US3542541A - Whisker reinforced alloys and method of making the same - Google Patents

Description

F. D. LEMKEY Nov. 24, 1970 WHISKER REINFORCED ALLOYS AND METHOD OF MAKING THE SAME Filed March 15, 1966 5 Sheets-Sheet 1 X200. HARGOLIN ENCE ECHANT FIG TRANSVERSE SECTION.

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DiRECTiON OF GROWTH MARGOLIN ENCE ECHANT LONGITUDINAL SECTION. X IOOO FIG INVENTOR Fem/r1 w 0. LEMlf'y ,5 152 ATTORNEYS NOV. 24, 1970 LEMKEY 4 3,542,541

WHISKER REINFORCED ALLOYS AND METHOD OF MAKING THE SAME Filed March 15, 1966 5 Sheets-Sheet 2 TRANSVERSE SECTION MAGNIFICATIONI 200 X MARGOLIN- ENCE ETCHANT FIG. 3

LONGITUDINAL SECTION MAGNIFCATIONI ZOOX MARGOLIN -ENCE ETCHANT FIG. 4

INVENTOR FPAN (UN D. LEMKEY ATTORNEYS Nov. 24, 1970 F. D. LEM KEY WHISKER REINFORCED ALLOYS AND METHOD OF MAKING THE SAME Filed March 15, 1966 5 Sheets-Sheet 3 MAGNIFICATIONI 200 X BLADE-LIKE WHISKERS FROM LOW-STRENGTH SPECIMEN FIG. 5

MAGNI FICATIONI 200 X NEEDLE-LIKE WHISKERS FROM HIGH-STRENGTH SPECIMEN FIG. 6

INVENTOR FPANKZM/ D. L EMKEY .W BY M ATTORNEYS Nov. 24, 1970 F. D. LEMKEY 3,542,541

. WHISKER REINFORCED ALLOYS AND METHOD OF MAKING THE SAME Filed March 15, 1966 5 Sheets-Sheet STRESS-STRAIN BEHAVIOR OF To-TogC EUTECTIC ALLOYS |eo,ooo I I 0 2 PASS UNIDIRECTlONALLY SOLIDIFIED (PRIMARILY NEEDLE-LIKE WHISKERS) M0000 El 2 PASS UNIDIRECTIONALLY SOLIDIFIEO N (PRIMARILY NEEDLE-LIKE WHISKERS) 1 PASS UNIDIRECTIONALLY SOLIDIFIED (PRIMARILY BLADE-LIKE WHISKERS) A A5 RECEIVED AND ANNEALEID |20,000

b w" 83 a: 80,000 a (X STRAIN,

FIG- 7 IINVENTOR FRANKLIN D. LEM/KEY ATTORNEY BY V 1970 F. D. LEMKEY 3,54

WHISKER REINFORCED ALLOYS AND METHOD OF MAKING THE SAME Filed March 15, 1966' 5 Sheets-Sheet 5 FIG.8

I/JV/iN'l ()ld. F/QANKL/N D. LEMKEY A T TORNEXS.

United States Patent Int. Cl. 75-1225 9 Claims ABSTRACT OF THE DISCLOSURE A polyphase alloy composition characterized by a microstructure of a eutectic composition embedded in a matrix and method of making the same. The microstructure includes needle-like lamellae of eutectic composition having a length to diameter ratio in excess of about 40 with the average non-length dimensions being not in excess of 25 microns.

This invention relates to a method of reinforcing alloys through the use of so-called whiskers which are needleshaped microcrystals embedded in a matrix phase, and to particular eutectic compositions embodying the invention.

In US. Pat. No. 3,124,452 to Kraft there is disclosed a method for unidirectionally solidifying alloys containing eutectic constituents and achieving the formation of eutectic blades or rods embedded in a solid matrix.

It has been found that the presence of needle-shaped eutectic constituents having a high aspect ratio, i.e. a length to diameter ratio in excess of about 40, with the average non-length dimensions preferably not in excess of 25 microns display a much higher tensile strength than corresponding materials showing a blade or plate-like configuration embedded in the matrix. Therefore, in the practice of this invention, there is disclosed a method of converting a blade-like eutectic constituent in a matrix to a need1e-like constituent having a sufiiciently high aspect ratio. In another embodiment of the invention there is a'particular alloy system which can be treated to yield the appropriate needle-like eutectic constituents in the matrix.

