Method for enhancing mechanical strength of a titanium alloy by aging

Info

Publication number: 20130139933
Type: Application
Filed: Dec 6, 2012
Publication Date: Jun 6, 2013
Applicant: NATIONAL CHENG KUNG UNIVERSITY (Tainan)
Inventor: National Cheng Kung University (Tainan)
Application Number: 13/706,377

Abstract

A titanium-molybdenum alloy having α″ phase as a major phase is subjected to an aging treatment, so that yield strength of the aged alloy is increased by 10% to 120% with elongation to failure thereof being not less than about 5.0%.

Description

Description

CROSS REFERENCE TO RELATED PATENT APPLICATIONS

The present patent application claims the benefit of U.S. provisional patent application No. 61/567,170, filed Dec. 6, 2011, the contents of which are hereby incorporated by reference in their entireties.

FIELD OF THE INVENTION

The present invention is related to a method for enhancing mechanical properties of a titanium-molybdenum alloy having α″ phase as a major phase by aging, and in particular to a method for enhancing mechanical properties of a medical implant of a titanium-molybdenum alloy by aging.

BACKGROUND OF THE INVENTION

Titanium and titanium alloys have been popularly used in many medical applications due to their light weight, excellent mechanical performance and corrosion resistance. The relatively low strength commercially pure titanium (c.p. Ti) is currently used as dental implant, crown and bridge, as well as denture framework. Other examples for use of commercially pure titanium include pacemaker case, heart valve cage and reconstruction devices. Nevertheless, due to its relatively low strength, c.p. Ti may not be used for high load-bearing applications.

The most widely-used titanium alloy for load-bearing applications is Ti-6Al-4V alloy (the “work-horse” titanium alloy). With a much higher strength than c.p. Ti, Ti-6Al-4V alloy has been widely used in a variety of stress-bearing orthopedic applications, such as hip prosthesis and artificial knee joint. Nevertheless, studies indicated that release of Al and/or V ions from Ti-6Al-4V might cause long-term health problems (Rao et al. 1996, Yumoto et al. 1992, Walker et al. 1989, McLachlan et al. 1983). Its poor wear resistance could further accelerate the release of such harmful ions (Wang 1996, McKellop and RoKstlund 1990, Rieu 1992).

Compared to conventional 316L stainless steel and Co—Cr—Mo alloy, titanium and titanium alloys have much lower elastic modulus values. The lower elastic modulus allows the titanium/titanium alloy to more closely approximate the stiffness of bone for use in orthopedic devices compared to alternative stainless steel and cobalt-chrome alloys in orthopedic implants. Thus, devices formed from the titanium alloy produce less bone stress shielding and consequently interfere less with bone viability.

It should be noted that, although the elastic modulus values of c.p. Ti and Ti-6Al-4V alloy (about 110 GPa) are much lower than the popularly-used 316L stainless steel and Co—Cr—Mo alloy (200-210 GPa), the modulus values of c.p. Ti and Ti-6Al-4V alloy are still much higher than that of the natural bone (for example, only about 20 GPa for typical cortical bone). The large difference in modulus between natural bone and implant is one major cause for the well-recognized “stress-shielding effect.”

More recently an Al and V-free, high strength, low modulus α″ phase Ti—Mo based alloy system (typically Ti-7.5M) has been developed in the present Inventors' laboratory (U.S. Pat. No. 6,726,787 B2), which demonstrates excellent mechanical properties and biocompatibility, and has a great potential for use as orthopedic or dental implant material.

Biocompatibility of this α″ type Ti-7.5Mo alloy was confirmed through cytotoxicity test and animal implantation study. The cell activity of this alloy is similar to that of Al₂O₃(control). Animal study indicates that, after 6 weeks of implantation, new bone formation is readily observed at alloy surface. It is interesting to note that, after 26 weeks, the amounts of new bone growth onto the surface of Ti-7.5Mo implants at similar implantation site are dramatically larger than that of Ti-6Al-4V implant, indicating a much faster healing process.

