EP0812924A1 - Titanium material, process for its production and use - Google Patents

Abstract

A titanium material contains more than 0.05 wt.% of one or more precipitation hardening elements (preferably Fe, Cu and/or Si) and more than 0.05 (preferably 0.20-0.35) wt.% interstitially dissolved oxygen, has a fine grained structure of grain size finer than 5, preferably finer than 8 (ASTM Standard E 112-88), and has been subjected to hot working, optional recrystallisation annealing, cold working and precipitation hardening. Also claimed is a titanium material production process involving (a) alloying with one or more precipitation hardening elements and oxygen if not already present in sufficient amounts in the sponge titanium or any alloying components, the oxygen dissolving interstitially; (b) hot working and optionally recrystallisation annealing to obtain a fine grained structure; (c) cold working with optional intermediate annealing; and (d) tempering to precipitate the precipitation hardening elements.

Description

Field of application of the invention

The present invention relates to a biocompatible titanium material with improved fatigue strength, a method for producing the titanium material and its preferred use.

Background of the Invention

The following requirements are placed on an implant material - starting with the top priorities: best possible biocompatibility and fatigue strength, high elastic limit or strength, minimum elongation of 10%, good cold formability, no warping after machining, mechanical machining, good machinability, unproblematic melting and cost-effective production .

Titanium and titanium alloys have become indispensable materials in medical technology for more than 20 years. The main applications concern joint replacement (eg hip joint endoprostheses, knee and arm prostheses), osteosynthesis products (eg bone plates and screws, intramedullary nails) and dental implants. The particular suitability of titanium as an implant material results from its excellent biocompatibility, which has not been achieved by any other metallic material. With the use of titanium alloys, highly resilient implants are obtained, such as Hip joint endoprostheses that have to withstand enormous, changing loads. In principle, this also applies to osteosynthesis products and dental implants. Titanium alloys with forged design and high fatigue strength are always used for the production of hip joint endoprostheses. In contrast, unalloyed titanium is advantageously used for the production of osteosynthesis products and dental implants (cf. SCHROEDER / SUTTER / BUSER / KREKELER: Orale Implantologie. Georg Thieme Verlag Stuttgart, 2nd ed. 1994, p. 37ff.).

Sauerstoff wird bei unlegiertem Titan als Beimengung zur Festigkeitssteigerung genutzt. Bei Grade 4 liegt spezifikationsgemäss der maximal zulässige Sauerstoffgehalt bei 0,40%; der Eisengehalt ist auf maximal 0,50% begrenzt. Die Gehalte an Kohlenstoff und Stickstoff werden bewusst niedrig gewählt, da diese Elemente die Duktilität des Titans stark herabsetzen.Differentiating unalloyed titanium in 4 degree, corresponding to the maximum of different proportions of oxygen (cf. the standard A merican S ociety for T esting and M aterials;. ASTM 67-89; Specification for unalloyed titanium for surgical implants). Insofar as titanium grade is mentioned below, the reference to ASTM 67-89 always applies. The term "unalloyed" only means that there are no metallic additives. After ZWICKER, U .: Titan and titanium alloys. Springer-Verlag Berlin (et al.) 1974, p. 220, the following approximate equation applies to the relationship between hardness (HB = Brinell hardness values) and the content of additives: $$\text{HB = 45}\sqrt{\text{\%}\mathit{\text{C.}}}\text{+158}\sqrt{\text{\%}\mathit{\text{O}}}\text{+ 196}\sqrt{\text{\%}\mathit{\text{N}}}\text{+20}\sqrt{\text{\%}\mathit{\text{Fe}}}\text{+ 57}$$

With unalloyed titanium, oxygen is used as an admixture to increase strength. In grade 4, the maximum permissible oxygen content is 0.40%; the iron content is limited to a maximum of 0.50%. The levels of carbon and nitrogen are deliberately chosen to be low, since these elements greatly reduce the ductility of the titanium.

In order to further increase the strength of unalloyed titanium in addition to alloying measures, cold forming with degrees of deformation up to over 90% is used. When setting high strengths, however, the ductility - especially the elongation measured in the tensile test - decreases sharply with an increasing degree of cold deformation and drops below the value of 10% required in practice. With the use of special titanium grades (grade 4 with increased oxygen content) or with special thermomechanical manufacturing processes, however, it was possible to achieve a balanced relationship between tensile strength, elongation and bending ability.

The fatigue strengths previously achieved with unalloyed titanium - mostly measured as alternating bending fatigue strength - are σ _bw ≈ 380 MPa, while values of σ _bw ≈ 540 MPa were achieved with titanium alloys. For dental implants, only unalloyed titanium is currently considered due to the required biocompatibility and better waxing behavior. The present invention is thus aimed at achieving fatigue strengths in unalloyed titanium which at least come close to the known implant titanium alloys. To improve the strength or fatigue strength, the oxygen content, the work hardening and, in some cases, a subsequent relaxation annealing have been used; However, it is important to perfect hardening mechanisms known per se to the metal scientist and to use them in a combination, as a result of which the mechanical material parameters can be further optimized.

Titanium has good corrosion resistance; its high resistance in oxidizing media is characteristic. For Titan, the strong affinity for oxygen with the immediate formation of an oxide layer - this inhibits further corrosion - is characteristic. The special suitability of titanium and titanium alloys as implant materials has been demonstrated using current density potential curves. Both unalloyed titanium and titanium alloys show no significant differences in corrosion removal in physiological saline at 37 ° C in the relevant application range between -200mV and + 300mV. Differences between unalloyed titanium or the titanium alloy TiAl6Nb7 and the high-iron-containing implant alloy TiAl5Fe2.5 only occur in the current density potential curves under more aggressive corrosion conditions in oxygen-free, two-molar hydrochloric acid, the breakthrough potential for the latter being 4.4 V (10 V for unalloyed titanium).

It has been proven that the corrosion rate of unalloyed titanium increases with increasing iron content. A test in 10% hydrochloric acid - these conditions never occur when an implant is used in the human body - showed a corrosion attack that was three times higher. In physiological saline, on the other hand, there is no significant difference between unalloyed titanium with iron contents of 0.05% and 0.15%. However, corrosion resistance is not the only decisive factor for the biocompatibility of a material. Not only a low rate of corrosion of the materials investigated in vivo is a good biocompatibility decisive, but also the material-specific, strongly divergent tissue compatibility.

As can be seen in Figure 1 , the corrosion resistance of titanium, titanium alloys, zircon, niobium, tantalum and platinum is at a high level, whereby these materials are inert in the fabric. The implant steel 316L (FeCrNiMo) and the cobalt-based alloy CoCrNiMo also have a high corrosion resistance, but these materials cause an undesirable tissue reaction in the body due to sequestration. The ratios of silver, gold, aluminum, molybdenum and iron are even more unfavorable, the latter having the lowest corrosion resistance. The elements cobalt, copper, nickel and vanadium are increasingly toxic as a result. This illustrates that a material with high corrosion resistance does not guarantee biocompatibility at the same time. As a consequence, ideal implant materials are titanium, zircon, niobium, tantalum and their alloys.

The very good biocompatible behavior of titanium is essentially due to the strong affinity for oxygen, which means that the titanium surface immediately covers itself with an oxide film of atomic thickness, which behaves neutrally in the electrolyte - ie in the blood. The electrically non-conductive oxide film acts as a strong barrier against further dissolution of the metal. Electrically neutral hydrolysis products do not develop an attempt to react with organic molecules. In the case of implant steel, on the other hand, there are twofold electrical-positive undesirable reaction products charged ions that can form toxic antigens with the proteins of the human body. Nickel is known for its allergic and toxic effects.

Evidenced by numerous investigations, unalloyed titanium with its oxide film has an ideal surface for forming a chemical connection with human bones. Only pure or commercially pure titanium with the permissible minor foreign components such as iron, nitrogen, carbon and oxygen allows the bone to grow on the implant and causes its "osseointegration".

