Titanium alloy member and production method therefor

Abstract

A titanium alloy member with high strength and high proof stress not only in the surface but also inside, using a general and inexpensive - type titanium alloy, and a production method therefor, are provided. The production method includes preparing a raw material made of titanium alloy, nitriding the raw material to form a nitrogen-containing raw material by generating a nitrogen compound layer and/or a nitrogen solid solution layer in a surface layer of the raw material, mixing the raw material and the nitrogen-containing raw material to yield a nitrogen-containing mixed material, sintering the nitrogen-containing mixed material to obtain a sintered titanium alloy member by bonding the material together and uniformly diffusing nitrogen in solid solution from the nitrogen-containing raw material to the entire interior portion of the sintered titanium alloy member, and hot plastic forming the sintered titanium alloy member.

Claims

1. A titanium alloy member having a fine deformed structure including curved grains and containing 0.02 to 0.13 mass % of nitrogen in solid solution, GOS.sub.3 of the titanium alloy member is 30% or more, when an average misorientation among all pixels in each grain by FE-SEM/EBSD (Electron Back Scatter Diffraction) method is represented by GOS (Grain Orientation Spread) and an area ratio of grains with not less than 3 of the GOS to the entire observation visual field is represented by the GOS.sub.3, the titanium alloy member has 0.2% bending proof stress of 1702 MPa or higher, and a bending strength of 2321 MPa or higher, wherein diameter of pores in the titanium alloy member having the fine deformed structure is less than 10 m, and the fine deformed structure is constructed without an coarse phase and without a coarse phase.

2. The titanium alloy member according to claim 1, wherein the titanium alloy member is made from an - type titanium alloy.

3. The titanium alloy member according to claim 1, wherein the titanium alloy member is produced by the following method: preparing a raw material made of titanium alloy; nitriding the raw material to form a nitrogen-containing raw material by generating a nitrogen compound layer and/or a nitrogen solid solution layer in a surface layer of the raw material; mixing the raw material and the nitrogen-containing raw material to yield a nitrogen-containing mixed material; sintering the nitrogen-containing mixed material to obtain a sintered titanium alloy member by bonding the material together and uniformly diffusing nitrogen in solid solution from the nitrogen-containing raw material to the entire interior portion of the sintered titanium alloy member; and hot plastic forming the sintered titanium alloy member.

4. A biological implant device comprising the titanium alloy member of claim 1.

5. The titanium alloy member according to claim 1, which has a bending strength of 2650 MPa or higher.

6. The titanium alloy member according to claim 1, wherein the GOS.sub.3 of the titanium alloy member is 40% or more.

7. The titanium alloy member according to claim 1, which has a 0.2% bending proof stress of 1810 MPa or higher.

8. The titanium alloy member according to claim 1, wherein the fine deformed structure is formed by hot plastic working without dynamic recrystallization occurring.

9. The titanium alloy member according to claim 1, wherein the amount of nitrogen in solid solution is 0.053 to 0.13 mass %.

10. A method for producing a titanium alloy member of claim 1, comprising: preparing a raw material made of titanium alloy; nitriding the raw material to form a nitrogen-containing raw material by generating a nitrogen compound layer and/or a nitrogen solid solution layer in a surface layer of the raw material; mixing the raw material and the nitrogen-containing raw material to yield a nitrogen-containing mixed material; sintering the nitrogen-containing mixed material to obtain a sintered titanium alloy member by bonding the material together and uniformly diffusing nitrogen in solid solution from the nitrogen-containing raw material to the entire interior portion of the sintered titanium alloy member; and hot plastic forming the sintered titanium alloy member.

11. The method for producing the titanium alloy member according to claim 10, wherein the raw material is made of an - type titanium alloy.

12. The method for producing the titanium alloy member according to claim 10, wherein the raw material is a titanium alloy fiber produced by a molten metal extraction method.

13. The method for producing the titanium alloy member according to claim 10, wherein the heating temperature of the sintered titanium alloy member in the hot plastic forming is in a range of 800 to 1200 C.

14. The method for producing the titanium alloy member according to claim 10, wherein the sintering is performed by one of hot pressing sintering, hot isostatic pressing sintering, and spark plasma pressure sintering.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1A and 1B show a metal fiber producing apparatus used in an embodiment of the present invention. FIG. 1A is a schematic side sectional view of the overall apparatus, and FIG. 1B is a sectional view of a circumferential edge of a disk used in the apparatus.

