US3490959A - Beryllium composite - Google Patents

Description

Jail; 20,. 1970 R. H. KRdcK ETAL 3,490,959

BERYLLIUM COMPOSITE Filed Feb. 11, 1966 2 Sheets-Sheet 1 SI LVER- BERYLLIUM PHASE DIAGRAM WEIGHT PER CENT BERYLLIUM 0.5 I I 2 3 4 5 7.5 IO I5 3O 4Q 6080 TEMPERATURE, C

o :0 2o .so 10 so I00 Ag ATOMIC PER CENT BERYLLIUM Be ATTORNEY Jan. 20, 1970 R. H. KROCK ETAL 3,490,959

BERYLLIUM COMPOS ITE Filed Feb. 11, 1966 2 Sheets-Sheet 2 INVENTORS RICHARD H. KROCK CLINTFORD RJONES ATTORNEY United States Patent 3,490,959 BERYLLIUM COMPOSITE Richard H. Krock, Peabody, and Clintford R. Jones, Arlington, Mass., assignors to P. R. Mallory & Co. Inc.,

Indianapolis, Ind., a corporation of Delaware Filed Feb. 11, 1966, Ser. No. 526,746

Int. Cl. C22c 25/00 U.S. Cl. 14822 6 Claims ABSTRACT OF THE DISCLOSURE A mixture consisting essentially of about 60 to about 85 weight percent beryllium and up to 2 percent of a lithium halogenide, the remainder being essentially silver, useful for making lightweight, strong, and ductile composites of beryllium-silver.

The present invention relates to prime composites of beryllium and more particularly to means and methods for providing such composites through liquid phase sintering.

Liquid phase sintering differs from the several other other types of sintering techniques in that the sintering of the compact is carried out in the presence of a liquid phase. Liquid phase sintering encompasses raising the temperature of the compressed powder metal constituents to a temperature wherein a predetermined amount of the liquid phase appears. In the liquid phase, one of the metal constituents, the solid, is progressively dissolved in the other metal constituent, the liquid. However, the quantities of these constituents are such that, at equilibrium, some solid phase always exists. It is thought that the liquid wets the solid so as to bring about favorable surface energies existing between the liquid and the solid thereby permitting solution into the liquid phase.

However, heretofore, when certain alloys were developed in accordance with liquid phase sintering, it was found that the solid expelled the liquids from the compact. For example, an alloy of beryllium-silver exhibits expulsion of the silver from the specimen using known liquid phase sintering techniques. It is thought that the unfavorable surface energy equilibrium causing expulsion of the liquid is due to a tough, tenacious film of beryllium oxide which is present on each particle of beryllium.

The present invention prevents the expulsion of the liquid from the specimen by using an agency to intervene in the sintering stage. The agency either breaks down the oxide film on the beryllium or segregates to the metal oxide interface and lowers the surface energy of the liquid metal with respect to the beryllium oxide film so that the liquid metal progressively dissolves the solid metal.

The agency can be called a fiuxing agent or flux, however, the agent has other characteristics which assist in wetting beryllium so as to surround the beryllium with a ductile envelope phase of a silver-beryllium alloy matrix metal thereby avoiding the expulsion of the liquid from the specimen.

Beryllium has several desirable physical features which make it attractive for a variety of applications such as lightweight gears, lightweight fasteners, airplane parts or the like. Some of the more desirable features of beryllium metal are a density of 1.82 grams per cubic centimeter as compared to 2.7 grams per cubic centimeter for aluminum, a high elastic modulus of 40 10 pounds per square inch as compared to 30x10 pounds per square inch for steel, a high melting temperature of 1285 centigrade, extremely high specific stiffness, good strength, excellent dimensional stability, low capacity for neutron absorption, most elfective metal for slowing and reflecting neutrons, and good corrosion resistance in air and water. It is seen that beryllium metal is lighter than aluminum metal and has a melting temperature that is about twice that of aluminum. In addition, beryllium is very transparent to X-rays. This factor, in conjunction with its high melting point, makes beryllium suitable for use as windows in X-ray tubes. However, beryllium has one major drawback which has seriously limited its commercial acceptance, that is, beryllium is inherently brittle at room temperature.

