Processing-related interface properties in titanium metal matrix composites

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This paper deals with the influence of processing-related residual stresses on the shear strength of the fiber/matrix interphase in a SiC/Ti MMC. For this purpose, three identical SCS-6/Timetal-21S composites were consolidated with different sets of processing variables and post-processing heat treatments. The evolution of residual stress fields in each composite throughout initial cool down from consolidation to room temperature is calculated using finite element method. In this analysis, the relaxation characteristics of the residual stresses at elevated temperature and their effects on the stress field at room temperature are identified. A series of fiber pushout tests on thin-slice samples of each composite were carried out to determine the load values at which partial and full debonding occurs. Finite element calculations of the stress field were employed to assess the interphase strength of the composite as function of temperature. In these calculations, the semi-infinite thickness and the traction-free surface effects of the pushout specimens on the corresponding stress field are considered. For each of these specimens, the distribution of shear stress along the fiber/matrix interface is determined in order to identify a region of stress localization which is taken in this study to be a measure of the interphase shear strength. This strength is then identified as the balance of forces at this localized field due to the traction-free surface of the composite section. Both contributions from process-induced residual stress and geometry-induced constraint of the traction-free surface to the strength are considered. The results of this study showed that the interphase shear strength decreases with an increase in temperature and processing-related residual stress contributes about 35 % to the interphase shear strength at room temperature. Furthermore, the interphase shear strength as calculated in this paper was found to be larger than that determined by considering uniformly distributed shear stress along a pushout fiber.

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