(2) FIGS. 2A and 2B show a fiberizing apparatus used in an embodiment of the present invention. FIG. 2A is a side view of the fiberizing apparatus and FIG. 2B is a partially enlarged view of the fiberizing apparatus.

(3) FIG. 3 is a side sectional view of an extruding apparatus used in an embodiment of the present invention.

(4) FIG. 4 shows photographs of structures of titanium alloy members in the Example.

(5) FIG. 5 shows a graph showing a relationship between amount of nitrogen and bending strength and 0.2% bending proof stress of titanium alloy members in the Example.

(6) FIG. 6 shows a graph showing a relationship between area ratio of crystal grain (GOS.sub.3) and bending strength and 0.2% bending proof stress of titanium alloy members in the Example.

(7) FIG. 7 shows a graph showing a relationship between amount of nitrogen and hardness of titanium alloy members in the Example.

EXPLANATION OF REFERENCE NUMERALS

(8) 100 denotes a metal fiber producing apparatus, 101 denotes a chamber, 110 denotes a material feeding portion, 120 denotes a material holding portion, 130 denotes a heating portion, 140 denotes a disk rotating portion, 141 denotes a disk, 141a denotes a circumferential edge, 142 denotes a rotating shaft, 150 denotes a temperature measuring portion, 160 denotes a high-frequency generating portion, 170 denotes a metal fiber receiving portion, 200 denotes a fiberizing apparatus, 210 denotes a material conveyer, 211 denotes a feed roller, 212 denotes a fiberizing mechanism, 213 denotes a conveyer, 214 denotes a belt, 300 denotes an extruding apparatus, 305 denotes an outer die, 310 denotes a container, 320 denotes a lower die, 330 denotes a die, 340 denotes a punch, 360 denotes a heater, A denotes a raw material, B denotes a nitrogen-containing raw material, A+B denotes a nitrogen-containing mixed material, C denotes a sintered titanium alloy member, M denotes a material, and Ma denotes a molten material.

EMBODIMENT OF THE INVENTION

(9) A method for producing the titanium alloy member of the present invention will be specifically described. It should be noted that the apparatuses used in the following method are merely an embodiment, and other apparatuses may also be used.

(10) (1) Preparing Step

(11) FIGS. 1A and 1B schematically show a metal fiber producing apparatus 100 for obtaining a raw material A (hereinafter called an apparatus 100). FIG. 1A is a schematic side sectional view of the overall apparatus 100, and FIG. 1B is a sectional view of a circumferential edge 141a of a disk 141 used in the apparatus 100. FIG. 1B is a side sectional view in a direction perpendicular to the plane of the paper of FIG. 1A.

(12) The apparatus 100 is a metal fiber producing apparatus using a molten metal extraction method. In the apparatus 100, an upper end portion of a rod-shaped material M melts, and the molten material Ma contacts the circumferential edge 141a of the rotating disk 141. Then, a portion of the molten material Ma is extracted toward an approximately tangential direction of the circumference of the disk and is rapidly cooled, and thereby a titanium alloy fiber is formed as a raw material A. For example, an - type titanium alloy such as Ti-6Al-4V is used as a material M for a raw material A, and a raw material A having a diameter of 10 to 200 m is produced. The diameter of the raw material A is not particularly limited and is appropriately selected according to the amount of nitrogen that should be contained in the titanium alloy member. For example, when a larger amount of nitrogen should be contained, the diameter of the raw material A may be thinner. In this case, the proportion of a nitrogen compound layer and/or a nitrogen solid solution layer which are formed by the nitriding can be increased with respect to the diameter.

(13) As shown in FIG. 1A, the apparatus 100 includes a sealable chamber 101 containing a material feeding portion 110, a material holding portion 120, a heating portion 130, a disk rotating portion 140, a temperature measuring portion 150, a high-frequency generating portion 160, and a metal fiber receiving portion 170.

(14) The chamber 101 is evacuated or is filled with an inert gas as an atmosphere so as to inhibit reaction of impurities such as oxygen and the molten material Ma. For example, an Ar (argon) gas can be used for the inert gas atmosphere. The material feeding portion 110 is located, for example, at the bottom of the chamber 101, feeds the material M toward the direction of the arrow b at a predetermined speed, and provides the material M to the material holding portion 120. The material holding portion 120 prevents movement of the molten material Ma toward a radial direction thereof and guides the material M to a suitable position of the disk rotating portion 140.

