US3676083A - Molybdenum base alloys - Google Patents

Abstract

A MOLYBDENUM BASE ALLOY IS DISCLOSED THAT HAS A RECRYSTALLIZATION TEMPERATURE ABOVE THAT OF MOLYBDENUM AND UPON BEING HEATED ABOVE ITS RECRYSTALLIZATION TEMPERATURE FORMS AN INTERLOCING GRAIN STRUCTURE. THE ALLOY COMPRISES, IN PARTS PER MILLION BY WEIGHT OF THE ALLOY, FROM ABOUT 30 TO ABOUT 180 PARTS PER MILLION OF ALUMINUM, FROM ABOUT 600 TO ABOUT 2500 PARTS PER MILLION OF SILICON AND FROM ABOUT 50 TO ABOUT 150 PARTS PER MILLION OF AN ALAKLI METAL. A PROCESS FOR PREPARING THE ALLOY IS DISCLOSED THAT INVOLVES FORMING A UNIFORM MIXTURE OF FINELY DIVIDED MOLYBDENUM DIOXIDE AND A SUFFICIENT AMOUNT OF A RECRYSTALLIZATION MODIFIER, REDUCING THE MOLYBDENUM DIOXIDE IN THE MIXTURE TO MOLYBDENUM COMPACTING THE MIXTURE INTO SHAPED ARTICLES AND THEREAFTER HEATING THE ARTICLES IN A REDUCING ATMOSPHERE UNDER CONTROLLED RATE OF HEATING BY SELF-RESISTANCE HEATING.

Description

3,676,083 MOLYBDENUM BASE ALLOYS Richard-F. Cheney and Donald S. Parsons, Towanda, Pa., assignors to Sylvania Electric Products Inc. No Drawing. Filed Jan. 21, 1969, Ser. No. 792,754 Int. Cl. B22f 3/00 U.S. c1. 29-182 5 Claims ABSTRACT OF THE DISCLOSURE A molybdenum base alloy is disclosed that has a recrystallization temperature above that of molybdenum and upon being heated above its recrystallization temperature forms an interlocking grain structure. The alloy comprises, in parts per million by weight of the alloy, from about 30 to about 180 parts per million of aluminum, from about 600 to about 2500 parts per million of silicon and from about 50 to about 150 parts per million of an alakli metal. A process for preparing the alloy is disclosed that involves forming a uniform mixture of finely divided molybdenum dioxide and a sufiicient amount of a recrystallization modifier, reducing the molybdenum dioxide in the mixture to molybdenum, compacting the mixture into shaped articles and thereafter heating the articles in a reducing atmosphere under controlled rate of heating by self-resistance heating.

BACKGROUND OF THE INVENTION This invention relates to molybdenum base alloys. More particularly it relates to new molybdenum base alloys having improved properties such as higher recrystallization temperatures and improved grain structure and to processes for preparing the alloys.

Commercially available molybdenum and molybdenum base alloys generally derive a major portion of their strength from work hardening, that is the development of strength as a result of the plastic deformation received by the grains during the shaping of the wrought product. This strengthening is achieved by the Working of the metal below its recrystallization temperature, generally below about 1400 C. When heated to above about 1400 C., molybdenum and the molybdenum base alloys recrystallize to forn'requiaxed grains. Such recrystallization considerably decreases the hardness and strength of the product and the equiaxed grain structure is generally brittle. It is believed, therefore, that a molybdenum base alloy that has a higher recrystallization temperature and therefore, is stronger and more ductile over a wider temperature range, would be an advancement in the art. Furthermore, an alloy that recrystallizes to a more ductile and interlocking grain structure would also be an advancement in the art.

SUMMARY OF THE INVENTION In accordance with one aspect of this invention, there is provided a molybdenum base alloy having a recrystallization temperature above that of pure molybdenum and upon being heated above the recrystallization temperature forms an interlocking grain structure. The foregoing molybdenumbase alloy comprises, in parts per million by weight of the alloy,-from about 40 to about 180 parts per million of aluminum, from about 600 to about 2500 parts 3,676,083 Patented July 11, 1972 per million of silicon, and from about 50 to about parts per million of an alkali metal. In accordance with another aspect of this invention, there is provided a method for preparing the alloy. The process for producing the alloy comprises (1) forming a relatively uniform mixture of a relatively pure and finely divided molybdenum dioxide powder and a sufficient amount of a recrystallization temperature modifier comprising an aluminum source, a silicon source and an alkali metal source, (2) reducing said molybdenum dioxide powder to molybdenum powder by heating in a hydrogen atmosphere, (3) compacting the uniform powder to form a shaped article, and (4) sintering said article by self-resistance electrical heating under controlled conditions, as hereinafter described, to form the molybdenum base alloy of this invention.

