Field of the invention

The invention relates to the field of biomechanical, specifically a polycrystalline alloy for orthopaedic implants and a method of producing thereof.

Background of the Invention

Currently used materials for the production of orthopedic implants are mostly made of stainless steel, cobalt-chromium (Co-Cr) alloys or titanium and titanium alloys. The use of titanium for the production of orthopedic implants is advantageous due to the excellent combination of properties such as high strength, low density, high corrosion resistance due to the formation of a thin passive TiOs oxide layer, increased biocompatibility and the ability of titanium alloys to bond to bone and other tissues. Titanium is a metal that occurs in two crystalline (allotropic) modifications, which are termed a and p. Due to the fact that titanium alloys achieve significantly higher strength properties than in the case of the use of pure metal, there are often used alloying elements such as aluminum, vanadium, tin or chromium. The most commonly used titanium alloy is Ti6AI4V.

Elastic moduli of the conventional biomedical alloys, such as stainless steels and Co-Cr alloys are known as around 200 GPa a while that of commercial pure titanium around 100 GPa. Thinking the elastic modulus of cortical bones is around 10 to 30 GPa, thus an order of magnitude lower than for most metals, the inhibition of the bone due to stress shielding cased from elastic moduli misfit between implant material and natural bone is a great issue. When a part of the bone is replaced with this orthopedic implant material, there is an uneven load transfer between the bone and the implant. This phenomenon is known as "stress-shielding", which inhibits the bone formation or reconstruction because the implant having high elastic modulus suffers the most of load and less on the natural bone. Most of the load is transmitted by the implant, which leads to unloading of the bone around it and thus leads to a significant weakening. This phenomenon is very undesirable because there is a high risk of breaking the implant and damaging the surrounding tissue. A combination of titanium, zirconium and niobium appears to be a suitable alloy with suitable properties. A number of alloys containing these elements in different proportions and in different amounts are known. Document CN 1 11676407 describes a medical composition comprising titanium diboride, titanium, niobium and zirconium. The composition has elastic moduli around 30 GPa. Document EP 0601804 describes a medical implant from a titanium alloy with low elastic moduli containing titanium, niobium and zirconium. Elastic moduli of this alloy is close to the values of the elastic moduli of the bone. In both documents, the amount of niobium in the alloys is above 10% by weight.

One of the important properties of alloys for biomedical use, especially for orthopedic implants, is the crystallographic structure. The description of each crystal is related to the reference coordinate axes, which are generally referred to as crystallographic axes or crystallographic axis crosses. They are usually parallel to the edges of significant crystal surfaces or perpendicular to the planes of symmetry of the crystal. In most crystal systems, the axes are usually referred to as a, b, c. Miller symbols or Miller indices are used to describe the crystal edges. Miller symbols define the plane of atoms in a crystal according to its intersections with the crystallographic axes by finding the intersections of the plane with the three basic crystallographic axes and denoting them as the lengths of the edges of the unit cell. The crystallographic direction of the a-axis has the Miller symbol [100], the direction of the b-axis has the Miller symbol [010], and the direction of the c-axis has the Miller symbol [001 ]. In the structure, nodal lines correspond to these directions, which differ from each other only in their orientation with respect to the coordinate axes, but do not differ in the density of occupancy by nodal points.

Miller symbolism uses several types of parentheses that have their specific meaning: (hkl) - the symbol indicates a specific plane (area on the crystal), [hkl] - the symbol indicates a specific line (edge on the crystal),

{hkl} - the symbol indicates a set crystallographically equivalent planes belonging to one crystal shape and

<hkl> - the symbol indicates a set of crystallographically equivalent lines.

Another important property of alloys is their arrangement in a single crystal or polycrystal. A single crystal is a crystal within which the particles are arranged so that their distribution in space is repeated periodically (NaCI, diamond). The regular arrangement of the particles gives the single crystals a regular geometric shape. A polycrystal is a crystal that consists of a large number of small crystals, ie grains that have dimensions from 10 pm to several millimeters. Inside the grains, the particles are arranged regularly, but the position of the grains is random. Most solids occur in this form.

