Patent Publication Number: US-2011070121-A1

Title: Beta-based titanium alloy with low elastic modulus

Description:
TECHNICAL FIELD 
     The present disclosure relates to a titanium alloy with a low elastic modulus, including no elements harmful to the human body, and more particularly, to a beta-based titanium alloy with a low elastic modulus, including titanium (Ti), niobium (Nb) and zirconium (Zr), and further including tantalum (Ta), hafnium (Hf), molybdenum (Mo), tin (Sn), and the like. 
     BACKGROUND ART 
     Titanium is widely used in the fields of aerospace, weaponry, nuclear power, sports and leisure, biomedicine and the like due to its high specific strength (strength/weight), high corrosion resistance, excellent mechanical properties including high temperature properties, and excellent biocompatibility. 
     Biomedical metals have been developed for use in implants for replacing bones, joints, teeth, and the like. The biomedical metals are used for manufacturing a variety of prostheses such as artificial bones, artificial joints, and dental prostheses. Accordingly, biomedical metals should be excellent in biocompatibility as well as mechanical properties, corrosion resistance, and chemical resistance. That is, biomedical metals should be non-toxic and not induce allergies in the human body. 
     Titanium and titanium alloys have been used as biomaterial for replacing stainless steel. In the beginning, pure titanium and titanium alloy such as Ti-6Al-4V were used as biomaterial. 
     However, since it came to light that aluminum can cause Alzheimer&#39;s disease and vanadium is cytotoxic in the human body, unceasing efforts have been made to develop a new biocompatible alloy based on titanium. 
     Widely known examples of biocompatible titanium alloys that have been developed to solve the problem of cytotoxicity are Ti-6Al-7Nb and Ti-5Al-2.5Fe, which are second-generation titanium alloys. 
     Since the 1990s, the problem of “stress shield effect” was newly raised. The stress shield effect is caused by elastic modulus difference between natural bone with a low elastic modulus and biocompatible material with a high elastic modulus. 
     For example, a metal implant with a high elastic modulus bears most of the load applied to the region around the implant, and the natural bone in the region does not bear any tension, compression and bending for a long time. As a result, the thickness and the weight of the natural bone are reduced gradually, causing serious problems such as osteoporosis around the implant. This phenomenon is referred to as the “stress shield effect.” As the natural bone weakens and the density of the osseous tissue of the cortex decreases, the bonding strength between the natural bone and the artificial implant also decreases, resulting in decreased service life of the implant. 
     As a result, a demand arose for a biomedical metal that satisfies bio-mechanical suitability requirements as well as bio-chemical suitability requirements including cytotoxicity. That is, a demand emerged for a metal that is not harmful to the human body and has an elastic modulus as low as bones of the human body. 
     Ti-13Nb-13Zr (ASTM F1713), Ti-12Mo-6Zr-2Fe (ASTM F1813), Ti-15Mo (ASTM F2066), and the like have been developed throughout the world to solve the above mentioned problems. In addition, a variety of alloys such as Ti-35Nb-5Ta-7Zr and Ti-16Nb-13Ta-4Mo in a similar composition range are being developed. 
     However, the titanium alloys hitherto developed have an elastic modulus of approximately 60 GPa to approximately 80 GPa, which is still much higher than the elastic modulus of natural bones that range from approximately 10 GPa to approximately 30 GPa. Accordingly, the problem of “stress shield effect” has not been completely solved yet. Therefore, there is a considerable demand for a material that is not harmful to the human body and, at the same time, has a lower elastic modulus. 
     Considering that an alloying element of a high melting point may make the alloying process more difficult and increase the manufacturing cost, the ease of the fabrication process should be taken into account in the development of a titanium alloy. 
     DISCLOSURE 
     Technical Problem 
     The present disclosure provides a titanium alloy composition that is not harmful to the human body, has an elastic modulus as low as bones of the human body, and at the same time, is melted and cast easily and cost-effectively. 
     Technical Solution 
     Noting that a beta phase generally has a low elastic modulus, the inventors selected alloying elements of titanium alloy on the basis of whether they can serve as a beta stabilizer in titanium alloy to lower the elastic modulus of titanium alloy. 
     Also, the inventors selected the alloying elements of titanium alloy on the basis of whether they are harmless to the human body in terms of biochemical suitability, and whether the density, melting temperature and boiling temperature thereof are economically suitable when compared to titanium. Resultantly, as beta stabilizers satisfying the above requirements, niobium (Nb) and zirconium (Zr) were selected. 
     Then, the inventors designed a titanium alloy composition having a low elastic modulus using a semi-experimental method for designing and developing an alloy. The method includes calculating the covalent bond order and the energy level of the electrons according to the content of each alloying element, using the electronic state, which is the core of the discrete variational (DV)-Xa molecular orbital method. 
     Most properties of a material are determined by the electronic state of the material except when a nuclear reaction is involved. Based on the electronic state determining micro-properties of the material on an atomic scale, we can estimate the macro-properties of the material by performing statistical-mechanical analysis. Here, the micro-properties of the material can be analyzed approximately from the electronic state of the material by interpreting the Schrodinger equation and the like. 