Tantalum forms a eutectic with carbon, the eutectic composition containing approximately 0.8% by weight carbon. When subjected to unidirectional solidification in accordance with the disclosure of the Kraft patent, a eutectic constituent develops in the manner indicated in the Kraft patent in the form of thin three-dimensional lamellae which are predominantly blade or plate-like. It has now been found that when the sample thus unidirectionally solidified to contain blade or plate-like eutectic constituents is again subjected to unidirectional solidification for a second pass, the micromorphology of the eutectic constituent changes radically from the blade configuration to a needle-like configuration having a very high aspect ratio.

It is preferred that in forming the needle-like eutectic whiskers from a starting untreated material, at least one subsequent pass through the unidirectional solidification apparatus be such that unidirectional solidification proceeds in the direction opposite from the direction in which the initial unidirectional solidification proceeded. Thus, when initial solidification of the tantalum-carbon system proceeds from top to bottom of a particular specimen, it is preferred that a subsequent unidirectional solid ification proceed from bottom to top. This can be accomplished conveniently, either by the use of two sets of apparatus, by use of a combined unit capable of accomplishing this result or by merely turning the specimen around and operating the apparatus in the same fashion as previously.

In the drawings:

FIG. 1 is a photomicrograph (200x) of a transverse microspecimen of a unidirectionally solidified tantalumtantalum carbide alloy where unidirectional solidification has occurred once.

FIG. 2 is a photomicrograph (IOOOX) of a longitudinal microspecimen of a unidirectionally solidified tantalum-tantalum carbide alloy where unidirectional solidification has occurred once.

FIG. 3 is a photomicrograph (200x) of a transverse microspecimen of a unidirectionally solidified tantalumtantalum carbide alloy where unidirectional solidification has occurred twice.

FIG. 4 is a photomicrograph (200x) of a longitudinal microspecimen of a unidirectionally solidified tantalumtantalum carbide alloy where unidirectional solidification has occurred twice.

FIG. 5 is a photomicrograph (200x) of the blade-like lamellae extracted from a specimen similar to that shown in FIG. 1.

FIG. 6 is a photomicrograph (200x) showing predominantly needle-like whiskers extracted from a composition similar to that shown in FIG. 3.

FIG. 7 is a graph showing the stress/ strain relationship of the alloys produced in accordance with this invention in contrast to those of the prior art.

FIG. 8 is a schematic diagram of an apparatus useful in carrying out the process.

In the products of the present invention, the needle-like whiskers embedded in the matrix are of eutectic composition. A eutectic composition or alloy may be defined as one in which two or more metallurgical phases freeze simultaneously at a fixed temperature, called the eutectic temperature, upon cooling from the liquid state.

While particularly useful results: are achieved when employing a tantalum-carbon eutectic composition in the method of this invention, useful results can be achieved with other alloy systems which upon one pass through a unidirectional solidification apparatus results in the formation of predominantly blade or plate-shaped eutectic alloy constituents embedded in a solid matrix. Representative eutectic alloy systems include the following: aluminumcopper, silver-copper, chromium-carbon, nickel-boron, copper-antimony, silver-aluminum, cadmium-zinc, copperphosphorous, cadmium-lead, bismith-cadmium, copper-oxygen, magnesium-tin, lead-tin, tin-zinc, berylliumnickel and uranium-nickel.

ent invention are selected from the class of eutectic alloys which can be controlled by unidirectional solidification techniques to give a microstructure which consists of fine three-dimensional plate-like lamellae of crystals of one of the phases embedded in another or second phase. The starting alloys may be true eutectic compositions or may deviate from true eutectic compositions. In any event, the lamellae formed will be of eutectic composition.

The proportions of eutectic in the alloys making up the products of the present invention can vary over wide limits. Preferably the eutectic portion of the alloys amounts to about between and 100 percent by weight of the alloys.

The following example with reference to the drawings illustrates the preferred mode of carrying out the present invention. v

A composition containing tantalum and carbon was prepared containing approximately 0.9 weight percent of carbon, approximately 1000 parts per million of impurities, and the balance elemental tantalum.