U.S. Pat. No. 6,726,787 B2 provides a process for making a biocompatible low modulus high strength medical device from a titanium alloy, which comprises preparing a titanium alloy having a composition consisting essentially of at least one isomorphous beta stabilizing element selected from the group consisting of Mo, Nb, Ta and W; and the balance Ti, wherein said composition has a Mo equivalent value from about 6 to about 9; casting or metal working the titanium alloy to form a work piece; and quenching the work piece which is the resulting hot cast having a temperature higher than 800° C. at a cooling rate greater than 10° C. per second, or heating the work piece resulted from said metal working to a temperature higher than 800° C. and quenching the work piece having a temperature higher than 800° C. at a cooling rate greater than 10° C. per second, so that the cooled work piece contains an α″ phase as a major phase, and can be used as a medical device which is biocompatible, and has a low modulus and high strength.

In this invention, said Mo equivalent value, [Mo]eq, can be represented by the following equation: [Mo]eq=[Mo]+0.28[Nb]+0.22[Ta]+0.44[W], wherein [Mo], [Nb], [Ta] and [W] are percentages of Mo, Nb, Ta and W, respectively, based on the weight of the composition.

A typical quenching method used in the process of the present application is water quenching; however, any methods known in the art which have a cooling rate greater than 10° C., preferably 20° C., per second, can also be used.

U.S. Pat. No. 6,409,852 and U.S. Pat. No. 6,726,787 disclosed a titanium alloy composition and a method for preparing a titanium alloy having α″ phase as a major phase.

One disadvantage for an α″ phase titanium alloy is that the alloy generally has a relatively low strength (e.g., compared to Ti-6Al-4V ELI).

SUMMARY OF THE INVENTION

A primary objective of the present invention is to provide an article made of a titanium-molybdenum alloy with enhanced mechanical properties.

Another primary objective of the present invention is to provide a method for enhancing mechanical properties of a titanium-molybdenum alloy having α″ phase as a major phase by aging.

In order to accomplish the aforesaid objective, a method for enhancing mechanical properties of a titanium-molybdenum alloy having α″ phase as a major phase by aging disclosed in the present invention comprises providing an article of a titanium-molybdenum alloy having α″ phase as a major phase; and aging said article, so that yield strength of said aged article is increased by 10% to 120%, based on the yield strength of said article, with elongation to failure of said aged article being not less than about 5.0%.

Preferably, the yield strength of said aged article is increased by at least 20%, preferably at least 50%, and more preferably at least 75%, based on the yield strength of said article, with elongation to failure of said aged article being not less than about 7.0%.

Preferably, the titanium-molybdenum alloy consists essentially of 7-9 wt % of molybdenum and the balance titanium. More preferably, the titanium-molybdenum alloy consists essentially of about 7.5 wt % of molybdenum and the balance titanium.

Preferably, the article provided is an as-cast article.

Preferably, the article provided is a hot-worked or cold-worked article.

Preferably, the article provided is a solution-treated article.

Preferably, the article provided is a hot-worked and then solution-treated article.

Preferably, the article provided is a cold-worked and then solution-treated article.

Preferably, the article provided is a solution-treated and then cold-worked article.

Preferably, the article provided is an as-cast and then cold-worked article.

Preferably, said aging is conducted at a temperature of 150-850° C., more preferably 200-800° C., and most preferably 250-750° C.

Preferably, said aging is conducted for a period of time depending on the temperature as follows:

250° C.: >5 m; preferably >15 m; more preferably >30 m;
350° C.: >5 m; preferably >10 m; more preferably >15 m;
400° C.: >1 m; preferably >5 m; more preferably >5 m and <960 m; most preferably >5 m and <480 m;
450° C.: >1m; preferably >3 m; more preferably >3 m and <960 m; most preferably >3 m and <480 m;
500° C.: >30 m; preferably >60 m; more preferably >120 m;
550° C.: >15 m; preferably >30 m; more preferably >60 m;
600° C.: >5 m; preferably >15 m; more preferably >30 m;
650° C.: >1 m; preferably >5 m; more preferably >15 m;
700° C.: >1 m; preferably >3 m; more preferably >10 m;
750° C.: >1 m; preferably >3 m; more preferably >5 m.

DETAILED DESCRIPTION OF THE INVENTION

It is discovered by the present inventors that an appropriate aging treatment can effectively increase the mechanical strength of α″ phase titanium alloy. To the knowledge of the present inventors, aging treatment has never been taught or tested to increase the mechanical strength of a titanium alloy having an α″ phase with an orthorhombic crystal structure.