In addition to the best possible biocompatibility, the static and dynamic, mechanical properties are also extremely important. The static, mechanical properties normally determined in the tensile test are the tensile strength R _m (maximum stress occurring in the stress-strain diagram) and the often more significant proof stress R _p (limit stress at which a material no longer behaves exclusively elastically and changes into the plastic state ). The technical proof stress is usually specified with 0.2% proof stress R _p0.2 . A high-strength material should have both a high R _m value and a high R _p0.2 value so that no plastic deformation occurs under real loads.

Table 1 below shows the characteristic strength values according to DIN standard 17 869 from Titan Grade 1, Titan Grade 4 and the titanium alloy TiAl6V4: Table 1 Strength values (guide values) Titan Grade 1 Titan Grade 4 Titanium alloy TiAl6V4 Tensile strength R _m [MPa] 350 640 930 0.2% proof stress R _p0.2 [MPa] 240 480 865 Elongation A ₅ [%] 45 25th 13

The specification values for the strength of titanium materials for implants are in the ASTM standards F 67-89, F 136-92, F 1295-92 and F 1341-92 as well as in ISO standards 5832 / II, 5832-3 and 5832 -11 fixed.

The following table 2 shows the characteristic strength values according to ASTM standard F 67-89 for bars made of annealed, unalloyed titanium of various degrees: Table 2 Titan grades Tensile strength R _m min. [MPa] 0.2% proof stress Rp _0.2 min. [MPa] Elongation A ₄ min. [%] Constriction Z min. [%] 1 240 170 24th 30th 2nd 345 275 20th 30th 3rd 450 380 18th 30th 4th 550 483 15 25th

Table 3 below shows the characteristic strength values for wire according to ASTM standard F 1341-92 annealed, unalloyed titanium of various grades: Table 3 Titan grades Diameter D [mm] Tensile strength R _m min. [MPa] 0.2% proof stress R _p0.2 min. [MPa] Elongation A ₄ min. [%] Constriction Z min. [%] 1 8.0-3.2 240 170 24th 30th <3.2-1.6 15 - <1.6-0.5 12th - <0.5-0.1 10th - 2nd 8.0-3.2 345 275 20th 30th <3.2-1.6 12th - <1.6-0.5 10th - <0.5-0.1 8th - 3rd 8.0-3.2 450 380 18th 30th <3.2-1.6 10th - <1.6-0.5 8th - <0.5-0.1 6 - 4th 8.0-3.2 550 483 15 25th <3.2-1.6 8th - <1.6-0.5 6 - <0.5-0.1 4th -

Both the ASTM standards F 67-89 and F 1341-92 as well as the ISO standard 5832 / II leave the possibility open to use work hardened titanium for surgical implants, whereby only the last standard with a strain A = 10% a minimum value for specifies the tensile strength with R _m ≥ 680 MPa. According to ASTM standard F 67-89, an elongation A ≥ 10% is required for rods with a measuring length L ₀ = 4 D, while ASTM standard F 1341-92 also allows elongation values below 10% for thin wires. For the rest, the ASTM standards leave the fixation of strength values to the coordination between the titanium producers and the implant manufacturers.

An essential property of a heavily loaded material is the remaining ductility, which e.g. if a component is overloaded, its abrupt failure is excluded. With work hardened materials, however, part of the ductility is lost through the work hardening process, i.e. With increasing degree of cold deformation, the elongation A or the ductility decreases.

The metallurgist can use various hardening methods in accordance with Table 4 below to influence the mechanical material parameters: Table 4 Hardening process Operation Mixed crystal hardening Substitution of titanium by other metallic elements soluble in it (e.g. zircon, niobium, tantalum) Hardening by adding interstitially dissolved elements Additive (e.g. oxygen, carbon, nitrogen) Cold forming Rolling, pulling, hammering Grain refinement Recrystallization annealing Precipitation hardening Additive (e.g. iron, copper, silicon) Cold forming and precipitation hardening Additive (e.g. iron, copper, silicon) and cold working

State of the art

The titanium alloy TiAl6V4 has been introduced as a standard alloy for decades.

A forgeable alloy for implants for bone and dental surgery is known from CH-A-539 118. This alloy consists of 3-50% by weight X and otherwise essentially Z, where X is one of the elements niobium, tantalum, chromium, molybdenum, tungsten, iron or aluminum or any mixture of these elements, while Z is one the elements are titanium or zirconium or any mixture of these elements. The vanadium present in the standard alloy was then either replaced by iron or in a further improvement by niobium, so that the alloys TiAl5Fe2.5 and TiAl6Nb7 were formed.

However, the titanium materials known in this respect cannot be regarded as optimal with regard to the totality of all requirements - ie biocompatibility, in particular "osseointegration", mechanical parameters, production expenditure and cold deformability. The compromises found so far are unsatisfactory, in particular it has so far not been possible to achieve the highest possible biocompatibility and "osseointegration", as well as high tensile strength R _m and fatigue strength. In addition, the manufacturing outlay for the materials is relatively high, and improved cold formability, for example for the production of long products such as bars and profiles, is also particularly desirable.

Object of the invention

• Gemäss der Elementarforderung nach bestmöglicher Biokompatibilität kommt für den zu kreierenden Titanwerkstoff allein unlegiertes Titan der Grades 1 bis 4 in Betracht.
• Eine höchstmögliche Ermüdungsfestigkeit bzw. Dehngrenze R_P0,2 sowie Zugfestigkeit R_m sind zu erreichen.
• Die Mindestdehnung A ≥ 10% muss gewährleistet sein, um die Spezifikation der ASTM-Norm F 67-89 zu erfüllen. Für Implantatwerkstoffe ist es besonders wesentlich, dass keine Zuginstabilität vorliegt. Die Spannung darf nach Überschreiten der 0,2%-Dehngrenze R_p0,2 nur langsam abfallen und nicht abrupt abreissen.
• Eine gute Kaltverfombarkeit ist unabdingbar, z.B. für das Walzen von Profilen.
• Im Werkstoff sollen innere Spannungen weitestgehend abgebaut sein, um eine verzugsfreie, spanende Weiterbearbeitung zu ermöglichen, wobei generell auf eine günstige Zerspanbarkeit zu achten ist.
• Insgesamt sind eine problemlose Erschmelzung und kostengünstige Herstellung des Titanwerkstoffs anzustreben.
• Einhaltung sonstiger relevanter nationaler und internationaler Normen.
• Für spezielle Anwendungsfälle - abgesehen von einem eventuellen Absinken der Biokompatibilität und des Erfordernisses der Neuzulassung des Titanwerkstoffs - soll eine zusätzliche Mischkristallhärtung möglich sein.

In view of the imperfections of previously known titanium materials - primarily for use in medical technology as implants - with regard to the material properties, the manufacturing effort and the further processability, the invention aims to create an improved titanium material and the associated manufacturing process. The development of the material and the manufacturing process were based on the following premises:

• According to the elementary requirement for the best possible biocompatibility, unalloyed titanium of grades 1 to 4 can be used for the titanium material to be created.
• The highest possible fatigue strength or _{proof stress} R _P0.2 and tensile strength R _m can be achieved.
• The minimum elongation A ≥ 10% must be guaranteed to meet the specification of ASTM standard F 67-89. It is particularly important for implant materials that there is no train instability. After exceeding the 0.2% proof stress R _{p0.2, the} tension may only drop slowly and not abruptly.
• Good cold formability is essential, for example for rolling profiles.
• Internal stresses in the material should be reduced as much as possible in order to enable distortion-free, subsequent machining, whereby general attention must be paid to good machinability.
• Overall, problem-free melting and cost-effective production of the titanium material are desirable.
• Compliance with other relevant national and international standards.
• For special applications - apart from a possible decrease in biocompatibility and the requirement for new approval of the titanium material - an additional mixed crystal hardening should be possible.

Essence of the invention

The essential features of the titanium material according to the invention result from patent claim 1, advantageous embodiment details being evident from claims 2 to 6. The principle of the manufacturing process can be found in claim 7, while claims 8 to 18 contain special process features. The preferred applications of the titanium material are defined in claim 19.