The .starting alloy was produced from tantalum, rated as 99.8 percent by weight pure, and tantalum carbide powdered compacts. The two components were double electron beam melted to a 3 inches diameter ingot, machined and extruded at 2350" F. at a 4:1 extrusion ratio. The resulting 1.5 inches diameter rod was annealed in argon at 2400 F. for 2 hours and subsequently swaged to 4 inch stock at 800 F. in several passes, each pass being a percent reduction. Stress/strain relationships on this stock were determined as described hereinafter.

The precise analysis of the starting material was determined by analyzing the top and bottom of the specimen. The two analyses are as follows:

Top, 0.93 We; bottom 0.87 We.

The rods thus produced were unidirectionally solidified using an electron beam zone melting apparatus as illustrated in FIG. 8. As an alternative, the apparatus shown in FIG. 8 of the Kraft patent can be employed.

FIG. 8 is a schematic diagram illustrating a particular apparatus useful for carrying out the method of this in vention. The basic apparatus is a commercially available MRC electron beam floating zone solidification apparatus which was designed primarily to zone refine high temperature materials, but which was modified to perform unidirectional solidification.

One specimen end clamp was replaced by a large cop- 7 th i t fl --I1 .s PP b s s las s as heat sink which establishes the unidirectional heat flow.

It may be positioned at either the top or the bottom of the specimen, depending on the direction of solidification desired.

Tantalum sheets 3 in two thicknesses spaced inch apart were used to radially shield the thermionic emitter 4 and the molybdenum pill box 5. The pill box encloses the emitter and acts as a reflector for electrons. If desired, total enclosure of the filament emitter and the pill box can be employed by shielding top and bottom of the molten zone 6 with tantalum heat shields, not shown.

The energy required to melt the specimen is provided by the kinetic energy of the electrons which are transformed into thermal energy. The flow of electrons is focused by the geometry of the filament andthat of the enclosure. I

A ribbon filament fabricated from thoriated tungsten sheet was found to be satisfactory. Wire filaments can also be utilized whenuniformly seated to the electric leads Within a machined seat rather than held in place by set screws. Also, because a wire filament does not completely close upon itself to allow the electrons to bombard the entire circumference, means 8 were provided for rotating the lower half of the specimen 7 at speeds of 10-200 r.p.m. v

Translation of the pill box and filament emitter upward or downward along the length of the specimen 9 was provided by a screw drive mechanism 10.

Means not shown are provided for evacuating the confined area in the vicinity of the specimen to preclude oxidation and insure removal of entrapped gases as they become liberated. In operation, the current and voltage applied to the apparatus and the rate of coolant were regulated to produce a solid-liquid interface in the specimen which extended across the entire cross-sectional area of the specimen in a direction substantially transverse 'to the longitudinal direction of the specimen. The temperature at the interface must be above the eutectic temperature of the alloy system in order to induce melting.

After liquification, the interface is subjected to cooling and hence solidification by translation of the pill box and emitter along the length of the specimen. As the interface moves along the length of the specimen, oriented eutectic lamellae are formed and grow coupled with the matrix which can be the alloy tantalum-carbon or another alloy of tantalum having a lesser carbon content. The ratio of the thermal gradient at the liquid interface to the solidification velocity man vary; generally it will be within the range from about 0.1 to 1000 C./cm. /hr., preferably from about 1 C./cm. /hr. to 300 C./cm hr., the precise conditions being best determined for each alloy composition by a process of trial and error.

At the end of one pass through the zone melting apparatus a section was taken and etched with a Margolin Ence etchant and observed under a microscope with the results shown in FIG. 1 in the case of the transverse section and in FIG. 2 in the case of a longitudinal section. These photomicrographs illustrated the predominance of a plate-like eutectic lamellae embedded in the matrix.

When unidirectional solidification was repeated in the reverse direction by first moving the melting zone upward through the specimen as an initial step and then reversing the specimen and causing the melting zone to pass downward through the specimen, the microstructure differed substantially as shown in FIGS. 3 and 4, which are transverse and longitudinal sections respectively, showing the predominance of needle-like eutectic lamellae embedded in the matrix.