Known alloys used for the production of orthopedic implants are in monocrystalline form. However, the production of a monocrystalline orthopedic implant sample is very complicated and takes a very long time, so the production of an alloy for orthopedic implants is very expensive, which is not advantageous for industrial applicability.

Therefore, the creation of a new method of preparing a polycrystalline alloy for orthopedic implants containing titanium, zirconium and niobium is the right solution to achieve a lower elastic modulus at a reasonable cost for actual use. The subject of the development is therefore not only various modifications of the design solutions of the shape of prostheses, but also the effort to create a material with a sufficiently low elastic moduli and thus approach the mechanical parameters of the bone, which is the object of the present invention. Therefore, it is necessary to develop new materials for biomedical use, especially for orthopedic implants, which have a modulus of elasticity close to the modulus of elasticity of the bone and at the same time sufficient strength.

Summary of the invention

The stated object is solved by the method of preparing a polycrystalline alloy for orthopedic implants containing titanium, zirconium and niobium according to the present invention. The essence of the invention is that the alloy containing 46 % w. Ti, 46 % w. Zr and 8 % w. Nb warms in a vacuum to deformation temperature 950 °C, whereby the alloy enters the beta phase and subsequently the warmed alloy is pressure deformed with a deformation rate of od 1 ,0x10 ^-2 s ^-1 to 5,0x10 ^-4 s ^-1 , thereby preparing a polycrystalline alloy with grains with an oriented microstructure in a direction <001 > parallel to the compression axis thereby reducing the modulus of elasticity while maintaining strength characteristics. It means that the modulus of elasticity has been reduced due to this texture. At the same time, the strength characteristics of the original material were preserved. The combination of high temperature and very slow deformation ensures the formation of a unique polycrystalline alloy with grains with an oriented microstructure in the crystallographic direction <001 >, which is parallel to the compression axis. This completely unique course of thermomechanical processing of a titanium alloy ensures the preparation of a new polycrystalline alloy with the above-mentioned properties thus, the lower modulus of elasticity caused by the <001 > texture while maintaining the strength characteristics. The generation of the oriented structure in the crystallographic direction <001 >, which is parallel to the compression axis of the Ti-46Zr-8Nb, uses high- temperature plastic deformations. Heating of the alloy to a higher temperature of the beta phase region and application of compressive deformation with a deformation rate from 1 1 ,0x10 ² s ^-1 do 5,0x10 ^-4 s ^-1 controls the grain boundary migration. The result is an oriented microstructure in the <001 > direction parallel to the compression axis. The driving force of the process is the deformation energy.

The invention also relates to a polycrystalline alloy for orthopedic implants prepared by the above method according to the invention. The essence of the invention is that it contains 46 % w Ti, 46 % w. Zr and 8 % w. Nb also referred to as Ti-46Zr-8Nb. The polycrystalline alloy has a modulus of elasticity of up to 50 GPa and its microstructure contains grains oriented in the crystallographic direction <001 > parallel to each other.

The advantages of the polycrystalline alloy for orthopedic implants are, in particular, that the modulus of elasticity is close to the modulus of elasticity of the bone, at the same time it has sufficient strength and its preparation is at a reasonable price for actual use.

