A micromechanical model of oxidation effects in SiC/Ti metal matrix composites

Abstract

Residual stresses develop in metal matrix composites (NMC) during cool down from processing temperatures and subsequent thermal fatigue loading due to a material mismatch between the fiber and the matrix. These residual stresses often initiate microcracks such as fiber/matfix debonding and matrix radial cracks. However, these thermal residual stresses alone are not critical in creating damage. Environmental effects, specifically oxidation, create additional residual stresses due to the growth of an oxide layer causing additional material mismatch between the oxide surface layer and the matrix. This interaction degrades the composite and inherently lowers its strength. Thus, a more realistic damage evolution mechanism includes residual stresses formed due to material mismatch and oxidation. Therefore, a micromechanical model is developed to analyze the stress states and properties of composites at elevated temperatures including the effects of surface oxidation. The model focuses on a representative volume element (RVE) for a single ply continuous fiber SiC/Ti-15V-3A]-3Sn-3Cr (Ti-15-3) composite system. Thermoelastic material models are assumed for the matrix, oxide layer, and fiber. The analysis shows that the oxide layer, when formed on the free surface of the composite, is initially in compression. However, small displacements initiate cracks perpendicular to the oxide/matrix interface before the matrix yields. In addition, a higher processing temperature delays the onset of oxide layer damage.