homogenization of the Ti-46Zr-8Nb alloy, under the conditions of a total strain of -1.0 and a temperature of 1123K, the strain rate is 1.0×10 -2 s -1 , 1.0×10 -3 s -1 and 5.0×10 -4 s -1 As a result of the high-temperature uniaxial compression changed to s, the deformation behavior and the formed structure were evaluated, and the following findings were obtained.

[0033] (1) In the as-cast _r material, the as-cast _m material, and the homogenized _s material, a distinct yielding phenomenon was confirmed. In all samples, a steady state with a strain of 0.1 to 0.8 was confirmed. This indicates that dynamic recrystallization and dynamic recovery occurred during deformation. (2) During high-temperature uniaxial compression, the logarithmic-logarithmic plot of the strain rate and the true stress at a strain of -0.5 was in a linear relationship, and the stress exponent n obtained from the slope was 2.65. This suggests that the deformation is dominated by the movement of dislocations that resist the solute atmosphere. (3) In the material at the time of melting, grain growth of <001>-oriented coarse grains was confirmed at any strain rate.

[0001] From the inverse pole figure, it was confirmed that at a strain rate of 1.0 × 10 -3 s -1 the pole density of <001> increased by more than 21.1 times that of the random orientation. (4) In the material at the time of homogenization, it was confirmed that the strain rate decreased and the pole density of <001> increased. At a strain rate of 1.0 × 10 -3 s -1 an increase in the pole density of <111> was also confirmed. (5) In all samples, the grain boundaries had a large convex shape, and it was confirmed that the proportion of low-angle grain boundaries became smaller as the strain rate was lower. Therefore, it is considered that grain boundary migration occurs by the convex mechanism, which leads to the formation of a {001} texture. (6) In the material at the time of melting, the {001} texture was more developed compared to the material at the time of homogenization. This is considered to be because the crystal grain size is small and the initial structure is easily replaced by the growth of <001>-oriented coarse grains by the convex mechanism.

Industrial Applicability

[0034] The polycrystalline alloy for orthopedic implants according to the present invention can be used particularly for the manufacture of orthopedic implants for the femur, tibia, or fibula.< / hkl>

Claims

1. A method for preparing a polycrystalline alloy for orthopedic implants containing titanium, zirconium, and niobium, an alloy containing 46 wt% Ti, 46 wt% Zr, and 8 wt% Nb is heated in a vacuum to a deformation temperature of up to 950 °C, thereby converting the alloy to the β phase, and then the heated alloy is -2 s -1 to 5.0 × 10 -4 s -1 pressure-deformed at a deformation rate of, thereby preparing a polycrystalline alloy having coarse grains with a microstructure oriented in the <001> direction parallel to the compression axis The method is characterized by this.

2. A polycrystalline alloy for orthopedic implants prepared by the method according to Claim 1, the alloy contains 46 wt% Ti, 46 wt% Zr, and 8 wt% Nb, has an elastic modulus of less than 50 GPa, and the microstructure of the alloy includes coarse grains of a microstructure oriented parallel to the <001> direction The polycrystalline alloy is characterized by this.

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