Int. J. Engng Sci. Vol. 33, No. 15, pp. 2327-2343, 1995
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MODELING OF OXIDATION IN METAL MATRIX COMPOSITES
D. C. L A G O U D A S , X. MA, D. A. MILLER and D. H. ALLEN
Center for Mechanics of Composites, Department of Aerospace Engineering, TexasA&M University, College Station, TX 77843-3141, U.S.A. Abstract--Oxidation in metal matrix composites (MMCs) is modeled by Fickian diffusion of oxygen in both the oxide layer and metal matrix. The oxidation process and the resulting immobilized oxygen at the interface is accounted for by the introduction of a jump discontinuity in the oxygen concentration across the interface. The problems of auniformly growing oxide layer from the surface of a semi-infinite solid, outward oxygen diffusion and oxidation from a cylindrical cavity, as well as inward oxygen diffusion and oxidation from the surface of a cylinder, are considered as benchmark problems in oxidation of MMCs. In addition to the modeling results, a series of experimental results of oxidation of a unidirectional SiC/Ti matrix MMC arepresented. The measured oxide thicknesses at different times and temperatures are used to calibrate and validate the model under development. While the present paper deals only with the oxidation problem, the coupled oxidationthermomech~mical problem, and especially the influence of oxidation on fatigue life of MMCs, will be presented in a future communication.
1. I N T R O D U C T I O N
1.1Damage and oxidation in MMCs
Metal matrix composites have been proposed for elevated temperature advanced applications due to their high strength and ability to retain their mechanical integrity at relatively high temperatures. Applications anticipated for structural components in advanced turbine engines and hypersonic aiLrcraft require the structural composite to withstand severe mechanicalloading and temperature variations. Most of the systems that have recently been investigated are the different SiC/Ti MMCs. In these systems mechanical failure is induced by a number of interacting damage modes, such as fiber cracking, interface debonding, matrix radial cracking, and slip banding. Damage is enhanced by the introduction of residual stresses that develop in MMCs during cool down fromprocessing temperatures and subsequent thermomechanical loading, due to the material mismatch between the fiber and the matrix [1-6]. Although these residual stresses, together with imperfections introduced during the manufacturing process, encourage the development of microcracks, there is another effect which is critical in creating damage. Environmental effects, such as oxidation, have beenshown experimentally to contribute significantly to damage development in SiC/Ti systems at elevated temperatures [6-10]. Oxidation severely degrades the composite due to the development of a brittle oxide layer on the titanium matrix at elevated temperatures. This oxide layer initiates and accelerates damage throughout the composite. Our own preliminary studies of surface oxidation on SiC/Ti-15-3MMC unidirectional laminates verify the formation of a brittle TiO2 layer, as shown in Fig. 1, with a closer look at the oxide layer shown in Fig. 2. The oxide scale was formed after exposure of a sample from a 4-ply unidirectional SiC/Ti-15-3 laminate in air at 650°C for 1.5 h. Under closer investigation the oxide layer has been found to be highly porous, allowing for oxygen diffusion and furtheroxidation of the Ti-15-3 matrix.
1.2 The high temperature oxidation of titanium
The oxidation behavior of titanium has undergone extensive investigation. The oxide layer formed on the surface of titanium has been studied by a number of researchers, all of whom have reported that the layer consists of TiO2 in the rutile form, although there are diverse