Patent Publication Number: US-2018044763-A1

Title: Alpha-ß TITANIUM ALLOY

Description:
TECHNICAL FIELD 
     The present invention relates to an α-β titanium alloy. More particularly, the present invention relates to an α-β titanium alloy with excellent machinability. 
     BACKGROUND ART 
     A high-strength α-β titanium alloy, typified by Ti-6Al-4V, can have its strength level changed easily by a heat treatment, in addition to being lightweight and having high strength and high corrosion resistance. For this reason, this type of α-β titanium alloy has been hitherto used very often, especially in the aircraft industry. To further make use of these characteristics, in recent years, applications of the α-β titanium alloy have been increasingly expanded into the fields of consumer products, including vehicle parts, such as engine members of automobiles or motorcycles, sporting goods such as golf goods, materials for civil engineering and construction, various working tools, and spectacle frames, the development fields of deep sea and energy, and the like. 
     For example, as such an α-β titanium alloy, Patent Document 1 mentions an α-β titanium alloy extruded material with excellent fatigue strength and a manufacturing method for the α-β titanium alloy extruded material. Specifically, the α-β titanium alloy extruded material includes specified contents of C and Al, and also includes 2.0 to 10.0% in total of one or more of V, Cr, Fe, Mo, Ni, Nb, and Ta, in which an area ratio of a primary α-phase is within a certain range, a direction of a major axis of each of 80% or more of primary a grains in the primary α-phase is positioned within a specified angle range, and an average minor axis of a grains in a secondary α-phase is 0.1 μm or more. 
     As the α-β titanium alloy with enhanced forgeability, Patent Document 2 mentions an α-β titanium alloy for casting that has higher strength and more excellent castability than a Ti-6Al-4V alloy. Specifically, this α-β titanium alloy mentioned includes specified contents of Al, Fe +Cr +Ni, and C +N +0, and further a specified content of V if needed, with the balance being Ti and inevitable impurities. 
     However, the α-β titanium alloy has extremely high manufacturing cost, and in addition, especially bad machinability, which interferes with the expansion of the applications of the α-β titanium alloy. The usage range is limited in practice. In view of such circumstances, various titanium alloys with improved machinability have been recently proposed. 
     For example, Patent Document 3 mentions a titanium alloy for a connecting rod that has improved the machinability while suppressing the reduction in toughness and ductility by containing rare earth elements (REM) and Ca, S, Se, Te, Pb, and Bi as appropriate to form granular compounds. Patent Document 4 mentioned a free-cutting titanium alloy that has improved the machinability by containing a rare earth element and improved the hot workability by containing B. 
     Patent Document 5 mentions a free-cutting titanium alloy that achieves the reduction in ductility of a matrix and the refinement of inclusions to improve the free cutting properties, while suppressing the reduction in the fatigue strength and ensuring hot workability, by adding P and S, P and Ni, or P, S and Ni, or further REM in addition to these elements as free-cutting component. 
     Further, Patent Document 6 mentions an α-β titanium alloy with excellent machinability and hot working. The α-β titanium alloy includes specified contents of C and Al and 2.0 to 10% in total of one or more elements selected from the group of β-stabilizing elements consisting of respective specified contents of V, Cr, Fe, Mo, Ni, Nb, and Ta, with the balance being Ti and impurities. In the titanium alloy, an average area ratio of TiC precipitates in a microstructure is 1% or less, and an average value of the average circle equivalent diameter of the TiC precipitates is 5 μm or less. 
     PRIOR ART DOCUMENT 
     Patent Document 
     Patent Document 1: JP 2012-52219 A 
     Patent Document 2: JP 2010-7166 A 
     Patent Document 3: JP 06-99764 B 
     Patent Document 4: JP 06-53902 B 
     Patent Document 5: JP 2626344 B1 
     Patent Document 6: JP 2007-84865 B 
     Disclosure of the Invention 
     Problems to be Solved by the Invention 