It is further discovered by the present inventors that, with an appropriate aging treatment process (with specific aging temperature and time ranges), the strength of α″ phase Ti alloys (particularly Ti—Mo based α″ phase Ti alloys) can be dramatically increased while other important properties (e.g., a reasonable elongation) of the alloys are maintained.

It is further discovered that the appropriate aging temperature/time ranges generally correspond to the ranges, in which the formation of hard, brittle omega phase is largely avoided.

It is further discovered that a mild aging treatment (with a temperature lower than about 250° C. and time less than about 30 m) can significantly increase the strength level while maintaining low modulus and reasonable elongation values of the alloy.

The α″ phase Ti-7.5Mo alloy samples for aging treatment may be prepared from directly casting the alloy into a mold from the molten state, from solution-treating (heated to beta-phase regime) a cast alloy followed by fast cooling, e.g., water quenching, or from solution-treating a mechanically or thermomechanically worked (e.g., through rolling, drawing, forging, extrusion, etc. at a room temperature or a high temperature) alloy followed by fast cooling, e.g. water quenching. The aging treatment can also be directly conducted on a mechanically or thermomechanically worked (e.g., through rolling, drawing, forging, extrusion, etc. at a room temperature or a high temperature) alloy followed by water quenching without first solution-treating said alloy.

EXPERIMENTAL EXAMPLES

Terminology:

AC: As-cast;
ST: Solution-treated at 900° C. for 5 min followed by water quench;
A: aged (Note: Aging was carried out in a quartz tube, which had been evacuated, followed by purging with inert (argon) gas. All aged samples were air-cooled to room temperature from the aging temperature.)
HR: Hot-rolled (Samples are heated to 900-1000° C. then taken out of furnace for rolling by about 65% reduction in thickness);
CR: Cold-rolled (Samples at room temperature are directly rolled at room temperature).

Preparation of α″ Phase Binary Ti—Mo Alloys:

The various Ti alloys were prepared from grade-2 commercially pure titanium (c.p. Ti) bars (Northwest Institute for Non-ferrous Metal Research, China) and molybdenum wire of 99.95% purity (Alfa Aesar, USA) by using a commercial arc-melting vacuum-pressure type casting system (Castmatic, Iwatani Corp., Japan). Prior to melting/casting, the melting chamber was evacuated and purged with argon. An argon pressure of 1.5 kgf/cm2 was maintained during melting. Appropriate amounts of metals were melted in a U-shaped copper hearth with a tungsten electrode. The ingots were re-melted at least three times to improve chemical homogeneity of the alloys. After each melting/casting, the alloys were pickled using HNO₃/HF (3:1) solution to remove surface oxide.

Prior to casting, the alloy ingots were re-melted again in an open-based copper hearth in argon under a pressure of 1.5 kgf/cm². The difference in pressure between the two chambers allowed the molten alloys to instantly drop into a graphite mold at room temperature. This fast cooling process generates a cooling rate of the alloy that is sufficient to form an α″ phase. Some of these as-cast alloy samples directly underwent cold working treatment to obtain a desired shape/thickness. Other cast samples, to further improve structural uniformity, were solution-treated to a beta phase regime (about 900-1000° C.), followed by fast cooling (water quenching) to transform the beta phase into α″ phase again. Thus obtained α″ phase alloys then underwent cold working treatment to obtain a desired shape/thickness. The XRD results confirm that the fast-cooled (water-quenched) samples have α″ phase as a major phase.

X-Ray Diffraction

X-ray diffraction (XRD) for phase analysis was conducted using a Rigaku diffractometer (Rigaku D-max IIIV, Rigaku Co., Tokyo, Japan) operated at 30 kV and 20 mA with a scanning speed of 3°/min. A Ni-filtered CuKα radiation was used for the study. A silicon standard was used for the calibration of diffraction angles. The various phases were identified by matching each characteristic peak in the diffraction patterns with JCPDS files.

Tensile Testing

A servo-hydraulic type testing machine (EHF-EG, Shimadzu Co., Tokyo, Japan) was used for tensile tests. The tensile testing was performed at room temperature at a constant crosshead speed of 8.33×10−6 m s−1. The average ultimate tensile strength (UTS), yield strength (YS) at 0.2% offset, modulus of elasticity (Modulus) and elongation to failure (Elongation) were taken from five tests under each process condition.

