A PRELIMINARY INVESTIGATION ON FIBERlMA1RIX INTERPHASE OXIDATION IN METAL MATRIX COMPOSITES USING ACOUSTIC MICROSCOPY Shoufeng Hu NRC Associate Materials Directorate, Wright Laboratory Wright-Patterson Air Force Base, Ohio 45433 Prasanna Karpur and Theodore E. Matikas Research Institute, University of Dayton Dayton, Ohio 45469-0127 INTRODUCTION Titanium matrix composites (TMCs) reinforced with silicon carbide fibers have received considerable attention due to their attractive specific strength and stiffness. In comparison with polymeric matrix composites, TMCs are much higher in strength and stiffness and can take much higher temperature. However, there are several distinctive phenomena that contribute to the material degradation. One of them is interphase oxidation. Oxidation is defined as a chemical reaction between a material and gaseous oxygen to form the oxide material. The oxide material can be either solid or gas, depending on whether the oxide evaporates. The oxidation of TMCs fiber/matrix interphase has been proven extremely harmful to the composite system. It can reduce the interphase strength and consequently reduce the strength and stiffness of the composite structures. It may also potentially introduce other forms of damage such as matrix microcracking initiated at oxidized interphase and fiber breakage. The studies for interphase oxidation in TMCs have been limited despite its importance. Marcus et al. [1] systematically studied the interface of metal matrix composites (Ti-6AI-4V/SiC) including interface oxidation. More recently, Das et al. [2] extensively studied the interphase oxidation under elevated temperature using scanning electron microscope. Blatt et al. [3] investigated the effect of thermal/mechanical fatigue loading on interphase oxidation. In their paper, the observation of the stress effect on oxidation rate was first indicated. In this study we focus our attention on the interphase oxidation mechanisms and the effect of stress level on the interphase oxidation rate. It is desirable that such an investigation will lead to a better understanding of the mechanisms of interphase oxidation as well as stress effect on interphase oxidation. ANALYSIS For the composite system of TMCs, there exists a possibility of oxidation of all the elements: titanium matrix, silicon carbide fibers, and fiber/matrix interphase. But if we limit the temperature to approximately 500' C to 700' C, the oxidation of titanium matrix and silicon carbide fibers do not happen or are not significant [2]. This allows us to eliminate the possibility of matrix and fiber oxidation, and consequently to assure that the ultrasonic scanning image is the image of interphase oxidation only. Review of Progress in QlUlIIIitative Nondestructive Evaluation, Vol. 14 Edited by D.O. Thompson and D.E. Chimenti, Plenum Press, New Yark. 1995 1263