Microscopic Examination of Titanium Matrix Composites Subjected to Testing Designed to Cause Varying Dominant Damage Mechanisms

Titanium Matrix Composites (TMC’s) like SCS-6/Timetal 21S are envisioned for use as a structural material for advanced aircraft and in the next generation of turbine engine. In general, damage in representative temperature and load regimes can be dominated by either mechanical, environmental or time dependent deformation and damage mechanisms. Mechanical deformation, present under all load conditions, includes mechanisms such as matrix yielding and fiber/matrix debonding. Time dependent deformation refers to either the creep or relaxation of the matrix and will typically occur at moderately high elevated temperatures. Environmental damage is also time dependent but is distinguished by the fact that it describes the chemical interaction of the composite and the environment. Environmental damage in SCS-6/Timetal 21S composites is commonly thought to be caused by the oxygen embrittlement of the matrix. This damage type becomes prevalent at high elevated temperatures which are near the operational limit of the material. Testing has been completed at Georgia Tech to separate the three damage types listed above. This test matrix involved isothermal constant amplitude fatigue tests at temperatures of 400, 500 and 650°C with hold times at the maximum stress varying from 1 to 100 seconds. Testing was conducted on laminates with stacking sequences of both [0/±45/90]s and [90/±45/0]s. The changes in stacking sequence yielded significant differences in cyclic lives for identical test conditions. Fractographic examination of the specimens indicate that the true cause for environmental attack is not the formation of brittle oxides, but the coarsening of the alpha grains in the microstructure. This form of damage is shown to affect the fracture morphology of specimens at all temperatures, but proper selection of the stacking sequence can reduce the importance of this effect and increase cyclic lives.