     In the methods like Patent Documents 3 and 4 mentioned above, metallic inclusions are precipitated by using REM. In the method like Patent Document 5 mentioned above, P is positively contained to form a P inclusion. In the method like Patent Document 6, the size of a TiC precipitate is controlled. However, in these methods, it is considered that precipitation of these precipitates and inclusions are more likely to be affected by the temperatures and cooling rates of melting to forging steps, thus making it difficult to control the size of the precipitate or the like. Furthermore, the shape or size of a raw material tends to cause variations in the size or the like of the precipitate or inclusion. Thus, to achieve the excellent machinability by precipitating inclusions of interest, there is a problem that strict control of a manufacturing process is necessary. 
     The present invention has been made in view of the foregoing circumstance, and it is an object of the present invention to achieve an α-β titanium alloy that has high strength and excellent hot workability of the level of the α-β titanium alloy, typified by the Ti-6Al-4V, while exhibiting more excellent machinability than the Ti-6Al-4V, without the necessity for the strict control or the like of the manufacturing process. 
     Means for Solving the Problems 
     An α-β titanium alloy according to the present invention, which can solve the above-mentioned problem, is characterized by including, in percent by mass: at least one element of 0.1 to 2.0% of Cu and 0.1 to 2.0% of Ni; 2.0 to 8.5% of Al; 0.08 to 0.25% of C; and 1.0 to 7.0% in total of at least one element of 0 to 4.5% of Cr and 0 to 2.5% of Fe, with the balance being Ti and inevitable impurities. 
     The α-β titanium alloy may further include, in percent by mass: more than 0% and 10% or less in total of one or more elements selected from the group consisting of more than 0% and 5.0% or less of V; more than 0% and 5.0% or less of Mo; more than 0% and 5.0% or less of Nb; and more than 0% and 5.0% or less of Ta. 
     The α-β titanium alloy may further include, in percent by mass, more than 0% and 0.8% or less of Si. 
     Effects of the Invention 
     Accordingly, the present invention can provide the a-8 titanium alloy that has high strength and excellent hot workability, such as forgeability, of the level of an α-β titanium alloy, typified by the Ti-6Al-4V, and also exhibits more excellent machinability than the Ti-6Al-4V, making it possible to ensure satisfactory lifetime of working tools. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a photomicrograph of a titanium alloy according to the present invention. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     The inventors have intensively studied to solve the foregoing problems. As a result, it has been found that especially, a specified content of at least one of Cu and Ni is contained in a titanium alloy, thereby significantly improving the ductility of the titanium alloy at high temperatures. In particular, thin chips are formed on the titanium alloy during a cutting process due to the reduction in deformation resistance, leading to a reduced cutting resistance, i.e., improving the machinability thereof. The composition of the α-β titanium alloy according to the present invention will be described in sequence below, starting from Cu and Ni, which are the features of the present invention. 
     At least one element of Cu: 0.1 to 2.0%, and Ni: 0.1 to 2.0% 
     These elements are solid-soluted into the α-phase and the β-phase in the alloy, thereby increasing its ductility at a high temperature and improving the hot workability. Thus, especially, the cutting resistance of the titanium alloy becomes lower, and the machinability thereof is improved. These elements may be used alone or in combination. If the content of each of these elements is less than 0.1%, the effect of improving the ductility is lessened. Thus, the content of each of these elements is set at 0.1% or more. The content of each of these elements is preferably 0.3% or more, and more preferably 0.5% or more. In contrast, if the content of each of these elements exceeds 2.0% by mass, the hardness of the titanium alloy is increased, thereby making it more likely to reduce the machinability and the hot workability, such as forgeability. Thus, the content of each of these elements is set at 2.0% or less. The content of each of these elements is preferably 1.5% or less, and more preferably 1.0% or less.
     Al: 2.0 to 8.5%   

     Al is an a-stabilizing element and thus is contained in the titanium alloy to form the α-phase. If the Al content is less than 2.0%, the formation of the α-phase is lessened, failing to obtain sufficient strength. Thus, the Al content is set at 2.0% or more. The Al content is preferably 2.2% or more, and more preferably 3.0% or more. Meanwhile, if the Al content exceeds 8.5% to become excessive, the ductility of the titanium alloy is degraded. Thus, the Al content is set at 8.5% or less. The Al content is preferably 8.0% or less, more preferably 7.0% or less, and still more preferably 6.0% or less.
     C: 0.08 to 0.25%   

     C is an element that exhibits the effect of improving the strength of the titanium alloy. To exhibit such an effect, the C content needs to be 0.08% or more. The C content is preferably 0.10% or more. Meanwhile, if the C content exceeds 0.25%, coarse TiC particles not solid-soluted in the α-phase will remain, thus degrading the mechanical properties of the titanium alloy. Therefore, the C content is set at 0.25% or less. The C content is preferably 0.20% or less.
     1.0 to 7.0% in total of at least one element of Cr: 0 to 4.5% and Fe: 0 to 2.5%   

     These elements are β-stabilizing elements. These elements may be used alone or in combination. To exhibit the above-mentioned effects, the total content of these elements needs to be 1.0% or more. The total content of these elements is preferably 2.0% or more and more preferably 3.0% or more. The lower limit of the total content of these elements only needs to be 1.0% or more as mentioned above, and the lower limit of the content of each of these elements is not limited specifically. Regarding the lower limit of the content of the individual element, for example, when Cr is contained in the titanium alloy, the lower limit of Cr content can be set at 0.5% or more, and further 1.0% or more. When Fe is contained in the titanium alloy, the lower limit of Fe content can be set at 0.5% or more, and further 1.0% or more. 
     In contrast, when the total content of these elements is excessive, the ductility of the titanium alloy is degraded. Thus, the total content of these elements is set at 7.0% or less. The total content of these elements is preferably 5.0% or less, and more preferably 4.0% or less. Even when the total content of these elements is within the above-mentioned total content range, if the Fe content is excessive, the degradation in the ductility becomes significant. Thus, the Fe content should be restrained to 2.5% or less. The Fe content is preferably 2.0% or less. Meanwhile, if the Cr content is excessive, the machinability of the titanium alloy is degraded. Thus, the Cr content is set at 4.5% or less. The Cr content is preferably 4.0% or less, and more preferably 3.0% or less. 
     The α-β titanium alloy according to the present invention contains the above-mentioned components, with the balance being Ti and inevitable impurities. The inevitable impurities may include P, N, S, O, and the like. In the α-β titanium alloy according to the present invention, the P content is restrained to 0.005% or less; the N content is restrained to 0.05% or less; the S content is restrained to 0.05% or less; and the O content is restrained to 0.25% or less. The α-β titanium alloy according to the present invention may further contain the following elements. 
     More than 0% and 10% or less in total of one or more elements selected from the group consisting of V: more than 0% and 5.0% or less, Mo: more than 0% and 5.0% or less, Nb: more than 0% and 5.0% or less, and Ta: more than 0% and 5.0% or less 
     These elements are p-stabilizing elements. These elements may be used alone or in combination. To form a β-phase, the total content of these elements is preferably 2.0% or more and more preferably 3.0% or more. As long as the total content of these elements is more than 0%, the lower limit of the content of the individual element is not limited specifically. Regarding the lower limit of the content of the individual element, for example, when V is contained in the titanium alloy, the lower limit of V content can be set at 0.5% or more, and further 2.0% or more. When Mo is contained in the titanium alloy, the lower limit of Mo content can be set at 0.1% or more, and further 1.0% or more. When Nb is contained in the titanium alloy, the lower limit of Nb content can be set at 0.1% or more, and further 1.0% or more. When Ta is contained in the titanium alloy, the lower limit of Ta content can be set at 0.1% or more, and further 1.0% or more. 
     In contrast, if the total content of these elements is excessive, the ductility of the titanium alloy is degraded. Thus, the total content of these elements is preferably 10% or less and more preferably 5.0% or less. Even when the total content of these elements is within the above-mentioned range, if the content of at least one element of them is excessive, the ductility of the titanium alloy is degraded. Thus, the upper limit of the content of any of these elements is preferably 5.0% or less. The content of any of these elements is more preferably 4.0% or less.
     Si: more than 0% and 0.8% or less   

