US3070468A - Method of producing dispersion hardened titanium alloys - Google Patents

Description

D. 25, N. J. GRANT METHOD OF PRODUCING DISPERSION HARDENED TITANIUM ALLOYS Filed 001'.. 29, 1958 ATTORNEY 3,070,468 Patented Dec. 25, 1962 tice 3,070,468 METHOD OF PRODUCING DSPERSION HARDENED TITANIUM ALLOYS Nicholas il. Grant, Leslie Road, Winchester, Mass. Filed Oct. 29, 1953, Ser. No. 770,392 13 Claims. (Cl. 14S-20.3)

use as a load carrying structural element at elevated temperatures. Unfortunately, this is not the case as wrought titanium and zirconium are characterized by low resistance to creep at relatively high temperatures.

With respect to titanium, it is subject to instability when used over cer-tain temperature ranges arising from the transformation of beta to alpha or intermediate phase products, whereby the physical properties are adversely aected. The alpha phase, which comprises an hexagonal-close-packed structure, is the desirable structure prop'- ertywise. structure, has lower strength properties and, therefore, is less desirable. Hence, the addition of alloying elements which tend to promote the formation of the beta .structure is avoided as far as it is possible while those which rtend to stabilize the alpha phase are preferred. However, only a few elements, such as aluminum or tin, are alpha stabilizers. Any method for strengthening titanium and titanium-base alloys Without relyingr on phase stabilization or transformation would be desirable, particularly any method which would increase the load carrying power up to 1200 F.

I have discovered a method whereby the high temperature strength properties of titanium group metals (i.e. Ti, Zr and Hf), particularly titanium and its alloys, can be greatly improved `by utilizing the principle of dispersion hardening. The hardening and strengthening of these metals are achieved by relying on -their propensity to absorb oxygen in solid solution. Titanium in particular has a high ainity and solubility for oxygen (about 15% by weight oxygen.) which normally has a deleterious effect on its properties.v My discovery resides in producing a titanium-base alloy containing a small amount of a rare earth metal in solid solution therewith, which alloy is hardened bv subsequent treatment under controlled oxidizing conditions to convert the rare earth metal to finely dispersed refractory oxide.

It is the object of this invention to provide a method for dispersion hardening titanium group metals and their alloys, in particular titanium and titanium-base alloys.

It is also an object to provide a dispersion hardened wrought compositori of titanium group metals and their alloys. l

Another object is to provide as an article of manufacture a titanium or titanium alloy composition characterized by improved streng-th properties at elevated temperatures.

These and other objects will more clearly appear from the -following disclosure and the accompanying drawing, wherein:

FIG. 1 is an equilibrium diagram of the titanium-rich binary showing the various phases that exist for cerium contents ranging up to about 6%.

FIG. 2 is an equilibrium diagram of the titanium-rich portion of the titanium-lanthanum binary showing the The beta phase, which is `body-centered-cubic l various phases that exist for lanthanum contents ranging up to about 6%.

FIG. 3 is a representation of a photomicrograph taken at 2500 times magnification showing a dispersion of CeO2 in an equiaxed alpha matrix of titanium after internally oxidizing a titanium alloy containing 0.7% Ce; and

FIG. 4 is a representation of a photomicrograph at 2500 times magnification of an internally oxidized Ti-Ce alloy which contained 0.7% Ce after being creep tested to fracture for 396 hours at 538 C. and a stress of 5000 p.s.1.

One aspect of the invention comprises forming an alloy of titanium with an amount of a rare earth metal ranging up to its solid solution maximum in the titanium alloy. In the case of cerium or lanthanum (note FIGS. l and 2), the amount may range up to about 5% by weight. The binaries of FIGS. l and 2 are derived from the work of E. N. Savitskii and G. C. Burkanov (Phase Diagrams for the Systems Titanium-Lanthanum and Titanium-Cerium; Academie of Science Journal of Inorganic Chemistry, vol. 2, No. 1l, 1957, page 2609). It is appreciated that these binaries may be subject to refinement through additional investigation and, therefore, the maximum of 5% given above could vary one way or the other. In any event, so long as the amount of rare earth metal employed ranges up to its equilibrium maximum in the solid solution, the results of the invention Will be achieved. z

I prefer that the amount of rare earth metal employed in forming the .solid solution, e.g. Ce or La, not exceed about 1% and, more preferably, range vfrom about 0.05 to 0.8% by weight. 'L

I then fabricate lthe alloy after which it is subjected to heat treatment at an elevated temperature in an oxygencontaining environment to effect absorption of oxygen by solution sucient to convert substantially all of the rare earth metal to nely dispersed rare earth metal oxide of high melting point.