• hervorragende Biokompatibilität;
• deutlich gesteigerte Ermüdungsfestigkeit σ_bw ≈ 500 MPa (Ausgangswert σ_bw ≈ 380 MPa), wie sie bisher nur von Titanlegierungen erreicht wurde;
• Warm- und Kaltverformbarkeit (z.B. beim Drahtziehen), die signifikant über den Werten von bekannten α+β-Titanlegierungen liegt. Die Kaltverformbarkeit von unlegiertem Titan nimmt mit steigendem Grade ab. Titan Grade 4 lässt sich aber, verglichen mit der Implantatlegierung TiAl6Nb7, weitaus besser und kostengünstiger kaltverformen, insbesondere wegen der geringeren Verfestigungsneigung von unlegiertem Titan gegenüber den α+β-Titanlegierungen. Dies bedeutet ein höheres Verformungsvermögen bis zur nächsten Zwischenglühung;
• Zugfestigkeit R_m von 900-1000 MPa, die der Titanimplantat-Legierung TiAl6Nb7 ebenbürtig ist;
• hohe Verfügbarkeit, da die chemische Zusammensetzung keine strengen Anforderungen an die zulässigen Verunreinigungen (Begleitelemente) stellt;
• Einstellbarkeit der Werkstoffeigenschaften Dehngrenze R_P, Zugfestigkeit R_m, Dehnung A und Ermüdungsfestigkeit σ_bw durch gezielte Anlassglühungen mit äusserst variablen Temperaturbereichen und Glühzeiträumen;
• durch die Kaltverfestigung herbeigeführte gute Zerspanbarkeit. Unlegiertes Titan hat bei der spanenden Bearbeitung an sich die äusserst negative Eigenschaft zu schmieren; dieses Verhalten wird durch die Kaltverfestigung reduziert. Ferner ergibt sich ein kürzerer Span beim Drehen, Fräsen oder Bohren sowie eine Reduzierung des Verschleisses an den Bearbeitungswerkzeugen mit der Folge günstiger Bearbeitungskosten;
• kein Verzug bei der mechanischen Bearbeitung, da spannungsfrei geglüht, was besonders bei Teilen mit engsten Masstoleranzen wünschenswert ist;
• der Titanwerkstoff ist amagnetisch - somit z.B. auch in Kernspintomographen einsetzbar;
• die Erschmelzung von unlegiertem Titan ist weitaus weniger aufwendig, da Seigerungen - diese können bei der Herstellung von Legierungen auftreten - hier ausgeschlossen sind;
• der gesamte Herstellungsprozess des Titanwerkstoffs, vom Schmelzen, über das Schmieden, Warmwalzen, Kaltwalzen, Ziehen bis zu den erforderlichen Zwischenglühungen sowie der Aushärtung, ist im Vergleich zur Implantatlegierung TiAl6Nb7 weniger kostenaufwendig und bringt damit für die Fertigprodukte einen Kostenvorteil.

Thanks to the invention, a titanium material is now available, which is characterized by the following:

• excellent biocompatibility;
• significantly increased fatigue strength σ _bw ≈ 500 MPa (initial value σ _bw ≈ 380 MPa), which was previously only achieved with titanium alloys;
• Hot and cold formability (eg in wire drawing), which is significantly above the values of known α + β titanium alloys. The cold formability of unalloyed titanium decreases with increasing degrees. However, compared to the TiAl6Nb7 implant alloy, titanium grade 4 can be cold worked much better and more cost-effectively, in particular because of the lower tendency of unalloyed titanium to harden compared to α + β titanium alloys. This means a higher deformability until the next intermediate annealing;
• Tensile strength R _m of 900-1000 MPa, which is on a par with the titanium implant alloy TiAl6Nb7;
• high availability, since the chemical composition does not place strict requirements on the permissible impurities (accompanying elements);
• Adjustability of material properties yield strength R _P , tensile strength R _m , elongation A and fatigue strength σ _bw through targeted tempering with extremely variable temperature ranges and annealing periods;
• good machinability brought about by work hardening. Unalloyed titanium itself has the extremely negative property of lubricating during machining; this behavior is reduced by work hardening. Furthermore, there is a shorter chip during turning, milling or drilling and a reduction in wear on the processing tools, with the result of favorable processing costs;
• no distortion during mechanical processing, as it is annealed without stress, which is particularly desirable for parts with the closest dimensional tolerances;
• the titanium material is non-magnetic - so it can also be used in magnetic resonance imaging, for example;
• the melting of unalloyed titanium is far less complex since segregations - which can occur during the production of alloys - are excluded here;
• the entire manufacturing process of the titanium material, from melting, forging, hot rolling, cold rolling, drawing to the required intermediate annealing and hardening, is less expensive than the implant alloy TiAl6Nb7 and therefore brings a cost advantage for the finished products.

Wire, rods, profiles, plates, sheet metal and strip of any desired dimensions can be produced from the material. The titanium material according to the invention is used primarily for use in medical technology for implants - e.g. Dental implants, root posts, posts, bone plates and screws, intramedullary nails, heart valves and prostheses -, for instruments and for devices. Another important area of application is the watch and jewelry industry, where the excellent material properties in contact with the skin are essential.

Representations and examples

Bild 1:

Korrosionswiderstand und Gewebsreaktionen von Reinmetallen und Legierungen (aus SCHROEDER/SUTTER/BUSER/KREKELER, a.a.O., S. 46);

Bild 2:

Einfluss von Sauerstoff und Stickstoff auf die mechanischen Eigenschaften von Titan bei Raumtemperatur (aus ZWICKER, a.a.O., S. 220);

Bild 3:

Einfluss von Sauerstoff auf die mechanischen Werte von unlegiertem Titan (aus BOYER/WELSCH/COLLINGS: Materials Properties Handbook. Titanium Alloys, ASM International - The Materials Information Society, 1994);

Bild 4:

Einfluss von Kohlenstoff auf die mechanischen Werte von unlegiertem Titan (aus BOYER/WELSCH/COLLINGS: a.a.O.);

Bild 5:

Einfluss von Stickstoff auf die mechanischen Werte von unlegiertem Titan (aus BOYER/WELSCH/COLLINGS: a.a.O.);

Bild 6:

Einfluss von Stickstoff, Sauerstoff, Kohlenstoff und Eisen in Atom-% bzw. Gew.-% auf die Vickers-Härte von unlegiertem Titan (aus BOYER/WELSCH/COLLINGS: a.a.O.);

Bild 7:

Verfestigungsschaubilder von Titan Grade 1 - Werkstoff 3.7025 und von Titan Grade 2 - Werkstoff 3.7035 (aus DIN-Norm 17869);

Bild 8:

Verfestigungsverhalten von Titan Grade 1 - Werkstoff 3.7025 und von Titan Grade 2 - Werkstoff 3.7035 nach Lage der Walzrichtung (aus DIN-Norm 17869);

Bild 9:

Verfestigungskurven von Titan Grade 1;

Bild 10:

Verfestigungskurven von Titan Grade 2;

Bild 11:

Verfestigungskurven von Titan Grade 4;

Bild 12a:

Gefügeausbildung von rekristallisierend geglühtem Stabmaterial;

Bild 12b:

Gefügeausbildung vom erfindungsgemäss gefertigtem Titanwerkstoff als Stabmaterial gleicher Abmessung wie Bild 12a;

Bild 13:

Zustandsdiagramm Fe-Ti (aus MURRAY,J.L.: Phase Diagramms of Binary Titanium Alloys. ASM International - Metals Park Ohio, 1987);

Bild 14:

Elektronenmikroskopische Aufnahme des Verformungsgefüges von Titan Grade 4, kaltverformt und angelassen bei 375°C/1h;

Bild 15:

Elektronenmikroskopische Durchstrahlungsaufnahme des Verformungsgefüges von Titan Grade 4, kaltverformt und angelassen bei 375°C/1h;

Bild 16:

Elektronenmikroskopische Durchstrahlungsaufnahmen und Beugungsaufnahme mit Überstrukturreflexen von Titan Grade 4, kaltverformt und angelassen bei 250°C/1 h;

Bild 17:

