DE2000256A1 - TITANIUM ALLOY - Google Patents

Description

The invention relates to a titanium alloy which is suitable for use at high temperatures under load.

Alpha-type titanium alloys have a high strength to weight ratio and good creep resistance even at high temperatures, heat resistance, toughness and other desirable properties.

Titanium alloys are widely used in the manufacture of aircraft and spacecraft, where material performance and efficiency are critical sizes. In this field of application, an alloy must have high strength and good creep resistance, resistance and toughness even at high temperatures. At the same time, it should be machinable, ductile and weldable. The influences of individual alloy elements on titanium are known to a certain extent. However, the overall effect of several alloying elements on the properties of titanium alloys is difficult to estimate in advance given the large number of alloying elements in question and the different amounts in which they can be added. For example, aluminum is known to be an alpha stabilizer and titanium provides strength even at high temperatures, but more than about 8% aluminum affects ductility and stability.

Zirconium and tin are generally cited as alpha stabilizers alike. They can, to some extent, be used as alpha stabilizers in place of aluminum to compensate for the fragility and instability that otherwise occur with high aluminum content. It is also known that generally beta-stabilizing additives such as Mo, V, Cb, Ta, W, etc. make titanium noticeably stronger and hardenable, but at the expense of specific gravity, oxidation resistance and usually creep resistance. It is also known that certain alloying additives such as silicon can be used to increase creep resistance, but above certain limits the toughness is compromised. It is therefore an object to find critical compositions and amounts of alloying additives which meet the demand for increasing heat resistance and creep resistance without reducing the original ductility and toughness which make the alloys primarily useful.

There is therefore a need for an alpha titanium alloy with low specific gravity, heat resistance, stability and creep resistance, as well as toughness, weldability and ductility, the combination of all these properties being said to outperform known titanium alloys.

To solve this problem, a titanium alloy with a specific weight of 4.5 to 4.6 kp / dm [high] 3, a tensile strength of at least 84 kp / mm [high] 2 at room temperature, good thermal stability and creep deformation of less than 0.2% after one

Load of 35.15 kp / mm² for 100 hours at 510 ° C with the following composition suggested

5.5 to 6.5% aluminum 1.5 to 4.0% tin 0.5 to 2.5% zirconium 0.5 to 1.5% molybdenum 1.0 to 2.0% tungsten 0.05 to 0 , 25% silicon up to 0.2% oxygen up to 0.4% total of other components including possible impurities, the remainder titanium.

According to the invention, aluminum is added between 5.5 and 6.5%, mainly in order to increase the high temperature and creep strength and to reduce the specific weight of the alloy. If the aluminum content is less than 5.5%, the strength-to-weight ratio will be decreased, and if the aluminum content is more than 6.5%, the high temperature and creep strength will be inferior. Aluminum within the limits provided according to the invention stabilizes the alpha phase and improves the resistance to oxidation. When 5.5 to 6.5% aluminum is present in the titanium alloy, the result is mainly alpha structure. Small amounts of molybdenum, tungsten and silicon do not significantly change the alpha character of this titanium alloy.

The tin content of 1.5 to 4.0% is critical. Up to about 2% tin increases the creep resistance in a titanium alloy with aluminum more effectively than a corresponding aluminum additive, but with only a third loss of stability. Above 3%, tin is only a third as effective as aluminum, increasing density and reducing elasticity to undesirable levels. Below about 1.5% tin, the creep resistance is reduced to an undesirable extent. On the other hand, tin works to solidify the solid solution, which remains effective at higher temperatures.

Zirconium works to solidify the solid solution in the alpha phase and in the small amount of beta phase that is present in the titanium alloy of the present invention. By and large, zirconium is not as effective as a strengthening element in the titanium alloy of the present invention as is aluminum and tin. It is quite useful in amounts up to about 2%. Above about 2.5%, its strength-increasing effect no longer outweighs its tightness-increasing effect.

In the alpha range, aluminum, tin and zirconium work in conjunction with one another. Of the quaternary Ti-Al-Sn-Zr alloys, the alloy Ti-6Al-2Sn-2Zr has an optimal high-temperature creep strength combined with a low density. However, the Ti-6Al-2Sn-2Zr-alpha-titanium alloy has too low a tensile strength to be useful. Small amounts of beta-stabilizing elements are known to increase tensile strength, but creep resistance, stability and other valuable properties are also improved by beta-stabilizing additives.

Molybdenum forms continuous series of solid solutions with beta titanium and its solubility in alpha titanium is less than 1%. The small amount of beta phase, which is stabilized by the addition of 1% molybdenum, serves three purposes, namely, on the one hand, to create a certain heat treatability and increase the strength up to high temperatures, secondly, to increase the creep resistance and, thirdly, to improve the ability of the alloy large to withstand static loads with notches with shape numbers small alpha [deep] K> 8. Additions of molybdenum significantly above about 1.5% cause a reduction in the high-temperature creep strength and weldability in undesirable degree. Less than about 0.25% does not impart the desired properties mentioned. The use of molybdenum alone undesirably lowers the creep resistance.

The addition of tungsten to the titanium alloy according to the invention, like molybdenum, stabilizes the beta-titanium phase, but in contrast to molybdenum, the beta-titanium stabilized by tungsten decomposes eutectoidally at about 28% tungsten and a temperature of about 720 ° C. Both alpha and beta titanium containing tungsten at the solubility limit can coexist at the end point of solid solution of tungsten in titanium. Therefore, alpha titanium alloys with additions of tungsten and below the beta transus, heat-treated, can consist of an alpha phase and a solid solution of tungsten. In creep environments, tungsten in alpha titanium alloys not only contributes to the solidification of the solid solution of any beta phase present, but also causes dispersion hardening after suitable heat treatment, which increases the creep resistance. Large additions of tungsten not only undesirably increase the specific weight of the alloy, but also increase instability and brittleness. The effective range of tungsten in the titanium alloy according to the invention should therefore be one to two%. Optimal creep properties in conjunction with other desirable properties are achieved in this area.

Silicon is a beta-eutectoid stabilizer of titanium like tungsten. But unlike tungsten, silicon forms solid bonds with titanium. So Ti [deep] 5Si [deep] 3, TiSi and TiSi [deep] 2 have been found. In the binary system, Ti [deep] 5Si [deep] 3 can coexist with solid alpha and beta solutions of silicon in titanium.

Silicon also forms compounds with zirconium, molybdenum and tungsten, but not with aluminum or tin. Heat treatments in the alpha-beta field enable molybdenum and tungsten to transition to the beta phase. Silicon improves creep resistance, especially in the alpha phase. A minimum of about 0.05% silicon is necessary to increase creep resistance. The creep instability sets in above about 0.25% silicon, so that toughness and ductility are lost in the event of creep deformation. The use of silicon in this alloy is important, especially in conjunction with tungsten. Together they lead to a creep hardening of the beta phase and the alloy back to the level of Ti-6Al-2Sn-2Zr with the common beneficial advantage of increased tensile strength. By heat treatment low in the alpha-beta phase range, the beta phase is enriched in molybdenum and tungsten. With subsequent hardening, the beta phase decomposes into alpha phase with a dispersion of tungsten. This has the further effect of eliminating the tendency to reduce creep strength in the presence of beta phase.

Oxygen should only be present in amounts up to about 0.20% in the titanium alloy according to the invention. For high temperature applications, the oxygen content should be up to 0.15%. Titanium and alloying elements suitable for making alloys and containing less than about 0.10% oxygen are available, but alloys of such purity are correspondingly more expensive. At 0.07% oxygen, the alloy has excellent toughness and stability with excellent other properties.

A preferred alloy composition from the claimed range specified above consists of 6% aluminum, 2% tin, 2% zirconium, 1% molybdenum, 1.5% tungsten,

0.1% silicon, up to 0.12% oxygen, the remainder titanium and usual impurities.

Unavoidable impurities from the titanium sponge and the alloy additives used can be present in the titanium alloy according to the invention, but should not total more than about 0.4%. With regard to each individual alloy element contained, the impurities should not impair the essential character of the alloy and its described properties.

The specific gravity of the titanium alloy according to the invention will be between 4.5 and 4.6 kp / dm [high] 3, it being dependent on the content of alloying elements with high and low specific gravity. The specific gravity of the above-mentioned preferred alloy from the claimed range is approximately 4.54 kp / dm [high] 3.

The characteristic strength of the titanium alloy according to the invention is essentially comparable or better than that of the commercially available Ti-5Al-2.5Sn or the Ti-6 Al-4V alloy. At a certain load above 455 to 593 ° C, however, the creep strength of the titanium alloy according to the invention will be up to about 1000 units better than that of a commercially available Ti-6Al-2Sn-4Zr-2Mo alloy, measured with a parameter that reflects the effects of temperature and time according to the formula (460 + ° F) * (20 + log hours) recorded in the manner explained in more detail below. This corresponds to an improvement over the Ti-6Al-2Sn-4Zr-2Mo alloy with regard to the creep strength of about 56 to 85 kgf / cm 2 at a temperature of about 455 and 593 ° C. and times on the order of 150 hours . At 593 ° C., the titanium alloy according to the invention is almost twice as resistant to creep as the one mentioned above well-known titanium alloy. The advantage in terms of creep resistance of the titanium alloy according to the invention over the known titanium alloy increases with the loading time, because the temperature-time parameter of the latter is P = (460 + [° F) * (15 + log hours).

The notched impact strength of the alloy according to the invention with a low oxygen content is 1.66 to 2.5 mkp at room temperature. The generally good toughness of the alloy according to the invention with a low oxygen content is further demonstrated by a notch tensile strength of about 148 kp / mm [high] 2 after a creep load with a shape number small alpha [low] K = 8 and a permanent deformation of 1.27 mm / min .. Samples with the same shape number subjected to a creep test usually withstand a load of 120 kp / mm [high] 2 for at least 5 hours.

A typical melt analysis of a titanium alloy from the range according to the invention with the nominal composition Ti-6Al-2Sn-2Zr-1Mo-1.5W-0.1Si showed 6.13% Al, 1.94% Sn, 1.98% Zr, 0, 99% Mo, 1.68% W, 0.107% Si, 0.067% O [deep] 2, 0.053% Fe, 0.002% N [deep] 2, the remainder being titanium. The beta transus of this alloy was found at 1000 ° C and the specific gravity was 4.54 kp / dm [high] 3. The modulus of elasticity changed with the temperature in the manner shown in Table 1:

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The following Table 2 shows the strength properties at room temperature and elevated temperature of samples of the composition indicated above after they had undergone various heat treatments which are indicated in the table.

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Table 2 shows the excellent room and high-temperature properties, namely strength and toughness in various heat treatments up to 593 ° C.

Table 3 below shows the excellent creep properties of the titanium alloy according to the invention after the heat treatments indicated in the table.

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Table 4 below gives additional values with regard to the creep properties and also the toughness of samples of the composition given above, which have been heat-treated in the following manner: 1066 ° C - 1/4 hour and air cooling, + 843 ° C - 1 hour and air cooling, + 593 ° C - 8 hours and air cooling.

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The preferred heat treatment consists essentially of cooling from the beta region to about 816 ° C., at which temperature the alloy is held until the alloy additives have effectively distributed between the alpha and beta phases. Alternatively, the alloy can be cooled to room temperature, then reheated and held at 816 ° C. Subsequent curing at 565 to 650 ° C serves to further equalize and stabilize the phases and to increase tensile and creep strength. The cooling rate is not critical, but higher cooling rates produce greater tensile strengths with smaller losses in creep strength and ductility.

