INVESTIGATION OF THE EFFECTS OF MATRIX MICROSTRUCTURE AND INTERFACIAL PROPERTIES 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 and damage in a metastable beta titanium matrix composite are reported. The effects of matrix microstructure are studied by heat treatment below the beta transus which promotes a transformation from beta to alpha + beta. Similarly, the effects of the fiber/matrix interface are investigated by annealing above the beta transus which promotes interfacial coarsening without significant changes in matrix microstructure. The effects of thermal exposure above and below the beta transus are rationalized by considering the diffusion of alpha and beta stabilizers. The initiation and evolution of damage in smooth specimens is also investigated under monotonic and cyclic loading using acoustic emission techniques. Tensile strength degradation and low-cycle/high-cycle fatigue lives are associated with the coarsening of interfacial dimensions during thermal exposure. Trends in the evolution of damage are also elucidated using filtered acoustic emission data and longitudinal sections of deformed specimens.