Method to Produce Metal Matrix Composite Articles From Lean Metastable Beta Titanium Alloys

The present invention relates to metal/fiber composite materials, and in particular, to titanium alloy matrix composites.

Pure titanium is relatively soft, weak and extremely ductile. Through additions of other elements, the base metal is converted to an engineering material having unique characteristics, including high strength and stiffness, corrosion resistance and usable ductility, coupled with low density.

Titanium is allotropic. Up to 785.degree. C., titanium atoms arrange themselves in a hexagonal close-packed crystal array called alpha phase. When titanium is heated above the transition temperature (beta transus) of 785.degree. C., the atoms rearrange into a body-centered cubic structure called beta phase. The addition of other elements to a titanium base will favor one or the other of the alpha or beta forms.

Titanium alloys are classified into three major groups depending on the phases present: alpha, beta, or a combination of the two, alpha-beta. The table below lists common titanium alloy additions. The elements which favor (stabilize) the alpha phase are termed alpha stabilizers, those which favor the beta phase are termed beta stabilizers, and those which do not show a preference for either phase, but promote one or more desirable properties are termed neutral. The alpha stabilizers raise the beta transus temperature, i.e., the temperature at which the atoms rearrange from the alpha form to the beta form, and beta stabilizers lower the beta transus temperature.

The so-called beta titanium alloys are, in general, metastable. That is, within a certain range of beta stabilizer content, the all-beta matrix can be decomposed by heating the alloy to a temperature below the beta transus temperature. Such decomposition can result in allotriomorphic alpha phase or an intimate eutectoid mixture of alpha and a compound. The beta stabilizers which exhibit the former type of reaction are called beta isomorphous stabilizers while those which provide the latter reaction are called beta eutectoid.

The metastable beta titanium alloys may be divided into two major groups, the rich metastable beta alloys and the lean metastable alloys. Broadly, the division of metastable beta titanium alloys is made as a result of processing and heat treatment practices: Lean metastable beta alloys retain the beta phase at room temperature only after relatively rapid cooling through the beta transus, such as by water quenching, while rich metastable beta alloys retain the beta phase at room temperature even after relatively slow cooling, such as air cooling.

The metastable beta titanium alloys may also be classified as lean or rich according to their valence electron density (VED). This value is obtained by multiplying the atomic percent of each element in the alloy by the number of its valence electrons, i.e., the number of elecrons available for combining with other atoms to form molecules or compounds, then dividing the sum of the products by 100. The alloys having a VED equal to or greater than about 4.135 may be classified as rich metastable beta alloys and those with a VED below about 4.135 may be classified as lean.

Another, perhaps more convenient method for classifying the metastable beta titanium alloys is to compare the weight percents of the beta stabilizers. In general, the metastable beta titanium alloys which contain less than about 14 weight percent total beta stabilizers may be classified as lean alloys while those which contain about 14 weight percent or more total beta stabilizers may be classified as rich.

Examples of metastable beta titanium alloys are given in the following table:

In recent years, material requirements for advanced aerospace applications have increased dramatically as performance demands have escalated. As a result, mechanical properties of monolithic metallic materials such as titanium often have been insufficient to meet these demands. Attempts have been made to enhance the performance of titanium by reinforcement with high strength/high stiffness filaments.

Titanium matrix composites have for quite some time exhibited enhanced stiffness properties which approach rule-of-mixtures (ROM) values. However, with few exceptions, both tensile and fatigue strengths are well below ROM levels and are generally very inconsistent.

These titanium composites are fabricated by superplastic forming/diffusion bonding of a sandwich consisting of alternating layers of metal and fibers. At least four high strength/high stiffness filaments or fibers for reinforcing titanium alloys are commercially available: silicon carbide, silicon carbide-coated boron, boron carbide-coated boron and silicon-coated silicon carbide. Under superplastic conditions, the titanium matrix material can be made to flow without fracture occurring, thus providing intimate contact between layers of the matrix material and the fiber. The thus-contacting layers of matrix material bond together by a phenomenon known as diffusion bonding. At the same time a reaction ocurs at the fiber-matrix interfaces, giving rise to what is called a reaction zone. The compounds formed in the reaction zone may include TiSi, Ti.sub.5 Si, TiC, TiB and TiB.sub.2. The thickness of the reaction zone increases with increasing time and with increasing temperature of bonding. Titanium matrix composites have not reached their full potential, at least in part because of problems associated with instabilities of the fiber-matrix interface. The reaction zone surrounding a filament introduces new sites for crack initiation and propagation within the composite, which operates in addition to existing sites introduced by the original distribution of defects in the filaments. It is well established that mechanical properties are influenced by the reaction zone, that, in general, these properties are degraded in proportion to the thickness of the reaction zone.

The lean metastable beta titanium alloys need to be solution treated below the beta transus temperature for good combination of strength and ductility. Due to the coarse grain size in foil produced by current rolled foil methods, a relatively high temperature is required for compacting rolled foil (typically above the beta transus temperature), which leads to further beta grain coarsening, which in turn degrades both compactability and post compaction matrix properties.

It is, therefore, an object of the present invention to provide improved titanium composites.

It is another object of this invention to provide an improved method for fabricating titanium composites.

Other objects, aspects and advantages of the present invention will be apparent to those skilled in the art from a reading of the following description of the invention and the appended claims.

In accordance with the present invention there is provided an improved titanium composite consisting of at least one filamentary material selected from the group consisting of silicon carbide, silicon carbide-coated boron, boron carbide-coated boron and silicon-coated silicon carbide, embedded in a lean metastable beta titanium alloy matrix.