The lack of ductility of beryllium is attributed to the crystal structure of beryllium which is hexagonal close packed. During deformation, the basal planes of the hexagonal close packed structure, being the easiest to slip, are aligned along the working direction. Since slip is crystallographically dilficult perpendicular to the basal plane, the ductility of beryllium perpendicular to the primary fabrication direction is practically nonexistent.

Several tentative solutions have been advanced in an attempt to make beryllium metal sufficiently ductile so as to permit a widespread commercial acceptance of the metal. Cross-rolling and cross-forging have been suggested as fabrication methods which might enhance the ductility of beryllium. These fabrication techniques reduced the number of basal planes along the direction of rolling and resulted in improved ductility. However, the degree of improvement was far from satisfactory. The fact remained that beryllium must be classifed as brittle at room temperature even utilizing the aforementioned method when ductility perpendicular to the fabrication temperature is considered. In addition, the abovementioned technique would not be feasible where the fabrication is, by nature, solely along one axis such as swaging, drawing, and extrusion.

In reecnt years, attention has been directed to the fabrication of beryllium alloys not having the inherent brittleness of beryllium itself but possesing various outstanding properties of the metal such as, for example, low density combined with high strength. It is thought that U.S. Patent 3,082,521 fabricated the first ductile beryllium-silver alloy by rapidly quenching the part from a temperature at which it was liquid. However, the beryllium content was not in excess of 86.3 atomic percent which is approximately 30 weight percent. Although the beryllium alloy was ductile, the density of the alloy was in excess of that of aluminum and about equal to that of titanium.

It has also been suggested that beryllium alloys might be fabricated by pressing and sintering a mix of metal powders. However, such a method results in expulsion of the matrix metal or metals from the beryllium specimen and the eventual freezing of the matrix metal or metals into globs on the surface of the solid specimen. It is thought that the expulsion of the matrix metal or metals is due to the surface energies of the solid beryllium and the various liquid formed. The unfavorable surface energy equilibrium is believed to be due to a tough, tenacious film of beryllium oxide which is present on each particle of beryllium.

A means and method have been discovered for preparing a composite of beryllium and a metal such as silver containing up to percent, by weight, of beryllium thereby producing a composite having a density less than that of aluminum, having high strength, and having good ductility. The ductility is due to the resulting microstructure of the composite. By surounding the beryllium particles with a ductile envelope phase, a composite is formed where, under load, the beryllium is so constrained by the ductile phase that it and the ductile phase deform continuously.

Alkali and alkaline earth halogenide agents such as lithium fluoride-lithium chloride or the like in a determined ratio are utilized to segregate to the solid interface of the beryllium particle and either break down the film on the particle of beryllium and/or alter the liquidsolid surface energy in the system.

Therefore, it is an object of the present invention to provide an agent to promote liquid phase sintering of a beryllium-silver mixture.

A further object of the present invention is to provide a ductile beryllium composite having a low density and high strength.

A further object of the present invention is to provide a ductile composite of beryllium in which beryllium is the predominate ingredient.

Another object of the present invention is to provide a means and method of producing a ductile composite of beryllium-silver whose microstructure consists of beryllium particles surrounded by a ductile envelope phase of a silver-beryllium alloy matrix metal.

Yet another object of the present invention is to provide a ductile composite of beryllium containing 60 percent, by weight, or more of beryllium.

Yet still another object of the present invention is to provide a ductile composite of beryllium-silver containing about 75 percent, by weight, beryllium, and the remainder silver.

A further object of the present invention is to pro-vide an agent which eliminates the expulsion of a matrix metal from a beryllium specimen.

Still another object of the present invention is to provide alkali and alkaline earth halogenide agents used in the fabrication of a beryllium composite.

