Abstract:
A method of making an article of a titanium alloy is disclosed, which includes i) quenching a work piece, which is made of a titanium alloy composition containing a) about 0.01-5 wt % Bi based on the weight of the composition; b) at least one alloy element selected from the group consisting of Mo, Nb, Ta, Zr and Hf; and c) the balance Ti, having a temperature higher than a beta transition temperature of the titanium alloy composition to a temperature lower than 500° C. at a cooling rate greater than 10° C./second between the beta transition temperature and 500° C., so that the quenched work piece contains beta phase as a major phase; and ii) plastically deforming the quenched work piece or polishing the quenched work piece so that a surface of the polished work piece has an average surface roughness less than about 0.1 μm.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a method of making a titanium alloy article, and more particularly to a method of making a titanium alloy article by plastically deforming at room temperature and/or polishing the surface thereof to has an average surface roughness less than about 0.1 μm. The titanium alloy article can be a dental casting or a medical implant.  
       BACKGROUND OF THE INVENTION  
       [0002]     Due to its lightweight, high strength-to-weight ratio, low elastic modulus, superior chemical corrosion resistance, and excellent mechanical properties at high temperature up to 550° C., titanium and its alloys have been widely used on aerospace, chemical, sports, and marine industries. Their superior biocompatibility also makes them ideal as the primary materials used in dental and osteological restorations or implants, such as artificial bone pins, bone plates, shoulders, elbows, hips, knees and other joints, and dental orthopraxy lines.  
         [0003]     A number of methods for fabricating titanium and its alloys with a desired shape have been developed. However, the titanium alloy, for example Ti-6Al-4V, is very difficult to be cold worked, i.e. the cold-worked titanium alloy has poorer mechanical properties or the cold-worked titanium alloy cracks after cold working.  
         [0004]     Precision casting has the advantage that the cast produced has a near net shape, which greatly decreases the titanium fabrication cost. Also, precision casting is particularly suitable for producing objects with a small volume, high size accuracy, and complicated shape, for example in dental and osteological fields. Titanium is inherently difficult to cast due to its high melting point and high reactivity. Its low density is another problem in casting.  
         [0005]     U.S. Pat. No. 6,572,815B1 discloses a technique to improve the castability of pure titanium by doping an alloying metal in an amount of 0.01 to 3 wt %, preferably 0.5 to 3 wt %, and more preferably about 1 wt %. Among various alloying metals used in this application bismuth is found the most promising element.  
         [0006]     US patent publication No. 2004-0136859 A1 discloses a technique to improve the castability of titanium alloys by doping an alloying metal in an amount of 0.01 to 3 wt %, preferably 0.5 to 3 wt %, and more preferably about 1 wt % of bismuth, the disclosure of which is incorporated herein by reference.  
         [0007]     U.S. Pat. No. 4,810,465 discloses a free-cutting Ti alloy. The basic alloy composition of this free-cutting Ti alloy essentially consists of at least one of S: 0.001-10%, Se: 0.001-10% and Te: 0.001-10%; REM: 0.01-10%; and one or both of Ca: 0.001-10% and B: 0.005-5%; and the balance substantially Ti. The Ti alloy includes one or more of Ti—S (Se, Te) compounds, Ca—S (Se, Te) compounds, REM-S (Se, Te) compounds and their complex compounds as inclusions to improve machinability. Some optional elements can be added to above basic composition. Also disclosed are methods of producing the above free-cutting Ti alloy and a specific Ti alloy which is a particularly suitable material for connecting rods. Bismuth up to 10% was suggested in this free-cutting Ti alloy. However, there is no teaching as to the improvement of castability or reducing surface tension of pure titanium or a titanium alloy.  