FIGS. 5 and 6 illustrate the plates and needles themselves respectively after extraction chemically from the matrix. Detailed operating conditions for several specimens of the same starting alloy composition are recorded in Table I.

TABLE I.OPERATING DATA N o. of unidirectional Beam Beam Solidifi- Transverse solidification voltage current Filament Microcation rate 1 passes End quench Kv. ma. current A Vacuum mm.Hg. structure direction Specimen:

15 Liquid N2 at 3 11.8.1. 1.4 410 12. 5 5.5X10- 3X10- blades Upward. 15 one do 1.5 390 23 1.2j 10 blades Down-1 war 15 {Liquid N; at 3 psi. 1. 4 400 16 0.8-1.2X10- blades"... Upward. 15 w HO, 0.85 g.p.m. 1. 45 400 17 2.6X10 1.X10- needles Upward. 15 Liquid N1 at 5 p.s.i 1. 35 400 24 11.7 10- b1ades Downtwo ward. 20 H2O 0.80 g.p.rn 1. 5 410 16 0.6XHL5X 10' needles. Upward. 50 Liquid N a at 3 psi. 1. 3 410 17 0.84) 10 blades Downtwo ward. pass 2 H1O, 0.75 g.p.m 1. 4 420 21 1.6Xl0- 2jX10- needles Upward.

1 Speed of travel of thermionic emitter in cm./hr. 2 Annealed at; 2,000 C.

3 See figures 1 and 2 for photomicrographs.

4 See figures 3 and 4 for photomicrograph.

5 Sample reversed so travel direction remained the same.

FIG. 7 illustrates the stress/ strain behavior of the materials produced in accordance with this invention in contrast with those produced from the prior art.

The room temperature tensile properties were determined using round specimens with a 1 inch gauge section, having a nominal diameter of 0.125 inch. Loading was applied parallel to the growth direction of the unidirectionally solidified specimens, using a Tinius Olsen 4-screw testing machine at a constant loading rate of 0.01 inch per minute using a 1 inch non-averaging extensometer to measure strain. Tensile properties are recorded in Table II with reference to specimens identified in time.

Table I. The preferred products of this invention are tantalum TABLE II.ROOM TEMPERATURE TENSILE TEST DATA Ultimate 0.2 percent Youngs tensile offset yield modulus strength, strength, Total X10 p.s.i. p.s.i. strain p.s.i. History (see Table I) Specimen:

A 72, 000 43, 500 18 1 31. 7 As received and annealed. B 81, 600 53, 300 2. 2 33. 3 Unidirectionally solidified once (platelets). E 155, 000 74, 000 1. 78 37. 5 Unidirectionally solidified twice in reverse passes. F 146, 000 79, 000 1. 44 39. 0 Unidirectionally solidified twice in reverse passes.

1 By extrapolation.

Sample D of Table I was tested for elevated temperature tensile properties at a loading rate of 0.0078 inch per minute at two different temperatures.

If additional strength is required, a third or further solidification operations can be performed in either direction.

The precise reason for the relatively low strength of the blade-like carbide containing material is not completely clear. It may be due to the imperfections evident on these bades as seen in FIG. 5. The needle-like carbide whiskers as shown in FIG 6, appear to exhibit the desirable tapered ends which minimize stress concentrations in the matrix near the fiber ends.

A study of the fracture mode operative in these systems revealed that the needles exerted typical whisker reinforcing behavior. Upon observation of the longitudinal sections of the tensile specimens near the fracture surface, it became apparent that many of the whiskers had broken indicating good load transfer. This was observed only near the fracture surface, indicating localized deformation. The fracture was noted to have propagated across the specimen with little whisker pull out noted. The fracture behavior of the specimens containing predominantly tantalum carbide blades as opposed to needles was somewhat diiferent. In general, the fractured suralloys in which tantalum carbon eutectic alloy needles are embedded in a matrix consisting essentially of tantalum or of a tantalum-carbon alloy of less than eutectic composition. The needles are characterized by an average length to diameter ratio in excess of about 40 with the non-length dimensions (e.g. diameter or in noncircular needles, the total of average width plus thickness) being preferably not in excess of about 25 microns. The needles are generally normal to the liquid-solid interface and are preferably oriented with their lengths parallel to the longest dimension of the specimen. Of course, the orientation of the needles with respect to the longest dimension of the specimen can be varied, depending upon the direction in which whisker reinforcement is desired. The length of the needles, preferably substantially parallel to one another (i.e. within about 5) is preferably at least about microns up to about 2 inches.