(15) The material holding portion 120 is a water-cooled metal tubular member and is located below the disk 141 between the material feeding portion 110 and the metal fiber forming portion 140. The heating portion 130 is a high-frequency induction coil that generates magnetic flux for melting the upper end portion of the material M and forming the molten material Ma. As a material for the material holding portion 120, a material that has high thermal conductivity for cooling effect by a cooling water and is non-magnetic to avoid effects of the magnetic flux generated by the heating portion 130 is preferable. Copper or copper alloy is preferable as a material for the material holding portion 120 for practical use.

(16) The disk rotating portion 140 produces a raw material A from the molten material Ma by the disk 141 which rotates around a rotating shaft 142. The disk 141 is made from, for example, copper or copper alloy having high thermal conductivity. As shown in FIG. 1B, a V-shaped circumferential edge 141a is formed on the circumferential portion of the disk 141.

(17) The temperature measuring portion 150 measures the temperature of the molten material Ma. The high-frequency generating portion 160 supplies high-frequency current to the heating portion 130. The power of the high-frequency generating portion 160 is controlled based on the temperature of the molten material Ma, which is measured by the temperature measuring portion 150, and thereby the temperature of the molten material Ma is maintained constant. The metal fiber receiving portion 170 receives the raw material A which is formed by the metal fiber forming portion 140.

(18) In the apparatus 100 constructed in this way, first, the material feeding portion 110 continually feeds the material M in the direction of the arrow b, thereby supplying it to the material holding portion 120. The heating portion 130 melts the upper end portion of the material M by induction heating, thereby forming the molten material Ma. Then, the molten material Ma is continually fed to contact the circumferential edge 141a of the disk 141 rotating in the direction of the arrow a, and a part thereof is extracted toward an approximately tangential direction of the circumference of the disk 141 and is rapidly cooled, whereby a raw material A is formed. The formed raw material A extends toward the approximately tangential direction of the circumference of the disk 141 and is received by the metal fiber receiving portion 170 which is located in the direction in which the raw material A extends.

(19) (2) Nitriding Step

(20) In the nitriding step, an aggregate of the raw material A produced in the preparing step is carried into a vacuum furnace, which is then evacuated and supplied with a nitrogen gas, and the raw material A is heated. In this case, an inert gas such as an argon gas may be supplied with the nitrogen gas for adjusting the density and the pressure of the nitrogen gas. The pressure and the temperature in the furnace and processing time are suitably selected according to amount of nitrogen which should be contained in a titanium alloy member.

(21) If the temperature in the furnace is too low, a very long time is required to form a nitrogen compound layer and/or a nitrogen solid solution layer. If the temperature in the furnace is too high, the processing time is difficult to control because the reaction rate is high, and a thick nitrogen compound layer is readily formed. The thick nitrogen compound layer requires a very long time for diffusing nitrogen in a subsequent sintering step. Thus, the temperature in the furnace is preferably 600 to 1000 C. for practical production. By the nitriding step, a nitrogen-containing raw material B in which a very thin TiN compound layer and/or nitrogen solid solution layer is formed in a surface layer of the raw material A is produced.

(22) (3) Mixing Step

(23) The raw material A and the nitrogen-containing raw material B are mixed together with predetermined percentage according to amount of nitrogen which should be contained in a titanium alloy member. As a mixing means, for example, a fiberizing apparatus 200 shown in FIG. 2A is used. Appropriate amounts of an aggregation of the raw material A and an aggregation of the nitrogen-containing raw material B are supplied to a material conveyer 210 and are moved to the exit side. A feed roller 211 is located at the exit of the material conveyer 210. A fiberizing mechanism 212 is located outside of the feed roller 211.

(24) As shown FIG. 2B, the feed roller 211 includes numerous feeding teeth provided on the circumference thereof, and the feeding teeth bite and feed the raw material A and the nitrogen-containing raw material B. The fiberizing mechanism 212 includes numerous fiberizing teeth provided on the circumference thereof, and the fiberizing teeth comb a part of the raw material A and the nitrogen-containing raw material B, which are fed from the feed roller 211, and drop it on a belt 214 of a conveyer 213. In this condition, the raw material A and the nitrogen-containing raw material B are cut and mixed and are piled up on the belt 214 as an aggregation of random fibers without orientation in a cross section in a direction approximately perpendicular to the belt 214, whereby a nitrogen-containing mixed material A+B is formed. As a mixing means other than the fiberizing apparatus 200 shown in FIG. 2A, various means can be appropriately used. For example, unwoven fabric forming machines, such as of the card type and the aeration type, and mixing machines such as mixers and mills can be used.