     The inventors calculated the bonding order, B o  and the energy level of the electrons, M d  of the above-described alloying elements through the DV-Xa molecular orbital method, and discovered a beta-based titanium alloy composition with a low elastic modulus from there-among. 
     In accordance with an exemplary embodiment, the titanium alloy with a low elastic modulus includes from 37 wt. % to 41 wt. % niobium (Nb), from 5 wt. % to 8 wt. % zirconium (Zr), and a balance of titanium, with unavoidable impurities. The titanium alloy has an elastic modulus of 55 GPa or lower. 
     As such, it is possible to realize a low elastic modulus of 55 GPa or lower, which is difficult to realize in a related art Ti—Nb—Zr ternary alloy and a related art quaternary alloy further including another element such as Ta. 
     Niobium (Nb), which is a major alloying element in the titanium alloy in accordance with the exemplary embodiment, is a soft, grey, ductile metal. Niobium is known as a biocompatible metal because it is stable and does not undergo toxic reactions with fiber cells, corrosion products, and bio-solutions in the human body. In addition, niobium is very stable at room temperature, and has very high corrosion resistance so that it is not corroded by oxygen and strong acids. It is preferable that niobium is contained in the titanium alloy in a weight percentage ranging from 37 wt. % to 41 wt. %. This is because the beta phase is difficult to form sufficiently outside this composition range, and thus the elastic modulus increases considerably to 70 GPa or higher. It is more preferable that niobium is contained in the titanium alloy in a weight percentage ranging from 38 wt. % to 40 wt. %. 
     Zirconium (Zr) has very high corrosion resistance in hot water under acidic or basic atmosphere. Zirconium forms oxide film even in air, showing high corrosion resistance. Zirconium is a biocompatible metal without the cytotoxic effect. It is preferable that zirconium is contained in the titanium alloy in a range from 5 wt. % to 8 wt. %. This is because the elastic modulus of the ternary alloy of titanium, niobium and zirconium increases considerably outside this range, so that it cannot be applied to a living body. It is more preferable that zirconium is contained in the titanium alloy in a range from 5 wt. % to 7 wt. %. 
     According to an exemplary embodiment, it is possible to lower the elastic modulus of the titanium alloy to 50 GPa or lower, as well as to 55 GPa or lower. 
     According to use, one or more elements selected from tantalum (Ta), hafnium (Hf), molybdenum (Mo), and tin (Sn) may be further added in the titanium alloy in a range of 3 wt. % or lower. It is preferable that they are added in a range from 1 wt. % to 3 wt. % in view of the elastic modulus factor. In this case, it is preferable that the content of niobium is from 37 wt. % to 39 wt. %, and the content of zirconium is from 5 wt. % to 7 wt. %. 
     Tantalum (Ta) is ductile, and has high mechanical strength even at high temperature. Tantalum forms a stable film with high electric resistance so that it is relatively free from oxidation in air. In addition, tantalum is highly resistant to acid, and has excellent compatibility with the human body, so that it can be used for cementing bones. Tantalum, when alloyed in titanium, serves as a major beta stabilizer. 
     Hafnium (Hf) has characteristics very similar to zirconium, and has excellent corrosion-resistance and bio-compatibility. It serves as a beta stabilizer when alloyed in titanium. 
     Molybdenum (Mo) has a relatively high melting point. However, it has excellent thermal conductivity, high corrosion resistance even in strong acid, and very favorable mechanical properties over a wide temperature range. It serves as a beta stabilizer when alloyed in titanium. 
     Tin (Sn) is stable in an air and has excellent ductility. It is soluble in acids and alkalis, and has a very low melting temperature of about 232° C. It is stable in the human body and thus widely used in the fields of table ware, plating and the like. It may also serve as a beta stabilizer when alloyed in titanium. 
     Addition of the above elements in an amount greater than 3 wt. % may affect the titanium-niobium-zirconium ternary system to increase the elastic modulus. Accordingly, the maximum content of the above-mentioned elements in the titanium alloy is set to 3 wt. % or lower. 
     The titanium alloy in accordance with the exemplary embodiments can be fabricated by various melting or casting methods such as vacuum induction melting (VIM), vacuum arc remelting (VAR), induction skull melting (ISM), plasma arc melting (PAM), electron beam melting (EBM) and the like. 
     ADVANTAGEOUS EFFECTS 
     The beta-based titanium alloy in accordance with the exemplary embodiments of the present invention has low elastic modulus and excellent mechanical properties. Therefore, it can be used in a variety of applications, for example, as a material for medical devices, such as artificial bones, artificial teeth and artificial hip joints, as a material for general civilian goods such as eyewear frames and headsets, and as a material for sports and leisure goods such as golf clubs. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a photograph of an ingot prepared by melting and casting a titanium alloy in accordance with an exemplary embodiment. 
         FIG. 2  is a photograph of a cylinder-shaped product prepared by drawing the ingot of  FIG. 1 . 
         FIGS. 3 and 4  are micrographs, each showing a microstructure of a titanium alloy in accordance with Embodiment 1. 
         FIG. 5  is a micrograph showing a microstructure of a titanium alloy in accordance with Embodiment 2. 
     