Another object of the present invention is to provide a composite of beryllium-silver that may be sintered to substantially theoretical density.

Yet another object of the present invention is to provide a means and method whereby a ductile beryllium composite may be succesfully fabricated in both a practical and economical manner.

A further object of the present invention is to provide a lithium fluoride-lithium chloride agent for promoting liquid phase sintering in a beryllium and silver mix.

Yet still another object of the present invention is to provide a lithium fluoride-lithium chloride agent wherein the constituents are used in a predetermined ratio.

The present invention, in another of its aspects, relates to novel features of the instrumentalities of the invention described herein for teaching the principal object of the invention and to the novel principles employed in the instrumentalities whether or not these features and principles may be used in the said object and/or in the said field.

With the aforementioned objects enumerated, other objects will be apparent to those persons possessing ordinary skill in the art. Other objects will appear in men by the forces of surface energy of solid beryllium and various liquids formed.

FIGURE 3 is a photomicrograph of a 25 percent, by weight, silver in beryllium composite illustrating a delta or a gamma intermediate phase surrounding the beryllium particles.

FIGURE 4 is a photomicrograph of a 25 percent, by weight, silver in beryllium composite illustrating the absence of the delta or the gamma intermediate phase.

Generally speaking, the means and method of the present invention relate to a ductile beryllium composite fabricated by liquid phase sintering. The composite contains from about 60 to percent, by Weight, of beryllium, and the remainder silver.

The method of producing the beryllium-silver composite by liquid phase sintering comprises the steps of mixing predetermined portions of powder beryllium and powder silver together with a predetermined portion of an agent selected from the group consisting of alkall and akaline earth halogenides. The portions are pressed in a die to form a green compact. The compact is then heated to the sintering temperature. At this temperature the agent provides a favorable surface energy equilibrium between the beryllium and the silver so that the silver progressively dissolves the beryllium at the sintering temperature. Thereafter, the composite is quenched or heat treated so as to substantially eliminate the formation of a gamma or of a delta phase in the alloy.

More particularly, the method of the present invention comprises mixing powder beryllium of about 60 to 85 percent, by Weight, and the remainder powder silver. An agent of lithium fluoride-lithium chloride in about 0.5 to 2.0 percent, by weight, of the total metal additions is mixed with the beryllium and the silver powders. The constituents of the agent are in about a one to one ratio, by weight. The beryllium, the silver, and the agent are pressed so as to form a green compact. The green compact is heated in a non-oxidizing atmosphere such as argon at a temperature of about 1050 centigrade to about 1250 centigrade. At the aforementioned temperatures, the agent provides a favorable surface energy equilibrium between the beryllium and the silver so that the silver progressively dissolves the beryllium. The microstructure of the resultant composite consists of beryllium particles surrounded by a ductile envelope phase of a silver-beryllium alloy matrix metal. The alloy is sintered to substantially its theoretical density. The alloy is then quenched or specially heat treated so as to substantially avoid the formation of a gamma phase or a delta phase in the alloy.

In carrying out the present invention, a beryllium base compact is fabricated by any suitable means such as powder metallurgy techniques. A suggested method utilizing this technique is to mix beryllium powder with powdered silver and an agent of equal parts of lithium fluoridelithium chloride. The powders are blended and mixed by ball milling the metal powders and the flux agent. The blended and mixed powders are compacted to form a green compact by accepted metallurgical methods such as by compacting within the confines of a die in a hydraulic or an automatic press or by placing the powders in a rubber or a plastic mold and compacting in a hydrostatic press. The green compact is sintered in a non-oxidzing atmosphere such as argon or the like at a temperature of about 1050 centigrade to about 1250 centigrade. It is seen that the range of the sintering temperatures is below the l277 centigrade melting point temperature of beryllium but above the 960.8 centigrade melting point temperature of silver. The silver will dissolve smaller beryllium particles and will dissolve the surfaces of the larger beryllium powder particles thereby surrounding the remaining beryllium particles with a ductile envelope phase of a silver-beryllium alloy.