         [0008]     U.S. Pat. No. 5,176,762 discloses an age hardenable beta titanium alloy having exceptional high temperature strength properties in combination with an essential lack of combustibility. In its basic form the alloy contains chromium, vanadium and titanium the nominal composition of the basic alloy being defined by three points on the ternary titanium-vanadium-chromium phase diagram: Ti-22V-13Cr, Ti-22V-36Cr, and Ti-40V-13% Cr. The alloys of the invention are comprised of the beta phase under all the temperature conditions, have strengths much in excess of the prior art high strength alloys in combination with excellent creep properties, and are nonburning under conditions encountered in gas turbine engine compressor sections. Bismuth up to 1.5% was suggested in this age hardenable beta titanium alloy. However, there is no teaching as to the improvement of castability or reducing surface tension of pure titanium or a titanium alloy.  
       SUMMARY OF THE INVENTION  
       [0009]     A primary object of the present invention is to provide a technique capable of making a titanium alloy article by cold working.  
         [0010]     Another object of the present invention is to provide a technique capable of making a titanium alloy article by casting a titanium alloy composition with an improved castability, and cold working.  
         [0011]     Another object of the present invention is to provide a technique capable of making a titanium alloy article having an enhanced fatigue life.  
         [0012]     The present invention includes (but not limited to) the following preferred embodiments:  
         [0013]     1. A method of making an article of a titanium alloy comprising  
         [0014]     i) quenching a work piece, which is made of a titanium alloy composition comprising a) about 0.01-5 wt % Bi based on the weight of the composition; b) at least one alloy element selected from the group consisting of Mo, Nb, Ta, Zr and Hf; and c) the balance Ti, having a temperature higher than a beta transition temperature of said titanium alloy composition to a temperature lower than 500° C. at an average cooling rate greater than 10° C./second between the beta transition temperature and 500° C., so that the quenched work piece contains P phase with a body-centered cubic crystal structure as a major phase; and ii) plastically deforming the quenched work piece.  
         [0015]     2. The method of Item 1, wherein said cooling rate is greater than 25° C./second.  
         [0016]     3. The method of Item 1, wherein the work piece to be quenched in step i) has a thickness less than 1.0 cm.  
         [0017]     4. The method of Item 1, wherein the work piece to be quenched in step i) has a thickness less than 0.5 cm.  
         [0018]     5. The method of Item 3, wherein said average cooling rate is greater than 25° C./second.  
         [0019]     6. The method of Item 1 further comprising iii) heating the deformed work piece to a temperature higher than 500° C.; and iv) cooling the heated deformed work piece.  
         [0020]     7. The method of Item 1, wherein the work piece has a temperature of 800-1200° C. before said quenching in step i).  
         [0021]     8. The method of Item 1, wherein said plastically deforming in step ii) is carried out at room temperature.  
         [0022]     9. The method of Item 1, wherein said titanium alloy composition comprises 0.1-3 wt % Bi.  
         [0023]     10. The method of Item 9, wherein said titanium alloy composition further comprises at least one eutectoid beta stabilizing element selected from the group consisting of Fe, Cr, Mn, Co, Ni, Cu, Ag, Au, Pd, Si and Sn.  
         [0024]     11. The method of Item 9, wherein said titanium alloy composition consists essentially of 0.1-3 wt % Bi, 10-50 wt % of at least one alloy element selected from the group consisting of Mo, Nb, Ta, Zr and Hf, based on the weight of the composition, and the balance Ti.  
         [0025]     12. The method of Item 11, wherein said titanium alloy composition consists essentially of 0.5-1.5 wt % Bi, 10-20 wt % of Mo, based on the weight of the composition, and the balance Ti.  
         [0026]     13. The method of Item 11, wherein said titanium alloy composition consists essentially of about 1 wt % Bi, about 15 wt % of Mo, based on the weight of the composition, and the balance Ti.  
         [0027]     14. The method of Item 6 further comprising v) polishing the cooled work piece so that a surface of the polished work piece has an average surface roughness less than about 0.1 μm.  
         [0028]     15. The method of Item 1 further comprising polishing the plastically deformed work piece so that a surface of the polished work piece has an average surface roughness less than about 0.1 μm.  
         [0029]     16. The method of Item 1, wherein said article is a dental casting.  
