Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C27/00—Alloys based on rhenium or a refractory metal not mentioned in groups C22C14/00 or C22C16/00
  • C22C27/02—Alloys based on vanadium, niobium, or tantalum

Definitions

• hafnium being substitutable for zirconium up to a total of from 0.48 to 1.5% hafnium, from about 0.04 to 0.10% nitrogen, to provide an atom ratio of zirconium equivalent to nitrogen of 0.8 to 1.2, the balance being tantalum, and incidental impurities.
• This invention relates to a precipitation hardenable tantalum base alloy which is characterized by high strength at elevated temperatures.
• Patent No. 3,243,290 discloses a tantalum base alloy including tungsten and zirconium and/or hafnium with at least 0.01% carbon and only residual amounts of nitrogen normally less than 0.005%, which accounts for tensile properties being inferior to the alloys of this invention.
• tantalum base alloys are precipitation hardenable by suitable heat treatment.
• this invention is directed to a tantalum base alloy having no appreciable amount of carbon above minimal residuals but having predetermined small amounts of nitrogen and zirconium or hafnium, or both of the latter, is responsive to precipitation hardening treatment, the alloy including selected amounts of certain solid solution hardening elements, and therefore results in tensile strength and elongation properties which are significantly superior to those of prior known tantalum base alloys.
• wrought members have been produced from a cast member comprising a multicomponent tantalum base alloy comprising (a) certain proportions of tungsten, with molybdenum and rhenium being substitutable for a portion of the tungsten, and (b) a small amount of at least one metal selected from a group consisting of zirconium and hafnium, along with a small but critical amount of nitrogen, and the balance being tantalum except for incidental impurities and substantially no carbon.
• Tungsten is present in the alloy in an amount of from 7 to 10 weight percent. Molybdenum and rhenium may be substituted for part of the tungsten. Zirconium is present in an amount varying from 0.25 to 0.75 weight percent. Hafnium may be substituted on an atom-toatom basis for all or a part of the zirconium; namely, up to a total of from 0.48 to 1.50 weight percent of hafnium. The term zirconium equivalent is the weight of the zirconium plus half the weight of hafnium. Nitrogen is present in an amount of from about 0.04 to 0.08 weight percent to provide an atom ratio of zirconium equivalent to nitrogen of 0.8 to 1.2, and preferably about 1. The remainder of the alloy is entirely tantalum exprise a total of less than 0.1% of carbon, oxygen, silicon,
• the impurities oxygen and carbon are preferably less than 100 parts per million.
• tungsten may be replaced, on an equal weight basis, by up to 2 weight percent of molybdenum and/ or up to 3 weight percent of rhenium to provide a modified alloy having at least about 5 weight percent tungsten.
• Molybdenum and rhenium in these proportions are functionally equivalent to tungsten.
• the total tungsten equivalent (when molybdenum and/or rhenium are present) should not exceed weight percent in order to maintain satisfactory fabricability.
• the optimum composition within the foregoing ranges comprises, in weight percent, about 8 percent tungsten, about 0.5 percent zirconium, about 0.08 percent nitrogen, (Zr/N atom ratio being about 0.96) and the balance is tantalum.
• This alloy composition may contain hafnium substituted for the zirconium up to a total of 0.95% hafnium.
• the alloy may be melted by one of several procedures which ensure homogeneity and a minimum of contamination.
• high purity tantalum together with the proper amounts of selected alloying elements may be fed into a conventional nonconsumable arc melting furnace containing an inert atmosphere, such as argon, or a vacuum.
• the desired nitrogen content is obtained by adding strips of tantalum nitride to the melt.
• the resulting ingot is then remelted, preferably by consumable arc melting, to achieve homogeneity, then it is hot worked to the desired shape.
• Material of similar quality can also be produced by consumable electrode arc melting or electron beam melting techniques.
• the alloy may also be prepared by pressing powders, pellets or even shavings of tantalum and the selected alloy components together into a rod and consumably arc melting the rod. Nitrogen is added as tantalum nitride.
• Alloy A comprises in weight percent about 5.3 tungsten, about 1.56 percent rhenium, about 0.65 percent molybdenum, about 0.52 percent zirconium, about 0.08 percent nitrogen, and the balance essentially tantalum with small amounts of incidental impurities.
• Alloy B comprises in weight percent, about 8.1 percent tungsten, about 0.52 percent zirconium, about 0.08 percent nitrogen (the zirconium-nitrogen atom ratio being approximately 1), and the balance essentially tantalum with small amounts of incidental impurities. Hardness tests of samples of alloys A and B of the invention were made at room temperature and at various elevated temperatures listed in Table I.
• the tensile properties for the alloy A are shown for tests conducted at various temperatures and are compared with corresponding properties for two commercially available high strength refractory metal alloys commonly known as T-222 (tantalum base) and TZC (molybdenum base) in Table III.
• the alloy T-222 has a composition of about 10 weight percent tungsten, about 2.5% hafnium, and about 0.01% carbon with the balance being tantalum and incidental impurities.
• the alloy TZC has a composition in weight percent of about 1.2% .titanium, about 0.25% zirconium, about 0.15% carbon, and the balance being molybdenum with incidental impurities.