Clarification of drawings

The invention will be explained in more detail by means of the drawings which illustrate: fig. 1 shows optical micrographs of central part of (a) as-cast, (b) as-rolled (RD) and (c) as- homogenized of Ti-46Zr-8Nb alloy, fig. 2 shows optical micrographs of lower end of (a) as-cast, (b) as-rolled (RD) and (c) as- homogenized of Ti-46Zr-8Nb alloy, fig. 3 shows the hardness test results of as-cast material, as-rolled material, and as- homogenized of Ti-46Zr-8Nb alloy, fig. 4 shows the XRD results of as-cast, as-rolled, and as-homogenized of Ti-46Zr-8Nb alloy, fig. 5 shows the IPF maps of as-cast and as-homogenized of Ti-46Zr-8Nb alloy, fig. 6 shows the true stress-true strain diagram of the high-temperature uniaxial compression test changed to 1.0 x 10 ^-2 s ^-1 , 1.0 x 1 ^{0 3} s ^-1 and 5.0 x 10 ^-4 s ^-1 , fig. 7 shows true stress vs. true strain rate relationship of Ti-46Zr-8Nb alloy compressed at 1223 K, fig. 8 shows the IPF map for RD mid plaine of (a) as-cast_r, (b) as-cast_m, (c) as-cast_m (d) as-homogenized_m (e) as-homogenized_s of Ti-46Zr-8Nb alloy, fig. 9 shows grain boundary map for RD mid plaine of (a) as-cast_r, (b) as-cast_m, (c) as- cast_m (d) as-homogenized_m (e) as-homogenized_s of Ti-46Zr-8Nb alloy, fig. 10 shows table 1 , fig. 11 shows IPF map for RD mid plaine of (a) as-cast_r, (b) as-cast_m, (c) as-cast_m (d) as- homogenized_m (e) as-homogenized_s of Ti-46Zr-8Nb alloy.

Examples of invention embodiment

The polycrystalline alloy for orthopedic implants was produced by melting at 950 °C and solidification, followed by rolling at a high temperature of 950 °C to obtain square bars with a diameter of 25 mm and a length of 300 mm. Rolling was omitted for small ingots for the uniaxial pressure test. This means that after melting and solidification, the samples were processed by homogenization, others without heat treatment. Samples for uniaxial compression are small samples without rolling. And some samples were used as castings, just after casting without heat treatment. Subsequently, samples with a diameter of 8 mm and a length of 12 mm were produced. Compression deformation was applied at a high temperature 950 °C at deformation rates of 1 ,0x10 ^-2 s ^-1 , 1 ,0x10 ^-3 s ^-1 a 5,0x10 ^-4 s ^-1 up to half the height of the sample, specifically to a height of 6 mm from 12 mm of the original sample height. After this deformation, the texture of the polycrystalline alloy appeared in the deformed samples. Since there was no significant problem with the heating rate, it was carried out as fast as the equipment, i.e. the furnace and the pressing machine, allowed. Shorter heating times reduce the likelihood of sample oxidation.

Optical microscopy

Fig. 1 show the results of optical microscope observations of the central parts of as-cast material (a), as-rolled material (b), and as-homogenized material (c). Fig. 2 show the results of optical microscope observations of the lower parts of as-cast material (a), as-rolled material (b), and as-homogenized material (c). A columnar crystal structure was observed at the lower end (cooling end) of the as-cast material, and an equiaxed crystal structure was observed at the center. In addition, the second phase was not confirmed in this observation. In the as-rolled material, the second phase, which is considered to be stress-induced martensite, was confirmed, which is consistent with the report by Hisada et al. [1 ]. In the as-homogenized material, an equiaxed crystal structure was observed, and coarsening of crystal grains was confirmed as compared with the as-cast material and the as-rolled material. [1 ] Y. Hisata, E. Kobayashil , T. Sato, Materials Transactions, Vol.56, No.9 (2015)1553-1557

Micro-Vickers hardness measurement

Fig. 3 shows the hardness test results of as-cast material, as-rolled material, and as- homogenized material. In the as-rolled material, the maximum hardness was 301 .1 HV due to the strain introduced by cold working and the second phase presumed to be stress-induced martensite. On the other hand, in the as-homogenized material, the strain during melting was eliminated and the crystal grains became coarse, so the hardness reached a minimum value of 242.3 HV.

Phase identification by XRD

Fig. 4 shows the XRD results of as-cast, as-rolled, and as-homogenized materials. Based on the results of histological observation, the peaks of p phase were identified for as-cast and as- homogenized materials, and the peaks of phase and a'phase were identified for as-rolled materials, and each peak was indexed.

EBSD analysis

Fig. 5 shows the IPF maps of as-cast and as-homogenized materials. Anisotropy was not confirmed in the as-cast material, and the particle size varied. The average crystal grain size was 0.37 mm. It was confirmed that the as-homogenized material had a non-anisotropic equiaxed crystal structure, and the average crystal grain size was 1.1 mm. It is considered that this is because the recrystallized grains grew after recrystallization occurred by using the strain introduced by cold rolling as a driving force.