I have found cerium to be preferably suitable as the solute element in carrying out my invention. It, like the other rare earth metals. has a higher free energy of formation of the oxide (CeO2) per gram atom of oxygen than the heat of solution of oxygen in the solvent metal titanium and therefore can be selectively oxidized in the presence of titanium. Oxygen has a very low .solubility in 0 cerium: in addition, the oxide of cerium is -substantially insoluble in titanium at fabrication and application temperatures, and furthermore is a very stable refractory oxide under operating condition.

In one specific embodiment of the invention, after the Ti-Ce alloy has been worked into the desired shape, the alloy is heated to a temperature at which the alpha phase prevails for the particular amount of rare earth metal eniploved, for example within the range of about 650 to 875 C., to insure substantially complete solution of the cerium in alpha titanium, after which thethus-heated alloy is then subjected to internal oxidation at a temperature ranging up to about 875 C., preferablv within the alpha region, e.g. from about 650 C. to 875 C., under controlled oxidizing conditions adapted to prevent the build up of an oxygen concentration gradient of an oxygen barrier. In other Words, the conditions are such as lto maintain a slow rate of oxygen solution into the titanium. This may be accomplished by using a system in which oxygen has a pressure ranging from about 0.1 to 200 microns. One means of accomplishing this is to carry out the oxidizing treatment in a vacuum system having a known leak rate of air, for example a leak rate adapted to maintain a vacuum ranging from about 0.5 micron to 200 microns, preferably from about 0.5 micron to 50 microns. Or the oxidizing treatment may be carried out in a low pressure oxygen atmosphere diluted with an inert 3 gas, e.g. argon, in which the oxygen partial pressure is between about 0.1 and 2O microns.

Another specific embodiment for converting the cerium, or other rare earth metal, to .a substantially uniormly dispersed phase of cerium oxide comprises subjecting the wrought Ti-Ce alloy to heating at a temperature within the alpha region of the Ti-Ce binary, e. g. the temperature range of about 650 C. to 875 C. to insure solid solution of cerium in the alpha titanium followed `by quenching to retain the solid solution. The alloy is Vthen reheated to a temperature in the alpha plus cerium region, for example over the temperature range of about 400 C. to 650 C. to precipitate cerium as .finely 'dispersed particles followed by internal oxidation at a temperature within the alpha plus cerium region, e.g. 400 C. to 650 C. One advantage kof having cerium soprecipitated throughout the alpha matrix prior to oxidation is that control of the particle size, -shape and distribution is achieved to yield more reproducible properties. Another advantage resides in the fact that lower kinternal oxidation temperatures can be utilized.

As illustrative of the invention, the following example is given:

High purity titanium sponge 'containing O;071% O2, v0.007% N2, 0.0013% H2, 0.062% Fe and 0.031% C was charged into a tungsten arc furnace under a positive pres- `sure of argon. Each amount of charge was suicient to form a 100 ngram casting. Cerium of 99.9 plus purity was employed in forming two charges, one `containing 0.12% 'and the other 0.7%. In order to vminimize Voxide pick-up as much as possible, each of the cerium additions -in the :form of cubes were carefully cleaned ofsurface oxides by polishing on all sides with line silicon carbide powder just prior Vto melting to minimize the addition of 'cerium as cerium oxide.