Elektronenmikroskopische Durchstrahlungsaufnahme und Dunkelfeldaufnahme im Lichte eines Überstrukturreflexes sowie zugehörige Beugungsaufnahme von Titan Grade 4, kaltverformt und angelassen bei 350°C/1 h;

Bild 18:

Elektronenmikroskopische Durchstrahlungsaufnahmen und Beugungsaufnahme mit Überstrukturreflexen von Titan Grade 4, kaltverformt und angelassen bei 450°C/1 h;

Bild 19:

Elektronenmikroskopische Durchstrahlungsaufnahme von Titan Grade 4, kaltverformt und angelassen bei 250°C/1 h, Ausscheidung von rundlichen Teilen, die von Spannungsfeldern umgeben sind;

Bild 20:

Elektronenmikroskopische Durchstrahlungsaufnahme von Titan Grade 4, kaltverformt und angelassen bei 450°C/1 h, Ausscheidung von ovalen Teilen mit umgebenden Spannungsfeldern an Korngrenzen;

Bild 21:

Einfluss der Glühtemperatur auf die Dehngrenze R_p0,2 von Titan Grade 4;

Bild 22:

Einfluss der Glühtemperatur auf die Zugfestigkeit R_m von Titan Grade 4;

Bild 23:

Einfluss der Glühtemperatur auf die Dehnung A₅₀ von Titan Grade 4;

Bild 24:

Typische statische und dynamische Festigkeitseigenschaften von unlegiertem Titan, Draht, geglüht;

Bild 25:

Einfluss des Sauerstoffgehalts und der Kaltverformung auf die Zug- und Ermüdungsfestigkeit von unlegiertem Titan;

Bild 26:

Vergleich der Umlaufbiegewechselfestigkeit zwischen konventionellen Titan Grade 4 und dem erfindungsgemässen Titanwerkstoff und

Bild 27:

Einfluss der Anlassbehandlung auf die Ermüdungsfestigkeit des erfindungsgemässen Titanwerkstoffs.

With reference to the attached pictures, the detailed description of exemplary embodiments regarding the nature and the manufacturing process of the titanium material according to the invention is given below. The individual treatment and transformation processes in the titanium material are discussed in accordance with the sequence of the manufacturing process. Show it:

Image 1:

Corrosion resistance and tissue reactions of pure metals and alloys (from SCHROEDER / SUTTER / BUSER / KREKELER, loc. Cit., P. 46);

Picture 2:

Influence of oxygen and nitrogen on the mechanical properties of titanium at room temperature (from ZWICKER, loc. Cit., P. 220);

Picture 3:

Influence of oxygen on the mechanical values of unalloyed titanium (from BOYER / WELSCH / COLLINGS: Materials Properties Handbook. Titanium Alloys, ASM International - The Materials Information Society, 1994);

Image 4:

Influence of carbon on the mechanical values of unalloyed titanium (from BOYER / WELSCH / COLLINGS: loc. Cit.);

Image 5:

Influence of nitrogen on the mechanical values of unalloyed titanium (from BOYER / WELSCH / COLLINGS: loc. Cit.);

Image 6:

Influence of nitrogen, oxygen, carbon and iron in atomic% or% by weight on the Vickers hardness of unalloyed titanium (from BOYER / WELSCH / COLLINGS: loc. Cit.);

Image 7:

Solidification diagrams of titanium grade 1 - material 3.7025 and of titanium grade 2 - material 3.7035 (from DIN standard 17869);

Image 8:

Hardening behavior of titanium grade 1 - material 3.7025 and of titanium grade 2 - material 3.7035 according to the position of the rolling direction (from DIN standard 17869);

Image 9:

Solidification curves of titanium grade 1;

Image 10:

Solidification curves of titanium grade 2;

Image 11:

Solidification curves of titanium grade 4;

Image 12a:

Microstructure formation of recrystallized annealed rod material;

Image 12b:

Structure formation of the titanium material produced according to the invention as a rod material of the same dimensions as Figure 12a;

Image 13:

State diagram Fe-Ti (from MURRAY, JL: Phase Diagrams of Binary Titanium Alloys. ASM International - Metals Park Ohio, 1987);

Image 14:

Electron micrograph of the deformation structure of titanium grade 4, cold worked and tempered at 375 ° C / 1h;

Image 15:

Electron micrograph of the deformation structure of titanium grade 4, cold worked and tempered at 375 ° C / 1h;

Image 16:

Electron microscopic radiographs and diffraction images with superstructure reflections of titanium grade 4, cold worked and tempered at 250 ° C / 1 h;

Picture 17:

Electron microscopic radiograph and dark field image in the light of a superstructure reflex as well as the associated diffraction pattern of titanium grade 4, cold worked and tempered at 350 ° C / 1 h;

Image 18:

Electron microscopic radiographs and diffraction images with superstructure reflections of titanium grade 4, cold worked and annealed at 450 ° C / 1 h;

Picture 19:

Electron microscopic radiograph of titanium grade 4, cold worked and tempered at 250 ° C / 1 h, excretion of rounded parts, which are surrounded by stress fields;

Image 20:

Electron microscopic radiograph of titanium grade 4, cold worked and tempered at 450 ° C / 1 h, separation of oval parts with surrounding stress fields at grain boundaries;

Image 21:

Influence of the annealing temperature on the _{proof stress} R _p0.2 of titanium grade 4;

Picture 22:

Influence of the annealing temperature on the tensile strength R _m of titanium grade 4;

Image 23:

Influence of the annealing temperature on the elongation A ₅₀ of titanium grade 4;

Image 24:

Typical static and dynamic strength properties of unalloyed titanium, wire, annealed;

Image 25:

Influence of oxygen content and cold deformation on the tensile and fatigue strength of unalloyed titanium;

Image 26:

Comparison of the circular bending fatigue strength between conventional titanium grade 4 and the titanium material according to the invention and

Image 27:

Influence of the tempering treatment on the fatigue strength of the titanium material according to the invention.

Create the melt

The starting point for the production of the titanium material is a melt with the chemical composition according to Table 5 , which corresponds to the requirements for titanium implant materials according to ASTM F 67-89: Table 5 element Mass content [%] O 0.30 Fe 0.15 C. 0.007 N 0.01 H 0.0031

Iron is an accompanying element or an impurity and can be contained in the titanium ore up to 20% by weight. It has to be extracted in a treatment process in order to achieve the desired low iron content in the titanium sponge. The sponge quality used depends on the goal. The iron content of commercially produced titanium can vary from 0.035 to 0.5% by weight, depending on the use of very clean to heavily contaminated titanium sponge. A raw material with a relatively high iron content is used in a targeted manner. A possibly too low oxygen content is compensated for by adding titanium oxide.

The above proportions of oxygen, carbon, nitrogen and hydrogen are fully soluble in titanium. Experience has shown that the oxygen content for unalloyed titanium can be increased to 0.40% by weight. At even higher values, however, the ductility drops rapidly, and the hot formability in the dimension range of a melting block or a billet is reduced to such an extent that pronounced crack formation means that reproducible production is no longer possible. The maximum value of 0.40% by weight for oxygen specified in the relevant titanium standards should therefore not be exceeded. Otherwise, the oxygen content is fixed in ASTM standard F 67-89 depending on the titanium grade. With unalloyed titanium grade 1 with good ductility, the oxygen content can be reduced to 0.035% by weight. For grade 4 titanium implants, where the highest possible tensile strength R _m or fatigue strength σ _{bw is} required, the oxygen content is increased up to 0.40% by weight, normally up to 0.35% by weight. It has proven to be advantageous here to limit the oxygen content to 0.35% by weight - oxygen contents up to this value improve the strength without reducing the ductility too much.

Hardening by adding interstitially dissolved oxygen Figure 2 shows the influence of the proportions of oxygen, nitrogen and carbon on the mechanical properties - proof stress, tensile strength, elongation and hardness. With an oxygen content, for example of 0.30% by weight, the yield strength of conventional titanium is R _P0.2 ≈ 500 MPa, a tensile strength R _m ≈ 650 MPa and an elongation A of just over 20%.

