SiC-fiber-reinforced TiAl composites (SiC/TiAl) have attractive attention due to their potential for replacing titanium and nickel-base alloys in aerospace systems such as advanced turbine engines and hypersonic vehicles where specific strength and stiffness at high temperature are critical. The interface layers of SiC/TiAl are believed to contribute strongly to its mechanical properties. A variety of interface modification, such as C, BN, W, and/or Mo coatings has been applied to the fiber to produce composites, together with the identification of several new interface modification systems, showing promise for high-performance fibers and improved composite properties. A research from a perfect cross-sectional view to get more direct and reliable interfacial information especially about bonding around the fiber circumference, has been limited partly due to the difficulty of preparing suitably thin TEM specimen without damaging the interface layers. We report our preparation method for a perfect cross-sectional TEM specimen of SiC/TiAl and the interface characterization results.
Single SiC-based Si-Ti-C-O fiber (SiTiC) ˜13 μm in diameter containing about 2% Ti to improve its thermal stability has been used as a reinforcement. Chemical or physical vapor deposition has been used to deposit C or TiAl layer. C layer has been deposited-uniformly around the fiber at a thickness of about 100 nm and TiAl coating at about 1400 nm. C layer consisted of large amounts of micro crystals of about 1-30 nm accumulated around the interface of C layer and fiber. TiAl layer consisted of crystals of about 60-360nm.
The specimen has been annealed at 1173 K for 2 hrs in Ar, and prepared by sandwiching and 3mm disks obtained by ultrasonic drilling and mechanical-polishing to ∼100 μm. Those disks have been further ground by a dimpler to ∼10 μm, and argon-ion-milling to get an electron-beam-transparent foil. H-9000UHR II TEM operating at 300 kV, equipped with EDX (prove 1nm) has been used.
Interfacial reaction and diffusion mainly occurred in the interface between the C layer and SiTiC fiber and the C and TiAl layers. No direct reaction or diffusion occurred between the SiTiC fiber and TiAl layer. Small amounts of needle-like compound assigned as Aluminum Titanium Carbide, (Ti3AlC)5C, propagating into the fiber has been found at the interface area. These compounds consist of C, Si, O, Ti, and Al, indicating they are reaction and diffusion products of SiTiC fiber and TiAl. Such Aluminum Titanium Carbide propagating into the fiber seems to induce new stress concentration and generate new crack in the fiber and degrade the fiber and the composite strength. Combined with the tensile testing of SiTiC/TiAl single fiber reinforced composites reported previously, we surmise that the large amounts of small crystals accumulating in the C layer contribute to resistance to interfacial reaction and diffusion between the fiber and TiAl layer.