Cold Rolling (Rolling Conducted at Room Temperature)

Cold rolling was conducted by using a two-shaft, 100 ton level rolling tester (Chun Yen Testing Machines Co., Taichung, Taiwan). After each pass, the thickness of the samples was reduced by about 5-15% from the last pass.

TABLE 1 Tensile properties of Ti-7.5Mo alloy under different aging conditions. (All α″ phase Ti-7.5Mo alloy samples for aging treatment are prepared directly from a fast-cooling casting process) Elonga- Aging condition UTS Modulus tion (Temp/Time) YS (MPa) (MPa) (GPa) (%) As-cast 540 879 80 29.1 AC-A (350° C./30 min) 639 892 74 18.9 AC-A (350° C./480 m) 728 1227 70 12.7 AC-A (400° C./30 m) 918 1111 106 11.2 AC-A (400° C./480 m) 990 1409 81 3.6 AC-A (450° C./15 m) 909 1123 98 11.1 AC-A (450° C./30 m) 973 1133 101 10.6 AC-A (450° C./480 m) 1025 1469 80 3.5 AC-A (500° C./15 m) 1291 1414 125 2.0 AC-A (500° C./30 m) 1239 1352 120 1.8 AC-A (550° C./5 m) 1399 1455 124 1.5 AC-A (550° C./10 m) 1403 1472 132 1.3 AC-A (550° C./15 m) 1317 1498 117 1.4 AC-A (550° C./30 m) 1133 1280 125 2.4 AC-A (550° C./60 m) 1119 1409 116 2.2 AC-A (550° C./240 m) 1021 1144 114 4.6 AC-A (550° C./480 m) 936 1112 130 7.8 AC-A (550° C./480 m) 917 1110 118 6.0 AC-A (600° C./30 m) 1049 1221 115 3.4 AC-A (600° C./480 m) 1035 1135 87 8.8 AC-A (650° C./15 m) 921 1037 120 13.4 AC-A (650° C./30 m) 927 1035 118 12.2 AC-A (650° C./60 m) 811 946 116 15.0 AC-A (650° C./480 m) 971 1035 85 11.0 AC-A (650° C./480 m) 758 830 106 21.0 AC-A (650° C./480 m) 725 878 113 19.7 AC-A (700° C./30 m) 827 907 114 14.7 AC-A (750° C./15 m) 810 935 123 14.6 AC-A (750° C./30 m) 912 1002 116 11.4 AC-A (750° C./60 m) 916 1064 104 11.4 AC-A (550° C./15 m) followed 911 1048 127 11.3 by A (750° C./60 m) (two-step aging)

TABLE 2 Tensile properties of aged Ti-7.5Mo alloy under different aging conditions. (All α″ phase Ti-7.5Mo alloy samples for aging treatment are prepared by solution treatment followed by water quenching) Elonga- UTS Modulus tion Aging condition (Temp/Time) YS (MPa) (MPa) (GPa) (%) As solution-treated 427 845 72 31.3 ST-A (350° C./30 m) 624 885 73 20.3 ST-A (350° C./45 m) 639 893 73 21.0 ST-A (350° C./60 m) 677 901 76 18.0 ST-A (350° C./120 m) 734 943 87 18.0 ST-A (350° C./240 m) 801 973 87 15.0 ST-A (350° C./480 m) 811 1031 94 13.0 ST-A (350° C./30 m) 804 965 99 18.3 ST-A (400° C./30 m) 963 1127 101 7.3 ST-A (400° C./240 m) 925 1143 104 5.0 ST-A (450° C./15 m) 916 1110 102 6.2 ST-A (450° C./30 m) 1002 1137 106 6.4 ST-A (450° C./240 m) 983 1083 106 3.0 ST-A (500° C./30 m) 1291 1371 125 1.2 ST-A (500° C./60 m) 1002 1098 118 2.1 ST-A (500° C./240 m) 872 983 110 5.0 ST-A (550° C./30 m) 1068 1213 118 4.2 ST-A (550° C./60 m) 902 1013 108 7.0 ST-A (550° C./240 m) 824 904 104 13.0 ST-A (600° C./30 m) 904 1074 110 7.2 ST-A (600° C./60 m) 827 931 104 9.7 ST-A (600° C./240 m) 768 863 102 12.0 ST-A (650° C./30 m) 866 975 110 13.9 ST-A (650° C./60 m) 795 929 104 14.4 ST-A (650° C./120 m) 717 869 102 16.7 ST-A (650° C./240 m) 730 885 107 16.0 ST-A (650° C./60 m) 856 975 109 11.0 ST-A (650° C./120 m) 815 911 104 14.4 ST-A (500° C./30 m) + A 872 993 108 9.1 (650° C./30 m) (two-step aging) ST-A (450° C./30 m) + A 884 1030 101 8.3 (650° C./5 m) (two-step aging) ST-A (650° C./30 m) + A 834 982 102 15.0 (450° C./30 m) ST-A (650° C./480 m) 694 823 109 20.8 ST-A (750° C./60 m) 813 978 113 14.0

