INVESTIGATION OF THE EFFECTS OF MATRIX MICROSTRUCTURE AND INTERFACIALPROPERTIES ON THE FATIGUE AND FRACTURE-BEHAVIOR OF A TI-15V-3CR-3AL-3SN SCS9 COMPOSITE

The results of a systematic study of the evolution of microstructure a nd damage in a metastable beta titanium matrix composite are reported. The effects of matrix microstructure are studied by heat treatment be low the beta transus which promotes a transformation from beta to alph a + beta. Similarly, the effects of the fiber/matrix interface are inv estigated by annealing above the beta transus which promotes interfaci al coarsening without significant changes in matrix microstructure. Th e effects of thermal exposure above and below the beta transus are rat ionalized by considering the diffusion of alpha and beta stabilizers. The initiation and evolution of damage in smooth specimens is also inv estigated under monotonic and cyclic loading using acoustic emission t echniques. Tensile strength degradation and low-cycle/high-cycle fatig ue lives are associated with the coarsening of interfacial dimensions during thermal exposure. Trends in the evolution of damage are also el ucidated using filtered acoustic emission data and longitudinal sectio ns of deformed specimens.