     Si acts to precipitate Ti 5 Si 3  in the titanium alloy. During cutting, stress is concentrated on the Ti 5 Si 3 , causing voids from Ti 5 Si 3  as a starting point, which makes it easy to separate chips. Consequently, the cutting resistance is supposed to be reduced. To efficiently exhibit this effect, the Si content is preferably 0.1% or more, and more preferably 0.3% or more. 
     Meanwhile, if the Si content is excessive, the strength of the titanium alloy becomes extremely high, whereby a working tool might be worn significantly or broken, which makes it difficult to cut the titanium alloy. Accordingly, the Si content is set at 0.8% or less. The Si content is more preferably 0.7% or less, and still more preferably 0.6% or less. 
     The titanium alloy according to the present invention has the microstructure at room temperature that is composed of the α-phase and the β-phase, or the α-phase, the β-phase, and a third-phase, such as Ti 2 Cu or Ti 2 Ni. When Si is contained in the titanium alloy, Ti 5 Si 3  is precipitated in the titanium alloy as mentioned above. 
     A manufacturing method for the α-β titanium alloy is not limited specifically. However, the α-β titanium alloy can be manufactured, for example, by the following method. That is, the α-β titanium alloy is manufactured by smelting titanium alloy material with the above-mentioned components, casting to produce an ingot, and then performing hot working, i.e., hot forging or hot-rolling on the ingot, followed by annealing as needed. The above-mentioned hot working involves: heating the ingot in a temperature range of a β-transformation temperature T β  to approximately (T β +250)° C., followed by rough forging or rough rolling at a processing ratio of approximately 1.2 to 4.0, which is represented by “original cross-sectional area/cross-sectional area after the hot working”; and then performing finish processing at a processing ratio of 1.7 or more in a temperature range of approximately (T β −50) to 800° C. After the above-mentioned finish processing, annealing may be performed at a temperature of 700 to 800° C. as needed. The annealing is performed, for example, for two to 24 hours. Then, an aging treatment may be performed as needed. 
     Note that the above-mentioned T β  is determined from the formula (1) below. The formula (1) below corresponds to formulas (1) to (3) mentioned in Morinaga et al., “Titanium alloy design using d electron theory”, Light metal, Vol. 42, No. 11 (1992), p. 614-621. 
         Boave= 0.326 Mdave− 1.95×10 −4   T   β +2.217   (1)
 
     In the formula (1), the respective symbols mean the following: 
         Boave=ΣXi ·( Bo ) I    (2)
 
         Mdave=ΣXi ·( Md ) I    (3)
 