TABLE 3 Tensile properties of Ti-7.5Mo alloy under different aging conditions. (All α″ phase Ti-7.5Mo alloy samples for aging treatment are prepared by solution treatment, followed by cold rolling with 50% reduction in thickness. Modu- YS UTS lus Elonga- Aging condition (Temp/Time) (MPa) (MPa) (GPa) tion (%) ST-CR 50% (no aging) 904 1149 64 20.5 ST-CR 50%-A (200° C./15 m) 1013 1193 66 14.6 ST-CR 50%-A (200° C./30 m) 919 1213 67 5.3 ST-CR 50%-A (250° C./30 m) 1006 1237 68 4.2 ST-CR 50%-A (250° C./240 m) 1044 1236 68 1.8 ST-CR 50%-A (350° C./30 m) 997 1263 76 0.7 ST-CR 50%-A (350° C./240 m) 731 1086 74 3.1

Results: Aging conditions of 200° C. for 15 minutes will enhance the yield strength (YS) of the cold-rolled α″ phase Ti-7.5Mo alloy by about 12% with the elongation to failure still being maintained at 14.6%. It can been from Table 3 that, for these cold-rolled samples, the period of time for aging is preferably no longer than 30 minutes for keeping the elongation to failure not less than 5% with the aging temperature being lower than 350° C.

TABLE 4 Tensile properties of Ti-7.5Mo alloy under different aging conditions. (All α″ phase Ti-7.5Mo alloy samples for aging treatment are prepared by hot rolling by 65% reduction in thickness followed by solution treatment) UTS Modulus Elongation Aging condition (Temp/Time) YS (MPa) (MPa) (GPa) (%) HR-ST-A (250° C./15 m) 463 784 86 24.3 HR-ST-A (250° C./30 m) 514 830 82 29.5 HR-ST-A (250° C./60 m) 496 800 81 26.8 HR-ST-A (250° C./240 m) 520 848 92 25.4 HR-ST-A (350° C./15 m) 664 869 95 12.2 HR-ST-A (350° C./30 m) 694 906 93 12.7 HR-ST-A (350° C./60 m) 705 900 98 11.8 HR-ST-A (350° C./240 m) 686 957 95 17.0 HR-ST-A (450° C./15 m) 791 1000 95 9.8 HR-ST-A (450° C./30 m) 882 1075 94 5.5 HR-ST-A (450° C./60 m) 941 1193 104 3.2 HR-ST-A (450° C./240 m) 1060 1178 125 0.5 HR-ST-A (550° C./15 m) 1087 1154 116 0.2 HR-ST-A (550° C./30 m) 1085 1150 113 0.8 HR-ST-A (550° C./60 m) 1028 1153 127 0.8 HR-ST-A (550° C./240 m) 888 1032 125 3.9 HR-ST-A (650° C./15 m) 865 1048 106 7.9 HR-ST-A (650° C./480 m) 694 823 108 20.8 HR-ST-A (750° C./15 m) 871 1042 108 13.5 HR-ST-A (750° C./60 m) 813 978 113 14.0 HR-ST-A (550° C./15 m) + A 882 1085 107 11.6 (750° C./15 m) (two-step aging) HR-ST-A (500° C./480 m) + A 897 1136 112 13.4 (750° C./15 m) (two-step aging)

TABLE 5 Comparison in tensile properties among selected aged Ti-7.5Mo alloy and popularly-used commercially pure titanium and Ti-6AI-4VELI. UTS Modulus Elongation Material YS (MPa) (MPa) (GPa) (%) c.p. Ti (Grade 2) 235 345 100 20 c.p. Ti (Grade 4) 483 550 100 15 Ti-6AI-4V (ELI) 795 860 114 10 (ASTM F136) AC-A (450° C./30 m) 973 1133 101 10.6 AC-A (600° C./480 m) 1035 1135 87 8.8 ST-A (650° C./30 m) 866 975 110 13.9 ST-CR 50%-A 1013 1193 66 14.6 (200° C./15 m)