Each of the castings obtained was subjected to cold working by cold press forging to 50% reduction in area and then .were cold rolled to `0.01 inch thickness, using a series of 14 passes Withno intermediate -anneal between passes. The rolled strips were sheared `into pieces one inch wide by four inches long. Torecrystallize andisolutonize any cerium which might be out of solution, the pieces were stacked in groups of 20, with astrip of pure molybdenum between each piece to prevent welding during heating, clamped between two heavy plates of titanium, and heated at 850 C. for two hours in a vacuum of 0.25 micron. The pieces were quenched from 850 C. to a `temperature below 300 C. by flushing purified helium through -the vacuum chamber 'at a rate'of 200 cubic -feet `rper hour. v

The recrystallized pieces were then subjected to-internal oxidation by suspending `them Vin a vacuum -furnace by iine titanium wire and heating to 850 C. at a-.leak rate of air to 'maintain a vacuum of ten microns. The strips containing `012% Ce were held at temperature -for -two hours while lthose `-which contained 0.7% -Ce'were held for four hours. IAll specimens were furnace I.cooled under vacuum.

EIn determiningthe extent to which vthecerium had been oxidized, `samples were taken `from each alloy group `and brominated at v200-C. At`thistemperature, titanium re- ;acts vwith bromine vapor to form TiBr., gas whichcan v.be :flushed from the system by 'passage of helium. The residue of this reaction contains al1 the unreacted com- 1pounds plus any-non-volatile bromides. T hus,vCeO2 particles -in the alloy would remain as CeO2 and any elemental cerium in Athe alloy would convert to CeBr3, which would yalso remain inthe residue. Titanium can never lbe completely removed by bromination at this temperature. `Since dissolved oxygenis not removed, titanium will be removed onlyas long as it is in excess of the amount necessary to form T102.

.X-ray patterns vfrom the bromine residue of the 4two alloys showed lineslfor T102 and CoD2-only. There were no .lines lfor .CeBr3, indicating .the virtual absence of elemental cerium in the structure. In other words, substantially al1 of the cerium was converted to CeOz by internal oxidation.

Stress-rupture specimens 4 inches x 1 inch with a 2 inch gage length and 'a 0.354 inch gage width, meeting 5 ASTM requirements, were stamped out of the 4 inches X 1 inch strips by means of a punch and die. Specimens were tested on standard creep rupture frames. Due to the thickness of the strips, actual loads were small and direct bottom loading was used rather than beam loading.

Specimens were tested -at these temperaures: 426, 538 and 648 C. (800, 1000 and 1200 l5.); and at stresses selected to yield rupture life values of less than one to 300 .hours and above. Control tests were also run on the unalloyed titanium melted in the same furnace and worked in the same manner.

The results of the stress rupture tests obtained on the unalloyed titanium and the Ti-Ce compositions are given in Tables I, II and III as follows:

TABLE I Titanium Stress in p.s.i. Te'npera- Life in Percent ture, C. y Hours Elongation 426 .0011 v426 .067 10.2 426 2.000 16.0 426, 61. 833 21. 9 426 122.00 44.5 s, 64s 0325 14. 2 V64s .0630 34.4 64s .34s 45.7 64s 3.57 53.6 648 40.08 12.4

TABLE II 'Titanium with 0.2% Ce (Internally Oxz'dzved) -Stress in p.s.. Tempera- Lite in l APercent ture, C. Hours l Elongation 426 .050 9.3 426 ..191 las 426 4.133 30.8 420 40.00 42.1 426 480. 20 57.9 53s .030 38.2 53s .67s 41.4 538 2. 583 47.3 53s 9.23 53.1 53s 90.45 62.8 648 .0636 31.3 64s .517 50.1 64s. s. 26 '68.7 .64s` 72.21 30.5 64s 629.45 20.7

TABLE III f5 Titanium with 027% Ce (Internally Oxdized) 0 Stress in p.s.i. 'Tempera- Life in Percent ture, C. Hours Elongation 426 .0167 15.00 426 .71 26.2 426 44. 90 37.5 426 661.2 45.4 426 430. 33 l4s. 6 53s .0206 27.1 V63a .484 36.3 53s 1.75 46.0 53s 10. 09 55.2 53s 4.15 64.2 53s 396, 02 66. 7 64s .039s 31.6 64s .224 40. 5 648 1.616 57. 5 64s s. 20 67.0 64s 43. 85 41.0 64s 287. 30 32,2

peratures as is evident from the following correlation of Table 1V:

1 Extrapolated.

The microstructures of the internally-oxidized alloys were composed of equiaxed alpha grains with a dispersion of ine phase particles within the grains and at grain boundaries. The dispersion, although concentrated somewhat preferentially at grain boundaries, appears to be well distributed as shown lin FIG. 3. Furthermore, there is no particle size gradient across the specimen. The appearance of some coarse particles was noted which was believedto .be due to the introduction of some cerium as CeOig into the melt on the surface of the cerium charge.

