US3434829A - Tungsten-base alloys - Google Patents

Description

March 25, 1969 w. H. CHANG TUNGSTEN-BASE ALLOYS Filed Dec. 21, 1966 m m P 0 wa d *4 at; wmmgmqr 3am 3500 HGE/NG TEMPEE/QTUPE, "F El 567 0F HGE/NG TEMPEPHTUPE' 0N HARD/V555 ITWQTWTQP wins ton H. Chang by 01% His Attovne9 United States Patent Ofiice 3,434,829 TUNGSTEN-BASE ALLOYS Winston H. Chang, Cincinnati, Ohio, assignor to General Electric Company, a corporation of New York Continuation-in-part of application Ser. No. 359,079, Apr. 13, 1964. This application Dec. 21, 1966, Ser. No. 609,702

Int. Cl. C22f 1/18; C22c 27/00 US. Cl. 75-176 3 Claims This application is a continuation-in-part of application Ser. No. 359,079, filed Apr. 13, 1964 now abandoned.

This invention relates to tungsten-base alloys, and more particularly to tungsten-rhenium alloys.

Alloys of tungsten with varying contents of rhenium have attracted some attention in recent years due to the apparent enhancement of ductility of the base metal by the presence of rhenium. However, such alloys with high contents of rhenium have proven relatively intractable due, in part, to rapid rates of work hardening, and to the formation of a brittle sigma phase between tungsten and rhenium. Further obstructions to the commercial utilization of such alloys have been due to the inability to vary in a controlled manner the mechanical properties and other characteristics of the alloys by heat treatment for precipitation or dissolution of a dispersed phase. An alloy which can be so treated is more amenable to optimized processing and subsequent development of greater hardness for load-bearing application. Difiiculties in heat treating or annealing previously known alloys are compounded by the brittle sigma phase present in the equilibrium diagram of tungsten and rhenium beginning at about 30% rhenium.

An object of the invention is to provide a tungsten-base alloy containing rhenium, the mechanical properties and characteristics of which alloy can be varied in a useful manner by heat treatment.

Another object of the invention is to provide a tungstenbase alloy with high levels of strength at elevated temperatures while still retaining a useful degree of ductility at room temperature.

Still another object of the invention is to provide sheet metal of a tungsten-rhenium alloy produced by fusion processes which has a controlled small grain size after recrystallization and after one hour at temperatures above 3500 F.

In the drawing:

FIG. 1 is a graph showing the effect of aging temperature on the hardness of an alloy of the invention.

FIG. 2 is a graph of the eifect of aging time on the hardness of an alloy of the invention.

I have discovered that useful alloys can be produced based on tungsten and containing substantial amounts of rhenium which can be heat treated to control a precipitate comprising columbium carbide or columbium and tungsten complex carbides, or both. Tests on such alloys show quite high strength levels at elevated temperatures, even at temperatures over 3000 F., along with the retention of substantial room temperature ductility. Such room temperature ductility has been observed in both the stress-relieved and recrystallized conditions.

In accordance with the invention, therefore, I provide a tungsten-base alloy containing substantial amounts of rhenium giving it valuable ductility and also containing columbium and carbon, with the amounts and proportions of each so selected as to give a useful precipitation-hardenable alloy.

More particularly, I provide a tungsten-base alloy containing from 20 to 30% by weight of rhenium, from 0.5 to 2% columbium, which has a low volatility at high temperatures and is capable of forming carbides which will 3 ,434,829 Patented Mar. 25, 1969 be stable at high temperatures, and from 0.01 to 0.2% carbon. Except where indicated otherwise, percentages given in this specification are by weight.