Tantalum alloys produced in accordance with this invention have utility in a great many applications in which high strength properties are desirable including aircraft applications and structural parts for other products.

Having thus described the invention that which is desired to be claimed and protected by Letters Patent is as follows:

1. A polyphase alloy composition characterized by a microstructure of between 5 and 100 percent by weight of the alloy of a eutectic composition comprising tantalum and about 0.8 percent by weight of carbon embedded in a matrix, the microstructure constituting needlelike lamellae of eutectic composition having a length to diameter ratio in excess of about 40 with the average non-length dimensions being not in excess of 25 microns, the matrix being constituted of at least one metallic component of the eutectic composition.

2. A polyphase alloy composition as in claim 1 in which the needle-like lamellae are substantially parallel to one another.

3. A polyphase alloy composition as in claim 1 in which the average length of the needles is at least about 100 microns.

'4; In a methodlof forming a polyphase alloy composition having a microstructure of lamellae of between 5 and 100 percent by weight of the alloy of a eutectic composition contained in a matrix said lamellae being predominantly in the formof a plate-like configuration in which at least one of criteria (a) and (b) apply: (a) the average length to diameter ratio of the lamellae is in excess of about 40; (b) the lamellae having an average transverse dimension not in excess of about 25 microns comprising establishing a eutectic 'alloy which is selected from that class of eutectics which solidify in the form of three-dimensional lamellae of one of the phases embedded in another phase, heating the composition to a temperature above the eutectic temperature to melt at least a portion thereof throughout its entire crosssectional area and to establish a liquid-solid interface, unidirectionally solidifying at the liquid-solid interface by moving the interface in a direction such as to give the desired lamellar orientation and subjecting the interface to a cooling medium While the interface is moving in said direction thereby solidifying the alloy at said interface, wherein the improvement which converts the platelike lamellar constituents to predominantly needle-like form with an average length to diameter ratio in excess of about 40 with transverse dimensions not in excess of about 25 microns, comprises re-establishing a liquidsolid interface and unidirectionally solidifying at the liquid-solid interface by moving the interface in a direction opposite to and substantially parallel to the direction of initial movement.

5. A method as in claim 4 wherein the starting alloy composition is an alloy of tantalum and carbon.

6. A method as in claim 5 wherein the eutectic composition contains tantalum and about 0.8 percent by weight of carbon. 1

7. A method as in claim 5 in which the initial alloy has a substantially homogeneous composition.

8. A method of forming an alloy of tantalum containing needle-like reinforcing whiskers of a eutectic composition comprised of tantalum and about 0.8 percent by weight of carbon embedded in a matrix comprised of tantalum, said method comprising forming an alloy of tantalum and carbon, unidirectionally solidifying said alloy along substantially its entire length by first heating each increment of length to a temperature above about 2800 C. to melt the eutectic composition and establish a liquidsolid interface and then cooling each heated increment to cause solidification and thereafter again unidirectionally solidifying the alloy.

9. A method as in claim 8 wherein the second unidirectional solidification proceeds in a direction reverse to the direction of the initial unidirectional solidification.

References Cited UNITED STATES PATENTS 3,084,421 4/1963 McDanels et a1. 29183.5 3,124,452 3/1964 Kraft -134 3,132,022 5/1964 Luborsky et al. 1481.6 3,226,225 12/1965 Weiss et al. 75-134 RICHARD O. DEAN, Primary Examiner U.S. Cl. X.R.

2233; UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 542 a 541 Dated November 24 1970 Inventofls) Franklin D. Lemkey It is certified that; error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 4, line 2, "unidirectional should read directional Column 6, line 28, "placed" should read played 3min MD SEALED M9 1971 GEAL Anew

Eamrammmhmhv m Attesting Officer Commissioner of Iateate

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