The agent, lithium fluoride-lithium chloride, either breaks down the oxide film on the beryllium or segregates to the metal oxide interface lowering the Surface energy of the liquid metal with respect to the beryllium oxide film. Simply, the agent causes the liquid to wet the beryllium.

Composites containing about 60 to 85 percent, by weight, of beryllium, and the remainder silver were successfully fabricated. The agent prevented the expulsion of the liquid silver-beryllium alloy from the compact by the forces of surface energy, that is, prevented the formation of very fine rounded droplets of the silver-beryllium alloy on the surface of the beryllium specimen. FIGURE 2 shows a beryllium specimen 20 having on the surface thereof an expelled alloy 21 of silver-beryllium. Specimens from which the silver-beryllium alloy has been expelled have gross porosity and as a result are weak, brittle, and of little commercial value.

The composition of the agent utilized is about 50* parts, by weight, of lithium fluoride to about 50 parts, by Weight, of lithuim chloride. The agent provides an action, such that, upon heating or sintering of the pressed powder mix to the temperature at which the liquid phase forms, expulsion of the melt from the specimen is eliminated. Furthermore, it was found that solution of the beryllium into the silver was enhanced as evidenced by the rounded particles or beryllium in the microstructure.

Where the beryllium is present in amounts greater than 75 percent, by weight, it was found that the amount by weight of lithium fluoride-lithium chloride agent should exceed 0.5 percent, by weight, of the total of all metal additions. It would appear that the optimum range of the agent is from about 0.5 percent to about 2.0 percent, by weight, of the total of all metal additions. It is believed that the quantity of lithium fluoride-lithium chloride agent required is related to the amount necessary to cover the total beryllium surface area. Hence, the minimum amount of agent needed would be a function of the surface area of the beryllium powder. The utilization of lithium fluoride-lithium chloride agent in other than equal parts is possible. It is thought, however, that an equal parts mixture achieves optimum results.

By using the methods of the present invention and the lithium fluoride-lithium chloride agent, a compact was fabricated containing up to 85 percent, by weight, of beryllium without the use of pressure during sintering. The composite was sintered to between about 95 and 99 percent of its theoretical density and had a density of between 2.19 and 2.29 grams per cubic centimeter. The good strength and low density characteristics of the beryllium were retained and the resulting beryllium-silver composite possessed good ductility. Thus, by substantially surrounding the beryllium particles with a ductile envelope phase of a silver-beryllium alloy matrix metal, the beryllium and the matrix metal deform continuously under load.

The beryllium-silver phase diagram of FIGURE 1 illustrates that'beryllium-silver mixtures having a beryllium content in excess of about 2.3 percent, by weight, form a melt and are in equilibrium with substantially pure beryllium at temperatures above about l0 centigrade. The composition of the silver-beryllium alloy melt is deterr'nined by the temperature of the melt and is independent of the percent, by weight, of beryllium while the relative amount of the solid beryllium and of the matrix metal at the sintering temperature is determined by the temperature itself, as well as by the percent, by weight, of beryllium relative to the percent, by weight, of silver. Beryllium-silver mixtures have been sintered at a plurality of temperatures between 1050 centigrade and 1250 centigrade. Liquid phase structures have been obtained at each of the temperatures at which the compact was sintered. It was noted that when the percent, by volume, of the liquid is less than about 5 percent, the sintering in the liquid phase is slow and porosity is apparent in the materials. It was also noted that when the percent of volume of the liquid exceeds about 35 percent, the solid beryllium particles are not capable of maintaining the structure intact and as a result thereof, sagging of the pressed compact may be observed. Hence, for a particular alloy, temperature ranges for sintering can be predicted from the phase diagram, and these temperature ranges have been corroborated by experimentation.

Microstructure calculations are given for both quenched, metastable and equilibrium structures in the following table.