(25) (4) Sintering Step

(26) The nitrogen-containing mixed material A+B is sintered by an apparatus that has a pressurizing mechanism and that can be evacuated or be purged with an inert gas. In the case of a vacuum HP (Hot Press) apparatus, a heating chamber is arranged in a vacuum vessel, and a mold is arranged within the heating chamber. In this case, a cylinder is provided at the upper side of the vacuum vessel, a press ram projected from the cylinder is vertically movable in the heating chamber, and an upper punch installed at the press ram is inserted into the mold. The nitrogen-containing mixed material A+B is charged into the mold of the vacuum HP apparatus as constructed above, the vacuum vessel is evacuated or purged with an inert gas, and the heating chamber is heated to a predetermined sintering temperature. Then, the nitrogen-containing mixed material A+B is compressed by the upper punch inserted into the mold, and is sintered.

(27) The sintering should be performed in a vacuum or an inert atmosphere to avoid contamination by impurities such as oxygen from the atmosphere into a titanium alloy member. The sintering temperature is preferably 900 C. or more, the sintering time is preferably 30 minutes or more, and the pressure of pressing is preferably 10 MPa or more. By sintering the nitrogen-containing mixed material A+B in such conditions, a sintered titanium alloy member C containing few pores can be obtained. Nitrogen contained in the nitrogen-containing raw material B uniformly diffuses into the entire interior portion of the sintered titanium alloy member C by solid solution during the sintering. Thus, the formed sintered titanium alloy member C contains no nitrogen compounds or contains very few nitrogen compounds, and has a plate-like structure composed of an - phase.

(28) (5) Hot Plastic Forming Step

(29) The sintered titanium alloy member C can be subjected to hot plastic forming. The hot plastic forming is performed by, for example, an extruding apparatus shown in FIG. 3. The extruding apparatus 300 has an outer die 305 containing a tubular container 310, and a lower die 320 is coaxially arranged at an end side of the container 310. A die 330 is arranged between the container 310 and the lower die 320, and a punch 340 can be slidably inserted into the container 310. A heater 360 is arranged around the container 310.

(30) The sintered titanium alloy member C is preliminarily heated by an outer furnace. After the sintered titanium alloy member C is charged into the container 310, the punch 340 moves down and compresses the sintered titanium alloy member C. The compressed sintered titanium alloy member C is extruded to the space inside the lower die 320 while being reduced in diameter by the die 330, whereby an extruded material is formed. The heating temperature of the sintered titanium alloy member C in the outer furnace may be 800 to 1200 C., the extrusion ratio may be 2 to 7, and the forward speed of the punch 340 may be 1 to 30 mm/second. By extruding the sintered titanium alloy member C in such conditions, all or almost all pores remaining after the sintering are removed, and a high strength and high proof stress processed member having a fine deformed structure composed of an - phase can be obtained.

(31) The conditions such as the heating temperature and the extrusion ratio in the extruding have a complicated relationship with each other in conjunction with the material composition of the titanium alloy and the effects of the contained nitrogen, and they are suitably determined by theory, experience, and experiment. Although the fine deformed structure is obtained by setting the heating temperature and the extrusion ratio in this embodiment, an appropriate heating temperature and appropriate processing parameters should be set to obtain the fine deformed structure according to the processing method when a processing method other than the extruding is used.

Examples

(32) The present invention will be described in detail by way of specific examples.

(33) 1. Production of Samples

(34) (1) Preparation of Raw Material (Preparing Step)

(35) A raw material with an average wire diameter of 60 m was produced from Ti-6Al-4V (corresponding to ASTM B348 Gr. 5) using the apparatus 100 shown in FIG. 1A.

(36) (2) Preparation of Nitrogen-Containing Raw Material (Nitriding Step)

(37) A part of the raw material was subjected to nitriding as follows. First, the raw material was carried into a vacuum furnace. After evacuating, a nitrogen gas was fed into the vacuum furnace, and the pressure in the furnace was set at 80 kPa. Then, the temperature in the furnace was increased to 800 C. and maintained for 1.5 hours to perform nitriding.