         [0030]     17. The method of Item 1, wherein said article is a medical implant.  
         [0031]     18. A method of making an article of a titanium alloy comprising  
         [0032]     I) quenching a work piece, which is made of a titanium alloy composition comprising a) about 0.01-5 wt % Bi based on the weight of the composition; b) at least one alloy element selected from the group consisting of Mo, Nb, Ta, Zr and Hf; and c) the balance Ti, having a temperature higher than a beta transition temperature of said titanium alloy composition to a temperature lower than 500° C. at an average cooling rate greater than 10° C./second between the beta transition temperature and 500° C., so that the quenched work piece contains β phase with a body-centered cubic crystal structure as a major phase; and II) polishing the quenched work piece so that a surface of the polished work piece has an average surface roughness less than about 0.1 μm.  
         [0033]     19. The method of Item 18, wherein said titanium alloy composition further comprises at least one eutectoid beta stabilizing element selected from the group consisting of Fe, Cr, Mn, Co, Ni, Cu, Ag, Au, Pd, Si and Sn.  
         [0034]     20. The method of Item 18, wherein said titanium alloy composition consists essentially of 0.1-3 wt % Bi, 10-50 wt % of at least one alloy element selected from the group consisting of Mo, Nb, Ta, Zr and Hf, based on the weight of the composition, and the balance Ti. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0035]     The invention will be described in conjunction with the following drawings wherein:  
         [0036]      FIG. 1  shows X-ray diffraction spectra of the specimens of Ti-15Mo-1Bi alloy subjected separately to water quenching, liquid nitrogen quenching, air cooling and furnace cooling, at a scanning speed of 3°/min;  
         [0037]      FIG. 2  is a plot showing the average cooling rates between 1000-300° C. of the specimens of Ti-15Mo-1Bi alloy shown in  FIG. 1 ;  
         [0038]      FIG. 3  is a plot showing the bending strength of the specimens of as-cast Ti-15Mo, as-cast Ti-15Mo-1Bi, as-rolled Ti-15Mo-1Bi, cold-rolled/annealed (5 min at 650° C.) Ti-15Mo—Bi, cold-rolled/annealed (5 min at 750° C.) Ti-15Mo-1Bi, cold-rolled/annealed (5 min at 850° C.) Ti-15Mo-1Bi, cold-rolled/annealed (5 min at 900° C.) Ti-15Mo-1Bi, and cold-rolled/annealed (5 min at 950° C.) Ti-15Mo-1Bi;  
         [0039]      FIG. 4  is a plot showing the elastic modulus of the specimens of as-cast Ti-15Mo, as-cast Ti-15Mo-1Bi, as-rolled Ti-15Mo-1Bi, cold-rolled/annealed (5 min at 650° C.) Ti-15Mo-1Bi, cold-rolled/annealed (5 min at 750° C.) Ti-15Mo-1Bi, cold-rolled/annealed (5 min at 850° C.) Ti-15Mo-1Bi, cold-rolled/annealed (5 min at 900° C.) Ti-15Mo-1Bi, and cold-rolled/annealed (5 min at 950° C.) Ti-15Mo-1Bi;  
         [0040]      FIG. 5  is a plot showing the ultimate tensile strength (UTS), yield strength (YS) and elongation of the specimens of as-cast Ti-15Mo-1Bi, as-rolled Ti-15Mo-1Bi, cold-rolled/annealed (5 min at 650° C.) Ti-15Mo-1Bi, and cold-rolled/annealed (1 min at 900° C.) Ti-15Mo-1Bi; and  
         [0041]      FIG. 6  is a plot showing the tensile modulus of the specimens of as-cast Ti-15Mo-1Bi, as-rolled Ti-15Mo-1Bi, cold-rolled/annealed (5 min at 650° C.) Ti-15Mo-1Bi, and cold-rolled/annealed (1 min at 900° C.) Ti-15Mo-1Bi.  
