Tension-tension fatigue behavior of a unidirectional titanium-matrix composite (SCS-6/SP-700) at elevated temperature [Comportamento meccanico in prove di fatica a caldo di compositi metallici a matrice di lega SP-700 rinforzata con fibre di Carburo di silicio tipo SCS-6]

Titanium based metal matrix composites (MMC) reinforced by unidirectional continuous SiC fibers are attracting considerable attention for potential use as structural materials for advanced engine and hypersonic aircraft applications because of their higher specific strength and stiffness at medium and elevated temperature with respect to monolithic materials such as superalloys. Before these MMCs can be used in real applications, the deformation and failure mechanisms in the environment under which they will operate must be fully characterized. For aircraft applications, this routinely includes fatigue loading at elevated temperature. This experimental work describes the manufacture and the mechanical characterization of a titanium matrix composite fabricated by a two step process sequence in which foils (also called monotapes), consisting of uniformly spaced continuous fibers in a porous alloy matrix, are produced using plasma spray methods and lay-ups of these monotapes are then consolidated to form near net shape composite using a process known as Hot Isostatic Pressing (HIP). The material investigated in this study was a unidirectional composite consisting of a titanium alloy reinforced with SCS-6 silicon carbide fiber. The matrix alloy was of composition Ti4.5Al-3V-2Mo-2Fe, commonly designated as SP-700. The monotapes were produced by Inert Plasma Spraying (IPS). The preforms in the as sprayed condition have been investigated by means of optical microscopy in order to evaluate their characteristics (closed internal porosity, presence of non completely fused particles). The same investigation was performed on the composite specimens in the as Hiped condition. The results of such tests showed that was achieved a high degree of consolidation, no intra-ply de laminations, negligible porosity and fiber swimming as well as a good interfacial bonding without an extended fiber -matrix reaction zone, thus confirming the quality of the process used. In order to assess the mechanical properties of the composite, tensile tests were conducted both at room temperature and at elevated temperature (300, 450, 600°C). It was possible to underline the improvement of mechanical strength of this composite system if compared with the properties of the unreinforced SP-700 at the same temperatures. The fatigue behavior of this titanium matrix composite was also investigated. Fatigue tests were performed in air at a range of temperatures relevant to gas-turbine compressor operation, namely 450°C and 600°C and at different maximum stresses (as obtained from tensile tests). Tests were performed under load control, at a frequency of 5Hz and R ratio of 0.1, with loading under a sinusoidal waveform. Post-test microscopy was conducted on the fracture surfaces and on the sectioned samples of the tested specimens. Fractographic analysis was performed on all test specimens using a scanning electron microscope (SEM). The fractured specimen was further polished parallel to the loading direction in order to reveal the damage near the fracture surface. It was possible to observe the different damage mechanisms as well as the importance of fiber bridging. The improvements observed in comparison with the unreinforced matrix alloy allow this composite system to be considered as a suitable material for future industrial applications.