For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims in connection with the above description of some of the aspects of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In accordance with one of the preferred embodiments, a new molybdenum base alloy is provided that comprises, in parts per million by weight, from about 40 to about ppm. of aluminum, from about 600 to 2500- p.p.m. of silicon, from about 50 to about 150 p.p.m. of an alkali metal and the remainder being primarily molybdenum. It is preferred to utilize an alkali metal selected from the group consisting of sodium and potassium, with potassium being especially preferred. It is also preferred to incorporate from about 150 to about 600 p.p.m. of a metal selected from the group consisting of chromium, iron, nickel, cobalt and mixtures thereof. In most instances it is preferred to use mixtures of chromium, iron and nickel at weight ratios of from about 1:10-24 to about 1:30:17 respectively. It is especially preferred to utilize ratios of the foregoing elements of from about 1:l1:4 to about l:13:6 respectively, when mixtures of the foregoing metals are incorporated into the powder mixtures to form the alloys of the present invention.

Although the metal additives can be added at any point prior to sintering and the benefits of this invention can be achieved, it is preferred to add either the metal salts or the metal oxides to the molybdenum dioxide and then heat the mixture under a hydrogen atmosphere, thereby converting the molybdenum oxide to molybdenum. For example, one method that will be described in more detail hereinafter is to incorporate appropriate amounts of potassium silicate and aluminum nitrate to the molybdenum dioxide. The potassium silicate is both an alkali metal source and a silicon source. The aluminum nitrate decomposes under heat to form an elemental aluminum or aluminum oxide. It is to be understood that when a metal salt is used it should be one that will decompose to the metal or to the refractory oxide.

The total amount of the additive, also referred to herein as the recrystallization temperature modifier, used in the practice of this invention sometimes does not remain in the molybdenum base alloy powder but rather is distilled from the matrix as the metallic oxide or as a volatile metal containing compound. The amount of the additive employed will be dependent upon several factors such as the particular metal source used, the purity of the molybdenum source, the heat applied and the time the heat is applied during the converting of M to M0 and during sintering. It is to be noted that while the amount of the source of metal used can vary depending upon several factors, it is necessary to produce an alloy containing amounts of the foregoing metals, that is, Al, Si, and the alkali metal within the ranges specified. In most instances an amount of the metal additive added prior to the sintering step is, at most, about 10 times the weight of the amount desired in the final product. However, the amount of the desired metals in the mixture will be at least as much as that desired in the final product.

In the process for preparing the alloys of this invention, a relatively uniform mixture of a relatively pure, finely divided molybdenum dioxide powder and the recrystallization temperature modifier is formed. As used herein, rela tively pure molybdenum means that at least 99% by weight of total material is molybdenum. By finely divided it is meant that the maximum particle size is about 50 microns and the average particle size is below about 5 microns as measured by the Sharples Micromerograph. A preferred method for providing the relatively uniform mixture is to form an aqueous slurry of the modifier and the molybdenum oxide and then evaporate essentially all of the water using conventional drying techniques. The preferred method of providing the mixture of the modifier and the molybdenum dioxide insures uniform distribution of the modifier throughout the molybdenum dioxide and enables closer control of the level of addition of the materials constituting the modifier. Other methods can be used such as spraying a slurry or solution of the modifier onto a bed of molybdenum dioxide that is under agitation, such as in a blender, a rotary drier and the like. Other methods of obtaining a relatively uniform mixture of the molybdenum dioxide and the modifier will be suggested to one skilled in the art. All that is necessary is that a relatively uniform mixture of the components of the alloy be obtained. The mixture of the modifier and molybdenum dioxide is heated in a reducing atmosphere, generally in a hydrogen atmosphere, for a time sufficient to reduce essentially all of the molybdenum dioxide to molybdenum. The reduction conditions will depend upon the particular equipment used, however, the conditions are in general those employed in the production of molybdenum by powder metallurgical techniques.