     where T β  is the β-transformation temperature (K). 
     When each element is represented as an element i in the formula (2), Boave is an average value of a bond order Bo of the element i, Xi is an atomic ratio of the element i, and (Bo) i is a value of the bond order Bo of the element i. 
     When each element is represented as an element i in the formula (3), Mdave is an average value of a d-orbital energy parameter Md of the element i, Xi is an atomic ratio of the element i, and (Md)i is a value of the d-orbital energy parameter Md of the element i. 
     The bond order Bo and the d-orbital energy parameter Md of each element are mentioned in Table 1 at p.616 of the above-mentioned reference. Xi is determined from the composition. From these data, Boave and Mdave of each element including Ti are determined and substituted into the above-mentioned formula (1), thereby making it possible to calculate a T β . Note that this reference does not have data on Bo and Md of C. However, since the C content in the present invention is small, C is neglected to calculate the T a . 
     This application claims priority based on Japanese Patent Application No. 2015-064275 filed on Mar. 26, 2015, and Japanese Patent Application No. 2016-009417 filed on Jan. 21, 2016, the disclosure of which is incorporated by reference herein. 
     EXAMPLES 
     The present invention will be more specifically described below by way of Examples, but is not limited to the following Examples. It is obvious that various modifications can be made to these examples as long as they are adaptable to the above-mentioned and below-mentioned concepts and are included within the scope of the present invention. 
     First Example 
     Test materials were fabricated in the following way. The titanium alloy with each composition shown in Table 1 below was processed by button arc melting to manufacture an ingot with a size of about 40 mm in diameter×20 mm in height. In any example, the P content was restrained to 0.005% or less; the N content was restrained to 0.05% or less; the S content was restrained to 0.05% or less; and the 0 content was restrained to 0.25% or less. In Table 1, the mark “-” means that the corresponding element was not contained. The ingot was heated to 1,200° C. and subjected to the rough forging at a processing ratio of 2.4, represented by the “original cross-sectional area/cross-sectional area after the hot working”, followed by forging at a processing ratio of 4.4 at 870° C. to perform finish processing. Thereafter, annealing was performed on the forged material by holding it at 750° C. for 12 hours, thereby producing a test material. Note that as shown in Comparative Example 7 of Table 1 below, a test material in which a crack occurred by the rough forging was not subjected to the finish forging. 
     Evaluation on Forgeability 
     In this example, the hot workability was evaluated by the hot forgeability. In detail, the presence or absence of a crack in each of forging steps, namely, the rough forging and the finish forging mentioned above, was evaluated. That is, the surface of the above-mentioned test material after each forging step was visually observed. The test materials having any crack were rated as NG, while the test materials having no cracks were rated as OK. Then, the test materials rated as OK in terms of both the rough forging and the finish forging were evaluated to have excellent forgeability. 
     Evaluation on Machinability 
     The test materials having good forgeability were evaluated for the machinability as follows. That is, a test specimen with the size below was taken out of the above-mentioned test material, and a cutting test was performed on the test specimen on the cutting conditions below. The machinability was evaluated as an average cutting resistance by measuring a cutting resistance in the cutting direction with a Kessler&#39; s cutting dynamometer, Model: 9257 B, from the start to the end of cutting and then determining an average value of the cutting resistance from the start to the end of the cutting. When performing the cutting test on Ti-6Al-4V as a general α-β titanium alloy on the same conditions, an average cutting resistance was 180 N. Because of this, in the first example, the test materials having an average cutting resistance of lower than 180 N were evaluated to be superior in the machinability, while the test materials having an average cutting resistance of 180 N or higher were evaluated to be inferior in the machinability. 
     Cutting Conditions 
     
         
         Test Specimen: 10 mm in height×10 mm in width×150 mm in length 
         Tool: Carbide tip S30T (nose 0.4 mm) manufactured by Sandvik Corporation 
       
    
     End mill R390 manufactured by Sandvik Corporation (20 mm in diameter, one blade)
     Cutting speed Vc: 100 m/min   Cutting amount in the axial direction: 1.2 mm   Cutting amount in the radial direction: 1 mm   Feeding speed: 0.08 mm/blade   Cutting length: 150 mm   Cutting oil: None   

     Measurement of Tensile Strength 
     The tensile strength of the α-β titanium alloy according to the present invention was also measured for reference. In detail, the titanium alloys of Examples 1 and 3, and Comparative Example 1 were used and subjected to the tensile test on the following conditions of the shape and testing speed of the test specimen. As a result, the test materials had a strength of 948 MPa in Example 1, 1, 125 MPa in Example 3, and 948 MPa in Comparative Example 1, all of these strengths being relatively high. Specifically, the strengths of these test materials exhibited higher strength than the strength of 896 MPa of an annealed material of Ti-6Al-4V as a general α-β titanium alloy.
     Shape of Test Specimen: ASTM E8/E8M  FIG. 8  Specimen 3   Test Speed: 4.5 mm/min   

     The evaluation result of the above-mentioned forgeability and an average cutting resistance are also shown in Table 1. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                   
                 Composition (% by mass) Balance  
                   
