Continuous monofilament titanium matrix composites (TMC) offer the potential for strong, stiff, lightweight materials at usage temperatures as high as about 1500° F. The principal applications for this class of materials would be for hot structure, such as hypersonic airframe structures, and for replacing superalloys in some portions of jet engines. The use of TMCs has been restricted by the high cost of the materials, fabrication, and assembly procedures.
Specialty Materials SCS-6 silicon carbide fiber is the most prevalent fiber used in continuous reinforced titanium matrix composites. SCS-6 is made in a manner very similar to boron fiber. A small diameter carbon substrate (1.3 mil in diameter) is resistively heated as it passes through a long glass reactor, and silicon carbide is chemically vapor deposited. A gradated carbon-rich SiC protective coating is then applied to help slow the interaction between the fiber and the titanium matrix, both during processing and later during elevated temperature service. If the metal matrix and the fiber surface interact extensively during processing or elevated temperature usage, the fiber surfaces can develop brittle intermetallic compounds, and even surface notches, which drastically lowers the fiber tensile strength. Typical properties of 5.6mil diameter SCS-6 fiber include a tensile strength of 550 ksi, a modulus of 58msi and a density of 0.11 lb/in.3
Two other smaller diameter SiC fibers, SCS-9 and Sigma, have also been evaluated as reinforcements for titanium matrix composites. The SCS-9 fiber, also made by Specialty Materials, is basically the same as SCS-6 except for its smaller diameter (3.2 mil). It is also deposited on a 1.3 mil carbon core. By way of comparison, the core comprises about 16% of the cross-sectional area of SCS-9 but only about 5% of the SCS-6 cross-section, so the SCS-9 has lower mechanical properties (tensile strength of 500 ksi and modulus of 47msi) but, being smaller in diameter, has better formability and its density is also lower (0.09lb/in.3). Sigma is a 4mil diameter fiber currently produced Defence Evaluation and Research Agency (DERA) in the UK. Unlike the SCS fibers, Sigma is deposited on a tungsten core rather than carbon. Sigma has a tensile strength of 500 ksi, a modulus of 60msi, and a density of 0.1 lb/in.3 Sigma SM1240, which contains a 1 ^m coating of TiB2 over a 1 ^m inner coating of carbon, was developed for titanium aluminide matrices, while SM1140+, which contains a thicker 4.5 ^m coating of only carbon, was developed originally for beta titanium alloys.16 Cross sections of an SCS-6 fiber and a Sigma fiber are shown in Fig. 9.16.
Ti-6Al-4V, the most prevalent titanium alloy used in the aerospace industry, was one of the first alloys to be evaluated as a matrix for TMC composites. However, Ti-6Al-4V, being a lean alpha-beta alloy has at least two serious shortcomings: (1) it has only moderate strength at elevated temperature; and (2) it is not very amenable to cold rolling into thin foil for foil-fiber-foil lay-ups, or for cold forming into structural shapes. To overcome the forming problem, a considerable amount of work was conducted with the beta alloy Ti-15V-3Cr-3Sn-3Al. Testing showed that the Ti-15-3-3-3 alloy performs well in all respects but one: its oxidation resistance at temperatures approaching 1300-1500° F is poor. The need for a matrix alloy with the good forming characteristics
Fig. 9.16. Silicon Carbide Monofilaments of Ti-15-3-3-3 but with improved oxidation resistance led Titanium Metals Corporation (Timet) to initiate a program to develop an alloy with improved oxidation resistance. Ti-15Mo-2.8Nb-3Al-0.2Si alloy was selected as the most promising. This alloy, subsequently designated Beta 21S, was found to be far superior not only to Ti-15-3-3-3 but also to alloys such as Commercially Pure and Ti-6Al-4V, from the standpoint of oxidation resistance. It should be noted that the addition of alloying elements such as vanadium, molybdenum, and aluminum has been found to reduce the tendency of the titanium matrix to degrade the fiber.17 Titanium aluminide matrices offer the potential of even higher temperature usage. However, the aluminides are both difficult to process and very expensive, so they may be prohibitive from a cost standpoint except where they are required to meet the most stringent temperature requirements.
The most prevalent method used to fabricate continuous silicon carbide fiber titanium matrix composites is the "foil-fiber-foil" method. In this method, a silicon carbide fiber mat is held together with a cross-weave of either molybdenum, titanium, or titanium-niobium wire or ribbon. The fabric is a uniweave system in which the relatively large diameter SiC monofilaments are straight and parallel, and held together by a cross-weave of metallic ribbon. The titanium foil is normally cold rolled down to a thickness of 0.0045 in. An example of a SiC uni-weave and two pieces of titanium foil are shown in Fig. 9.17. The foil surfaces must be cleaned prior to lay-up to remove all volatile contaminates. The thin foils should also be uniform in thickness to avoid uneven matrix flow during
diffusion bonding. A fine grain size in the foil will enhance diffusion bonding by facilitating creep and possibly superplastic deformation. Since extremely thin foils with good surface finishes are required, the final rolling conditions are cold rolling. For TMC this dictates the use of beta titanium alloys that can be cold rolled to thin gauges. The plies are cut, layed-up on a consolidation tool as shown in Fig. 9.18, and then consolidated by either vacuum hot pressing or HIP. A disadvantage the foil-fiber-foil process is the rather poor fiber distribution with some fibers touching, which has a detrimental effect on mechanical properties, especially fatigue crack nucleation.15
Plasma spraying has also been evaluated for SiC/Ti composites. In vacuum plasma spraying, metallic powders of 20-100 ^m are fed continuously into a plasma where they are melted and propelled at high velocity onto a single layer of fibers, which have been wound onto a drum. One potential disadvantage of plasma spraying is that titanium, being an extremely reactive metal, can pickup oxygen from the atmosphere potentially leading to embrittlement problems. This method has been primarily evaluated for titanium aluminide matrix composites, due to the extreme difficulty of rolling these materials into thin foil.
