US3441391A - Tungsten-base alloys - Google Patents

Description

Unite States te 3,441,391 TUNGSTEN-BASE ALLOYS Stephen Foldes, San Jose, Calif., assignor to General Electric Company, a corporation of New York No Drawing. Continuation-impart of application Ser. No.

423,389, Jan. 4, 1965. This application Jan. 26, 1967,

Ser. No. 611,818

Int. Cl. 1322f 3/10 U.S. Cl. 29--182.5 3 Claims ABSTRACT OF THE DISCLOSURE Powder-metallurgy alloys of W with -20% M0, 0.1- 1.0% Ti and 200-1000 p.p.m. oxygen are ductile at low temperatures and strong at intermediate temperatures. They may be used to line die casting dies.

CROSS REFERENCES TO RELATED APPLICATIONS This is a continuation-in-part of application Ser. No. 423,389, filed Jan. 4, 1965, now abandoned.

BACKGROUND OF THE INVENTION SUMMARY OF THE INVENTION It is an object of the present invention to provide tungsten-base alloys combining moderate to good high temperature tensile strength with room temperature ductility, thereby providing tungsten-base alloys having increased utility.

Another object of the invention is to provide such tungsten-base alloys which can be produced by powder metallurgical means, and which do not require constituents much more expensive than pure tungsten alone, thereby providing an improved alloy at no great increase in cost.

Briefly stated, the present invention in one form provides tungsten-base alloys produced by powder metallurgy techniques and containing, by weight, about l025% molybdenum, 0.1-1.0% titanium, 200 to 1000 parts per million by weight (p.p.m.) oxygen, and incidental impurities, the balance consisting essentially of tungsten. (Percentages herein are by weight except where stated otherwise.) Specific embodiments of the invention include such alloys in which the molybdenum content is about 18%, titanium is present at about 0.3%, and the oxygen content is about 600 ppm. Incidental impurities may include the interstitial elements carbon, hydrogen, nitrogen, and diverse other elements in quantities not so large as to substantially nullify the commercial utility of alloys of the invention. The distribution and morphology of the interstitial elements, particularly in combination with titanium, can profoundly influence their effects on alloys of the invention.

Although the upper and lower limits of the constituents necessary to give the desirable properties of alloys of the invention are known approximately, their exact values are not precisely known and would, in any event, depend more or less on the thermal and mechanical treatment to which each alloy is subjected. The word about is used herein to indicate that the precise composition limits of alloys of the invention may vary slightly from the stated limits.

DESCRIPTION OF THE PREFERRED EMBODI- MENTS Powder-metallurgical techniques are much more suitable for producing fine-grain-size metal than are techniques which involve melting such as are casting and electron beam melting. The initial fine grain size of powdermetallurgical alloys of the invention is important in dispersing the dispersoid phase over larger areas to avoid embrittlement which might occur with relatively large-grainsize cast alloys of the same compositions. Thus, for achieving the advantages of the invention, powder-metallurgical alloys are needed to give the desired ductility at relatively low temperatures.

In investigations leading to the present invention, I have found that certain low levels of addition of titanium together with small but important quantities of oxygen are unique in their effects on the low temperature ductility of moderately high strength tungsten-base alloys containing substantial amounts of molybdenum. The fact that I have also found that zirconium and hafnium do not produce effects similar to those of titanium in such tungstenmolybdenum alloys indicates that the beneficial effects of titanium do not come merely as a result of internal gettering or deoxidation, since that would be expected to occur at least to the same extent in the alloys containing zirconium or hafnium as in the titanium-bearing alloys. Although one cannot definitely state the reason for the beneficial properties conferred on alloys of the invention by titanium, one of my present hypotheses is that titanium increases the ductility of these alloys at lower temperatures by the mechanism of free dislocation source multiplication in the vicinity of very fine dispersoids. The titanium in these alloys appears capable of lowering the levels of interstitial oxygen by forming an extremely fine dispersoid phase having a morphology well adapted to providing sources for the multiplication of free dislocations, thereby leading to easier slip and deformation of the metal and greater ductility at lower temperatures. Also, these particles of dispersoid phase can be located in grain boundaries and sub-grain boundaries in discontinuous form to serve to inhibit the propagation of microcracks. Although other metallic additives, such as zirconium and hafnium do form dispersoid phases by similar mechanisms, titanium appears to be unique in this system in that the dispersion formed is sufiiciently fine and well-dispersed to substantially promote ductility in the alloy.