         [0042]      FIG. 7  is a plot showing the fatigue lives (numbers of cycles to failure) of cold-rolled (78% reduction in thickness)/annealed (900° C., 1 min) Ti-15Mo-1Bi specimens with different surface roughness values at 900 MPa loading. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0043]     The inventors of the present application find that Ti-15Mo, though being a beta phase alloy, cannot withstand excess cold metal working (plastic deformation). With addition of 1 wt % Bi, not only the castability of the alloy is largely improved (as shown in US patent publication No. 2004-0136859 Al application), it is discovered in this invention that its workability (especially cold workability—which is very critical for industrial application/fabrication) can also be dramatically improved. Further, this excellent cold workability is critically dependent on the cooling rate of Ti-15Mo—Bi alloy, i.e. the phase structure thereof. Therefore, the Ti-15Mo—Bi alloy must have a configuration, e.g. thickness, enabling a fast cooling rate to obtain the desired β phase of the cooled Ti-15Mo—Bi alloy. In one of the preferred embodiments of the present invention, the specimens of Ti-15Mo-1Bi alloy were heated to a temperature higher than its beta transition temperature (about 850° C.), and cooled with water quenching, liquid nitrogen quenching, air cooling and furnace cooling to provide different cooling rates, and X-ray diffraction (XRD) for phase analysis of the cooled specimens was conducted.  
         [0044]     Compared with Ti-6Al-4V (the most popularly-used Ti alloy for medical implant), Ti-15Mo-1Bi (cold-worked or cold-worked/annealed) has at least following advantages:  
         [0045]     (a) More biocompatible (without Al and V—especially V).  
         [0046]     (b) Ti-15Mo-1Bi exhibits excellent cold workability. On the other hand, Ti-6Al-4V cannot be cold-worked but has to be hot-worked (typically at 900-1000° C.), that largely limits its applications and increases complexity and cost in processing.  
         [0047]     (c) Thermomechanically-treated Ti-15Mo-1Bi demonstrates a similar (or even higher) mechanical strength with an acceptable (for cold-worked alloy) or higher (cold-worked and annealed alloy) elongation.  
         [0048]     Metal working methods include (but not limited to) such common methods as rolling, forging, swaging, drawing, and extrusion, etc.  
         [0049]     Preferred embodiments according to the present invention will be described in the following examples that are intended as illustrative only since numerous modifications and variations therein will be apparent to those skilled in the art.  
       EXAMPLE 1  
       [0050]     Ti-15Mo-1Bi alloy (15 wt % Mo and 1 wt % Bi) was prepared from a commercially pure titanium (c.p. Ti) bar, molybdenum of 99.95% and bismuth of 99.5% in purity using a commercial arc-melting vacuum-pressure type casting system (Castmatic, Iwatani Corp., Japan). The melting chamber was first evacuated and purged with argon. An argon pressure of 1.5 kgf/cm 2  was maintained during melting. Appropriate amounts of the c.p. Ti bar, molybdenum and bismuth were melted in a U-shaped copper hearth with a tungsten electrode. The ingot was re-melted three times to improve chemical homogeneity.  
         [0051]     A specimen having an outer diameter of 7 mm and a length of 29 mm was prepared from the Ti-15Mo-1Bi alloy, at one end of which was further provided with a hole having a diameter of 3.5 mm and a depth of 12 mm for mounting a K-type thermocouple therein. A titanium in the form of a sponge was received in a quartz tube and fixed at a bottom thereof by a quartz cap, and the specimen equipped with the thermocouple was inserted into the quartz tube and hermetically mounted inside the quartz tube with one end of the thermocouple being connected to a temperature recorder (Sekonic SS-250F, Sekonic, Japan). The quartz tube at the sealed end was further equipped with a vacuum pump, and a vacuum meter. The quartz tube was vacuumed for five minutes, and placed in an air furnace (S19, Nabertherm®, Germany) preheated at 1000° C. for 30 minutes. The quartz tube was removed from the air furnace, and the specimen together with the thermal couple was subjected to water quenching, liquid nitrogen quenching or air cooling. The average cooling rates recorded are shown in  FIG. 1  and Table 1, in which the average cooling rate of a furnace-cooled specimen is also shown.  