• the tensile properties of alloy A of this invention are superior tothose of alloys T-222 and TZC when tested at room temperature, 2000,F., and 2400 F.
• Alloys A and C of this invention were then tested to determine their creep properties at 1315 C.; for comparison with those of the alloys T-222 and TZC. The results are listed in Table IV below.
• the alloy C has a composition of 7.1% tungsten, 1.56% rhenium, 0.13% zirconium, 0.25% hafnium, about 0.04% nitrogen, and the balance being tantalum with incidental. impurities.
• the atom ratio of the total of hafnium and zirconium .to nitrogen is approximately 1. It was solution annealed at 2000 C. and not precipitation hardened before testing. Of course, precipitation hardening occurred during the test conducted at 1315 C.
• Alloys A and C exhibit equal superior resistance to creep deformation as compared to the commercially available alloys T-222 and TZC under the conditions of test listed in Table IV.
• the improved creep resistance exhibited by the alloys of this invention is attributable primarily to the dispersed secondary phase of the nitrides of hafnium and/ or zirconium.
• the strengthening of the tantalum base alloys of this invention is achieved by the solid solution strengtheners tungsten, molybdenum, and rhenium.
• the precipitation hardening is due to the interaction of the reactive metal additions; that is, hafnium and. zirconium, with nitrogen, which forms a coherent precipitate within the tantalum alloy matrix.
• a coherent precipitate is a particle that has registry with the parent lattice and no discrete interface exists between the particle and the matrix, thereby preventing dislocation movement until very high stresses are applied.
• carbon is present the benefits of optimum strength due to nitrogen derived by heat treatment are minimized because carbon forms a noncoherent precipitate located at a discrete interface between the particle and the matrix.
• a non-coherent precipitate does not strengthen to the same degree as a coherent precipitate.
• the interaction of both carbon and nitrogen with zirconium and hafnium forms a dispersed secondary phase that can be termed precipitation strengthening
• the precipitation kinetics for the carbides and the nitrides are different. Accordingly, it is critical to the alloys of this invention that they contain only up to 100 p.p.m. of oxygen and carbon as residuals. To be properly effective the amount of nitrogen should bev greater than 0.04 w/o.
• the alloys are primarily suitable for forging applications, but they exhibit excellent fabricability as demonstrated by their being readily processed to sheet form.
• the alloys of this invention are amenable to heat treatments to render the material in the overaged condition which permits fabrication using standard practices. Thereafter subsequent thermal treatment can be used to obtain a material having optimum strength properties.
• compositions of the alloys of this invention should be within the range listed above.
• the atomic ratio of the reactive metals, that is, hafnium and zirconium, to nitrogen should be maintained at substantially one for optimum resistance to creep deformation. Alloys of this invention having compositions in which the components are above the range listed are characterized by degradation of fabricability.
• the amount of oxygen be restricted to an amount of 100 parts per million or less.
• the tantalum base alloys of this invention provide for the attainment of superior tensile properties by precipitation hardening of a novel tantalum base alloy.
• the alloys have excellent hot fabricability and can be readily produced in sheet form.
• Subsequent precipitation hardening thermal treatment can be applied to the worked and shaped alloy to obtain optimum strength properties for such uses as turbine components and as high temperature testing machine parts.
• the material is susceptible to cold rolling from room temperature to about 500 F. for finishing into plate, bar, rods, or wire or other forms even in solution annealed condition.
• a precipitation hardened tantalum base alloy consisting of, by weight, at least one metal selected from the group consisting of tungsten, molybdenum, rhenium and mixtures thereof, the amount of tungsten being at least 5%, the molybdenum, when present not exceeding 2%, the rhenium, when present not exceeding 3%, the sum of tungsten plus molybdenum plus rhenium being at least 7% and not exceeding 10%, at least one metal selected from the group consisting of zirconium, hafnium and mixtures thereof, the total zirconium equivalent being from 0.25% to 0.75%, from about 0.04% to 0.08% nitrogen, with the atom ratio of zirconium equivalent to nitrogen being within the range of from 0.8 to 1.2, and the balance tantalum with incidental impurities, the alloy being characterized by a microstructure consisting of coherent zirconium equivalent nitride precipitate particles having registry with the lattice matrix and being substantially free of discrete interfaces between the particles and the
• the alloy of claim 1 in which there is about 7% tungsten, about 1.5% rhenium, about 0.1% zirconium, about 0.25% hafnium, about 0.04% nitrogen, and the balance being tantalum.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Manufacture And Refinement Of Metals (AREA)
• Heat Treatment Of Sheet Steel (AREA)

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Cited By (4)

Citations (4)

• 1966
  • 1966-12-29 GB GB58142/66A patent/GB1115760A/en not_active Expired
• 1967
  • 1967-01-13 FR FR91132A patent/FR1507927A/fr not_active Expired
  • 1967-01-13 BE BE692608D patent/BE692608A/xx unknown
• 1968
  • 1968-04-11 US US720453A patent/US3498854A/en not_active Expired - Lifetime

Patent Citations (4)

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