The microstructure after testing at these temperatures consists of recrystallized alpha grains characterized by irregular grain boundaries and a sub-structure within the grains. A point of interest in these microstructures was the distribution of the/ dispersed phase, which no longer exists primarily at grain boundaries but rather is found clustered in lines or groups running through grains and across boundaries. FIG. 4 is typical of this type of structure. Note that the grain size is still very tine. The fact that the particles have not coalesced after long times at temperatures shows the stability of Ce02 in titanium and the absence of elemental cerium in the structure.

The results of the internal oxidation of cerium in titanium indicate that significant benefits in the strength of titanium can be achieved for service up to at least l200 F. Lower oxidation temperatures are indicated to produce more of the much finer Ce02 particles and smaller interparticle spacing for optimum high strength properties.

In order to optimize the strengthening obtained by dispersion hardening through internal oxidation, it is preferred that the thickness of the titanium material being treated not exceed about 0.25 inch, Whether the material be in the form of particles, strip, sheet, rods, etc. Larger sizes can be internally oxidized depending upon the amount of time employed.

Where the material is desired for the powder metallurgy production of titanium articles, the wrought alloy is comminuted below 1A inch in size and preferably comminuted to yield particles ranging in size from about l mesh down to 100 mesh or lower followed by internal oxidation as described herein. The internally oxidized powder is then consolidated and followed by sintering and/ or extrusion to the desired shape.

The advantage of the powder metallurgy method of production is that it enables the production of thick shapes in excess of one quarter inch section having a uniform dispersion throughout of the rare earth metal oxide. This is because each particle already contains a random dispersion of the oxide achieved through internal oxidation. In utilizing the powder metallurgy technique, the internally oxidized comminuted material is consolidated by pressure into a slug of green strength at least about 60 to 80% of true density and the slug sintered if necessary in a non-reactive environment to further strengthen it. The slug is then subjected to hot working, such as by hot extrusion, by encasing it in a metal sheath, such as iron or nickel, and the whole reduced in size at an extrusion ratio of at least l5 to 1, and preferably from about 20 to l to 25 to 1, with extrusion pressures ranging up to 250 tons per square inch for temperatures ranging from about 950 C. to 1350 C., the amount of extrusion pressure employed depending on the temperature used. After completion of the working, the sheath metal is removed by pickling, grinding or other means. Thus, large sized rounds can be produced or rectangular shapes, angles and other contours while at the same time being dispersion hardened throughout.

While the invention has been described with respect to improving the strength properties of titanium metal, it is also applicable to the strengthening of titanium alloys containing at least titanium. Examples of such alloys include l0 to 20% molybdenum and the balance titanium, 8% manganese Kand the balance titanium, titanium containing 2% copper, and a Ti-Si alloy containing 2% silicon, and others. The expressions titanium-base alloys or titanium substantially the balance lare meant to cover such composition as well as titanium itself.

Because of the chemical and metallurgical similarities between titanium and zirconium, the invention is also applicable. to zirconium and likewise to hafnium, both of which fall in the titanium metal group. The invention also includes alloys based on these metals, that is alloys containing at least 80% of one or more of the titanium group metals Ti, Zr and Hf.

Examples of other rare earth metals which may also be employed in carrying out the invention besides Ce land La are Pr, Nd, Il, Sm, Eu, Gd, etc. 0f these gadolinium and lanthanum are particularly attractive along with cerium. Thorium likewise may be employed. Depending on the extent to which these elements go into solid solution with titanium or titanium-base alloys, alloys containing these elements may be similarly treated as the cerium-containing titanium alloy described hereinbefore.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art Will readily understand, and such modifications and variations are considered to be within the purview and scope of the invention and the appended claims.