(38) (3) Preparation of Nitrogen-Containing Mixed Material (Mixing Step)

(39) The raw material and the nitrogen-containing raw material were supplied to the fiberizing apparatus 200 shown in FIGS. 2A and 2B and mixed together, whereby a nitrogen-containing mixed material was obtained. The weight percentage (Wf) of the mixed nitrogen-containing raw material is shown in Table 1.

(40) (4) Preparation of Sintered Titanium Alloy Member (Sintering Step)

(41) The nitrogen-containing mixed material was charged into a carbon mold, and a sintered titanium alloy member having a thickness of 28 mm was obtained by a vacuum HP apparatus. After the vacuum chamber was evacuated to the degree of vacuum of 110.sup.2 Pa or less and purged with an argon gas so as to be at 80 kPa, the sintering was performed at a temperature increasing rate of 10 C./minute, a sintering temperature of 1100 C., and a pressure of press of 40 MPa for 1.5 hours, and cooling was then performed in the furnace. The carbon mold and the nitrogen-containing mixed material as well as the sintered titanium alloy member which is a sintered body of the mixed material are reactive under the high temperature conditions described above. In view of this, a release plate as a liner made from Al.sub.2O.sub.3 (alumina, purity of 99.5% or more) was preliminarily installed to the carbon mold.

(42) (5) Preparation of Titanium Alloy Member Sample (Hot Plastic Forming Step)

(43) The sintered titanium alloy member was formed into a shape with a diameter of 25 mm and a height of 90 mm by machining and was heated in an outer furnace. Then, the sintered titanium alloy member was hot plastic formed by the extruding apparatus 300 shown in FIG. 3, whereby a processed titanium alloy member was prepared (Samples Nos. 101 to 125). The heating temperature (T.sub.E) in the outer furnace was 700 to 1200 C., the temperature of the container was 300 C., the forward speed of the punch was 10 mm/second, and the extrusion ratio (R) was 1.5 to 10. The sintered titanium alloy member was preliminary applied with an antioxidizing lubricating agent (DeltaGlaze349 manufactured by Acheson Japan) before heating. The time from taking out the sintered titanium alloy member from the outer furnace to starting to advance the punch was approximately 30 seconds. The extruded titanium alloy member was water cooled right under the lower die.

(44) The result of extruding is also shown in Table 1. In Table 1, broken means that the member was broken during extrusion and titanium alloy material which is an extruded material was not obtained, rough/crack means that extruded titanium alloy material in which surface roughness and/or cracking were observed was obtained, and superior means that superior titanium alloy member having no roughness and cracking was obtained. The samples of superior were further evaluated.

(45) (7) Preparation of Comparative Sample

(46) For comparison, a rod of an expanded material of Ti-6Al-4V (corresponding to ASTM B348 Gr. 5) was prepared as a comparative sample 10. In addition, this expanded material was subjected to the hot plastic forming with the same conditions as described above, whereby a comparative sample No. 11 was prepared. Result of extrusion of the comparative sample 11 is also shown in Table 1.

(47) TABLE-US-00001 TABLE 1 Extrusion Sample Wf (%) T.sub.E ( C.) R result Sample 101 5 1100 4 Superior Sample 102 10 1100 4 Superior Sample 103 15 1100 4 Superior Sample 104 20 1100 4 Superior Sample 105 25 1100 4 Superior Sample 106 30 1100 4 Superior Sample 107 35 1100 4 Superior Sample 108 40 1100 4 Superior Sample 109 20 700 4 Broken Sample 110 20 750 4 Rough/crack Sample 111 20 800 4 Superior Sample 112 20 850 4 Superior Sample 113 20 900 4 Superior Sample 114 20 1000 4 Superior Sample 115 20 1150 4 Superior Sample 116 20 1200 4 Superior Sample 117 20 1100 1.5 Superior Sample 118 20 1100 2 Superior Sample 119 20 1100 3 Superior Sample 120 20 1100 5 Superior Sample 121 20 1100 6 Superior Sample 122 20 1100 7 Superior Sample 123 20 1100 8 Rough/crack Sample 124 20 1100 9 Broken Sample 125 20 1100 10 Broken Comparative sample 10 Comparative 1100 4 Superior sample 11
2. Evaluation Items and Evaluation Methods

(48) Evaluation items and evaluation methods will be described hereinafter. The evaluation results are shown in Table 2.