Additions of 20-30% rhenium to tungsten, and particularly 25-30%, are known to have the effect of significantly increasing strength by solid solution mechanisms, accompanied by lowering the ductility transition temperature. Excessive additions of carbon to such an alloy base would have an effect of depressing the melting point which would be deleterious to high temperature strength, since both tungsten-carbon and rhenium-carbon are eutectic systems. Columbium has a sufficiently high melting point, low enough volatility at the melting point of tungsten, and forms stable enough carbides to be quite beneficial in alloys of the invention. When columbium is supplied at the 1% level, and carbon is provided in an amount of 0.1%, the atomic ratio of the carbide forming element columbium to carbon is 1.3:1. Preferred atomic ratios of columbium to carbon in alloys of the invention are from 1:1 to 10:1.

EXAMPLES An electrode was prepared by powder metallurgical techniques to approximate the alloy composition of 27% rhenium, 1% columbium, 0.1% carbon, balance tungsten. The actual amounts of material added to the powder which were expected to give a melted ingot of the above composition were 28% rhenium, 1.2% columbium and 0.11% carbon, balance tungsten.

The powder was hydrostatically pressed at 55,000 p.s.i. and vacuum sintered at 3750 F. for two hours to produce six-inch long pieces subsequently joined by TIG (tungsten inert-gas) welding to produce an electrode 0.8 inch in diameter, 40 inches long and weighing about ten pounds. The powders used to blend the powder lot from which the electrodes were pressed were as follows: The tungsten had a particle size of -325 mesh, and contained 0.0080% oxygen and 0.002% carbon; the rhenium was of a -200 mesh particle size and had an oxygen content of less than 0.1500% and 0.003% carbon; the columbium had a particle size of -200 mesh, an oxygen content of 0.1400%, a nitrogen content of 0.0037% and 0.003% carbon; and the carbon was Grade 38 powder supplied by the Acheson Colloids Company of Port Huron, Mich. The first sintered electrode had a density of about 67% which was insufficient for satisfactory melting and resulted in over-heating and breakage of the electrode during an attempted melt. Because of the melting difficulties, the electrode was resintered for about two hours at about 4400 F. by self-resistance. This raised the electrode density to approximately of theoretical. The entire electrode, some 36 inches long, melted into a 2% inch mold, using unalloyed tungsten chips as the starting material. In view of the electrode overheating encountered previously, a lower starting current of 1800 amps. was used, which was subsequently increased to 2400 amps. as the melting proceeded. The melting was generally successful. However, gross cavities were found near the bottom and a heavy scull was formed at the top of the ingot. The cavities indicated a lack of fluidity due probably to the low initial current density. With hot-topping procedures the extended scull at the top of the ingot should be eliminated. Straight polarity direct current melting was used with an average voltage of 28 volts and a maximum pressure reached of 1 micron. The cast ingot had a composition of about 29.06% rhenium, 1.31% columbium, 0.O-273%, carbon, 0.0027% oxygen, 0.0017% nitrogen, and 0.0004% hydrogen. The atomic ratio of columbium to carbon was about 6:1.

The ingot was then extruded to sheet bar. Before extrusion, the ingot, ground to a diameter of 2.2 inches and having a length of 2.15 inches, was homogenized at 3750" F. for two hours to dissolve the grain-boundary phases and the suspected sigma phase. The homogenized ingotwas jacketed in a molybdenum cylinder to make up the 3-inch diameter required for the extrusion facility utilized. To achieve solutioning and processing case, an extrusion temperature of 3750 F. and an extrusion ratio of 5 to l were selected. The jacketed billet was extruded to a bar measuring 1.72 inches by 0.45 inch by about inches long (not including molybdenum jacket). The maximum extrusion force was 500 tons. The extruded bar, by visual examination, appeared to be in good condition. Preliminary microscopic examination indicated a warm worked structure wth a hardness of 549 Vickers (VHN).

After removing the molybdenum jacket, a small section of the extruded material (approximately 0.4 inch thick) was used to devise a procedure for rolling to 0.05 inch sheet. No difliculty was encountered upon rolling at 2750 F. for However, lowering the temperature to 2500 F. failed to permit significant further reduction in thickness, even with an intermediate anneal at 300 F. The apparently high resistance to deformation made it necessary not only that the entire rolling be done at 2750 F., but also required an intermediate annealing at 3000 F. for one hour after the first 60% reduction in thickness. Using this procedure, the extruded material was converted to 0.045 inch thick sheets, two of which each measured 3.5 inches Wide by 11 inches long. In spite of the high rolling temperature, the sheet surface appeared to be exceptionally smooth and free of cracking or other visible defects. This procedure resulted in greater than material yield during the rolling processing.