The extent to which titanium increases the ductility of tungsten-base alloys relative to other additives is herein expressed in two ways. The percentage amount of plastic elongation in the gauge length of tensile specimens is a measure of the amount of ductility at a certain temperature. Also, in the case of the subject alloys, there exists a ductile-to-brittle transition temperature above which temperature the metal or alloy exhibits at least a certain amount of ductility in terms of plastic elongation, and below which temperature the metal or alloy fractures without more than a specified small amount of elongation. The transition temperature is herein defined as the temperature at which 5% plastic elongation is obtained in the alloys in a recrystallized condition.

Alloys of the present invention possess unusual ductility at room temperature (about F.) in combination with substantial strength at intermediate temperatures such as in the range around 2000 F. In fact, the strength of these alloys in the intermediate temperature range is much greater than that of most known molybdenum-base alloys. This combination of properties makes alloys of 3 the invention quite useful for modern commercial applications such as for lining die-casting molds.

After essentially identical processing, and using the same strain rate during tensile testing, alloys of the invenhomogeneity due to slow diffusion rates. Reactive metals such as titanium, zirconium and hafnium were added as hydrides. Analyses in p.p.m. by weight and particle sizes of the powders are given in Table II.

TABLE II.CHEMIOAL ANALYSES AND PARTICLE SIZE Impurity Zr Hf l 99.98 min. purity.

1 99.88 min. purity.

3 Percent.

tion were found to have a substantially lower transition temperature and greater ductility at temperatures below the transition temperature than did similar alloys outside the invention. The results of this testing are presented in Table I below. The testing was performed on double-step button-head specimens 0.113 inch in diameter by 0.5 inch gauge length that were carefully ground to size and configuration and then electropolished until n trace of the grinding marks was visible at 80X magnification. The electropolishing removed about 0.005 inch from the surfaces of the specimens. At temperatures up to 225 C., the samples were immersed in an oil bath for heating; above 225 C., they were heated in air in a resistance furnace. Testing was performed in an Instron testing machine at a strain rate of 2X l0- /second. In Table I, the percentages of additives are expressed as atomic percent for a more direct comparison of effects. The additives were added to an alloy base composition of 70 atomic percent tungsten, 30 atomic percent molybdenum. In alloys of the invention, for titanium, 0.3 atomic percent is equal to about 0.09 weight percent, 1 atomic percent equals about 0.58 weight percent and 5 atomic percent equals about 1.57 weight percent. These titanium-bearing alloys had oxygen contents, respectively, of 455 p.p.m., 652 p.p.m., and 635 p.p.m.

Alloying additive 0.3 1.0 5.0

Comparable transition temperatures found for unalloyed tungsten and tungsten-30 atomic percent molybdenum were 175 C. and 150 C., respectively. It can be seen from this table that titanium, especially in an amount of about 1 atomic percent and together with 500-600 p.p.m. oxygen, lowers the transition temperature of the alloy of tungsten containing about 30' atomic percent molybdenum (about 18.2 weight percent) far more than did additions of the other elements used. The transition temperature of the 1 atomic percent titanium alloy was about 55 C. In addition, this alloy had a ductility of about 0.6% plastic elongation at room temperature (24 0.).

Examples Alloys of the invention and alloys compared in Table I with those of the invention were prepared by essentially identical techniques beginning with metal powders. Small particle size metal powders were used to minimize the possibility of second phase formation and resultant in- Freshly prepared tungsten and molybdenum powders and other alloying additions were mixed together in a twin-shall blender and then subsequently blended for four hours in a tungsten-lined rod mill. The powders were formed into compacts about one inch diameter by 2 /2 inches long by isostatic pressing at 35,000 p.s.i. The compacts were presintered in a vacuum furnace for two hours at 1050 C. and then finally sintered in another vacuum furnace for two hours at 2800 C. The sintering was done under a dynamic vacuum of 10" to 10* torr, and temperature measurements were made under black body conditions with a micro-optical pyrometer. The alloys acquired their oxygen content from the residual metallic oxides always present on the surfaces of refractory metal particles. During sintering, this oxygen can react with active metal additives, e.g., titanium, to form dispersed oxide phases.

