In situ characterization of metal matrix composites processing

The high temperatures and pressures used for the processing of fiber reinforced metal matrix composites (MMC's) can result in the bending and fracture of fibers, and the development of residual stresses in both the fibers and surrounding metal matrix. These phenomena adversely affect the properties of MMC's. Methods for their nondestructive measurement are therefore needed both to better understand the process induced damage mechanisms and to ensure that composites are not placed into service with unacceptable fiber damage and/or residual stresses.; A fiber optic luminescence approach based upon the frequency shift of the R lines emission of doped sapphire fibers was used to determine the residual stresses in both Ti/Al₂O₃ and Ti/SiC composites. To investigate the significance of the creep relaxation effects, residual stresses were measured for sapphire fibers embedded in Ti-6Al-4V plates that had been cooled at different rates. The compressive stresses in the fiber are consistent with the coefficients of thermal expansion (CTE) of sapphire being less than Ti-6Al-4V. A multiple concentric cylinder model was used to predict the residual stress state. The model results confirmed that the creep relaxation was induced responsible for the lower stress in the slowly cooled samples and suggest that cooling rate is important to control during processing.; To test the notion of the use of a sapphire fiber as a ‘witness to’ the stress state in an MMC, a sapphire fiber was inserted into a Ti-6Al-4V coated SIGMA (SiC) fiber bundle prior to its consolidation. A generalized method of cells (GMC) model was used to develop a relationship between the stress state within the sapphire witness fiber and that of the surrounding Ti-6Al-4V matrix and the SIGMA fibers.; Fiber fracture during the hot isostatic processing (HIP) consolidation of titanium matrix composite was measured using an in-situ acoustic emission approach. For process cycles in which pressure was applied prior to increasing temperature a greater number of acoustic events were detected, while process cycles in which heating was applied before pressure, few events were detected. The occurrence of detected acoustic emission events correlated well with post consolidation fiber fracture analysis, SEM images obtained from the samples and the predictions of recently developed models. The use of higher process temperatures prior to the application of pressure reduced the metal matrix flow resistance and thus decreases the number of fiber fractures. (Abstract shortened by UMI.)...