Comparable values for the yield strength R _P0.2 and the elongation A can be seen in _Figures 3 to 5 . The carbon and nitrogen contents shown in Figures 4 and 5 are sometimes outside the maximum values permitted by ASTM standard F 67-89 (for carbon up to 0.10% by weight and for nitrogen between 0.03% by weight and 0.05% by weight depending on the titanium grade). The carbon and nitrogen fractions that actually occur in unalloyed titanium are ≈ 0.01% by weight. At higher proportions, there is a risk of the formation of titanium carbides and nitrides, which can lead to material breaks. The carbon and nitrogen fractions are considered to be normal admixtures and not as alloying elements.

The influence of the additions of nitrogen, oxygen, carbon and iron on the Vickers hardness as a function of their proportions in atomic% or in% by weight is shown in Figure 6 . The curves clearly show that nitrogen has the greatest influence on hardness, followed by oxygen, carbon and iron. The hardness can be doubled by adding 0.1% by weight of nitrogen or 0.2% by weight of oxygen and increased by 30% when adding 0.15% by weight of iron. These values apply to annealed titanium at 700 ° C / 1h in vacuum. At 700 ° C, the highest solubility for iron is in the α-lattice of titanium, which is not a hardened state.

thermal treatment

When manufacturing and processing titanium, particular attention must be paid to the fact that pure titanium undergoes a phase change at 882 ° C. Above this temperature the cubic space-centered β phase is stable and below the hexagonal α phase. The so-called β-transus at 882 ° C leads to a multitude of microstructures that range from globular β or α microstructures to martensitic constructions during rapid cooling from the β region. In the widely used α + β alloys, both phases occur side by side. The different phases not only have metallographically different appearances, but they also have completely different physical and mechanical properties.

When processing titanium from the melting block through the forging billets to the thin wire, this is always the case Ensure that hot forming temperatures, intermediate annealing operations or the final heat treatment are carried out exactly at the specified temperatures. Basically, however, the temperature control for unalloyed α-titanium and the α + β alloys usually takes place below the β-transus after the primary forming of the melting block.

Table 6 below gives an overview of the possible heat treatments for titanium and titanium alloys for setting specific material properties with the most important types of annealing - soft annealing, stress relieving, solution annealing and aging and the associated annealing temperatures ϑ (see also DIN standards 17869 and 65084). Within the entire process sequence for the production of the titanium material according to the invention, heat treatments known per se are carried out in some cases. Table 6 material (Soft) glow ϑ [° C] Stress relieving ϑ [° C] Solution annealing and aging ϑ [° C] Titan Grade 1 650-750 / air 450-550 / air --- Titan Grade 4 650-750 / air 450-550 / air --- TiAl6V4 700-850 / air 500-600 / air 820-900 / water and 480-600 / air

Softening

If the cold-formed titanium is annealed at elevated temperatures, the increase in strength achieved by the cold-forming process is more or less reversed. Complete softening down to the initial state is achieved by annealing above the recrystallization threshold (soft state). At annealing temperatures below the recrystallization threshold, only partial softening - called recovery - is brought about. The transition between recovery and recrystallization can be demonstrated most impressively by means of X-ray or electron microscopic examinations.

glow

The term "annealing" in the heat treatment of titanium denotes the reversal of a hardening caused by forming or hardening. As a rule, the annealing is carried out above the recrystallization temperature.

Recrystallization

A prerequisite for recrystallization - this means a complete recrystallization of the material - is a previous cold working. The recrystallization starts at the most disturbed places of the crystal lattice. No recrystallization occurs below a critical degree of deformation, which depends on the thermal and mechanical pretreatment, depending on the material. The lower recrystallization temperature (recrystallization threshold) decreases with increasing degree of deformation. In addition, the number of bacteria increases so that a more fine-grained structure is created.

recreation

In contrast to recrystallization, no new grains form during the recovery. It is a matter of changing places of atoms and empty spaces a reduction in dislocation density and polygonization. The temperatures used for a recovery anneal are below the recrystallization temperature and therefore only lead to a partial degradation (40% -60%) of the strain hardening. The yield strength R _p and the tensile strength R _m decrease, but the elongation at break A is increased.

Stress relieving

It serves to reduce residual stresses, e.g. after welding, machining or straightening. The process is carried out at low temperatures without changing the structure.

Solution annealing and outsourcing

This annealing is carried out in order to enrich the mixed crystal on the alloy components which are to be precipitated during the subsequent aging. The alloy components are dissolved in the supersaturated state during solution annealing. A prerequisite for the hardening of an alloy is a decrease in the solubility of alloy components as the temperature drops. A prerequisite for solution annealing is rapid cooling or quenching at the end of the annealing in order to keep the alloy components in the supersaturated solution. Table 6 does not contain temperature information for solution annealing and aging for titanium grades 1 and 4, since unalloyed titanium of this type cannot be hardened due to the normally low proportion of admixtures. No hardening alloy components are specifically added. The elements oxygen, nitrogen, carbon and hydrogen are fully soluble in titanium up to the specified maximum values. The iron originating from the titanium ore as an impurity is tolerated, for example, in titanium grade 4 up to 0.5% by weight (cf. ASTM standard F 67-89) and normally occurs in the form of coarse, spherical precipitations, preferably at grain boundaries.

Strain hardening

   mit a als einer Konstanten in MPa und n als dem Verfestigungsexponenten.Cold forming, which is an essential part of the process, is connected to the previous hot working with the optional recrystallization annealing, which is part of the manufacturing process of the titanium material. The titanium material treated in this way is now subjected to a cold deformation in the plastic area. This leads to an increase in the 0.2% proof stress R _p0.2 , the tensile strength R _m and the hardness, while the elongation at break A and the constriction Z decrease. Solidification depends on several factors. This is mainly influenced by the degree of deformation, the alloy composition, the microstructure, the type of deformation (eg drawing, rolling, hammering), the speed and temperature of the deformation. The following empirical approximation equation applies to the relationship between the deformation strength k _f [MPa] and the deformation ϕ (logarithmic value): $${\text{k}}_{\text{f}}{\text{= a ϕ}}^{\text{n}}$$

with a as a constant in MPa and n as the hardening exponent.

The hardening diagrams according to Figure 7 (Titan Grade 1 - material 3.7025 and Titan Grade 2 - material 3.7035) illustrate that both the yield strength R _p0.2 and the Tensile strength R _m steadily increase with the specified degrees of deformation. The elongation A ₅ initially drops steeply at degrees of deformation between 30% and 40% and then approaches values of 14% to 16%.

The hardening exponent n, as the degree for the hardening tendency of a material, is shown in Figure 8 (Titan Grade 1 - material 3.7025 and Titan Grade 2 - material 3.7035) depending on the position of the rolling direction. It becomes clear that with hexagonal titanium there is a strong directional dependency of the hardening exponent n with respect to the rolling direction. In the longitudinal direction, the highest values are measured for the mean hardening exponent n with 0.17 or 0.15 and for the transverse direction (90 ° to the rolling direction) the lowest values for the hardening exponent n ≈ 0.12.

The use of cold working enables the tensile strength R _m and yield strength R _p0.2 to be increased. At very high strain hardening, however, the elongation A drops to values <10%. For safety reasons, however, a residual ductility of <10% is not acceptable. It is therefore difficult to cold-work Titan Grade 1 and Titan Grade 4 in such a way that tensile strength values R _m = 860 MPa in combination with a minimum elongation A ≥ 10% are achieved. EP-B-0 436 910 provides a method for cold-forming unalloyed titanium with an intermediate annealing, this taking place below the recrystallization temperature and without the formation of cell structures. In this process, the tensile strength R _{m is} obtained through cold working steps increased from 30% each with intermediate annealing at temperatures ϑ <500 ° C to values between 900 MPa and 1000 MPa. The elongation A drops only moderately between the individual hardening steps, as a result of which the required 10% limit can be maintained.