Summary of the Results:

(1) Aging treatment can significantly increase the strength level of Ti-7.5Mo alloy. For example, at comparable modulus and elongation levels, the YS and UTS values of Ti-7.5Mo samplecoded “AC-A(450° C./30 m)” are higher than those of Ti-6Al-4V(ELI) (ASTM F136) by 22% and 32%, respectively. The YS and UTS values of Ti-7.5Mo sample coded “AC-A(600° C./480 m)” are higher than those of Ti-6Al-4V(ELI) (ASTM F136) by 30% and 32%, respectively.

(2) A mild aging treatment (with a temperature lower than about 250° C. and time less than about 30 m) can significantly increase the strength level while maintaining low modulus and reasonable elongation values of the alloy. The YS and UTS values of Ti-7.5Mo sample coded “ST-CR 50%-A (200° C./15 m)” are higher than those of Ti-6Al-4V (ELI) (ASTM F136) by 27% and 39%, respectively, while maintaining an elongation higher than Ti-6Al-4V (ELI) (ASTM F136) by 46% and a modulus lower than Ti-6Al-4V (ELI) (ASTM F136) by 42%. These data result in amazing YS/Mod and UTS/Mod ratios (important performance indices for orthopedic implants) which are higher than Ti-6Al-4V (ELI) (ASTM F136) by 120% and 140%, respectively.

(3) The best aging temperature range: 150-850° C.; preferably 200-800° C.; more preferably 250-750° C.;

(4) The best aging time range: depending on aging temperature, as follows:

250° C.: >5 m; preferably >15 m; more preferably >30 m;

350° C.: >5 m; preferably >10 m; more preferably >15 m;

400° C.: >1 m; preferably >5 m; more preferably >5 m and <960 m; most preferably >5 m and <480 m;

450° C.: >1 m; preferably >3 m; more preferably >3 m and <960 m; most preferably >3 m and <480 m;

500° C.: >30 m; preferably >60 m; more preferably >120 m;

550° C.: >15 m; preferably >30 m; more preferably >60 m;

600° C.: >5 m; preferably >15 m; more preferably >30 m;

650° C.: >1 m; preferably >5 m; more preferably >15 m;

700° C.: >1 m; preferably >3 m; more preferably >10 m;

750° C.: >1 m; preferably >3 m; more preferably >5 m.

Claims

1. A method for enhancing mechanical strength of an article of a titanium alloy by aging comprising providing an article of a titanium-molybdenum alloy having α″ phase as a major phase; and aging said article, so that yield strength of said aged article is increased by 10% to 120%, based on the yield strength of said article, with elongation to failure of said aged article being not less than about 5.0%.

2. The method of claim 1, wherein the yield strength of said aged article is increased by at least 20%, preferably at least 50%, and more preferably at least 75%, based on the yield strength of said article, with elongation to failure of said aged article being not less than about 7.0%.

3. The method of claim 1, wherein the titanium-molybdenum alloy consists essentially of 7-9 wt % of molybdenum and the balance titanium.

4. The method of claim 3, wherein the titanium-molybdenum alloy consists essentially of about 7.5 wt % of molybdenum and the balance titanium.

5. The method of claim 1, wherein the article provided is an as-cast article.

6. The method of claim 1, wherein the article provided is a hot-worked or cold-worked article.

7. The method of claim 1, wherein the article provided is a solution-treated article.

8. The method of claim 1, wherein the article provided is a hot-worked and then solution-treated article.

9. The method of claim 1, wherein the article provided is a cold-worked and then solution-treated article.

10. The method of claim 1, wherein the article provided is a solution-treated and then cold-worked article.

11. The method of claim 1, wherein the article provided is an as-cast and then cold-worked article.

12. The method of claim 1, wherein said aging is conducted at a temperature of 150-850° C.

13. The method of claim 12, wherein said aging is conducted at a temperature of 200-800° C.

14. The method of claim 13, wherein said aging is conducted at a temperature of 250-750° C.

15. The method of claim 14, wherein said aging is conducted for a period of time depending on the temperature as follows: