US3667940A - Columbium base alloy - Google Patents

Abstract

A COLUMBIUM BASE ALLOY IS DISCLOSED HAVING HIGH STRENGTH AND OXIDATION RESISTANT PROPERTIES PARTICULARLY AT ELEVATED TEMPERATURES AND ALSO ADEQUATE LOW TEMPERATURE DUCTILITY. THE ALLOY CONTAINS 20 TO 33% HAFNIUM, 11 TO 20% TUNGSTEN AND 0.5 TO 2% ZIRCONIUM, AND MAY CONTAIN SMALL AMOUNTS OF ONE OR MORE OTHER ELEMENTS INCLUDING ALUMINUM, IRON, CARBON AND TITANIUM TO MODIFY THE PROPERTIES OF THE ALLOY FOR VARIOUS APPLICATIONS, AND MAY ALSO CONTAIN UP TO ABOUT 4% OF TANTALUM WITHOUT MATERIALLY EFFECTING THE PROPERTIES OF THE ALLOY EXCEPT TO INCREASE ITS DENSITY.

Description

United' States Patent 01 ice 3,667,940 Patented June 6, 1972 Teledyne, Inc., Albany, 'Oreg. No Drawing.Filed Apr. 10, 1969, Ser. No. 815,219

Int. (:1. C22c 27/00 us. or. 75-174 9 ClalmS ABSTRACT OF THE DISCLOSURE A columbium base alloy is disclosed having high strength and oxidation resistant properties particularly at elevated temperatures and also adequate low temperature ductility. The alloy contains 20 to 33% hafnium, 11 to 20% tungsten and 0.5 to 2% zirconium, and may contain small amounts of one or more other elements including aluminum, iron, carbon and titanium to modify the properties of the alloy for various applications, and may also contain up to about 4% of tantalum without materially effecting the properties of the alloy except to increase its density.

BACKGROUND OF INVENTION The invention relates to an alloy which has both high strengths and good stress rupture properties as well as high resistance to oxidation at elevated temperatures and at the same time has sutficient low temperature ductility to enable them to be fabricated.

It is difiicult to obtain all of the properties referred to above. Prior alloys for use under stress at elevated temperatures have usually been deficient in one or more of the desired properties. Thus, for example, it has been necessary to deleteriously sacrifice oxidation resistance for high tensile strength or acceptable stress rupture properties or vice versa. In this application the term elevated temperatures is intended to mean temperatures above red heat, i.e. temperatures ranging from about 900 up to about 2400 I In accordance with the present invention an alloy containing between about 20 to 33% hafnium, about 11 to 20% tungsten, and about 0.5 to 2% zirconium by weight with the remainder essentially columbium provides a balanced combination of the desired properties and, in general, has greater oxidation resistance than previous columbium base alloys. This basic'alloy may be modified by the addition of small amounts of other elements to enhance certain of the properties of the alloy.

The hafnium in the columbium base alloy of the present invention provides high oxidation resistance and increased tensile strength at elevated temperatures when present in an amount at least equal to about 20%. An increase in the hafnium content above about 33%, however, tends to cause the tensile strength at elevated temperatures to again decrease. Tungsten also provides increased strength at elevated temperatures and is employed in amounts at least equal to about 11% but an increase in tungsten content above about 20% decreases low temperature ductility. The zirconium also increases high temperature strength and oxidation resistance when employed in amounts at least equal to about 0.5%. However, an increase in zirconium content above about 2% causes the alloy to become brittle and difiicult to fabricate.

The outstanding properties of the alloy can be characterized as that of high oxidation resistance and toughness throughout a range of temperatures from room temperatures to at least 2400 F., the term toughness being employed to mean a combination of high tensile strength, high impact strength and ductility. This toughness persists even in the range of 1200 to 1600 F. at which columbium base alloys are usually deficient in ductility. Also the stress rupture properties are adequate for all usual purposes.

It is therefore an object of the invention to provide an improved high temperature alloy having not only resistance to oxidation and excellent stress rupture properties at elevated temperatures but also having a characteristic toughness including high tensile and impact strengths and ductility over a range of temperatures from room temperature and below up to about 2400 F.

V DESCRIPTION OF PREFERRED EMBODIMENTS The amounts of the elements in the various forms of the alloy disclosed herein are given in percent by weight.

A general purpose alloy may, for example, contain 28% hafnium, 17% tungsten and 1.2% zirconium, the balance being essentially columbium. An alloy approximating this composition Will, in general have room temperature ductility of the order of 20% elongation with yield strengths of the order of 130,000 p.s.i. and ultimate strengths of 140,000 p.s.i. At 1400 F. the ductility as measured by elongation is greater than 10% with a yield strength of 80,000 p.s.i. and an ultimate strength of 85,000 p.s.i. or greater and at 2400 F. the yield strength is of the order of 35,000 p.s.i. and an ultimate strength of 40,000 p.s.i. and ductility of the order of 50% elongation. The oxidation resistance of the alloy will, in general, be sufiiciently great that the surface recession will be less than .006" and the oxide layer thickness less than .035" when the alloy is heated in a current of air at 2400 F. for 24 hours.

As indicated above each of the various properties of the new alloy can be materially enhanced by additions of small amounts of other elements while retaining the other properties at acceptable values.

It should first be pointed out, however, that tantalum frequently occurs as an impurity in columbium and and is difficult to remove therefrom. It can be tolerated in any of the various modifications of the alloy up to' about 4% and, in general, can be considered a diluent which reduces the amount of tungsten which can be employed in an alloy of given density, thus decreasing high temperature strength. It does provide a small improvement in stress rupture properties and toughness but not enough to justify its deliberate addition to the alloy. It, however, increases the density of the alloy and if a light weight metal is required, any appreciable amount of tantalum is undesirable.

Of the other elements referred to above, the addition of aluminum improves oxidation resistance at temperatures up to and including 2400 F. An amount of aluminum in excess of about 1% deleteriously impairs low temperature ductility and tensile strength at elevated temperatures as well as stress rupture properties at 2200 to 2400 F. The optimum amount of aluminum is about 0.2% to 0.25%.

The addition of small amounts of iron improves stress rupture properties at 2200 to 2400 F. Additions of iron in amounts above about 0.15% tend to cause the alloy to be brittle. The optimum amounts of iron for improvement of stress rupture properties is about 0.07%.

The addition of small amounts of carbon prow'des a heat treatable alloy, the carbon apparently going into solution above 3000 F. and precipitating out as finely dispersed complex carbides of the various metals upon cooling, the precipitated carbide being primarily hafnium carbide. The addition of carbon provides increased tensile strength at elevated temperatures as high as 2400 F. as well as low temperature ductility and increases toughness throughout the temperature range of room temperature to 2400 F. Increasing the carbon content above and stress rupture properties when the alloy is proc essed to provide the best low temperature ductility. Excess carbon also impairs oxidation resistance. The optimum amount of carbon for the purposes described above is about 0.1%.

The addition of titanium to the alloy improves fabricability at low temperatures or a few hundred degrees above room temperatures. It also improves oxidation resistance particularly when relative large amounts of carbon are present in the alloy and increases yield strength up to about 1400 F. It also reduces the density of the alloy but additions of titanium in excess of about 5% deleteriously decreases stress rupture properties.