The titanium alloy of the present invention can be made in any convenient conventional manner in which titanium and alloying elements are melted to a substantially homogeneous composition and either cast into blocks or sprayed into a cooling chamber to produce granules, followed by further processing. According to a preferred melting technique, the alloy elements are mixed in crushed form with titanium sponge and shaped under pressure to form electrodes, which are melted into blocks in a water-cooled copper crucible under an arc in a vacuum. The first blocks that are melted in this way are descaled in acid and remelted as self-consuming electrodes in a vacuum to increase the homogeneity and remove impurities. Forming, such as forging or rolling, is preferably carried out in the beta area before the heat treatments mentioned above are carried out.

Fig. 1 shows a graph of the Larsen-Miller parameter for a plastic deformation of 0.2% of the preferred alloy according to the invention, i.e. the alloy Ti-6Al-2Sn-2Zr-1Mo-1.5W-0.1Si compared to a commercial alloy Ti-6Al-25n-4Zr-2Mo.

Fig. 2 is a similar graph plotting the change in the Larsen-Miller parameter as a function of the amount of creep load on the alloy.

In these drawings, the creep load small delta in kp / mm [high] 2 is plotted on the ordinate and the Larsen-Miller parameter on the abscissa. The Larsen-Miller parameter P is calculated using the following formula:

Where T ° [low] R is the temperature during the loading in ° Rankine, t is the loading time in hours for a preselected amount of creep deformation and C is a constant of the material being investigated. The constant C = 20 for the alloy examined.

The creep test results, which are plotted in the diagrams, were obtained by subjecting the samples to a creep load at different temperatures and under different loading conditions in order to achieve the preselected degree of deformation in time t at each temperature of the test. Before the creep tests, the samples were heat-treated as follows: 1066 ° C-1/4 hour - air cooling + 843 ° C - 1 hour - air cooling + 593 ° C - 8 hours - air cooling.

1 shows that, apart from in the range between 28 and 46 kp / mm [high] 2, an improvement in the Larsen-Miller parameter of 1000 units by the preferred alloy within the claimed range compared with the above-mentioned known titanium alloy used for comparison achieved could be. In the range between 28 and 46 kp / mm [high] 2, the improvement was approximately 750 Larsen-Miller parameter units.

The following Table 5 shows the effect on creep resistance and toughness when changing the contents of alloy elements in the titanium alloy according to the invention. To obtain the values for this table, samples of each of the alloys listed in the table were heat-treated at 1066 ° C. for a quarter of an hour with subsequent air cooling to room temperature. Individual samples of this were then heat-treated for one hour at temperatures which differed from one another in the range from 788 to 899 ° C. by intervals of 38 ° C. each, with subsequent air cooling. All samples were then heat treated for 8 hours at 593 ° C followed by air cooling. All samples were then examined for their creep and then tensile properties after a creep load at 538 ° C. under a load of 31.5 kp / mm [high] 2 for 150 hours. From each group of alloys tested, the sample that exhibited the best combination of creep and toughness properties was selected and these values were entered in Table 5 for each alloy tested.

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From the above values it follows that titanium alloys according to the invention with contents of aluminum, tin, molybdenum, tungsten, zirconium and silicon within the claimed limits under the creep load conditions specified above have a creep deformation of about 0.2% and less and good tensile strength properties according to the Exhibited creeping.

Claims (2)

1. Titanium alloy with a specific gravity of 4.5-4.6 kp / dm [high] 3, a tensile strength of at least 84 kp / mm [high] 2 at room temperature, good thermal stability and creep deformation of less than 0.2% a load of 35.15 kp / mm [high] 2 for 100 hours at 510 ° C, characterized by the following composition: 5.5 to 6.5% aluminum 1.5 to 4.0% tin, 0.5 to 2.5% zirconia 0.5 to 1.5% molybdenum 1.0 to 2.0% tungsten 0.05 to 0.25% silicon up to 0.2% oxygen up to 0.4% total of other components including possible impurities, the remainder titanium. 2. Titanium alloy according to claim 1, characterized by 6% aluminum, 2% tin, 2% zirconium, 1% molybdenum, 1.5% tungsten, 0.1% silicon, up to about 0.12% oxygen, the remainder titanium as well usual impurities.