After the molybdenum dioxide is reduced to molybdenum, the bulk density of the resulting powder can be as low as about 1.5 g./cc. and it is preferred to increase the bulk density to at least about 1.80 g./ cc. Any mixers can be used that will yield the equivalent bulk density. In most instances, a high intensity mixer will be preferred. After the mixing, the powder is compacted into shaped articles, generally bars having relatively uniform dimensions. Bars made in this manner are the preferred geometric shape for self-resistance electrical heating. Relatively high pressures are generally required for compacting, that is, pressures in excess of about 20,000 pounds per square inch. Although the foregoing bulk density increase is preferred for ease of subsequent steps, the density in-' crease is not required.

After the articles are formed, the material is subjected to self-resistance heating, in a reducing atmosphere, preferably hydrogen. The electric current applied is slowly increased from an initial current equivalent to 8.3 amperes/ square foot to 41 amperes/square foot over a period of about to 12 minutes, and held at the upper level for about 9 to 11 minutes. The current is slowly decreased to zero in about 1 to 2 minutes. The foregoing current densities are based upon the original dimensions of the material prior to self-resistance heating. The bars are then allowed to cool for a period of from about 8 to 10 minutes in a reducing atmosphere. After the above heating cycle is completed, the articles are suitable for conventional swaging and drawing operations that are generally used to make wire, strip, sheet and the like. Some modifications to the foregoing techniques can be employed to attain a satisfactory density for the particular application, however, in general, the procedure will not deviate in any material respect.

The alloy produced by the foregoing process has a recrystallization temperature above that of molybdenum without the additives. For example, a 30 mil diameter wire produced from the above alloy and subjected to conventional swaging, drawing and annealing steps recrystallizes to an interlocking grain structure at 1800 C. The tensile strength of the wire produced by the process'of the present invention is considerably greater than that of wire produced from molybdenum powder that does not have the recrystallization temperature modifiers. For example, at 1300 C. a wire without the modifier has a tensile strength of about 40 grams/mg./ 200 mm. while a similar diameter wire produced from the alloy of this invention has a tensile strength of about 62 grams/mg./ 200 mm. Similar increases in tensile strength are noted at temperatures of 1400 C., 1550 C. and 1650 C. when wires having the same diameter are compared. It is to be noted that the smaller the diameter of the wire produced from the alloy of this invention, the higher the recrystallization temperature. For example, an 8 mil diameter wire has a recrystallization temperature of about 1900 C. and a 7 mil diameter wire has a recrystallization temperature of about 2000 C. To further illustrate this invention, the following non-limiting example is given. All parts, proportions and percentages are by weight unless otherwise specified.

EXAMPLE About 400 parts of molybdenum dioxide powder containing about 23.5% oxygen is slurried with sufficient potassium silicate and aluminum nitrate to yield the following percentages by weight of recrystallization modifiers calculated as the equivalent oxide, and based upon the weight of molybdenum dioxide: 0.1% K 0, 0.2% SiO and 0.02% A1203.

The molybdenum dioxide powder containing the recrystallization modifiers is dried and then heated to about 2100 F. for about 2.5 hours under a hydrogen atmosphere. The powder is then screened through a 250 mesh screen to remove lumps. The powder is then subjected to high intensity mixing using a Prodex-Henschel mixer, Model 35JSS for about 2 minutes to increase the bulk density to about 1.83 g./cc. The average particle size of the powder is about 3.85 microns as measured by the Fisher Sub-Sieve Sizer.

The densified powder is compacted into bars using a pressure of about 20,000 pounds/in The bars are heated to about 11 00" C. for about 3 minutes and then transferred to an electrical self-resistance heating furnace and are placed in a vertical position. An initial current density of about 8.3 amperes/ft.'- is applied and is increased over a period of about 10 /2 minutes to about 41 ampere7ftfi. The current density is then decreased to 0 in about 1.5 minutes and thereafter the position of the bar is reversed, end for end, and the current density is increased to about 37.3 amperes/ft. in about 3.5 minutes. The current density supplied to the bars is left at about 37.3 amperes/ft. for about 10 minutes and then decreased to 0 in about 1.5 minutes and the bars allowed to cool under atmospheric conditions for about 10 minutes.