The results of our own investigations to determine the hardening behavior of Titan Grade 1 are documented in Figure 9 , Titan Grade 2 in Figure 10 and Titan Grade 4 in Figure 11 . According to this, titanium grade 1 does not exceed the desired value of the tensile strength R _m of 860 MPa even at high degrees of deformation of 70%, but with degrees of deformation> 40%, the elongation A falls below the 10% limit. Titan Grade 2 behaves similarly. With 50% cold forming, the desired tensile strength R _m = 860 MPa has not yet been reached here, and with degrees of deformation> 40%, the elongation A also falls below the 10% limit.

Titan Grade 4 behaves differently with an oxygen content of 0.30% by weight, an iron content of 0.15% by weight and a basic strength of ≈ 700 MPa in the annealed condition. With a degree of deformation of 15%, the tensile strength R _m already exceeds the limit of 860 MPa and the elongation A is still ≈ 13%. It is a very fine-grained material with a grain size of ≈ 10 µm (measured according to ASTM standard E 112-88). The grain boundaries provide an additional consolidation contribution because they represent an obstacle to dislocations that build up in front of the grain boundaries and make further deformation more difficult (cf. BÖHM, H .: Introduction in the metallurgy. Bibliographisches Institut, Mannheim / Zurich, 1968).

   mit σ₀ und k als Konstante
stellt eine Beziehung zwischen der Dehngrenze σ_s und dem Korndurchmesser d her. Es wird ersichtlich, dass sich die Dehngrenze σ_s mit abnehmender Korngrösse erhöht. Dies ist eine fundamentale Voraussetzung für das erfindungsgemässe Verfahren. Die Korngrösse hat nicht nur einen positiven Einfluss auf das Verfestigungsverhalten - d.h. auf das Ansteigen der Dehngrenze R_p0,2, sondern auch auf die Ermüdungsfestigkeit σ_bw.The greatest dependence of the mechanical properties on the grain size is at the beginning of a plastic deformation, since here the influence of the grain boundary is most striking, while it decreases with increasing deformation - i.e. with increasing dislocation density. The Hall-Petch relationship (from BÖHM: loc. Cit., P. 103): $${\text{σ}}_{\text{s}}{\text{= <figure-callout id="0" label="σ" filenames="imgf0002.png,imgf0004.png" state="{{state}}">σ</figure-callout>}}_{\text{0}}{\text{+ kd}}^{\text{-½}}$$

with σ ₀ and k as a constant
establishes a relationship between the proof stress σ _s and the grain diameter d. It can be seen that the yield strength σ _{s increases} with decreasing grain size. This is a fundamental requirement for the method according to the invention. The grain size not only has a positive influence on the hardening _behavior - ie on the increase in the yield strength R _p0.2 , but also on the fatigue strength σ _bw .

Microstructure

Unalloyed titanium - it is an α modification at room temperature - has a mostly globular, single-phase α structure after hot or cold forming with subsequent recrystallization, which, depending on the deformation process, has a pronounced longitudinal extension. The own micrographs according to Figures 12a and 12b confirm the globular structure, whereby in Figure 12b the titanium material according to the invention shows an extremely fine-grained, elongated structure with a grain size of 10-11 (grain size according to ASTM standard E 112-88). The extremely fine-grained The structure results on the one hand from the relatively high iron content of 0.15% by weight - compared to the qualities usually used in medical technology with <0.05% above the highest solubility of iron in titanium lattice at ≈ 700 ° C - and on the other hand the intensive work hardening.

Iron normally separates at grain boundaries, especially at grain boundary coils, and vehemently hinders grain growth (see HÜLSE / KRAMER / BREME / SCHMIDT: Influence of small additions of Fe, Cr, Ni on the recrystallization behavior of commercially pure titanium.6th World Conference on Titanium, Cannes, 1988, p. 1975ff.). It has been found that iron additives severely hinder grain growth at 750 ° C ≤ ϑ ≥ 870 ° C annealing temperatures. While an extremely low iron content of 0.009% by weight achieves a maximum grain size of approx. 70 µm (corresponds to 4.5 according to ASTM standard E 112-88), the grain size with an iron content of 0.14% by weight % only a maximum of 20 µm (corresponds to 8 according to ASTM standard E 112-88). It is proven that the TiFe phase preferentially precipitates at grain boundaries and thus hinders growth during recrystallization.

A fine grain size of 10 (according to ASTM standard E 112-88) results in optimal consolidation and an increase in the 0.2 _{proof stress} R _p0.2 according to the Hall-Petch relationship. The fine-grain structure also increases the fatigue strength σ _bw . Only with the increased iron content of> 0.08% by weight, preferably from 0.10% by weight to 0.20% by weight, does the full inhibitory effect on grain growth come about Carry. The extreme fine granularity is assumed in the method according to the invention to achieve the desired mechanical properties and is used consistently.

Precipitation hardening

The procedure with the necessarily preceding steps concludes with the tempering treatment in which the precipitation hardening elements are excreted. However, these finest particles cannot be represented in the photoelectron microscopic image.

The separation of supersaturated mixed crystals is known to have a strong influence on the physical and mechanical properties of a material. Since segregation processes are generally associated with a significant increase in the _{proof stress} R _p0.2 , the tensile strength R _m and the hardness of an alloy, this is referred to as hardening. A precondition for a hardenable alloy is a dependency of the dissolving power for an added element on the temperature in the alloy system.

As can be seen in the state diagram of the two-component system titanium-iron (see Fig . 13 ), the maximum solubility for iron in titanium in the temperature range of approx. 700 ° C is 0.05% by weight. As the temperature drops, the solubility decreases sharply and is only 0.006% by weight at 400 ° C. At even lower temperatures, the solubility of titanium for iron should be less than 0.001% by weight. With the solubility limit of ≈ 0.05% by weight at ≈ 700 ° C, there is always in unalloyed titanium with higher iron contents TiFe excretions. Their shape is determined by the mass fraction in% by weight, the manufacturing process and the size range of the end product.

Mostly, unalloyed titanium is used in the annealed condition. Due to the needs of the chemical industry, Titan Grade 2 is clearly the most frequently used material. For higher demands on the mechanical properties, titanium grade 4 annealed is used. In order to achieve even higher tensile strengths R _m > 680 MPa (according to ISO standard 58321 / II), cold forming is necessary. The relevant standards do not refer to precipitation hardening of unalloyed titanium. This is certainly because the titanium grades Grade 1 to Grade 4 apart from oxygen - this is up to 0.4% interstitially dissolved - no metallic alloy element is added. The iron contained in the titanium ore rutile up to 20% by weight as an impurity is elaborately removed.

In order to be able to use precipitation hardening in the titanium-iron two-component system at all by separating the intermetallic TiFe phase, the prerequisite must be created in the form of thermal treatment as annealing or also as hot deformation in the temperature range of maximum solubility. This is followed by aging at temperatures far below 700 ° C in order to remove as much of the TiFe phase as possible.

When tempering supersaturated mixed crystals, coherent precipitates occur in the first decay stage, ie the phase which has separated out has the same lattice structure as that Matrix, but differs in composition. Coherent particles require a lower formation energy and, depending on the alloy, separate out flat, plate, spherical or needle-shaped. The second type of segregation is the partially coherent separation, in which there is coherence at least with an interface with the matrix. Partially coherent excretions are usually not the equilibrium phase, but rather metastable intermediate phases. According to the higher interfacial energy, the nucleation work in partially coherent phases is greater than in coherent phases. The last stage of the decay of a supersaturated mixed crystal is the elimination of an incoherent phase as a continuous or discontinuous elimination. The reason for the occurrence of coherent, partially coherent or incoherent phases lies in the different energy that has to be applied to form the interfaces.

In contrast to incoherent excretions, the easier nucleation in the intermediate phases leads to a higher nucleation frequency and consequently to a finer distribution of the excretions, which is associated with a greater hardening. Due to their higher nucleation work, incoherent phases are preferably eliminated at lattice defects such as dislocations and grain boundaries. The strong acceleration of segregation processes due to a previous cold working is therefore due to the increase in the number of foreign germs as a result of the increased dislocation density. These more detailed explanations of the different types of segregation processes are intended to lead to the processes of the method according to the invention.