    
    
     BEST MODE 
     Hereinafter, specific embodiments will be described in detail with reference to the accompanying drawings. However, it should be understood that the description of the embodiment is merely illustrative and should not be taken in a limiting sense. 
     Embodiment 1 
     Ti—Nb—Zr ternary alloys having compositions as listed in Table 1 were prepared by a vacuum arc remelting (VAR) process. 
     In order for uniform alloy composition, process convenience, process economy, time and energy savings and the like, a Ti—Nb master alloy was used to cast beta-based titanium alloys. 
     The titanium alloys melted by the VAR process in accordance with the embodiment were cast into ingots as shown in  FIG. 1 . Then, the ingots were processed into bars having a diameter of 15 mm as shown in  FIG. 2 , through a drawing process. 
     The ingot had an excellent appearance. Surface crack, fracture and the like that are often generated during the drawing process were not observed in the surface of the bar. Accordingly, it can be concluded that the titanium alloys in accordance with the embodiment have good formability and good workability. 
     The alloy bar fabricated in accordance with the embodiment was cut into a section perpendicular to the drawing direction and a section parallel to the drawing direction. The cut surface was first macro-polished with abrasive papers of up to 2400 grit and then micro-polished with a diamond paste. 
     After the mechanical polishing, the cut surface was etched with Kroll etchant (H 2 O 100 ml+HNO 3  5 ml+HF 3 ml) and then the microstructure of the cut surface was observed using an optical microscope. 
       FIG. 3  is a micrograph (at 200× magnification) of a surface of the specimen No. 4 (Table 1) cut perpendicular to the drawing direction.  FIG. 4  is a micrograph (at 200× magnification) of a surface of the specimen No. 4 (Table 1) cut parallel to the drawing direction. Referring to  FIGS. 3 and 4 , the beta-based titanium alloy fabricated in accordance with the embodiment had uniform grain size, and showed no segregations and no defects. 
     Four test specimens were taken from the titanium alloy in accordance with the embodiment. Then, an elastic compression test was performed according to ASTM E9-89a specifications. The average elastic moduli of the specimens obtained from the elastic compression test are given in Table 1. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Composition of alloy 
                 Elastic modulus 
                   