The two primary consolidation procedures for continuous fiber TMCs are VHP and HIP. Diffusion bonding is attractive for titanium because titanium dissolves its own oxide at temperatures above about 1300° F18 and exhibits extensive plastic flow at diffusion bonding temperatures. High temperature/short time roll bonding was evaluated some years ago but only to a very limited extent. Typical fiber contents for continuous fiber TMC laminates range from 30 to 40 volume percent.
In the VHP technique, the lay-up is sealed in a stainless steel envelope and placed in a vacuum hot press. After evacuation, a small positive pressure is applied by the press platens. This pressure acts to hold the filaments in place during the initial 800-1000° F soak used to decompose any volatile organics and remove them under the action of a dynamic vacuum. The temperature is then gradually increased to a level where the titanium flows around the fibers under full pressure and the foil interfaces are diffusion bonded together. A typical VHP cycle is 1650-1750° F at 6-10 ksi pressure for 60-90 min.
Hot isostatic pressing has largely replaced vacuum hot pressing as the consolidation technique of choice. The primary advantages of HIP consolidation are: (1) the gas pressure is applied isostatically, alleviating the concern about uneven platen pressure; and (2) the HIP process is much more amenable to making complex structural shapes. Typically, the part to be HIPed is canned (or a steel bag is welded to a tool), evacuated, and then placed in the HIP chamber. A typical HIP facility is shown in Fig. 9.19. For TMC, typical HIP parameters are 1600-1700° F at 15 ksi gas pressure for 2-4 h. Since HIP processing is a fairly expensive batch processing procedure, it is normal practice to load a number
of parts into the HIP chamber for a single run. The vertical HIP units can have large thermal gradients from top to bottom. The temperature is slowly ramped up and held until all points on the tool are at a uniform temperature. The gas pressure is then increased after the tool has reached this uniform temperature. The pressure then deforms the steel bag and plastically consolidates the laminate when everything is soft. The hold time must be sufficient to insure complete diffusion bonding and consolidation.
Diffusion bonding consists of creep flow of the matrix between the fibers to make metal-to-metal contact, and then diffusion across the interfaces to complete the consolidation process.19 As shown in Fig. 9.20, obtaining complete flow in the interstices between the fiber mid-plane and the foil segments on either side is difficult. Fine grained foil materials and high temperatures where the matrix is either very soft, or superplastic, can help. However, high processing temperatures can cause fiber-to-matrix reactions which cause degradation of the
fiber strength. Thermal expansion mismatches between the fiber and matrix can also cause high residual stresses, resulting in matrix cracking during cool down.19
The selection of the diffusion bonding parameters can have an effect on the occurrence of structural defects such as fiber breakage, matrix cracking, and interfacial reactions.20 For example, reducing the levels of fiber breakage and matrix cracking would dictate high processing temperatures and low pressures, while minimizing interfacial reactions would dictate exactly the opposite: low processing temperatures and high pressures. Higher consolidation temperatures promote creep and diffusion processes that contributes to void closure but, at the same time, can result in excessive interfacial reactions and grain growth in the matrix. A low consolidation temperature leads to long processing times and requires higher pressures, which can result in fiber cracking.
Another method for fabricating TMCs is to apply the matrix directly to the SiC fibers using physical vapor deposition (PVD). A single PVD matrix coated SiC fiber is shown in Fig. 9.21. An evaporation process used for fabrication of monofilament reinforced titanium involves passing the fiber through a region having a high vapor pressure of the metal to be deposited, where condensation takes place to produce a coating. The vapor is produced by directing a high power (~ 10 kW) electron beam onto the end of a solid bar feedstock. Typical deposition rates are ~ 5-10 ^m/min. Alloy composition can be tailored, since differences in evaporation rates between different solutes are compensated by changes in composition of the molten pool formed on the end of the bar, until a steady state is reached, in which the alloy content of the deposit is the same as that of the feedstock.1 Electron beam evaporation from a single source is possible if the vapor pressures of the elements in the alloy are relatively close to each other, otherwise multiple source evaporation can be used if low vapor pressure elements, such as niobium or zirconium, are present in the alloy.21
Composite fabrication is completed by assembling the coated fibers into a bundle and consolidating by hot pressing or HIP. Very uniform fiber distributions can be produced in this way, with fiber contents of up to about 80%, as shown in the Fig. 9.22 photomicrograph. The fiber volume fraction can be accurately controlled by the thickness of the deposited coatings, and the fiber distribution is always very homogeneous. The main advantages of this process are: (1) the fiber distribution and volume percentage are readily controllable, (2) the time required for diffusion bonding is shorter, and (3) the coated fiber is relatively flexible and can be wound into complex part shapes.
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