The bars are then swaged and drawn using standard equipment to produce wire. The tensile strength of a .wire having 52. mil diameter is measured as being from about 45 to about 55 g./mg./200v mm. All of the wire is drawn to a diameter of about 30 mils. About /3 of the wire is removed for testing and one-half of the remainder is drawn to about 0.008 inch diameter and the other onehalf is drawn to about 0.007 inch diameter. Spectrographic analysis of a sample of the wire drawn to 0.032 diameter indicates the presence of:

all tmeperatures and particularly at temperatures above 1500 C.

-Pm. While there have been shown and described what is Al 93 at present considered the preferred embodiments of this 51 1290 5 invention, it will be obvious to those skilled in the art K 54 that various changes and modifications can be made there- The three dilferent diameter wires are subjected to a in Without departing from i Scope of the invention as standard series of anneals as is the standard practice for defined PS the pp clalmsmolybdenum wire. Samples of each diameter wire after 1 What is claimed is: I annealing are measured for room hardness, tensile strength 1. A molybdenum base alloy cons1st1ng essenually of and percent elongation. 'Results of the tests are shown in molybdenum and a recrystalhzation temperature modlfier Table I. consisting essentially of, in parts per million by weight TABLE I 30 mil 8 mil 7 mil Tensile Tensile Tensile strength strength strength Hardness (gmS./mg./ Elongation Hardness (gms./mg./ Elongation Hardness (gms/mg Elongation Temperature (DPH-l kg.) 200 mm.) (percent) (DPH-l kg.) 200 mm.) (percent) (DPH-l kp.) 200 mm. (percent) The hardness measurements are made by a Tukon Tester, Model MO-143, using a diamond pyramid indenter and following ASTM E92-57. The tensile strength and percent elongation measurements are determined on an Instron Tensile Tester, Model TI'Cl. ASTM E857T is followed with the exceptions that a strain rate of 1 inch/ inch/ min. is used and the percent elongation is determined from cross-head travel. The guage section is 1 inch long.

It is generally accepted that upon recrystallization the tensile strength will drop suddenly to values below about 40 gms./mg./200 mm. and hardnesses to values below about 240 DPH-l kg. The sudden drop is indicative of the formation of interlocking grains, whereas a slow gradual drop corresponds to the formation of the weak equiaxed recrystallized structure.

Photomicrographs of samples, each of the 30, 8 and 7 mil wire indicates the recrystallization temperature is above about 1800* C., on all samples, and a recrystallized interlocking grain structure is developed.

Wire produced in essentially the same manner except the amount of aluminum is lowered to about 34 p.p.m. exhibits essentially the same properties as the wires of the above example.

Additionally, wire produced from material that was processed without reversing the bars in the self-resistance heating furnace, exhibits essentially the same properties, however, uniformity of the wire is improved by the reverse heating technique.

Wire produced without the modifiers and wire produced from material to which the modifiers were added to the powder but were subsequently partially removed by washing the powder with hydrofluoric and hydrochloric acid, each had significantly lower recrystallization temperature, and lower hardness and tensile strength at of the alloy, from about 40 to about 180 parts per million of aluminum, from about 600 to about 2500 parts per million of silicon and from about 50 to about 15 0' parts per million of an alkali metal, said alloy having a recrystalization temperature above that of essentially pure molybdenum processed similar to said alloy and upon being heated above its recrystallization temperature, which is above about 1800" (3., forms an interlocking grain structure.

2. An alloy according to claim 1 wherein said alkali metal is selected from the group consisting of sodium and potassium.

3. An alloy according to claim 2 wherein said alkali metal is potassium.

4. An alloy according to claim 2 wherein said alloy contains from about 150 to about 600 parts per million of a metal selected from the group consisting of chrommium, iron, nickel, cobalt and mixtures thereof.

5. An alloy according to claim 4 wherein said alloy contains chromium, iron and nickel at weight ratio of from about 1:10z4 to about 1:30:17 respectively.

References Cited UNITED STATES PATENTS 2/1953 Ramage 148l1.5 1/1954 Bechtold 148-115 OTHER REFERENCES The Metal Molybdenum, American Society for Metals, 1958, pp. 299-305, 341, 539541.

US. Cl. X.R. -l76