It will be apparent that the range of compositions of the alloy of the present invention is about as follows:

In almost all instances the composition of the alloy of the present invention will fall within approximately the following ranges:

CbBalance Feto 0.15% Hf.25 to 32% 0-0 to 0.15% W13 to 20% Ti-O to Zr-0.7 to 1.8% Ta-0 to 4% Al-O to 0.5%

The composition of the alloy exhibiting the most desirable balance of properties will, in general, fall within approximately the following preferred ranges:

CbBalance Fe-0 to 0.07% Hf28 to 30% C-() to 0.1% W13 to 17% Ti0 to 5% Zr-0.7 to 1.8% Ta0 Al-O to 0.25%

As indicated above the various properties of the alloy can be enhanced for the particular applications of the alloys. For example, gas turbine blades such as those employed in jet engines must withstand high tensile stresses and have high oxidation resistance at elevated and intermediate temperatures and musthave excellent stress rupture properties as well as low temperature ductility. An example of the composition of the alloy which has about the lower acceptable limit of stress rupture properties is as follows:

Example I CbBalance Fe--0 Hf28% C-0 W-15.1% Ti--0 Zr-1.2% Ta3 .5 Al-0 The alloy having this composition had a stress rupture life of 52.5 hours at 2200 F. at 20,000 p.s.i. stress. This is substantially the basic alloy containing Hf 28%, W 17% and Zr 1.2% first referred to herein except for the 3.5% tantalum and lower amount of tungsten. If the tantalum had not been inthe coluinbium so as to appear in the final alloy, the tungsten percentage could have been raised to approximately 17%. to provide greater high temperature tensile strength. Nevertheless, the alloy had excellent oxidation resistance and toughness including good high temperature strengths and intermediate and lower temperature ductility.

Another example of composition of an alloy having higher stress rupture properties is as follows: A

Example I! CbBalance ;Fe 0.05% Hf28% C0.13% W14.6% Ti-O Zr-l.2% :Ta.-0.1% (residual) Al-O' 1:

This example also had excellent oxidation resistance properties and toughness at temperatures from room temperature to 2400 F. and had a stress rupture life at 2400 F. and 20,000 p.s.i. from two to three times that of a similar alloy in which no iron was present.

From a consideration of the compositions and properties Alloy Composition A Q Gas turbine vanes are not-subjected to as high stresses as turbine blades and it is therefore possible to enhance the oxidation resistant properties 'to a somewhat greater extent than in, an alloy for turbine blades. An example of the composition of the alloy which exhibited higher oxidation resistance with acceptable values of other properties primarily due to the addition of. aluminum is as follows:

' Example III CbBalance Fe-O A Hf30% C0.28% W--13% Ti,0 Zr-1.1% Ta--3.7% Al -0.25%

When oxidized at 2400 in air for hours, the surfacerecessio'n. was approximately .002" with an oxide, layer thickness of .035". This example exhibited a yield tensile strength'at 3%elong'ation at room temperature after having been annealed at 2400 F. for 6 hours of 132,000 p.s.i. and an ultimate tensile strengthof 136,- 000 p.s.i. At 1400 F. the yield tensile strength .at 3% elongation was 78,700 p.s.i. and the ultimate tensile strength 93,400. p.s.i. The, stressrupture life, after annealing at 2400 F. for onehour and measured at 2200 and 20,000 p.s.ifwas 11.6,hours 'with a 36% elongation. The.

'1 plus 2400 Rim 6 hoursandmeasured at 2000 F. and

20,000 p.s.i. was 138.9 hours with 22.3% elongation.

Although this example of the. alloy is acceptable for turbine vanes, studies of other alloys show that a lower carbon content will increase the stress rupture life and 7 this. reduction. will a1so increase" oxidation .resistance.

' Again the tantalum was presentin the materials from which the .alloy was made with consequent reduction of tungsten which could be advantageously employed.

Anotherexample of the compositionof an alloy which canheemployed forv turbine vanes is as follows:

oxide layer thic kness of .023". After annealing at 2400" F. for 6, hours, ithad a. yield tensile strength at room Percent Composl- Composition B tion Balance Balance 28 28 15 15 1. 2 1. 2 0 0. 25 0 0 0. 1 0. 1 5 0 0 0 The carbon in both of these compositions is at the approximate optimum value of 0.1% to provide an alloy of the greatest toughness throughout a temperature range from room temperature to 2400 F. with very good oxidation resistance. The difference between these compositions is that composition C contains 0.25% aluminum and composition B contains 5% titanium, both of which increase oxidation resistance, and low temperature tensile strengths, the titanium being particularly effective when carbon is present in the alloy. The titanium also decreases the density of the alloy and improves fabricability.