(49) (1) Structure

(50) Each sample was cut into a suitable size and embedded in a resin so that the cross sectional structure perpendicular to the axial direction could be observed. Then, the embedded sample was mirror finished by mechanical polishing and was etched by an etching fluid (2 wt % of hydrofluoric acid and 4 wt % of nitric acid), and the structure was observed by an optical microscope (NIKON ME600). FIG. 4 shows typical microscope photographs of the samples. A fine deformed structure composed of an - phase is represented by the symbol A and an equiaxed structure composed of an - phase is represented by the symbol B in Table 2.

(51) (2) Existence of TiN Compound Phase (TiN Phase)

(52) The crystal structure was analyzed by an X-ray diffractometer (Rigaku X-ray Diffractometer RINT2000) using Cu tube target, and peak of TiN compound phase was observed.

(53) (3) Porosity

(54) Each sample was cut into a suitable size and embedded in a resin so that the cross sectional structure perpendicular to the axial direction could be observed. Then, the embedded sample was mirror finished by mechanical polishing. Then, pores were observed by a field-emission-type scanning electron microscope (FE-SEM, JSM-7000F, manufactured by JEOL). Magnification of observation was 100 times (visual field area 1.1 mm.sup.2). The number of pores observed in the magnitude was counted at freely selected thirty positions per each sample, and the number of pores per unit area was calculated as porosity (porosity (number/mm.sup.2)=number of all pores/(visual field area30)).

(55) (4) Amount of Nitrogen (N Amount)

(56) Amount of nitrogen was measured by inert gas melting-thermal conductivity technique and solid state type infrared absorption method (LECO TC600).

(57) (5) Area Ratio of Grains with not Less than 3 of Average Misorientation in Grain (GOS.sub.3)

(58) A GOS (Grain Orientation Spread: An average misorientation among all pixels in a grain) map was formed by FE-SEM/EBSD (Electron Back Scatter Diffraction) method (JEOL JSM-7000F, TSL solutions OIM-Analysis Ver. 4.6) at 1000-times magnification. Then, an area ratio of grains with not less than 3 of GOS to the entire observation visual field (GOS.sub.3) was calculated.

(59) (6) Hardness (HV)

(60) Hardness of the vicinity of the surface and the center of each sample in a cross section perpendicular to the axial direction were measured by a Vickers hardness tester (FUTURE-TECH FM-600). The test load was 10 gf. The hardness of the vicinity of the surface was measured at 10 points at 1 mm below the outer circumferential surface and the center hardness was measured at 10 points at the center and the vicinity of the center of the cross section, and the averages were calculated.

(61) (7) Bending Strength (.sub.b), 0.2% Bending Proof Stress (.sub.b0.2)

(62) A three-point bending test was performed by a 300 kN universal testing machine (INSTRON 5586 type). The test piece had a width of 6 mm, a length of 17 mm, and a thickness of 1 mm, and the distance between fulcrums was 15 mm. An average of bending strength (maximum bending stress) and an average of 0.2% bending proof stress were calculated by testing three pieces of each sample at a rate of 6 mm/minute.

(63) TABLE-US-00002 TABLE 2 Nitrogen Wf T.sub.E TiN Porosity amount GOS.sub.3 HV .sub.b .sub.b0.2 Sample (%) ( C.) R Structure phase (number/mm.sup.2) (mass %) (%) Surface Center (MPa) (MPa) Sample 101 5 1100 4 A Non 0.00 0.022 42.2 379 369 2613 1611 Sample 102 10 1100 4 0.00 0.034 38.4 377 372 2650 1702 Sample 103 15 1100 4 0.00 0.053 34.1 379 379 2683 1778 Sample 104 20 1100 4 0.00 0.075 38.6 390 394 2708 1832 Sample 105 25 1100 4 0.00 0.089 35.8 410 414 2727 1853 Sample 106 30 1100 4 0.00 0.105 41.8 424 421 2625 1890 Sample 107 35 1100 4 0.00 0.122 45.7 440 435 2323 1910 Sample 108 40 1100 4 0.00 0.138 49.9 445 454 2022 Not reached Sample 111 20 800 4 0.00 0.069 52.3 410 399 2780 1867 Sample 112 20 850 4 0.00 0.077 48.7 388 389 2754 1861 Sample 113 20 900 4 0.00 0.080 43.2 395 395 2736 1835 Sample 114 20 1000 4 0.00 0.072 40.0 396 403 2711 1810 Sample 104 20 1100 4 0.00 0.075 38.6 390 394 2708 1832 Sample 115 20 1150 4 0.00 0.074 38.0 388 384 2715 1841 Sample 116 20 1200 4 0.00 0.078 30.4 396 386 2635 1799 Sample 117 20 1100 1.5 0.65 0.072 21.5 372 376 2311 1520 Sample 118 20 1100 2 0.00 0.066 32.2 389 383 2321 1620 Sample 119 20 1100 3 0.00 0.073 34.2 390 388 2554 1711 Sample 104 20 1100 4 0.00 0.075 38.6 390 394 2708 1832 Sample 120 20 1100 5 0.00 0.070 42.5 401 396 2716 1865 Sample 121 20 1100 6 0.00 0.069 40.1 399 389 2782 1957 Sample 122 20 1100 7 0.00 0.077 46.5 410 400 2770 1922 C. Sample B 0.00 0.005 1.3 312 308 1896 1023 C. Sample 4 A 0.00 0.008 46.2 358 364 2640 1575
3. Results
(1) Heating Temperature

