US 3375109 A
Description (OCR text may contain errors)
United States Patent Ofiice 3,375,109 Patented Mar. 26, 1968 3,375,109 PROCESS FOR PREPARING RHENIUM- REFRACTORY ALLOYS John E. Peters, Wolcott, Conn., assignors to The Chase Brass & Copper Company, Incorporated, Cleveland,
Ohio, a corporation of Connecticut No Drawing. Filed June 24, 1966, Ser. No. 560,098
7 Claims. (Cl. 75-212) This invention relates to a process of preparing refractory metal alloys and to prealloyed powdersof such metals. More particularly, the invention pertains to alloys of rhenium in combination with tungsten, molybdenum and admixtures thereof, and to such metals in prealloyed powder form useful for fabricating products by powder metallurgy techniques.
Theme of powder metallurgy in the fabrication of very high temperature metals is of course a well-known technique and has been widely used for production of tungsten-rhenium, molybdenum-rhenium and tungstenmolybdenum-rhenium alloys. In general the conventional process comprises thoroughly mixing elemental powders of the metals to be alloyed by blending, screening, ball or rod milling, or the like; then pressing the mixed powders in dies to form a compacted shape that is reasonably selfsupporting; and finally subjecting the compacted shape to high temperature treatment which serves both to density and compact the metal powders and cause alloying thereof by solid state diffusion. High temperatures are required for sintering the compacted powder, for example around 2550 C. for tungsten alloys, using conventional techniques, otherwise considerable difiiculty is encountered in respect to lack of homogeneity in the resulting product. A substantial period of time at the elevated temperature, usually several hours, is likewise necessary. Such treatment definitely tends to cause enlarged grain size in the resulting alloys. Large grain size is highly undesirable for applications such as the production of fine wires, for example, which is drawn from alloy slugs or ingots produced by the sintering process.
Other methods of alloying heretofore employed have included co-reduction of mixed oxides or salts as well as elemental forms of one or more of the alloying components, followed by compacting and sintering. But again difiiculties are experienced with respect not only to homogeneity and enlarged grain size, but with such other factors such as poor density and undesirable change in the shape and size distribution of the powder particles. This latter characteristic is frequently "very critical in successful fabrication of parts by powder metallurgy.
It has now been found that substantially improved alloys can be obtained by coating the individual particles of a powder of a refractory metal, such as tungsten or molybdenum or a mixture of these, with a decomposable source of rhenium in a particular manner. Briefly, the invention comprehends thoroughly wetting tungsten, molybdenum or mixtures thereof in powder form with a solution of perrhenic acid. The slurry t-hus prepared is dried and reduced in a suitable atmosphere such as hydrogen at temperature up to about 1000 C. This produces a metal powder in which the individual powder particles are coated with elemental rhenium and in which some actual alloying takes place. This prealloy powder is then pressed into'a compact, pre-sintered at about 1000' C., then fully sintered to effect a solid state diffusion by heating for several hours in a hydrogen atmosphere at elevated temperatures from about 2000 C. to a maximum of about 2550 C.
An alternate and generally preferred technique involves neutralizing the perrhenic acid in situ on the powder after it has been wetted to produce ammonium perrhenate directly on the powder particles. This treated powder is then dried, reduced and sintered as previously described.
The process here disclosed produces homogeneous compacts or articles that are characterized by small grain size. This result is obtained primarily because the sintering temperatures required are several hundred degrees lower than those required for producing equivalent alloys by the conventional compacting and sintering of simple admixed elemental metal powders; also the time of sintering is no greater and generally is less than conventionally required. The resulting greater homogeneity at lower sintering temperatures and times is also of substantial importance for many applications, in that the reduction or total elimination of an extraneous or sigma phase in the product greatly eliminates a source of weakness heretofore encountered unless more elevated temperatures are employed with concomitant disadvantages of excessive grain growth inherent in such treatment. For example, tungsten-rhenium alloy products of the invention produced by sintering at 2000 C. exhibit a higher degree of diffusion and less than 1% sigma phase, as compared to 10% of such phase present when producing prealloyed powders of the same metal by conventional methods utilizing the same sintering temperature. By increasing the sintering temperature above 2000, e.g. to 2250" C., no sigma phase is detectable in the invention alloys, and grain growth is held within desirable limits providing the sintering time is not prolonged unnecessarily. This represents a temperature reduction of about 300 C. as compared to the conventional process, and is a very significant decrease. There is of course a temperature-time relationship to be observed to obtam a balance between greater homogeneity (lower sigma phase) and grain growth.