The requirements for turbine blades and turbine vanes are similar and the same composition of the alloy can be employed for both purposes, if the stress rupture properties are adequate for turbine blades. Thus the composition of an alloy which can be employed for both turbine blades and turbine vanes is as follows:

Example V CbBalance Fe0' Hf-29% C0.1% W14.5% Ti-0 Zr1.1% Ta-3.7 Al-O This example has exhibited a stress rupture life in excess of 30 hours when tested at 2200 F. under a 20,000 p.s.i. after annealing at 3400 F. for 2 hours plus 2400 F. for 6 hours and has exhibited room temperature ductility up to 33% elongation after annealing at 2400 F. for 6 hours. The alloy having this composition, when annealed at 2400 for 6 hours, also developed a yield tensile strength of 124,000 p.s.i. and an ultimate tensile strength of 133,000 p.s.i. at room temperature, and a yield tensile strength of 70,900 p.s.i. and an ultimate tensile strength of 93,600 p.s.i at 1400 F. It also had excellent oxidation resistance properties.

The tantalum content can be eliminated from the composition of Example V and the tungsten content increased to provide increased stress rupture properties. Also the hafnium content is desirably slightly decreased. A recommended alloy composition suitable for both turbine blades and vanes is therefore as follows:

Alloy Composition D Large bore gun barrels or their liners require high tensile strengths at elevated temperatures as well as high oxidation temperatures. 'Thealloy compositions of Examples III and IV can be employed for this purpose. Thus both of these examples exhibit superior oxidation resistance and excellent tensile-strengths at 1400 F, the alloy composition of Example III being somewhat better so far as oxidation resistance is concerned, as the oxide layer on this alloy is particularly hard and tenacious. The alloy composition of Example IV is somewhat better so far as low temperature ductility is concerned.

As in previous examples, the tantalum content of these examples can be eliminated, and the tungsten content increased. The hafnium content is also preferably decreased and the zirconium content increased-Also the carbon content of Example I11 can be brought to an optimum amount of about 0.1%. These recommended alloy compositions for gun barrels are approximately a follows:

The titanium content in alloy composition E and the aluminum content in alloy composition F provide enhanced oxidation resistance.

Another use of the alloy of the present invention is for sound suppressors for jet engines. These suppressors must withstand high tensile stresses and have high oxidation reistance and also the alloy from which they are made must exhibit ease of fabrication. The alloy composition of Example IV can thus be employed. The titanium and carbon content of this alloy composition provides good fabricability and the titanium enhances the oxidation resistance. As discussed above, the alloy having this composition exhibited a low temperature elongation of 13%, yield tensile strength of 129,000 p.s.i. and an ultimate tensile strength of 133,000 p.s.i. at room temperature and excellent oxidation resistance characteristics.

Another example of the composition of an alloy which can be employed for a sound suppressor is as follows:

Example VI Cb-Balance Hf-30% Ta-.3 6 (residual) The alloy having this composition had a surface recession ranging from .002 to .004" and an oxide layer thickness of .030" when heated at 2400 F. in air for 24 hours. The relative high carbon content imparted improved ductility to the alloy. Thus the alloy of this example, when tested for elongation at room temperature, had an elongation greater than 20% and retains ductility as determined by elongation tests of 6% and 7% after forging and before annealing. As in the alloy of Example IV, the titanium content also enhances oxidation resistance.