         [0052]     X-ray diffraction (XRD) for phase analysis of the cooled specimens was conducted using a Rigaku diffractometer (Rigaku D-max IIIV, Rigaku Co., Tokyo, Japan) operated at 30 kV and 20 mA. A Ni-filtered CuK α  radiation was used for this study. A silicon standard was used for calibration of diffraction angles. Scanning speed of 3°/min was used. The phases were identified by matching each characteristic peak in the diffraction pattern with the JCPDS files. The results are shown in  FIG. 2 , and are summarized in Table 1.  
                                         TABLE 1                                   Average cooling rate,               ° C./sec   Phase                                    Water-quenched   211   β phase with a bcc*               crystal structure       Liquid N 2 -quenched   26   β phase with a bcc               crystal structure       Air-quenched   9   β phase with a bcc               crystal structure       Furnace-cooled   0.05   α + β               (α phase dominates)                 *body-centered cubic             
 
       EXAMPLE 2  
     Ti-15Mo and Ti-15Mo-1Bi Cold Rolling  
       [0053]     Ti-15Mo (15 wt % Mo) and Ti-15Mo-1Bi (1-5 wt % Mo and 1 wt % Bi) alloys were prepared from a commercially pure titanium (c.p. Ti) bar, molybdenum of 99.95% and bismuth of 99.5% in purity using a commercial arc-melting vacuum-pressure type casting system (Castmatic, Iwatani Corp., Japan). The melting chamber was first evacuated and purged with argon. An argon pressure of 1.8 kgf/cm 2  was maintained during melting. Appropriate amounts of the c.p. Ti bar, molybdenum and bismuth were melted in a U-shaped copper hearth with a tungsten electrode. The ingot was re-melted three times to improve chemical homogeneity.  
         [0054]     Specimens having a thickness of 5.0 mm, a width of 13 mm and a length of 70 mm were prepared from the Ti-15Mo and i-15Mo-1Bi alloys using a graphite mold. The specimens removed from the mold were water-quenched, and surface-finished before being subjected to cold rolling. The cold rolling was carried our at room temperature using a 100-ton rolling machine (VF PCAK-P1, Toshiba Corp., Japan), wherein the specimen was rolled through a gap between two rollers several times with different deforming magnitudes by adjusting the gap.  
         [0055]     When the cold rolling was conducted with reductions in thickness of 1.5 mm, 0.9 mm, 0.9 mm, 0.3 mm and 0.3 mm in sequence (with a total reduction in thickness of 78%), all the specimens of Ti-iSMo-1Bi could be cold-rolled to the final thickness (with a total reduction in thickness of 78%) without breaking down or showing any cracking on the surfaces or edges of the specimens. However, the specimens of Ti-15Mo either showed deformation bands on the surfaces or cracking on the edges of the specimens, or even broke down during rolling. When such cold-rolled Ti-15Mo-1Bi specimens were bending-tested (using the same method as described in Example 3), all the specimens could be bent to the preset deflection limit of 8 mm, while most of the cold-rolled Ti-15Mo specimens failed prematurely. It can be understood from this example that the addition of 1 wt % Bi into Ti-15Mo alloy can significantly enhance the cold-rolling workability of the alloy.  
       EXAMPLE 3  
     Bending Strength and Elastic Modulus of Ti-15Mo and Ti-15Mo-1Bi Alloys  
       [0056]     Three-point bending tests were performed using a desk-top mechanical tester (Shimadzu AGS-500D, Tokyo, Japan) operated at 0.5 mm/sec. Reduced size (36×5×1 mm) specimens were cut from the-as-cast Ti-15Mo, as-cast Ti-15Mo-1Bi, and cold-rolled Ti-15Mo-1Bi. The cold-rolled Ti-15Mo-1Bi specimens were prepared according to the method described in Example 2.  