               
               
                 Specimen No. 
                 (wt. %) 
                 (GPa) 
                 Remarks 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 1 
                 Ti—34Nb—11Zr 
                 68 
                 Comparative 
               
               
                 2 
                 Ti—35Nb—8.2Zr 
                 72 
                 Comparative 
               
               
                 3 
                 Ti—37.9Nb—7.4Zr 
                 41.5 
                 Experimental 
               
               
                 4 
                 Ti—38.9Nb—5.5Zr 
                 38.9 
                 Experimental 
               
               
                 5 
                 Ti—39Nb—6Zr 
                 40 
                 Experimental 
               
               
                 6 
                 Ti—40.9Nb—5Zr 
                 40 
                 Experimental 
               
               
                 7 
                 Ti—42.4Nb—5.5Zr 
                 74 
                 Comparative 
               
               
                 8 
                 Ti—43Nb—12Zr 
                 81 
                 Comparative 
               
               
                   
               
            
           
         
       
     
     As can be seen from the measured elastic modulus data in Table 1, contents of niobium and zirconium in the comparative examples were similar to those in the experimental examples. However, the elastic moduli of the comparative examples were 80% to 100% greater than those of the experimental examples. 
     That is, by minimizing the addition of the alloying elements through a new alloy design for restricting the amount of the alloying elements, the ternary titanium alloy in accordance with the embodiment can achieve the ultra-low elastic modulus, which has been difficult to achieve even in related art quaternary titanium alloys. 
     MODE FOR INVENTION 
     Contrary to Embodiment 1, a titanium alloy in accordance with Embodiment 2 further includes tantalum (Ta) as shown in Table 2, so as to improve mechanical properties while still maintaining the low elastic modulus and including no elements harmful to the human body. The titanium alloys were melted by the vacuum arc remelting (VAR) process, cast into ingots, and then drawn into bars, as described in Embodiment 1. 
     Specimens were cut from the alloy bars and polished mechanically. After etching the specimen, the microstructure was observed at a magnification of 50× using an optical microscope. As shown in  FIG. 5 , there were no segregations and no defects visible in the microstructure of the alloy. 
     Further, the elastic compression test was performed four times according to ASTM E9-89a specifications to measure the elastic modulus of the alloy. The average elastic moduli of the specimens obtained from the elastic compression test are given in Table 2. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 Specimen 
                 Composition of alloy 
                 Elastic 
                   
               
               
                 No. 
                 (wt. %) 
                 modulus (GPa) 
                 Remarks 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 9 
                 Ti—37.3Nb—5.8Zr—2.9Ta 
                 43 
                 Experimental 
               
               
                 10 
                 Ti—39Nb—6.5Zr—1.5Ta 
                 39 
                 Experimental 
               
               
                   
               
            
           
         
       
     
     As shown in Table 2, the titanium alloys in accordance with Embodiment 2 were not increased in the elastic modulus in comparison with the titanium alloys in accordance with Embodiment 1. Accordingly, the titanium alloy in accordance with Embodiment 2 can be used to achieve the required mechanical properties as well as the elastic modulus.