In studies on this material directed toward identifying the precipitated phases, the dispersed phases in the as-cast condition were extracted electrolytically in a solution consisting of milliliters H 80 in 900 milliliters of ethyl alchol. Further studies were made on the asrolled condition by X-ray diffraction directly from the solid sample after electrolytic polishing and etching to render the dispersed phases in relief, and by electron diffraction of the dispersed-phase particles extracted from a heavily etched specimen by a double resin-carbon replica-tion technique. These studies indicated rather conclusively that two dispersed phases were present in the alloy. Coordinating the identification results with metallographic studies, these phases were determined to be a finely dispersed, rather uniform particulate phase of (Cb, W)C, a complex carbide of columbium and tungsten which was basically columbium carbide with tungsten dissolved therein, and a larger, more diffuse, less prevalent dispersoid of W-Re sigma phase.

In contrast to the as-cast condition, the face-centeredcubic phase gave considerably stronger X-ray diffraction lines in the residue from the as-rolled material. This phase has been identified as CbC containing small amounts of other metals (tungsten or possibly rhenium) in solid solution. Furthermore, both X-ray and electron diffraction data obtained from the as-rolled condition identified the existence of the W-Re sigma phase.

Studies made of the effects of aging on hardness of the alloy in the form of 0.05 inch thick sheet show substantial aging peaks, both with an increase in temperature and with an increase in time. The results of these studies are shown in FIGS. 1 and 2. FIG. 1 shows the effect on hardness in Vickers hardness number units (VHN) of aging for one hour at the various temperatures indicated on the abscissa after a solution treatment of one hour at 4000 F. FIG. 2 shows similar effects on hardness of aging for various periods of time at three different temperatures, 2500" F., 2750 F. and 3000 F., after a solution treatment of one hour at 4000 F.

The one hour aging treatments were conducted between 2000 F. and 3500 F., generally at 250 F. increments. The aging studies at varied times of the three temperatures shown in FIG. 2 were performed for up to 16 or 5O hours. The hardness of the alloy exhibited a medium-size one-hour peak at about 2750 F. Similar increases in hardness were obtained by aging at 2500 F. for five hours. Further aging at the latter temperature produced a maximum at about 16 hours, followed by over-aging at longer times. The effect of prolonged aging at higher temperatures was characterized by a hardness decrease. Metallographic studies of the microstructures of the aged materials indicated that the sigma phase predominant in the as-rolled condition was essentially completely dissolved at 4000 F. after one hour. The phase did not reappear readily upon aging. In fact, even a fifty-hour aging at 2750 F. or a sixteen-hour aging at 3000 F. only caused a moderate precipitation of the sigma phase, mostly at the grain boundaries, No precipitation of this phase was observed upon aging at higher temperatures which approached the equilibrium solutioning temperature of the sigma phase.

Samples of the rolled material were annealed for one hour between 1500 F. and 4000 F. generally at 250 F. intervals. The hardness and grain-size changes are given in Table I. The as-rolled condition has a hardness of 623 VHN which decreases with annealing temperature until 4000 F. at which a slight increase in hardness appeared to have taken place. The temperature giving complete recrystallization in one hour was about 3500 F., but the recrystallized grain size was still quite small, even after one hour at 3750 F.

Hardness, VHN

Annealing temp, F. Grain size, mm.