For example, the production of grade 4 titanium rods with a diameter of 5 mm and the chemical composition according to Table 5 proceeds as follows. The melt is melted, forged, hot rolled and drawn into wire using conventional techniques. The thermal or thermomechanical treatment necessary for optimal curing takes place in the temperature range from 650 ° C to 750 ° C. This is followed by rapid cooling in order to keep as much iron as possible in solution. Annealing between 200 ° C and 500 ° C with steps of 25 ° are carried out.

No excretions of titanium-iron particles can be found in light microscopic images. The separated phases can be detected with electron microscopic radiographs in light and dark fields and with diffraction images. Figure 14 shows a typical electron micrograph of the deformation structure of material tempered at 375 ° C / 1h. Figure 15 shows a recovered deformation structure with the first recrystallization nuclei and some coarse titanium-iron precipitates in a spherical or plate shape, which is probably the body-centered cubic TiFe phase.

In order to determine the temperature range in which segregation processes take place, cold-hardened samples were left at 250 ° C, 350 ° C and 450 ° C for one hour each and examined in cross-section. The results are documented in Figures 16 to 18 . It is a non-uniform structure from areas with high dislocation density (strong cold deformation) and with very fine-grained recrystallized areas. The initial state before tempering therefore consisted of a partially recrystallized structure and the subsequent annealing at 450 ° C / 1 h obviously caused further recrystallization.

The diffraction images within the images 16 to 18 of all three heat treatment states show strikingly pronounced superstructure reflections that occur as a result of finely separated coherent titanium-iron particles. The particles were detected by dark field imaging in the light of a superstructure reflex (see Figure 17 ). At the low tempering temperature of 250 ° C, the particles have a rounded shape (see Fig . 19 ) and change to an oval shape at 450 ° C (see Fig . 20 ).

The orientation relationship between the hexagonal α-phase of titanium and the finely separated intermetallic titanium-iron phase has not yet been clearly clarified.

The very fine precipitates occurring in the process according to the invention, which have an orientation relationship to the hexagonal matrix lattice of α-titanium which is proven on the basis of the diffraction recordings, obviously require a high level of nucleation work, since in cold-formed titanium they preferentially distinguish themselves from lattice defects - for example dislocations - and grain boundaries. These places of higher energy are probably also necessary in order to produce this kind of fine excretions at all. The described excretion behavior can therefore be not cold-formed titanium are not expected in the same way.

An examination of the titanium alloy TiCu2 also showed the appearance of a very fine phase as the result of a precipitation annealing. The Ti-Si state diagram shows another alloy system with eutectoid segregation. Silicon additives of 0.2% by weight are already used in the high-strength α-titanium alloys to improve creep resistance and are generally suitable for a biocompatible, hardenable material - also for implants.

Annealing also has a significant share in the mechanical properties of the titanium material created. Comparative experiments on temper annealing with conventional implant material (iron content 0.05% by weight) and the inventive titanium grade 4 material, cold-formed, rod material, with an iron content of 0.15% by weight yielded the following results, which are shown in Figures 21 to 23 are shown. The annealing temperatures from 200 ° C to 500 ° C cover the entire curing range. It can be seen that for both materials the 0.2% proof stress R _p0.2 initially increases from ≈ 750 MPa to values of 800 MPa at annealing temperatures up to ≈ 250 ° C. Above this temperature, the yield strength R _{p0.2 of} the conventional material _{drops sharply} compared to the material _according to the invention (see Figure 21 ).

There is a similar difference in quality with regard to the tensile strengths R _m (see Figure 22 ) with a difference of ≈ 140 MPa. While the tensile strength R _{m of} the material according to the invention in the non-tempered state is 950 MPa and only falls below the desired specification value of 860 MPa at a tempering temperature of ≈ 400 ° C, this value is far from being achieved by the conventional material even in the non-tempered state.

Figure 23 shows a comparison of the conditions with regard to the elongation A ₅₀ , which is ≈ 10% for both materials when not tempered. Tempering temperatures ≥ 200 ° C can significantly increase the strain values A ₅₀ , especially for the conventional material, to 20%.

The improvements in the material according to the invention are clearly visible. The tensile strength R _m in the non-tempered state is 950 MPa, so confidently above the required specification limit of 860 MPa. Tempering reduces the tensile strength R _m by ≈ 100 MPa even at temperatures up to 400 ° C, while the conventional material only has a strength of ≈ 700 MPa at the same tempering temperature. The problem with every grade of work hardened titanium is always the ratio of tensile strength R _m to elongation A. If too much cold deformation is used, the desired tensile strength R _m can be achieved, but the required one The minimum elongation is not reached. This disadvantage is eliminated by tempering treatment, provided that there is a sufficiently high level of strength, which is reduced by tempering.

Precipitation hardening has an enormous influence on the fatigue strength σ _bw . Surgical implants, for example, are mainly stressed by fatigue. This is why this parameter is particularly important. This applies equally to hip joint endoprostheses, to osteosynthesis products and to dental implants, which of course are also subject to bending, torsional and shear forces.

The flexural fatigue strength σ _bw is by definition the stress that a material withstands 10 ⁷ load cycles without breaking. It is known to increase the fatigue strength of titanium by adding oxygen. In literature there are values for the fatigue fatigue strength σ _bw of titanium grade 4, work hardened in the range from 357 MPa to 430 MPa. These values concern alternating bending fatigue strengths, which were determined with electropolished sample surfaces. IMI Titanium Ltd. published values are shown in Figure 24 . Figure 24 shows that both the tensile strength R _m and the fatigue strength σ _bw of soft-annealed samples increase with increasing oxygen content, with the curve for the tensile strength R _m increasing more. The values for notched samples are ≈ 25% lower than those of the smooth comparison samples.

Figure 25 shows the influence of the oxygen content and the cold deformation on the tensile strength R _m and fatigue strength σ _bw . In both cases, an increase in the mechanical parameters can be observed due to the increase in the oxygen content and the cold deformation. The fatigue bending fatigue strength σ _bw of the most cold-formed material is 380 MPa and thus coincides with the other values known from the literature.

The circular bending fatigue strength σ _{bw of} the grade 4 titanium material according to the invention was determined and is shown in Fig. 26 in comparison with the conventional material for which σ _bw = 380 MPa was measured. As can be seen, the titanium material according to the invention has a fatigue strength of ≈ 500 MPa, which means an increase of ≈ 30%.

The improvement in fatigue strength achieved by the hardening _treatment has occurred completely unexpectedly, since the tensile strength R _m and the yield strength R _{p0.2 drop sharply} , especially for the conventional titanium material at annealing temperatures of 375 ° C (see Figures 21 and 22 ). Understandably, the elongation A increased accordingly at the relatively high tempering temperature.

When hardening work-hardened material, two different processes, the effects of which are opposite, take place. One process is softening through recovery processes that lead to a reduction in dislocation density and thus to polygonization, which is associated with a drop in strength. The second process concerns the hardness-increasing effect of the precipitation of coherent or partially coherent particles of the TiFe phase, as evidenced by electron micrographs. The separated phases therefore preferentially form at higher energy points, such as dislocations and grain boundaries, and block their mobility with the result of increased fatigue strength.

The further test results on the influence of the tempering temperature and the tempering duration on the fatigue strength are shown in Figure 27 . After a tempering period of one hour, there is apparently a maximum for the fatigue strength σ _bw in the temperature range between 350 ° C and 450 ° C. Surprisingly, the fatigue strength increases with a lower tempering temperature of 325 ° C after a longer tempering time of 10 hours.

As an option, solid solution hardening can be used within the manufacturing process for special use of the material. Mixed crystal hardening replaces titanium atoms with other metallic atoms that are soluble in titanium. Zircon, for example, is particularly suitable for this, since it can be completely dissolved in both the β phase and the α phase. Niobium and tantalum dissolve completely in β-titanium, but only to a limited extent in the α-phase, so that in this case α + β-titanium alloys are obtained. Mixed crystal hardening can advantageously be used to improve tensile strength Raise R _m decisively without having to accept an excessive loss of ductility. The inventive precipitation hardening can consequently also be applied to a two-, three- or multi-material system, where zirconium, niobium or tantalum or any mixture thereof are added to the base material titanium.