PHASE IN BERYLLIUM-SILVER ALLOY [60 percent by weight, Beryllium and 40 percent by weight Silver] 1,050 29. 1 10. 5 89. 5 2. 29 1,100 33. 3 15. 8 84.2 2. 29 1,150.-. 39.6 23.8 76. 2 2. 29 1,200.-. 50. 0 36. 8 63. 2 2. 29 1,225 62. 3 52. 7 47. 3 2. 29 Room temp. equivalent 5. 45 94. 5 2 29 percent by weight, Beryllium and 15 percent by weight, Silver] 17. 7 5. 8 94. 2 2.07 20. 3 8. 7 91. 3 2. 07 23. 5 12.6 87. 4 2. 07 30. 0 20. 0 80. 0 2. 07 37. 5 29. 0 71.0 2.07 75. 0 68. 0 32. 0 2. 07 Room temp. equivalent 2. 96. 8 2. 07

It will be noted that the density values of the composite are between the density of beryllium and the density of aluminum. Composites containing from about 60 to about 75 percent, by weight, of beryllium may be sintered from about 96 to about 99 percent of density by a single sinter. Composites containing about 85 percent, by weight, of beryllium require a double pressing and sintering operation to attain about 95 percent of theoretical density.

It was noted that upon cooling from the sintering temperature of the beryllium, the beryllium particles react with the silver rich liquid through a peritectic reaction whereby a new phase, delta, is formed below a temperature of about 1010 centigrade. The delta phase which is in equilibrium with the solid beryllium between about 10l0 centigrade and 850 centigrade contains about 18 percent, by weight, of beryllium. Additional cooling to a temperature between abut 850 and about 760 centigrade results in reaction of the delta phase with solid beryllium particles to form a gamma phase in equilibrium with the beryllium particles. The gamma phase contains about 12 percent, by weight, of beryllium. At about 760 centigrade, the gamma phase reacts with the beryllium particles so as to form substantially solid silver in equilibrium with substantially pure beryllium.

Since solid state reactions are generally slow, it is possible during normal cooling of the composite to retain either the gamma or the delta phase at room temperature due to sluggish diffusion. Since the presence of either the gamma phase or the delta phase in the microstructure of the composite would have a deleterious effect thereon from a ductility standpoint, either an isothermal hold of a predetermined time duration at 750 centigrade or a reheat to 750 centigrade is required to dissolve the gamma or the delta phase present in the microstructure. It has been found that the gamma or the delta phase may be dissolved by maintaining or reheating the alloy to 750 centigrade for about 24 hours. Also, it was found that it was possible to retain the elevated temperature structure and composition by quenching the alloy from a temperature above 1010" centigrade. It is seen that quenching substantially eliminates the heat treatment step, however, quenching involves very rapid cooling rates.

Attention is directed to FIGURE 3, wherein a photomicrograph of 500 magnifications shows a composite of 25 percent, by weight, silver'in beryllium after being etched by any suitable etching means such as a dilute solution of ammonium hydroxide and hydrogen peroxide. The areas 10 are beryllium particles. The dark areas 11 are the intermediate delta or gamma phase surrounding the beryllium particles.

FIGURE 4 shows the appearance of the composite of 25 percent, by weight, silver in beryllium after heat treating at 750 Centigrade in an argon atmosphere for about 24 hours. Note that the delta or the gamma phase has been removed. In the figure, the areas 10 are the sintered beryllium particles and the area 12 is the ductile silverberyllium alloy matrix which surrounds the sintered beryllium particles.

Example 1 shows the expulsion of the liquid from a beryllium specimen and Examples 2 to 4 are illustrative of the preparation of beryllium-silver composites by liquid phase sintering.

EXAMPLE 1 Expulsion of the liquid silver-beryllium alloy from the solid beryllium specimen when the agent of lithium fluoride-lithium chloride is not used in the preparation of a beryllium-silver composite.