                 Forgeability 
                 Average cutting  
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 being Ti and inevitable impurities 
                 T β   
                 Rough  
                 Finish  
                 resistance 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 Cu 
                 Ni 
                 Si 
                 Al 
                 C 
                 Cr 
                 Fe 
                 (° C.) 
                 forging 
                 forging 
                 (N) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Example 1 
                 0.5 
                 0.5 
                 — 
                 4.5 
                 0.10 
                 2.5 
                 1.2 
                 976 
                 OK 
                 OK 
                 148 
               
               
                 Example 2 
                 1.0 
                 1.0 
                 — 
                 4.5 
                 0.10 
                 2.5 
                 1.2 
                 974 
                 OK 
                 OK 
                 170 
               
               
                 Example 3 
                 2.0 
                 2.0 
                 — 
                 4.5 
                 0.10 
                 2.5 
                 1.2 
                 969 
                 OK 
                 OK 
                 167 
               
               
                 Example 4 
                 — 
                 0.5 
                 — 
                 4.5 
                 0.10 
                 — 
                 1.2 
                 1,010 
                 OK 
                 OK 
                 129 
               
               
                 Example 5 
                 0.5 
                 0.5 
                 — 
                 4.5 
                 0.10 
                 — 
                 1.2 
                 1,011 
                 OK 
                 OK 
                 148 
               
               
                 Example 6 
                 — 
                 1.0 
                 — 
                 4.5 
                 0.10 
                 — 
                 1.2 
                 1,006 
                 OK 
                 OK 
                 140 
               
               
                 Example 7 
                 1.0 
                 1.0 
                 — 
                 4.5 
                 0.10 
                 — 
                 1.2 
                 1,009 
                 OK 
                 OK 
                 146 
               
               
                 Example 8 
                 2.0 
                 2.0 
                 — 
                 4.5 
                 0.10 
                 — 
                 1.2 
                 1,004 
                 OK 
                 OK 
                 155 
               
               
                 Comparative  
                 — 
                 — 
                 — 
                 4.5 
                 0.10 
                 2.5 
                 1.2 
                 979 
                 OK 
                 OK 
                 199 
               
               
                 Example 1 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Comparative  
                 — 
                 3.0 
                 — 
                 4.5 
                 0.10 
                 2.5 
                 1.2 
                 957 
                 OK 
                 OK 
                 229 
               
               
                 Example 2 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Comparative  
                 3.0 
                 3.0 
                 — 
                 4.5 
                 0.10 
                 2.5 
                 1.2 
                 965 
                 OK 
                 OK 
                 241 
               
               
                 Example 3 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Comparative  
                 4.0 
                 — 
                 — 
                 4.5 
                 0.10 
                 2.5 
                 1.2 
                 989 
                 OK 
                 NG 
                 — 
               
               
                 Example 4 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Comparative  
                 6.0 
                 — 
                 — 
                 4.5 
                 0.10 
                 2.5 
                 1.2 
                 994 
                 OK 
                 OK 
                 234 
               
               
                 Example 5 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Comparative  
                 4.0 
                 4.0 
                 — 
                 4.5 
                 0.10 
                 2.5 
                 1.2 
                 960 
                 OK 
                 OK 
                 261 
               
               
                 Example 6 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Comparative  
                 6.0 
                 6.0 
                 — 
                 4.5 
                 0.10 
                 2.5 
                 1.2 
                 950 
                 NG 
                 — 
                 — 
               
               
                 Example 7 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                   
               
            
           
         
       