Numbered process example and product

• a) warmverformt und
• b) kaltverfestigt und
• c) ausscheidungsgehärtet ist und
• d) enthält:
  • da) 0,15% Eisen als ausscheidungshärtendes Element und
  • db) 0,30% interstitiell gelösten Sauerstoff und
• e) ein feinkörniges Gefüge der Korngrösse feiner als 5, vorzugsweise feiner als 8, aufweist (Gefüge gemäss Bild 12b; Korngrösse definiert nach ASTM-Norm E 112-88).

The production of the titanium material according to the invention and the parameters achieved in the process are described below using a concrete numerical example, the process sequence taking place as a result of the work steps listed below:
1.) Production of the titanium melt
The starting point was a titanium melt with the following chemical composition, which meets the requirements for titanium implant materials in accordance with ASTM F 67-89: Table 5 element Mass content [%] O 0.30 Fe 0.15 C. 0.007 N 0.01 H 0.0031
The melting block was melted twice in a vacuum arc furnace.
2.) Hot forming
To produce the raw material, the block was forged or rolled in several heat at 1000 ° C on billets with a diameter of 100 mm and then Hot-rolled on a wire mill to dimensions between 5.5 mm and 20 mm in diameter with falling temperatures between 880 ° C and 700 ° C.
3.) Cold forming
The present wire rods were subjected to cold forming by drawing with inserted intermediate annealing in the area or just below the recrystallization limit, the following mechanical values being achieved (see Table 7 ): characteristics Reading 0.2% proof stress R _P0.2 752 MPa Tensile strength R _m 950 MPa Elongation A ₅₀ 10.2% Constriction Z 39%
After drawing, the wire was straightened, divided to rod lengths = 3000 mm and ground to precision tolerance h7.
4.) Starting treatment
The tempering was carried out at 375 ° C / 1h air / air. The following mechanical values were then achieved for 5 mm rods (see Table 8 ): characteristics Reading 0.2% proof stress R _P0.2 794 MPa Tensile strength R _m 933 MPa Elongation A ₅₀ 13.2% Constriction Z 41% Fatigue strength σ _bw 500 MPa
The quality improvements of the titanium material achieved through the process are manifest. Compared to the measurements in the work-hardened state, the 0.2% proof stress R _{P0.2 rose} by ≈ 40 MPa, while the tensile strength R _m only decreased by ≈ 20 MPa. The elongation A ₅₀ improved by 3% to over 13% and is thus surely above the required specification value of 10%.
The grade 4 titanium material obtained (grade defined in accordance with ASTM standard F 67-89) can be characterized by the fact that it

• a) thermoformed and
• b) work hardened and
• (c) is age hardened and
• d) contains:
  • da) 0.15% iron as a precipitation hardening element and
  • db) 0.30% interstitially dissolved oxygen and
• e) has a fine-grain structure with a grain size finer than 5, preferably finer than 8 (structure according to Figure 12b ; grain size defined according to ASTM standard E 112-88).

Claims (19)

Titanium material, characterized in that this a) hot worked and optionally recrystallized and b) work hardened and (c) is age hardened and d) contains: da) one or more precipitation hardening elements in an amount of more than 0.05% by weight and db) interstitially dissolved oxygen in an amount of more than 0.05% by weight, preferably 0.20% to 0.35% by weight and e) has a fine-grained structure of grain size finer than 5, preferably finer than 8 (structure according to Figure 12b ; grain size defined according to the American Society for Testing and Materials standard - ASTM standard E 112-88). Titanium material according to claim 1, characterized in that the precipitation hardening elements are iron, copper and silicon and the titanium material thereof contains an element or these elements as a two or three component mixture. Titanium material according to one of claims 1 or 2, characterized in that it contains: a) 0.15 wt .-% iron and b) interstitially dissolved oxygen, namely in: Grade 1 titanium material: 0.07% by weight Grade 2 titanium material: 0.13% by weight Grade 3 titanium material: 0.20% by weight Grade 4 titanium material: 0.30% by weight (Grades defined according to the American Society for Testing and Materials standard - ASTM standard F 67-89). Titanium material according to one of claims 1 to 3, characterized in that it additionally contains: a) one or more mixed crystal hardening elements in an amount of up to 30 wt .-% per element, wherein b) the mixed crystal hardening elements are contained in a total amount of 50% by weight. Titanium material according to claim 4, characterized in that the mixed crystal hardening elements are zirconium, niobium and tantalum and the titanium material thereof contains one element or these elements as a two- or three-component mixture. Titanium material according to one of claims 1 to 5, characterized in that it contains: a) 0.15 wt .-% iron and b) 0.10% by weight of interstitially dissolved oxygen and c) as mixed crystal hardening elements up to 15% by weight of zirconium, niobium and tantalum, the titanium material of which contains one element or these elements as a two- or three-component mixture. Process for the production of a titanium material, characterized by the treatment steps in the sequence: a) alloying one or more precipitation-hardening elements and oxygen, provided that sufficient amounts of both are not already present in the titanium sponge or possible alloy components, the oxygen dissolving interstitially; b) hot forming and optionally recrystallization annealing to produce a fine-grained structure; c) cold working with possible intermediate annealing and d) Tempering treatment in which the precipitation hardening elements are excreted. A method according to claim 7, characterized in that a) iron, copper and silicon are alloyed as precipitation hardening elements, whereby b) only one element or a two- or three-component mixture of these elements is alloyed in until a proportion of more than 0.05% by weight is reached. Method according to one of claims 7 or 8, characterized in that oxygen up to a proportion of more than 0.05 wt .-%, preferably up to a proportion of 0.20 wt .-% to 0.35 wt .-% , is introduced, the oxygen going into solution interstitially. Method according to one of claims 7 to 9, characterized in that the following are alloyed: a) iron until a proportion of 0.15 wt .-% is reached and b) oxygen until a proportion of: 0.07% by weight for grade 1 titanium material 0.13% by weight for grade 2 titanium material 0.20% by weight for grade 3 titanium material 0.30% by weight for grade 4 titanium material (Grades defined according to the American Society for Testing and Materials standard - ASTM standard F 67-89). Method according to one of claims 7 to 10, characterized in that the following are additionally alloyed: a) one or more mixed crystal hardening elements until a proportion of at most 30% by weight of an element is reached, wherein b) the mixed-crystal-hardening elements are alloyed in until a total proportion of at most 50% by weight is reached. A method according to claim 11, characterized in that the mixed crystal hardening elements are zirconium, niobium and tantalum and an element or a two- or three-component mixture is alloyed therefrom. Method according to one of claims 7 to 12, characterized in that the following are alloyed: a) iron until a proportion of 0.15 wt .-% is reached and b) interstitially dissolving oxygen until a proportion of 0.20% by weight is reached and c) zirconium, niobium and tantalum as mixed-crystal-hardening elements until a proportion of 15% by weight is reached, one element or these elements being added as a two- or three-component mixture. A method according to claim 7, characterized in that the fine-grained structure produced by means of hot forming and optionally recrystallization annealing has a grain size finer than 5, preferably finer than 8 (structure according to Figure 12b ; grain size defined according to the American Society for Testing and Materials - ASTM standard). Standard E 112-88). A method according to claim 7, characterized in that the total degree of deformation is> 5%, preferably> 20%, which is used in the cold deformation following the hot deformation and optionally the recrystallization annealing. A method according to claim 7, characterized in that the tempering temperature ϑ _Anl > 100 ° C, preferably 250 ° C ≤ ϑ _Anl ≤ 450 ° C. A method according to claim 7, characterized in that the following applies for the starting time t _Anl : 0.5h ≥ t _Anl ≥ 24h, preferably 0.5h ≤ t _Anl ≤ 10h. Method according to claims 7 or 16 and 17, characterized in that the tempering time t _Anl ≈ 1h and the tempering temperature ϑ _Anl ≈ 375 ° C. Use of the titanium material according to one of the preceding claims in: a) medical technology as an implant, instrument or device and b) the watch and jewelry industry.

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