(64) Influence of heating temperature is obvious from the samples 104 and 109 to 116. It is understood that the titanium alloy member can be produced by setting the heating temperature of the sintered titanium alloy member at 750 to 1200 C. in the case in which the extrusion ratio of extrusion is 4, from Tables 1 and 2. Lower heating temperature is desirable in order to realize high strength and high proof stress since larger stein can be introduced. However, in the sample 110 in which heating temperature was 750 C., cracking was generated on the member surface since the strain induced was too large. Therefore, it is obvious that the heating temperature of the sintered titanium alloy member is desirably set at 800 C. or more. In a case in which the heating temperature was more than 1200 C., coarse phase may be deposited at grain boundary, and equiaxed structure may be formed by dynamic recrystallization depending on an extrusion ratio condition. As a result, ductility and strength may be deteriorated.

(65) However, from the samples Nos. 104 and 117 to 125, it is obvious that the titanium alloy member is difficult to be produced if the extrusion ratio is too large even in the case in which the heating temperature is 1100 C. Furthermore, it is understood that the titanium alloy member can be appropriately produced by setting the heating temperature at 1100 C. and the extrusion ratio at 4 even in the case in which nitrogen amount is varied, from the samples Nos. 101 to 108. As explained, the conditions such as the heating temperature and the extrusion ratio in the extruding have a complicated relationship with each other in conjunction with the material composition of the titanium alloy and the effects of the contained nitrogen. Therefore, in the titanium alloy member of the present invention, it is desirable that extrusion is performed with controlling the extrusion ratio at the heating temperature of 800 to 1200 C. in order to realize high strength and high proof stress.

(66) (2) Structure

(67) The samples Nos. 101 to 108, and 111 to 122 were subjected to the extruding and had a fine deformed structure composed of an - phase. FIG. 4 (picture 1, sample No. 104) shows an example of fine deformed structure. The comparative sample No. 11 was also subjected to the extruding and also had a fine deformed structure composed of an - phase as shown in FIG. 4 (picture 3, comparative sample No. 11). On the other hand, the comparative sample No. 10 is a generally available expanded material, and has an equiaxed structure composed of an - phase as shown in FIG. 4 (picture 2, comparative sample No. 10). Since titanium alloy has low workability, the alloy is typically processed by a hot processing to a final product shape. Therefore, the comparative sample No. 10 which is a commercially available expanded material is shown in FIG. 4 (picture 2, comparative sample No. 10).

(68) (3) TiN Compound Phase

(69) According to the results of the X-ray diffraction, the peaks of nitrogen compounds such as the TiN compound phase were not detected in all of the samples. It indicates that the contained nitrogen did not form nitrogen compounds and were solid solved in the matrix. Existence of nitrogen compounds having a great difference in hardness (or elastic strain) from the matrix is undesirable because the boundary between the nitrogen compound phase and the matrix tends to be origins of fractures and can thereby cause decrease in the fatigue strength. Therefore, the titanium alloy member of the present invention is suitable with respect to fatigue in which repeated stress is loaded, because there is no boundary between a nitrogen compound phase and the matrix, which makes a great difference in the hardness and tends to be an origin of fractures.