     
     Table 1 shows the following. All Examples 1 to 8 satisfied the composition specified by the present invention and were found to enable good forging and to have excellent forgeability. Furthermore, these examples were found to have a lower average cutting resistance than that of Ti-6Al-4V as a general α-β titanium alloy and also to have good machinability. 
     In contrast, all Comparative Examples 1 to 7 did not satisfy the composition specified by the present invention and thereby were consequently inferior in forgeability or machinability. In detail, in Comparative Example 1, neither Cu nor Ni was contained, resulting in a high average cutting resistance. Comparative Example 1 had the same composition as that mentioned in Patent Document 6. The comparison of the above-mentioned Examples 1 to 3 with Comparative Example 1 in which the constituent elements, other than Cu and Ni, and their contents are the same as those in Examples 1 to 3 shows that in order to surely obtain good machinability by sufficiently decreasing the average cutting resistance, it is necessary to contain a specified content of at least one of Cu and Ni, as mentioned in the present invention. 
     In Comparative Example 2, which contained Ni, the Ni content was excessive. In Comparative Example 5, which contained Cu, the Cu content was excessive. In both examples, the average cutting resistance was higher than 180 N, resulting in bad machinability. 
     In Comparative Examples 3 and 6, the respective contents of Cu and Ni were excessive. In both comparative examples, the average cutting resistance was higher than 180 N, resulting in bad machinability. 
     In Comparative Example 4, since the Cu content was excessive, the forgeabililty was degraded. In Comparative Example 7, since the respective contents of Cu and Ni were drastically excessive, cracking occurred at the stage of the rough forging, resulting in degradation in the forgeability. 
     Second Example 
     In the second example, the influence of the Si content, especially, on the machinability were studied. As shown in Table 2, various ingots with different Si contents were manufactured to produce test materials in the same way as that in the first example. In any example, the P content was restrained to 0.005% or less; the N content was restrained to 0.05% or less; the S content was restrained to 0.05% or less; and the 0 content was restrained to 0.25% or less. In Table 2, the mark “-” means that the corresponding element was not contained. 
     Each of the above-mentioned test materials was used to confirm the presence or absence of a precipitation phase, as mentioned below, and the Vickers hardness of the test material was measured as an index of strength in the second example. Furthermore, the forgeability of the test material was evaluated in the same way as that in the first example, and the machinability thereof was evaluated as mentioned below. For reference, the tensile strength of test material No. 3 in Table 2 was measured in the same way as that in the first example. This test material No. 3 had a tensile strength of 968 MPa, which was higher than a strength, i.e., 896 MPa of an annealed material of Ti-6Al-4V as the general α-β titanium alloy. 
     Evaluation on Presence or Absence of Precipitation Phase 
     The cross section of the test material was polished to a mirror-smooth state, followed by acid treatment using hydrofluoric acid to an extent that crystal grain boundaries could be seen, and then visually observed at ten field of views, each field of view having a size of 40 μm×40 μm, with a field emission-scanning electron microscope (FE-SEM) at a magnification of 4,000 times. The test materials in which the precipitation phase with a circle equivalent diameter of 2 μm or more was recognized at five or more of the above-mentioned ten field of views in total were evaluated to be in the “presence” of the precipitation phase. The test materials in which the precipitation phase was recognized at four or less of the above-mentioned ten field of views in total were evaluated to be in the “absence” of the precipitation phase. Note that the above-mentioned precipitation phase was separately recognized as Ti 5 Si 3  by an X-ray diffraction (XRD). 
       FIG. 1  shows one example of a photomicrograph observed with the above-mentioned microscope.  FIG. 1  is one obtained by measurement of the test material No. 3 shown in Table 2, with an arrow indicating one precipitation phase. 
     Measurement of Vickers Hardness HV 
     A Vickers hardness HV was measured at five sites of each test material on the condition of a load 10 kgf, and the measured values were averaged. In this way, an average value of the Vickers hardness was determined. 
     Evaluation on Machinability 
     The test materials evaluated to have good forgeability in the same way as that in the first example, that is, all examples shown in Table 2 were evaluated for the machinability as follows. That is, a test specimen with the size mentioned below was taken out of the above-mentioned test material, and a cutting test was performed on the test specimen on the cutting conditions below. The machinability was evaluated as an average cutting resistance by measuring a cutting resistance in the cutting direction by the Kessler&#39; s cutting dynamometer, Model: 9257 B, from the start to the end of cutting and then determining an average value of the cutting resistance from the start to the end of the cutting. When performing the cutting test on Ti-6Al-4V as the general α-β titanium alloy on the same conditions, an average cutting resistance was 122 N. Because of this, in the second example, the test materials having an average cutting resistance of lower than 122 N were evaluated to be superior in the machinability, while the test materials having an average cutting resistance of 122 N or higher were evaluated to be inferior in the machinability. 
     Cutting Conditions 
     