High temperature uni-axial compression test

True stress-strain curves

For as-cast and as-homogenized materials, the strain rate was 1.0 x 10 ^-2 s ^-1 , 1.0 x 1 ^{0 3} s ^-1 and 5.0 x 10 ^-4 s ^-1 under the conditions of total strain -1 .0 and temperature 1223 K. Fig. 6 shows the true stress-true strain diagram of the high-temperature uniaxial compression test changed to 1.0 x 10 ² s ^-1 , 1.0 x 1 ^{0 3} s' ¹ and 5.0 x 10 ^-4 s’ ¹ . For both as-cast and as-homogenized materials, the deformation stress tended to decrease as the strain rate increased. A clear yield phenomenon was confirmed in the as-cast_r material, as-cast_m material, and as- homogenized_s material. Yield at high temperature deformation suggests dynamic recrystallization. Steady state was confirmed in all samples with strains between -0.1 and -0.8. Deformation stress increased after an apparent steady state with a strain of -0.8 or higher. This is because the sample was barrel-shaped and the surface of the sample and the anvil contacted and rubbed. Speed of compression was varied, 1.0x10-2 s-1 , 1.0x10-3 s-1 and 5.0x10-4 s-1. They correspond “r”, “m” and “s” respectively, stands for “Rapid compression”, “Medium compression” and “Slow compression”.

Fig. 7 shows a log-log plot of both true stress and strain rate at strain-0.5, where a state close to a steady state is obtained on the stress-strain curve. From Fig. 7, it was found that there was a linear relationship, and the stress index n obtained from the gradient of the straight line was 2.65. This suggests that the deformation is dominated by the drag motion of the solute atmosphere.

Texture analysis by EBSD

(a) IPF map and grain boundary map

Fig. 8 shows the IPF map of the sample subjected to high temperature uniaxial compression under the conditions of total strain -1.0, temperature 1223 K, strain rate 1.0 x 10 ^-2 s ^-1 to 5.0 x 10 ^-4 s ^-1 . Fig. 9 shows the grain boundaries in the field of view of .4.8. In Fig. 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 grains with an orientation near <001 > and the large bulging grain boundaries were observed at all strain rates. This suggests the grain boundary movement due to the bulging mechanism. In the as-homogenized material, small crystal grains were formed around the large crystal grains. At both strain rates, swelling was confirmed at the grain boundaries.

Table 1 in fig. 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 grain boundaries was the minimum at a strain rate of 1.0 x 10' ³ s ^-1 . In the as-homogenized material, the proportion of small grain boundaries decreased significantly at a strain rate of 5.0 x 10' ⁴ s ^-1 . (b) [001] inverse pole figures

Reverse pole figure of a sample subjected to high temperature uniaxial compression under the conditions of total strain -1.0, temperature 1223 K, strain rate 1.0 x 10 ^-2 s ^-1 to 5.0 x 10 ^-4 s ^-1 shown in Fig. 1 1 . In the as-cast material, the extreme density of <001 > increased at all strain rates. At a strain rate of 1.0 x 10 ^-3 s’ ¹ , the polar density was more than 21 .1 times the random orientation. Even in the as-homogenized material, the extreme density of <001 > increased in both as-homogenized_m material and as-homogenized_s material, and in the as- homogenized_s material with a small strain rate, it was 6.3 times or more compared to the random orientation. The extreme density of <001 > was confirmed. At a strain rate of 1.0 x 1O ^{_ 3} s’ ¹ , an increase in the extreme density of <11 1 > was also confirmed.

Effect of deformation mechanism and primary microstructures

In the as-cast material, the <001 > oriented grains became coarse and the extreme density increased at any strain rate. The extreme density of <001 > also increased in the as- homogenized material, but it was smaller than that of the as-cast material. The reason for this is thought to be the difference in the initial crystal grain size.