(70) (4) Porosity

(71) As a result of observation of the pores, there were not pores observed in the samples except for the sample No. 117. This result shows that the pores which slightly remained after sintering were extinguished by the extrusion processing, whereby there were no pores or almost no pores. The porosity was 0.65 number/mm.sup.2 in the sample No. 117 in which the extrusion ratio was small and thus the pores remained, thereby deteriorating the 0.2% bending proof stress. In addition, as a result of observing the pores of the sample No. 117 in detail by a high magnitude, the diameter of the pores were 6.5 to 23.6 m (the longer diameter was measured in a case in which pore is not circle), that is, the result indicates that there are no pores having diameter not less than 6.5 m in the samples in which no pores were observed by the 100 times observation magnitude, except for the sample No. 117. A pore having diameter 10 m or more may easily be an origin of fractures in fatigue in which repeated stress is loaded. Therefore, present invention is desirable for increasing fatigue strength since pores having diameter 6.5 vim or more do not exist.

(72) (5) Amount of Nitrogen

(73) FIG. 5 is a graph showing a relationship of the amount of nitrogen and bending strength and 0.2% bending proof stress. The Sample No. 101 containing 0.022% of nitrogen had slightly less bending strength than the comparative sample No. 11. However, the Sample No. 101 had greater 0.2% bending proof stress than the comparative sample No. 11 and is suitable for a part to be used under conditions which cause fatigue, because it can be used under higher stresses. As the amount of nitrogen increased, the bending strength and the 0.2% bending proof stress increased until the amount of nitrogen was 0.089% as in the Sample No. 105. However, in the Sample No. 106 containing 0.105% of nitrogen, the 0.2% bending proof stress further increased, but the ductility decreased, whereby the bending strength decreased to the level of the comparative sample No. 11. This tendency observed when the amount of nitrogen was 0.105% or more increased with the increase in the amount of nitrogen, and the Sample No. 108 containing 0.138% of nitrogen broke before reaching the 0.2% bending proof stress. When the amount of nitrogen was less than 0.02%, the strength and the proof stress were not effectively improved compared with those of the comparative sample No. 11. Accordingly, the titanium alloy member of the present invention preferably contains 0.02 to 0.13% of nitrogen in solid solution.

(74) (6) Area Ratio of Grains with not Less than 3 of Average Misorientation in Grain (GOS.sub.3)

(75) As shown by the results of the samples subjected to the extruding, the values of GOS.sub.3 as a parameter for the amount of strain accumulation were much greater than those of the comparative sample No. 10, which had an equiaxed structure. This result suggests that the fine deformed structure is work hardened due to strain accumulation, and the fine deformed structure contains a great number of grain boundaries that are perpendicular or curved with respect to directions of crack growth, thereby having a great effect for preventing crack growth by stopping and curving cracks, whereby the fatigue resistance can be improved. Furthermore, as shown in the samples Nos. 104, 117 to 122 of Table 2, the GOS.sub.3 becomes larger as the extrusion ratio is larger, and the bending strength and the 0.2% bending proof stress, that is, fatigue resistance is improved more as the GOS.sub.3 becomes larger as is obvious from FIG. 6 which shows relationship between the GOS.sub.3 and the bending strength and the 0.2% bending proof stress. The GOS.sub.3 of a sample in which 0.2% bending proof stress of 1600 MPa or more was obtained, which was more than the comparative sample 11, was 30% or more.

(76) (7) Hardness

(77) As shown in Table 2, in all of the samples Nos. 101 to 108 and 111 to 122, the hardness of the surface and the hardness of the center were approximately the same. The samples had hardness that is equal to or greater than the hardness of the comparative sample No. 11 subjected to the extruding in the same manner. As shown in FIG. 7, there was a close relationship between the amount of nitrogen and the hardness, and the hardness improved in accordance with the increase in the amount of nitrogen. Thus, according to the present invention, the titanium alloy member can be greatly strengthened in the entirety at the interior portion, and a necessary degree of strength can be obtained.

(78) The conditions in the sintering and the extruding are not limited to the conditions described in this Example and can be appropriately set in view of obtaining high strength and high proof stress. That is, the densified amount and the degree of diffusion of nitrogen in the sintering and the amount of strain introduced in the plastic forming greatly depend on complicated relationships such as the material composition, the temperature, and the processing rate, and can be determined by appropriately setting conditions based on theory, experience, and experiment.

(79) The titanium alloy material of the present invention is applicable for materials used for aircraft and automobiles required to be light in weight and have high strength, and materials for biological implant devices.