An overview of intermetallic matrix composites based on Ti3Al and TiAl

This paper reviews developments in intermetallic matrix components based on Ti3Al and TiAl. By reinforcing intermetallic alloys with continuous long fibers such as SCS-6 and SM1240, the objective is to lighten these materials and to improve the high temperature strength and stiffness beyond the monolithic levels. Compared to components based on TiAl, composites using the Ti3Al matrix have been more extensively investigated and the mechanical properties have been tested. Unidirectional composites exhibit outstanding longitudinal properties although properties transverse to the fiber direction are poor and generally lower than those of the corresponding monolithic matrices. This anisotropy results from the weak fiber/matrix interface. These composites are also submitted to environmental degradation. Concerning the TiAl/SCS-6 system, the plasma spraying process has been specifically developed to manufacture it. However, large tensile stresses, generated during cooling from the fabrication temperature or during thermal cycling, damage the matrix. In both composites, the major problem with continuous long SCS-6 fibers is the mismatch between the thermal expansion coefficient of the fiber and that of the matrix. If this mismatch cannot be reduced due to the limited range of currently available fibers, an alternative approach might consist in using alumina fibers or in developing a new fiber. Efforts are in progress to solve some of the problems inherent in the choice of the fiber, matrix and interfacial characteristics. The successful implementation of intermetallic matrix composites will provide substantial weight savings in aerospace systems.