What is claimed is:

l. A method of dispersion hardening a matrix metal comprising essentially at least about 80% of at least one element selected from the group consisting of Ti, Zr and Hf, said matrix metal being characterized by an alpha to beta transformation temperature which comprises, forming an alloy of said matrix metal and cerium with up to about 5% of cerium but not exceeding the maximum solubility of said cerium in the alpha solid solution of the matrix metal, and then subjecting said alloy to internal oxidation at an elevated temperature not exceeding the temperature at which the alpha phase transforms to beta in an oxidizing atmosphere corresponding to an oxygen pressure not exceeding the amount at which oxygen build up occurs as an oxygen gradient at the alloy interface, whereby to promote a controlled rate of oxygen diffusion into said alloy and convert selectively substantially said contained cerium to refractory metal oxide finely dispersed throughout the matrix metal.

2. A method of dispersion hardening a matrix metal comprising essentially at least about 80% titanium, said matrix metal being characterized by an alpha to beta transformation temperature which comprises, forming an alloy of said matrix metal and cerium with up to about 5% cerium but not exceeding the maximum solubility of said cerium in the alpha solid solution of the matrix metal, and then subjecting said alloy to internal oxidation at an elevated temperature not exceeding the temperature at which alpha phase transforms to beta in an oxidizing atmosphere corresponding to an oxygen pressure not exceeding the amount at which oxygen build up occurs '7 as an oxygen gradient at the alloy interface, whereby to promote a controlled rate of oxygen diffusion into said alloy and convert selectively substantially said contained cerium to refractory metal oxide finely dispersed throughout the matrix metal.

3. The method of claim 2, wherein said matrix metal is comprised essentially of titanium, wherein cerium ranges up to about 1% by weight, and wherein prior to internal oxidation the alloy is formed into a shape having a thickness of up to about 0.25 inch and is heated to an elevated temperature to effect solution of said cerium in the alpha solid solution of the alloy.

4. The method of claim 2, wherein the amount of cerium ranges from about 0.05 to 0.8% by weight, where in prior to oxidation the alloy shape is heated to an elevated temperature ranging from amount 650 C. to 875 C. within the alpha solid solution of the alloy `to effect solution of the cerium in the `solid solution, and wherein the internal oxidation following thereafter is conductin internal oxidation following thereafter is conducted lat an elevated temperature not exceeding about 875 C.

5. The method of claim 4, wherein after the alloy is heated to the temperature within the alpha solid solution region it is quenched to retain said solid solution followed by heating the alloy to a temperature outside said alpha solid solution to effect precipitation of finely divided cerium particles prior to subjecting the alloy to internal oxidation.

6. The method of claim 5, wherein the temperature for precipitating finely divided cerium particles ranges from about 400 C. to 650 C.

7. A method of dispersion hardening a matrix metal comprising essentially at least about 80% titanium, said matrix metal being characterized by an alpha to beta transformation temperature which comprises, forming an alloy of said matrix metal and cerium with up 'to about 5% of cerium but not exceeding the maximum solubility of said cerium in the alpha solid solution of the matrix etal, comminuting said alloy, and subjecting said comminuted alloy to internal oxidation at an elevated temperature not exceeding the temperature at which the alpha phase transforms to beta in an oxidizing atmosphere corresponding to an oxygen pressure not exceeding the amount at which oxygen build up occurs as an oxygen gradient at the alloy interface, whereby to promote a controlled rate of oxygen diffusion into said alloy and convert selectively substantially said contained cerium to refractory metal oxide finely dispersed throughout the matrix of the comminuted alloy.

8. The method of claim 7, wherein the matrix metal u is comprised essentially of titanium and wherein ythe amount of cerium ranges up to about 1% by weight.

9. The method of claim 8, wherein prier to internal oxidation the comminuted alloy is heated to an elevated temperature within the alpha solid solution region of the alloy, wherein the alloy is quenched to retain said solid solution, and wherein said comminuted alloy is heated to an e'evated temperature outside said alpha solid solution to effect precipitation of finely divided cerium lparticles.

10. A .dispersion hardened matrix metal comprised essentially of at least about of vat least one element selected from the group consisting of Ti,'Zr and Hf and containing an amount of cerium metal ranging up to about 5% by weight but not exceeding 'the maximum solubility of said cerium in the alpha solid solution ot the matrix metal, said cerium being dispersed throughout the 'matrix metal in 'the lform of finely divided refractory oxide.

11. The dispersion hardened metal Yof claim '10, where` in the matrix metal is comprised essentially of titanium and wherein the cerium'rang'es vup to about 1% by weight.