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

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

Therefore, it is considered that {001} texture formation was formed by the grain boundary movement by the bulging mechanism driven by the difference in stored energy. On the other hand, in Fig. 6, a decrease in yield accompanied by peak stress was confirmed, and it is considered that dynamic recrystallization also competed with each other at the same time as dynamic recovery. As mentioned in the previous chapter, recrystallized grains weaken the strength of the texture. From Fig. 8, it is considered that the small crystal grains with random orientation seen in the as-cast material and the small crystal grains existing at the grain boundaries of the large crystal grains of the as-homogenized_m material were generated by recrystallization. However, in the as-cast material, the number of <001 > oriented grains increased significantly, and it is considered that the dynamic recovery was predominant at 1223 K.

The initial crystal grain size of the as-cast material was 0.37 mm, while that of the as- homogenized material was as large as 1.1 mm. As mentioned earlier, in high temperature uniaxial compression at 1223 K, dynamic recovery and dynamic recrystallization are considered to compete. In the {001} texture formation by high-temperature uniaxial compression, the <001 > oriented grains of the initial structure and the <001 > oriented grains generated by recrystallization are stable against slip deformation and maintain the orientation during deformation. This is due to the fact that <001 > oriented grains grow due to the bulging mechanism. When the initial grain size is small, the length of grain boundaries per unit area increases. In addition, when the viscous motion of dislocations that drag the solute atom atmosphere dominates the deformation, it becomes difficult for dynamic recrystallization to occur all at once in the entire sample, and as can be seen in Fig. 8 (d), the grains. Dynamic recrystallized grains are formed in the field. Therefore, a small initial crystal grain size increases the extreme density of <001 > because it promotes the growth and dynamic recrystallization of <001 > oriented grains by the bulging mechanism. Therefore, it is considered that the {001 } texture was developed in the as-cast material as compared with the as- homogenized material.

Summary

In this chapter, the strain rate is 1.0 x 10 ^-2 s’ ¹ , 1.0 x 10 under the conditions of total strain of - 1 .0 and temperature of 1123 K for as-cast material and as-homogenized material of Ti-46Zr- 8Nb alloy. As a result of high-temperature uniaxial compression changed to -3 s ^-1 and 5.0 x 10 ^-4 s’ ¹ , the deformation behavior and the formed structure were evaluated, and the following findings were obtained. (1 ) A clear yield phenomenon was confirmed in the as-cast_r material, as-cast_m material, and as-homogenized_s material. In all samples, a steady state was confirmed with a strain of 0.1 to 0.8. This indicates that dynamic recrystallization and dynamic recovery occurred during the deformation.

(2) During high-temperature uniaxial compression, the log-log plots of strain rate and true stress at strain -0.5 had a linear relationship, and the stress index n obtained from the gradient was 2.65. This suggests that the deformation is dominated by the motion of dislocations that drag the solute atmosphere.

(3) In the as-melt material, grain growth of <001 > oriented grains were confirmed at any strain rate. [001] From the reverse pole figure, it was confirmed that the polar density of <001 > increased by a maximum of 21.1 times or more at the strain rate of 1.0 x 10-3 s-1 compared to the random orientation.

(4) In the as-homogenized material, it was confirmed that the strain rate decreased and the extreme density of <001 > increased. At a strain rate of 1.0 x 10 ^-3 s’ ¹ , an increase in the extreme density of <1 11 > was also confirmed.

(5) In all the samples, the grain boundaries had a large bulging shape, and it was confirmed that the lower the strain rate, the smaller the proportion of low-angle grain boundaries. Therefore, it is considered that the grain boundary movement was caused by the bulging mechanism, which led to the formation of {001 } texture.

(6) Compared with the as-homogenized material, the {001 } texture was more developed in the as-melt material. It is considered that this is because the crystal grain size is small and the initial structure is easily replaced by the growth of <001 > oriented grains by the bulging mechanism.

Industrial applicability

Polycrystalline alloy for orthopaedic implants according to this invention can be used especially for the production of orthopedic implants of the femur, tibia or fibula.