Another major advantage of the novel process and product resulting therefrom is that the distribution of different powder particle sizes and shapes in a given lot of powder is relatively undisturbed, significantly less so than occurs unavoidably in prior alloying practices. A controlled distribution of size and shape of particles is important and sometimes. critical to the achievement of a satisfactory product in powder metallurgy practice. There is, moreover, as a result of greater homogeneity in the products of the invention, better density and higher strength. And the invention makes possible the use of more commonly available furnaces or ovens which are incapable of the very high temperatures needed for achieving alloyed products using the conventional procedures described above. A substantial economy is thereby effected. The present process also offers substantial advantages from a cost standpoint compared .toother systems of alloying refractory metals heretofore practiced in respect to the type and criticali-ty of the operations to be performed.
Further advantages and other objects of the invention will become apparent from the description of several specific examples given below.
Example 1 For comparison. purposes, a control lot of tungsten-25% rhenium alloy powder and sintered slugs or ingots of this control lot were prepared employing powders of tungsten and rhenium in elemental form. The metal powders were admixed, pressed and sintered at various temperatures, reported below, and the results compared to two other alloys, designated Lots A1 and A-2, prepared in accordance with the present invention.
Alloy powder in Lot A-l was prepared by mixing 75 grams of commercially available tungsten powder, capable of passing through a 200 mesh screen, with an aqueous solution of pure perrhenic acid containing the equivalent of 25 grams of rhenium. The particular concentration of 3-zone reduction furnace employing a hydrogen atmosv phere. The impregnated powder was progressed successively through the furnace to effect reduction of the rhenium values (principally in oxide form) while avoiding voltatilization of the oxides and consequent loss of the rheniu-m content. Thus, the initial temperature range was kept fairly low, on the order of 400 C., and was increased in successive zones to a maximum of 800 to At this point the rhenium oxides formed on the tungsten powder during preliminary drying is reduced to elemental rhenium and it appears also from analysis that some actual alloying of the component metals takes place at this stage. Before compacting the pre-alloyed powder into a self-supporting slug, it is generally helpful to crush the powder lightly, sift through a 200 mesh screen and then TABLE 1.-PROPERTIES OF 0.5" DIAMETER PELLETS SINTERED AT VARYING TEMPERATURES [Stntering time-32 hours] Sinterlng Sintered Sintered Percent Average Batch Temp., Hardness, Density, 2nd Grain C. Rc Percent Phase Size, )1
Control 2,000 23 85. 7 9.9 12 2, 200 29-30 91. 7 4. 1 24 2, 400 34-36 97. 4 Trace 39 2, 550 84-35 98. 0 0 64 A1 2, 000 31-32 89. 4 2. 2 14 2, 200 34 93. 4 0 24 2, 400 36-37 97. 0 0 47 2, 550 3536 98. 6 0 03 A-Z 000 33-34 91. 8 0. 5 10 2, 200 36 94. 8 0 18 2, 400 3638 98. 3 0 32 2, 550 37-38 99. 3' 0 37 In addition, two test bars each were prepared from material of Lot A2;and the control lot for strength and ductility measurements. These bars measured one-quarter inch square by 12 inches long and were prepared by pressing the powder of the several lots at 30 t.s.i. The pressed bars were then pre-sintered at 1000 C. for 30 minutes, followed by sintering at 2250 C. for 3.5 hours. The bars were tested for transverse rupture strength and deflection at spaced points along their lengths in order to get an average value. The results of the tests are given in Table 2.