The method of this invention comprises the steps of providing a rapidly-solidified foil made of a lean metastable beta titanium alloy, fabricating a preform consisting of alternating layers of the rapidly-solidified foil and at least one of the aforementioned filamentary materials, and applying heat and pressure to consolidate the preform, wherein consolidation is carried out at a temperature below the beta-transus temperature of the alloy, thereby reducing the amount of reaction zone between the fiber and the alloy matrix.

In the drawing,

FIG. 1 is a 500x photomicrograph illustrating a portion of a SCS-6/Beta III composite structure;

FIG. 2 is a 1000x photomicrograph of the fiber/metal interface of the composite of FIG. 1; and

FIG. 3 is a 1000x photomicrograph showing the interface between Borsic fiber and rapidly solidified Beta III foil.

The titanium alloys employed according to the present invention are lean metastable beta titanium alloys. Suitable lean beta alloys include Ti-11.5Mo-6Zr-4.5Sn (Beta III), Ti-3.5Fe, Ti-1OV-2Fe-3Al, Ti-10Mo and Ti-6.3Cr.

Several techniques are known for producing rapidly-solidified foil, including those known in the art as Chill Block Melt Spinning (CMBS), Planar Flow Casting (PFC), melt drag (MD), Crucible Melt Extraction (CME), Melt Overflow (MO) and Pendant Drop Melt Extraction (PDME). Typically, these techniques employ a cooling rate of about 10.sup.5 to 10.sup.7 deg-K/sec and produce a material about 10 to 100 microns thick, with an average beta grain size of about 2 to 20 microns, which is substantially smaller than the beta grain size in materials produced by ingot metallurgy methods.

The high strength/high stiffness filaments or fibers employed according to the present invention are produced by vapor deposition of boron or silicon carbide to a desired thickness onto a suitable substrate, such as carbon monofilament or very fine tungsten wire. This reinforcing filament may be further coated with boron carbide, silicon carbide or silicon. To reiterate, at least four high strength/high stiffness filaments or fibers are commercially available: silicon carbide, silicon carbide-coated boron, boron carbide-coated boron, and silicon-coated silicon carbide.

For ease of handling it is desirable to introduce the filamentary material into the composite in the form of a sheet. Such a sheet may be fabricated by laying out a plurality of filaments in parallel relation upon a suitable surface and wetting the filaments with a fugitive thermoplastic binder, such as polystyrene. After the binder has solidified, the filamentary material can be handled as one would handle any sheet-like material.

The composite preform may be fabricated in any manner known in the art. For example, alternating plies of alloy foil and filamentary material may be stacked by hand in alternating fashion. The quantity of filamentary material included in the preform should be sufficient to provide about 25 to 45, preferably about 35 volume percent of fibers.

Consolidation of the filament/sheetstock preform is accomplished by application of heat and pressure over a period of time during which the matrix material is superplastically formed around the filaments to completely embed the filaments. Prior to consolidation, the fugitive binder, if used, must be removed without pyrolysis occurring. By utilizing a press equipped with heatable platens and a vacuum chamber surrounding at least the platens and press ram(s), removal of the binder and consolidation may be accomplished without having to relocate the preform from one piece of equipment to another.

The preform is placed in the press between the heatable platens and the vacuum chamber is evacuated. Heat is then applied gradually to cleanly off-gas the fugitive binder without pyrolysis occurring, if a fugitive binder is used. After consolidation temperature is reached, pressure is applied to achieve consolidation.

Consolidation is carried out at a temperature in the approximate range of 10.degree. to 100.degree. C. (18.degree. to 180.degree. F.) below the beta-transus temperature of the titanium alloy. For example, the consolidation of a composite comprising Beta III alloy, which has a beta transus of about 745.degree. C. (1375.degree. F.), is preferably carried out at about 730.degree. C. (1350.degree. F.). The pressure required for consolidation of the composite ranges from about 10 to about 100 MPa (about 1.5 to 15 Ksi) and the time for consolidation ranges from about 15 minutes to 24 hours or more. Consolidation under these conditions permits retention of the fine grain size of the alloy matrix.

The following example illustrates the invention:

A composite preform was prepared as follows:

Beta III ribbons produced by the pendant drop melt extraction (PDME) process, having a width of 2 mm., an average thickness of 63 microns and an average beta grain size of 5 microns, were cut into segments of about 1 inch length. A layer of such segments was placed into a carburized steel cup lined with CP titanium foil. SCS-6 fibers were placed on top of the ribbon segments. Another layer of the ribbon segments was placed over the fibers. Finally, a CP titanium foil cover was placed over the preform. A plug of carburized steel was fitted into the cup and the entire assembly was fitted into a die for hot pressing.

The preform was compacted at 750.degree. C. (1350.degree. F.) at 10 Ksi for 24 hours. The resulting composite is shown in FIG. 1 which illustrates complete bonding between the SCS-6 fiber and the Beta III ribbon. The fine grain structure of the rapidly solidified ribbon (average grain size 5 microns) may also be seen. FIG. 2 illustrates the fiber/alloy interface of this composite at higher magnification. No reaction zone is visible at the higher magnification.

FIG. 3 illustrates the interface between Beta III and Borsic fiber of a composite prepared using rapidly solidified Beta III ribbon and consolidated as described above. As before, no reaction zone is visible.

In contrast, a composite prepared using rolled Beta III foil and SCS-6 fiber, and consolidated at 925.degree. C. (1700.degree. F.)/8 Ksi/2 hr had a reaction zone about 1 micron wide.

Various modifications may be made in the present invention without departing from the spirit of the invention or the scope of the appended claims.