The sintered compacts, ranging in density from to of theoretical, were machined into l-inch diameter billets and extruded at temperatures in the range of 1950- 2200 C. The 0.3 atomic percent titanium alloy was extruded at 20 50" C., and the other two titanium-bearing alloys were extruded at 2150 C. on a Dynapak machine. An extrusion ratio of 10:1 was used for all of the alloys except those of W-Mo-Zr which was extruded at a ratio of 5:1. In each case, part of the billet was left unextruded in order to stop the downward movement of the extruded bar with minimum damage.

Metallographic studies of the extruded alloys generally indicated fully recrystallized, equiaxed microstructures except for a few alloys having a few large, elongated grains. The zirconium-bearing alloys contained relatively large stringers of ZrO while the hafnium-bearing alloys contained oxide particles of somewhat smaller size. In contrast, very fine dispersoids were seen in the grain boundaries and sub-grain boundaries of titanium-bearing alloys. It is assumed from the depression of the transition temperature and increased ductility that titanium in some way interacts with the oxygen content of these alloys to render it beneficial. Beneficial ductility effects could result from extremely fine precipitates of desirable morphology by several possible mechanisms including free dislocation source multiplication and the blocking of incipient microcracks and fracture patterns. The results of tests performed to locate the ductile-to-brittle transition temperature of several of the alloys produced are presented in Table I above. The W 18.2%-Mo 0.31%- Ti specimen exhibited 28% tensile elongation at C., whereas unalloyed tungsten exhibits very low ductility under comparable testing. Fractographic studies of this and other specimens revealed that ductility was accompanied by discontinuous precipitates, apparently titanium oxide, in the grain boundaries and sub-grain boundaries.

Table III presents tensile, hardness, transition temperature, and grain size data in the as-extruded condition at room temperature (about 24 C.) for alloys of the invenomissions and substitutions may be made within the true spirit and scope of the invention as defined in the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A powder-metallurgical sintered tungsten-base alloy consisting essentially of, by weight, about: -25% molybdenum, from 0.1l.0% titanium, 200 to 1000 p.p.m. oxygen, and incidental impurities, the balance being essentially all tungsten said alloy containing dispersed titanium oxide phases. Tran- 2. A powder-metallurgical tungsten-base alloy accord- Sition Grain ing to claim 1 consisting essentially of, by weight, about: 02% YSI 5 5: ,233; flffia 18% molybdenum, 0.1-1.0% titanium, 200 to 1000 ppm. oxygen, and incidental impurities, the balance be- K s.i.

tion and other alloys for comparison. UTS means ultimate tensile strength, K s.i. means thousands of pounds per square inch, 0.2% YS means the 0.2% offset yield strength, and VPH means Vickers pyramid hardness measured in kilograms per square millimeter. It should be noted that titanium, zirconium, and hafnium are not equivalent in their effect on the ductile-brittle transition temperature of tungsten-base alloys, nor on the room temperature strength of such alloys.

IIL-ROOM TEMPERATURE DATA FOR W- 10 ATOMIC PERCENT Mo-BASE ALLOYS Tensile Properties Ksi.

TAB LE Nominal Composition, Atomic Percent ntm w e s a i m S b O S .9 E 1 E n wm T .3 m c m .m& N sY,e HP e E .gHmh C n Tun nau S 50 A r R 0.0.! Y B E m m hn tm F mnvt.m CT CHM a E S.1.t nA N et :wm n e m.m re R to 3 e m Pmm m D R34 m i E66 066 me 99 9 T m t I11 11 O H. U n ar n w w 23 62 yklM-IS U11 1 wmbmm .E iNZ 51 05 a t 49 28 n l 12 600 B mnl. SA aa 63 005 s m y 14 20 e I2 94 oo e 1 3 6 .m fi

640906 249533 5995 22991 mmmmmmmnmmmmmammmmmmmmmmmm 37139nb58B7826w 271851366707 6 u 1 70777 .76 .0628 891366 21: n 7- n 1 0 1 1111 m Niobium, Tantalum, Molybdenum and Tungsten, About 80 pct recrystallized, others completely recrystallized. Elsevier Publishing New York, pp 320623 It has been demonstrated that alloys of the invention and 332 relied OIL have useful strengths and greatly improved ductilities CHARLES N. LOVELL, Primaly Examiner.

While specific examples have been given of alloys of the invention, it will be understood that various changes, 40 -176 and transition temperatures in relation to unalloyed tungsten and other of its alloys.