TABLE 2.-TRANSVERSE RUPTURE TESTS [54" x M x 12 bars sintered at 2,250 C. for 3.5 hours] Transverse Average Bar Test. Rupture Deflection ,Point; Stren h, [2" Strength, Deflection p.s.i. p.s.l. [2"
Contr. 1- A 248, 000 063 C 247, 000 063 Contr. 2-. A 255, 000 037 B 267, 000 063 253, 000 000 O 270, 000 075 D 260, 000 063 A-2-1 A 273, 000 075 r B 290, 000 094 282, 000 089 C 282, 000 094 D 284, 000 094 A;-22 A 286, 000 125 B 284, 000 094 284, 000 094 C 282, 000 063 D 285, 000 094 blend several batches obtained by dividing the powder initially into more than one boat, to obtain a homogeneous product. v
This prealloyed powder was compacted under a pressure of 30 t.s.i. to form a self supporting slug or ingot. This was pre-sintered at about 1000 C. for 30 minutes, followed by sintering for 3.2 hours in an electric furnace under a hydrogen atmosphere at selected temperatures of from 2000 to 2550 C. The results are compared in Table 1 which follows with the control lot andth-at of Lot A-2.
Example 2 In a preferred modification of the process, a second batch, designated Lot A2, of 75%25% tungstenrhenium powder pre alloy slurry was prepared as in Example 1. However after the concentrated perrhenic acid was mixed with the tungsten powder and preliminarily heated until the mixture was at approximately 100 C., sufiicient ammonium hydroxide was added, while agitating the slurry, to effect neutralization or slight basicity of the slurry. This was determined using pH paper as an indicator. The slurry was then heated to dryness, crushed and sifted as before, and subjected to the successive reduction steps in hydrogen atmosphere as described in Example 1.
Again the resulting pre-alloyed powder was compacted at 30 ts.i. into self-supporting slugs and sintered at the aforesaid range of temperatures.
A comparison of the three lots of material thus described is shown in Table l.
Tungsten-rhenium powders of different alloy proportions than that of the foregoing Examples 1 and 2 can be prepared in the same manner by varying the pro portions accordingly. However there is a practical maximum limit of about 26% rhenium in such pre-alloyed powder, since above that level homogeneity of the resulting alloy products is difficult and there is a concurrent appearance of substantial amounts of sigma phase. The more dilute alloys present no problem, and one of practical importance at the present time contains about 97% tungsten, 3% rhenium. Other alloys which are still more dilute in respect to rhenium, as well as those having rhenium contents at levels intermediate the limits mentioned, are readily possible and such alloys can be prepared in accordance with the methods described in either of Examples 1 or 2 above by proper porp'ortioning of the amount of tungsten powder and perrhenic acid. employed. In the case of the more dilute alloys, it may be. necessary to addwater to the concentrated perrhenic acid solution used in the previous examples in order to provide. sufiicient volume of liquid to effect uniform wetting of the tungsten powder.
Pro-alloyed powders of molybdenum and rhenium may also be prepared in a similar manner. The following is illustrative.
Example 3 A pre-alloyed powder containing 53% molybdenum,- 47% rhenium was prepared by admixing 53 grams of molybdenum powder with aqueous perrh'enic acid in amount sufiicient to provide the equivalent of 47 grams of rhenium. Using a perrhenic acid solution as before containing the equivalent of 1.5 grams of rhenium per milliliter, 31.3 ml. of solution is required.
Either of the procedures outlined in Examples 1 and 2 above may be followed in further treatment of the molybdenum-rhenium pre-alloyed powder preparation. Using the preferred procedure of Example 2, wherein the perrhenic acid impregnated molybdenum powder is neutralized with ammonium hydroxide prior to the reduction step, and sintering at 2250 C. for 3 hours in hydrogen after compacting at 30 t.s.i., the resulting slugs had the properties shown for Lot A-3 in Table 3. For comparison, a control lot of powder prepared in the conventional manner using elemental powder mixtures of 53% molybdenum and 47% rhenium was tested and the properties of this control material is likewise reported in Table 3.