A mixture of about 75 percent, by weight, of beryllium having a particle size of 200 mesh or finer was ball mill mixed with about 25 percent, by weight, of silver powder of suitable particle size. The milled mixture was pressed by any suitable means such as by an automatic press at a suitable pressure to provide a green compact sturdy enough to be handled. It was found that pressures of from about 15,000 to 20,000 pounds per square inch resulted in a green compact having a density from about 50 to 60 percent of theoretical density and sufiiciently strong to be handled. Sintering of the compact was carried out in an argon atmosphere at about 1150 centigrade for about 1 hour. This technique, due to the surface energies of the solid beryllium and the liquid formed, resulted in the expulsion of the liquid from the specimen and its eventual freezing into rounded globs on the surface of the specimen.

EXAMPLE 2 A composite of about 60 percent, by weight, beryllium and about 40 percent, by weight, of silver.

A mixture of about 60 percent, by weight, of beryllium having a particle size of 200 mesh or finer was ball mill mixed with about 40 percent, by weight, of silver powder of suitable particle size. Also ball mill mixed with the beryllium powder and the silver powder was about 1.0 percent, by weight, of the total metal additions equal parts of an agent of lithium fluoride-lithium chloride. Mixtures of beryllium and silver powders were also prepared with the agent having 0.5 and 2.0 percent, by weight of the total metal additions. The milled mixture was pressed by any suitable means such as by an automatic press at a suitable pressure to provide a green compact sturdy enough to be handled. It was found that pressures of from about 15,000 to 20,000 pounds per square inch resulted in a green compact having a density offrom about 50 to 60 percent of theoretical density and sufficiently strong to be handled. Sintering of the compact was carried out in an argon atmosphere at about 1l50 centigrade for about 1 hour. An individual composite was prepared using the abovementioned procedure at each of the hereinafter enumerated temperatures of: l050, 1100, 1200, 1225, and 1250 centigrade. Each of the composites was heat treated using the methods disclosed hereinafore.

EXAMPLE 3 A composite of about percent, by weight, beryllium and about 25 percent, by weight, silver.

The procedure of Example 2 was followed using 75 percent, by weight, of beryllium and 25 percent, by weight, of silver. An individual composite was prepared at each of the following temperatures of about l050, 1100", ll5 (;l, 1200", 1225 and 1250" centigrade using the aforementioned procedure.

EXAMPLE 4 A composite of about percent, by weight, beryllium and about 15 percent, by weight, silver.

The procedure of Example 2 was followed using about 85 percent, by weight, of beryllium and about 15 percent, by weight, of silver. An individual composite was prepared and heated to one of the following temperatures of about l050, 1100", 1150, 1200, 1225 and 1250 centigrade.

The present invention is not intended to be limited to the disclosure herein, and changes and modifications may be made in the disclosure by those skilled in the art without departing from the spirit and the scope of the novel concepts of this invention. Such modifications and variations are considered to be within the purview and scope of the invention and the appended claims.

Having thus described our invention, we claim:

1. A mixture consisting essentially of about 60 to about 85 weight percent beryllium and up to 2 weight percent of a lithium halogenide, the remainder being essentially silver.

2. The mixture as claimed in claim 1, wherein said lithium halogenide is lithium fluoride-lithium chloride.

3. The mixture as claimed in claim 2, wherein said lithium fluoride-lithium chloride is about 0.5 weight percent to about 2 weight percent.

4. The mixture as claimed in claim 2, wherein said beryllium is about 75 weight percent, said lithium fluoridelithium chloride about 1 weight percent, the remainder said silver. t

5. The mixture as claimed in claim 2, wherein said lithium fluoride to said lithium chloride is in a ratio of about one to about one.

6. A mixture as claimed in claim 2, wherein said beryllium is beryllium powder having a particle size of about 200 mesh or finer.

References Cited UNITED STATES PATENTS 3,264,147 8/1966 Bonfield 75l5() 2,287,251 6/1942 Jones 29182 2,244,608 6/1941 Cooper 75-150 L. DEWAYNE RUTLEDGE, Primary Examiner W. W. STALLARD, Assistant Examiner

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