Bend-test studies of the rolled alloy were conducted at room temperature for both the stress-relieved condition (2200 F./l hour) and the recrystallized condition (3500 F./1 hour). The minimum bend radius of the stress-relieved condition was 4 T. This means that the minimum radius about which a bend specimen could be bent through degrees at room temperature in the stress-relieved condition was four times the thickness of the sheet. For purposes of comparison, the bend transition temperature of rolled sheet of the binary tungsten alloy containing 27% rhenium, determined on the basis of 4 T radius and 15 angle, has been reported to be 32 F. after 2000 F. stress relief and 100 F. after 2400 F. stress relief. Recrystallization at 3500 F. raised the minimum bend radius of the alloy herein investigated to above 9 T at room temperature, at which radius the alloy cracked after having been bent through an angle of 50. The retention of room temperature ductility indicates that the alloy herein studied by way of example has retained an unexpectedly large and useful amount of ductility at room temperature.

The tensile properties of the alloy of the example stressrelieved at 2500 F./l hour and recrystallized in some cases at 3500 F./l hour are shown in Table II below. The letters SR indicate the stress-relieved material and the letters RX indicate the recrystallized material. K s.i. means thousands of pounds per square inch. The testing was conducted on sheet tensile specimens with a gauge length of 0.5 inch, /8 inch wide, cut from 0.05 inch thick sheet, the major surfaces of which were as-pickled in the rolled condition. Testing was done on Instron apparatus at a nominal strain rate of 0.05 per minute with the elevated temperature tests being done in a vacuum of about 10- torr.

TABLE II.'IENSILE PROPERTIES OF ROLLED ALLOY Tensile Tensilo/ 0.2% Elonga- Tesi; Condition strength, density yield tion, temp., F. K s.i. ratio strength, percent in X10- K s.i.

78 SR 234. O 334 8.1 RX 172. 2. 600 SR 199.0 284 12. 2 RX 159. 5 11.0 1,000 SR 185.0 204 10. 4

RX 1,500 SR 171.8 245 8 12 5 RX 2,000 SR 149. 2 213 124. 2 13. 8 RX 143. 3 72. 3 18. 3 2,500 S R 104. 0 149 77. 0 36. 0 RX 111. 1 66. 7 21. 5 3,000 SR 47. 5 68 31. 3 68. 8 RX 41. 9 38.1 77. 2 3,500 SR 21. 3 30 18. 1 144. 0

The sheet material sustained a tensile elongation of 8% at room temperature, giving an ultimate strength of 234,000 p.s.i. The tensile strength at 3000 F. was 47,500 p.s.i. which decreased to about 20,000 p.s.i. at 3500 F., due probably to the onset of recrystallization. The strengths at 3000 F. and 3500 F. correspond to strength-to-Weight ratios of about 68,000 and 30,000 inches.

Although the examples described above utilized consolidation by are melting, other consolidation methods such as electron-beam melting and powder metallurgy may be used for at least some applications.

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

1. A tungsten-base alloy consisting essentially of, by weight, from about 20% to about rheniurn, about 0.5-2.0% colurnbium, and about 0.01-0.'2% carbon, balance tungsten.

2. A tungsten-base alloy according to claim 1 consisting essentially of, by Weight, about 30% rhenium, 1.3% colurnbiurn, and 0.027% carbon, balance tungsten.

3. A tungsten-base alloy according to claim 1 consisting essentially of about 27% rhenium, about 1% columbium, and about 0.1% carbon, balance tungsten.

References Cited Dickinson et al. -176 OTHER REFERENCES Tungsten Alloys of High Melting Point, Kieffer et al., Journal of the Less Common Metals, v01. 1, 1959, relied on pp. 19-33.

Nb, Ta, Mo and W, Quarrell Elsevie-r Pub. Co., New York, 1961, pp. 376-377.

CHARLES N. LOVELL, Primary Examiner.

Claims (1)

1. A TUNGSTEN-BASE ALLOY CONSISTING ESSENTIALLY OF, BY WEIGHT, FROM ABOUT 20% TO ABOUT 30% RHENIUM, ABOUT 0.5-2,0% COLUMBIUM, AND ABOUT 0.01-0.2% CARBON, BALANCE TUNGSTEN.

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