         [0057]     Some of the Ti-15Mo-1Bi cold-rolled specimens were subjected to a further heat treatment and water quenching before the bending test. The heat treatment was conducted by sealing the specimen in a quartz tube and at the sealed end was further equipped with a vacuum pump, and a vacuum meter. The quartz tube was evacuated for five minutes, and placed in a tube-type furnace heated at a predetermined temperature for 5 minutes. After the heat treatment, the quartz tube was removed from the furnace, and the specimen was subjected to water quenching.  
         [0058]     All the specimens for bending test were polished using sand paper to a #1000 level. The bending strengths were determined using the equation, 
 
σ=3 PL/ 2 bh   2 , 
 
 where σ is bending strength (MPa); P is load (Kg); L is span length (mm) (L=30 mm); b is specimen width (mm) and h is specimen thickness (mm). The modulus of elasticity in bending was calculated from the load increment and the corresponding deflection increment between the two points on a straight line as far apart as possible using the equation, 
 
 E=L   3   ΔP/ 4 bh   3 Δδ, 
 
 where E is modulus of elasticity in bending (Pa); ΔP is load increment as measured from preload (N); and Δδ is deflection increment at mid-span as measured from preload. The average bending strength and modulus of elasticity in bending were taken from at least six tests under each condition. 
 
         [0059]     The comparison of the bending strength and elastic modulus of the Ti-15Mo as-cast specimens, the Ti-15Mo-1Bi as-cast specimens, and the Ti-15Mo-1Bi cold-rolled specimens with and without heat treatment are shown in  FIGS. 3 and 4 .  
         [0060]     It can be seen from the data shown in  FIGS. 3 and 4  that the bending properties of the Ti-15Mo-1Bi alloy can be largely modified through mechanical and/or thermal treatments. The cold-rolled Ti-15Mo-1Bi specimens with or without a further heat treatment have a bending strength higher and a comparable elastic modulus than/to the as-cast Ti-15Mo-1Bi specimens.  
         [0061]     In addition to the above-mentioned 5-minute heat treatment conditions, short-term heat treatments including 3-minute, 1-minute and 0.5-minute at 900° C. were also applied on the Ti-15Mo as-cast specimens before the bending test. The bending strength and elastic modulus of the Ti-15Mo-1Bi as-cast specimens, the cold-rolled Ti-15Mo-1B specimens with and without heat treatment are listed in Table 2.  
                                           TABLE 2                           Bending strength and elastic modulus of the cold-rolled Ti—15Mo—1Bi       specimens with and without heat treatment                Bending strength (MPa)   Bending modulus (GPa)                        As-cast   1200   77       Ti—15Mo—1Bi       cold-rolled   2200   76       Ti—15Mo—1Bi       cold-rolled   1480   84       Ti—15Mo—1Bi,       900° C., 3 min       cold-rolled   1630   77       Ti—15Mo—1Bi,       900° C., 1 min       cold-rolled   1860   84       Ti—15Mo—1Bi,       900° C., 0.5 min                  
 
         [0062]     The data in Table 2 reveal that the cold-rolled Ti-15Mo-1Bi specimens have the highest bending strength. With increasing the heat treatment time at 900° C., the bending strength of the specimens decreased. However, as will be shown in the tensile test data (Example 4), the ductility of the alloy increases with increasing the heat treatment time.  
       EXAMPLE 4  
     Tensile Test of Ti-15Mo-1Bi Alloys  
       [0063]     The tensile test was conducted on the reduced-size (40 mm in length, 10 mm in width and 1 mm in thickness with 10 mm in gauge length and 3 mm in gauge width) as-cast Ti-15Mo-1Bi specimens, the cold-rolled Ti-15Mo-1B specimens without heat treatment, cold-rolled Ti-15Mo-1B specimens with heat treatments (650° C., 5 min; and 900° C., 1 min). A Shimadzu Servopulser system (Shimadzu, Japan) with a crosshead speed of 0.5 mm/min was used for the tensile test. The specimens were prepared and heat-treated as in Example 3. The results are shown in  FIGS. 5 and 6 .  