The tantalum content in either of these Examples V and VI serves no useful purpose and can be eliminated and in the alloy of Example V I, the tungsten content is preferably increased. Thus two recommended alloy com-. positions for sound suppressors are as follows:

In common with other alloys for use at elevated temperatures the fabricated alloy of the present invention can be coated with primary coatings to improve their oxidation resistance. There are a number of commercial coatings, most ofwhich' consist largely of silicon and which can be applied as a powder to heated surface areas, or applied as ,a slurry to cold surfaces and after drying heated in an inert atmosphere to about 2400 F. These coatings are of assistance in preventing oxidation of the exposed surfaces of the fabricated alloy, but good oxidation resistance of the alloy itself is still of major importance. Thus pin holes may be present or develop in the coating or portions of the coating may be accidentally removed to expose the alloy to oxidizing conditions at high temperature. This can result in rapid destruction of elements fabricated from an alloy unless the alloy itself has high oxidation resistance at such temperatures.

The alloy of the present invention can be prepared by conventional methods usually involving the employment of electron beam or electric arc melting, or a combination of the two, under vacuum conditions. The alloying materials should be of high purity so far as undesired elements are concerned, although small amounts of residual impurities will inevitably be present. 7 I

I claim: I,

1. A columbium base alloy consisting essentially by weight of about 28 to 33% hafnium, 11 to 20% tungsten, 0.5 to 2% zirconium, up to about 1% aluminum, up to about 0.15% iron, up to about 0.33% carbon, 'up to about titanium and up to about 5% tantalum, the balance being essentially columbium.

2. An alloy in accordance with claim 1 in which the amount of hafnium is about 28 to 32%, the amount of tungsten is about 13 to 20%, the amount of zirconium is about 0.7 to 1.8%, the amount of aluminum is up to about 0.5%, the amount of iron is up to about 0.15 the amount of carbon is up to about 0.15 the amount of titanium is up to about 5%, the amount of tantalum is up to about 4% and the balance is essentially columbium.

3. An alloy in accordance with claim 1 in which the amount of hafnium is between about 28 and 30%, the amount of tungsten is between about .13 and 17%, the amount of zirconium is between about 0.7 and 1.8%, the amount of aluminum is up to about 0.25%, the amount of iron is up to about 0.07%, the amount of carbon is up to about 0.1%, the amount of titanium is up to about 5%, and the balance is essentially columbium.

4. The alloy in accordance with claim 3 in which the amount of hafnium is about 28%, the amount of tungsten is about 17%, the amount of zirconium is about 1.2%, the amount of iron is about 0.07%, and the balance is essentially columbium.

S. The alloy in accordance with claim 3 in which the amount of hafnium is about 28%, the amount of tungsten is about 15%, the amount of zirconium is about 1.2%, the amount of carbon is about 0.1%, the amount of titanium is about 5%, and the balance is essentially columbium.

6. An alloy in accordance with claim 3 in. which the amount of hafnium is about 28%, the amount of tungsten is about 15%, the amount of zirconium is about 1.2%, the amount of aluminum is about 0.25%, the amount of carbon is about 0.1%, and the balance is essentially columbium.

7. An alloy in accordance with claim 3 in which the amount of hafnium is about 28%, the amount of tungsten is about 17%, the amount of zirconium is about 1.2%, the amount of carbon is about 0.1%, and the bal ance is essentially columbium. v

8. An alloy in accordance with claim 3 in which the amount of hafnium is about 28%, the amount of tungsten is about 15%, the amount of zirconium is about 1.8%, the amount of carbon is about 0.1%, the amount of titanium is about 5%, and the balance is essentially columbium.

9. An alloy in accordance with claim 3 in which the amount. of hafnium is about 28%, the amount of tungsten is about 15%, the amount of zirconium is about 1.8% the amount of aluminum is about 0.2% the amount of carbon is about 0.1%, and the balance is essentially columbium.

References Cited UNITED STATES PATENTS 3,125,445 3/1964 Lottridge l74 3,152,891 10/1964 Begley 75174 3,173,784 3/1965 Wlodek et a1. 75l74- 3,317,314 5/1967 VVlodek et a1. 75l74 3,341,370 9/1967 Bradley et al. 148-433 OTHER REFERENCES Development of Columbium and Tantalum Alloys for Elevated-Temperature Service, Bureau of Mines Report 6558, 1964, pp. 6 and 7.

CHARLES N. LOVELL, Primary Examiner