         
         Test Specimen: 10 mm in height×10 mm in width×60 mm in length 
         Tool: Carbide tip S30T (nose 0.4 mm) manufactured by Sandvik Corporation 
       
    
     End mill R390 manufactured by Sandvik Corporation (20 mm in diameter, one blade)
     Cutting speed Vc: 100 m/min   Cutting amount in the axial direction: 1.2 mm   Cutting amount in the radial direction: 1 mm   Feeding speed: 0.08 mm/blade   Cutting length: 15 mm   Cutting oil: None   

     These results are also shown in Table 2. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                   
                 Composition (% by mass) Balance being  
                   
                   
                   
                 Forgeability 
                 Average cutting  
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                 Ti and inevitable impurities 
                 T β   
                 Precipitation  
                   
                 Rough  
                 Finish  
                 resistance 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 No. 
                 Cu 
                 Ni 
                 Si 
                 Al 
                 C 
                 Cr 
                 Fe 
                 (° C.) 
                 phase 
                 HV 
                 forging 
                 forging 
                 (N) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 1 
                 0.5 
                 0.5 
                 — 
                 4.5 
                 0.10 
                 2.5 
                 1.2 
                 976 
                 Absent 
                 298 
                 OK 
                 OK 
                 111 
               
               
                 2 
                 0.5 
                 0.5 
                 0.1 
                 4.5 
                 0.10 
                 2.5 
                 1.2 
                 993 
                 Present 
                 316 
                 OK 
                 OK 
                  99 
               
               
                 3 
                 0.5 
                 0.5 
                 0.3 
                 4.5 
                 0.10 
                 2.5 
                 1.2 
                 1,027 
                 Present 
                 320 
                 OK 
                 OK 
                 105 
               
               
                 4 
                 0.5 
                 0.5 
                 0.8 
                 4.5 
                 0.10 
                 2.5 
                 1.2 
                 1,110 
                 Present 
                 335 
                 OK 
                 OK 
                 112 
               
               
                 5 
                 0.3 
                 0.3 
                 0.3 
                 4.5 
                 0.10 
                 2.5 
                 1.2 
                 1,028 
                 Present 
                 316 
                 OK 
                 OK 
                 106 
               
               
                 6 
                 2.0 
                 2.0 
                 0.5 
                 4.5 
                 0.10 
                 2.5 
                 1.2 
                 1,054 
                 Present 
                 365 
                 OK 
                 OK 
                 120 
               
               
                 7 
                 2.0 
                 2.0 
                 1.0 
                 4.5 
                 0.10 
                 2.5 
                 1.2 
                 1,137 
                 Present 
                 360 
                 OK 
                 OK 
                 134 
               
               
                 8 
                 2.0 
                 2.0 
                 2.0 
                 4.5 
                 0.10 
                 2.5 
                 1.2 
                 1,303 
                 Present 
                 397 
                 OK 
                 OK 
                 Measurement was  
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 impossible due to  
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 damage to a working  
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 tool 
               
               
                   
               
            
           
         
       
     
     Table 2 shows the following. That is, as clearly shown, the test material No. 1 having the same composition as that in Example 1 of Table 1 were compared with test materials No. 2 to 6, particularly, test materials No. 2 to 4 in which the contents of elements other than Si were the same as those in Example 1 of Table 1. Based on the comparison, the arrangement that contains Si in the titanium alloy made it possible to further reduce the average cutting resistance and to ensure the sufficiently high machinability, compared to a case in which Si was not contained. In contrast, when the Si content was excessive, like the test materials No. 7 and No. 8, the hardness of the titanium alloy becomes extremely high, increasing the average cutting resistance and also causing inconveniences, such as a damage of a working tool.