         [0064]     As shown in  FIG. 5 , the Ti-15Mo-1Bi as-cast specimens have the lowest average ultimate tensile strength (UTS) of 819 MPa with an average yield strength (YS) of 560 MPa. The cold-rolled Ti-15Mo-1Bi specimens (78% reduction in thickness) have an average UTS of 1300 MPa, and an average YS of 735 MPa, both of which decline after the heat treatments, but the UTS is still higher than that of the as-cast specimens. As to the elongation (%), the cold-rolled Ti-15Mo-1Bi specimens have the lowest average elongation of 6.7%, which is much lower than the as-cast specimens (about 30%). However, the heat treatment enables the cold-rolled Ti-15Mo-1Bi specimens more ductile, wherein the cold-rolled Ti-15Mo-1Bi specimens with a heat treatment of 900° C., 1 min have an average elongation of 25.7%.  
         [0065]     The average tensile modulus shown in  FIG. 6  is changing from 78 GPa (as-cast) to 75 GPa (as-rolled), and to the lowest 65 GPa (cold-rolled, 900° C., 1 min), indicating that the cold rolling does not significantly affect the tensile modulus, and the heat treatment will further decrease the tensile modulus. (Note: a high strength and low modulus is often desirable for a medical implant.)  
       EXAMPLE 5  
     Bending Fatigue Test of Ti-15Mo-1Bi Alloy  
       [0066]     The inventors of the present application have conducted a bending fatigue test on the cold-rolled Ti-15Mo-1Bi specimens (78% reduction in thickness) with a heat treatment of (900° C., 1 min). A servo-hydraulic type testing machine (EHF-EG, Shimadzu Co., Tokyo, Japan) was used for the fatigue test on smooth plate specimens with dimensions of 40 mm in length, 5 mm in width and 1.5 mm in thickness. The smooth plate specimens were subjected to fatigue loading with a sinusoidal waveform at room temperature in air at a frequency of 4 Hz with a stress ratio R=0.1. Four different levels of surface roughness were prepared: (1) surface roughness of Ra=0.9-1.1 μm (the Ra value is measured according to ISO 4287: 2000 method) obtained from #60 sand paper; (2) surface roughness of Ra=0.1-0.2 μm obtained from #1000 sand paper; (3) surface roughness of Ra&lt;0.1 μm obtained from #1500 sand paper, followed by mechanical polishing using 1 μm, 0.3 μm and 0.05 μm alumina powder in sequence; and (4) surface roughness of Ra&lt;0.1 μm obtained from #1500 sand paper, followed by chemical polishing for 5 seconds in a solution containing 5 vol % HF, 15 vol % HNO 3  and 80 vol % water.  
         [0067]     It is discovered from the fatigue test data that the fatigue life/fatigue resistance is critically dependent on the surface roughness of the specimen being tested. As indicated in  FIG. 7 , the fatigue lives (numbers of cycles to failure) of all the five specimens prepared from #60 sand paper (Ra=0.9-1.1 μm) are between about 4×10 3  and 10 4  cycles; the fatigue lives of all the five specimens prepared from #1000 sand paper (Ra=0.1-0.2 μm) are between about 10 4  and 6×10 4  cycles.  
         [0068]     It is worth noting that the mechanically polished specimens and the chemically polished specimens (both with Ra&lt;0.1 μm) have dramatically increased fatigue lives. In each group, four out of six specimens tested demonstrate fatigue lives longer than 10 6  (specimens did not fail after 10 6  cycles). This result suggests that, for practical application, it is critical for the Ti-15Mo-1Bi alloy to be prepared with a surface roughness of Ra&lt;0.1 μm. Any cyclic load-bearing device made from this kind of material with surface roughness larger than 0.1 μm can have a risk of premature fatigue failure.  
         [0069]     Although the present invention has been described with reference to specific details of certain embodiments thereof, it is not intended that such details should be regarded as limitations upon the scope of the invention except as and to the extent that they are included in the accompanying claims. Many modifications and variations are possible in light of the above disclosure.