TABLE 3.-PROPERTIES or 0.5" diameter 57% MOLYBDENUM- 47% RHENIUM ALLOY PELLETS SINIERED AT 2250 C. FOR 3 HOURS There was no evidence of sigma phase in either of the foregoing lots but a notable increase in density of the invention lot was obtained and, quite importantly, there was much lower grain growth encountered. Pre-alloyed powders or slugs or ternary, aswell as the foregoing binary alloys, may also be prepared in accordance with the process of the invention by proper proportioning of the amounts of tungsten and molybdenum with the perrhenic acid. Ternary alloys currently of interest prepared in this manner comprise 46.4% tungsten, 18.2% molybdenum and 35.4% rhenium, by weight; also, 52.0% tungsten, 18.2% molybdenum and 29.8% rhenium.
In comparing densities of the slugs prepared in the examples reported above, the values given are based on measurements made by the water immersion method. Second phase content or sigma was measured by traversing a suitably prepared sample and taking four representative photomicrographs. The area of sigma inclusions was measured with a planimeter and expressed as a percentage of the total area photographed. Grain size determinations were made in accordance with standard methallographic examining procedures.
As previously mentioned, one of the chief advantages of the invention is that of minimizing any disturbance,
of the metal powder particle size and shape distribution. This is important for good sintering properties; i.e. good density, powder flow, pressing characteristics. It is especially important where the amount of exposed surface in alloy powder is critical, as in thermionic engines for example. In general, the refractory metal powders in the examples given above had an average starting particle size, as measured by a Fisher sub-sieve sizer (F.S.S.S.), of about 2 to 3 microns, and a Scott density of around 40 grams per cubic inch. The particle size of the prealloyed powders, before sintering, produced in accordance with the invention remained substantially the same and thus represents a notable improvement over prior lrnown techniques,
What is claimed is:
1. The process of preparing a pre-alloyed powder of rhenium and a refractory metal selected from the group consisting of tungsten, molybdenum and mixtures thereof, which method comprises wetting an elemental powder of said refractory metal with an aqueous solution of purified perrhenic acid, drying the wetted powder and reducing the rheniu'm content thereof to rhenium metal intimately adhered to and at least in part alloyed with individual particles of said refractory metal by subjecting it to a hydrogen atmosphere at temperatures up to 1200-" C. to complete reduction of the rhenium content to elemental form without significantly affecting particle size characteristics of the starting powder mixture.
2. The process as defined in claim 1, wherein the temperature employed during the reduction step is a maximum of from 800 to 1200" C. 1
3. The process as defined in claim 1, which includes the further step of neutralizing said perrhenic acid wetted refractory metal powder with ammonium hydroxide before subjecting it to said drying and reducing steps.
4. The method of preparing a homogeneous binary alloy of rhenium and a refractory metal selected from the group consisting of tungsten and molybdenum wherein the tungsten-rhenium alloy contains up to about 25% rhenium and the molybdenum-rhenium alloy contains up to about 47% rhenium, which method comprises completely wetting a powder of said refractory metal in elemental condition with an aqueous solution of perrhenic acid, drying the wetted powder and subjecting this to a hydrogen atmosphere at temperatures up to 800 to 1200 C. to reduce the rhenium content to elemental rhenium intimately adhered to and at least in part alloyed with individual particles of said refractory metal powder, compacting the thus treated powder and sintering the resulting compact at temperatures of from about 2000 C. to 2550 C. in a hydrogen atmosphere to produce a homogeneous alloy of said rhenium and refractory metal by solid state diffusion.
5. The method of producing a homogeneous alloy as defined in claim 4, which includes the further step of neutralizing said perrhenic acid wetted refractory metal powder with ammonium hydroxide prior to subjecting it to said drying and reducing steps.
6. The method of producing a homogeneous alloy as defined in claim 5, wherein said rhenium-treated refractory metal powder is compacted at a pressure of approximately 30 t.s.i. to form a self-supporting compact, and is presintered at atemperature of about 1000 C.
7. The method of producing a homogeneous alloy as defined in claim 6, wherein said presintered compact is further sintered at temperatures of from 2000 C. to
References Cited UNITED STATES PATENTS 3,024,522 3/1962 Cacciotti 117-417 x 3,236,699 2/1966 Pugh 15-207 X 3,300,285 .1/1967 Pugh 7s 207 x BENJAMIN R. PADGE'IT, Primary Examiner.
A. J. STEINER, Assistant Examiner.