12. As an article vof manufacture, a dispersion Vhardened 'wrought alloy product comprised essentially yof titanium containing up to 'about 1% 'cerium 'dispersed throughout ithe matrix 'o f 'said 'alloy'in the :form Vo f finely divided cerium oxide.

13. The articlje 'of'rnanu'facture as .defined yin'claim V12, wherein the 'cerium content ranges `from Iabout 0.0510 0.8% by'weight.

References Cited in the file of this patent yUNITED STATES .PATENTS 2,539,298 Doty et al Jan. 23, 1951 2,669,512 Doty et al Feb. '16, 1954 2,678,268 Ham May 1 1, 1954 2,766,113 Chisholm et al. Oct. v9, 1956 2,818,336 ,Swazy etal. ..'^Dec. 31, 195- 7 .2,823,988 Grant Feb. V1'8, 1958 2,894,838 Gregory July 14, 1959 FOREIGN PATENTS 645,681 'Great Britain Nov. '8, 1950 OTHER REEERENCES Dispersion Hardening of Sintered vTitanium Alloys by Refractory Metal Powder Additions, PB 131, 93'7, March 5, 1958, pp. 12 and 13.

Hansen, Constitution of Binary Alloys, copyright 1958 by McGraw-Hill, pp. V463, 893, 'and 1.23.2.

December 25V 1962 UNHE ST @E 'HHC Patent No 3O'ZO468 Nicholas Io Grant .rs in the above numbered ebters Patent should read as It is hereby Certified that error appea peteniJ requiring correction and that the said L corrected below Column 'M line 12Mz for the Claim reference numeral @'2 read 3 --g lines 19 and 20XI sirike our "Wis Conduct-f in internal oxidation following ihereaftermp Signed and sealed 'this ith dey o June i963.a

(SEAL) Attest: ERNEST w. swiDER DWD L MDD Commissioner of Patents ttesting Officer UNTTED STATES PATENT oTTToE CERTIFICATE 0F CORRECTIN Patent Noo 3O'ZOl8 December 25V 1962 Nioholae JI, Grant It is hereby certified. that error appears in the abo'veinumbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 'Z line 13x7 for the Claim reference numeral ,"2"

rjead 3 --5 lines 19 and 207 strike out Vvis ooxiductm in internal oxidation following thereafteo Signed and sealed vthis 4th dey of June M963@ (SEAL) Attest:

DAVID L. LADD -ERNES'I W. SWIDER Commissioner of'vPatentrs Attesting Officer

Claims (1)

1. A METHOD OF DISPERSION HARDENING A MATRIX METAL COMPRISING ESSENTIALLY AT LEAST ABOUT 80% OF AT LEAST ONE ELEMENT SELECTED FROM THE GROUP CONSISTING OF TI, ZR, AND HF, SAID MATRIX METAL BEING CHARACTERIZED BY AN ALPHA TO BETA TRANSFORMATION TEMPERATURE WHICH COMPRISES, FORMING AN ALLOY OF SAID MATRIX METAL AND CERIUM WITH UP TO ABOUT 5% OF CERIUM BUT NOT EXCEEDING THE MAXIMUM SOLUBILITY OF SAID CERIUM IN THE ALPHA SOLID SOLUTION OF THE MATRIX METAL, AND THEN SUBJECTING SAID ALLOY TO INTERNAL OXIDATION AT AN ELEVATED TEMPERATURE NOT EXCEEDING THE TEMPERATURE AT WHICH THE ALPHA PHASE TRANSFORMS TO BETA IN AN OXIDIZING ATMOSPHERE CORRESPONDING TO AN OXYGEN PRESSURE BUILD UP OCCURS AS AN OXYGEN GRADIENT AT WHICH OXYGEN BUILD UP OCCURS AS AN OXYGEN GRADIENT AT THE ALLOY INTERFACE, WHEREBY TO PROMOTE A CONTROLLED RATE OF OXYGEN DIFFUSION INTO SAID ALLOY AND CONVERT SELECTIVELY SUBSTANTIALLY SAID CONTAINES CERIUM TO REFRACTORY METAL OXIDE FINELY DISPERSED THROUGHOUT THE MATRIX METAL.

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