An experimental study of residual stresses in SiC-fiber-reinforced Ti-based composites

Composites of two Ti-based alloys (Ti14Al21Nb and Tiv6Al4V (weight per cent)) containing SCS-6 SiC fibers have been characterized microstructurally and are used as model systems in which to study the influence of fiber volume fraction and post-consolidation deformation on residual axial fiber strains. The magnitude and extent of local chemical strengthening of the matrix due to interstitial C diffusion have also been measured. A novel matrix etching technique was used to measure the axial fiber residual stress in the composites, and analysis of electron backscattered channeling patterns was employed as a qualitative means of measuring the extent of the local plastic yielding in the matrix. The measured fiber residual strain was found to depend on fiber volume fraction and was reduced significantly by the application of a mechanical load in excess of the matrix yield stress. Significant fiber-to-fiber variation in axial residual strain is measured in the (Ti14Al21Nb)SiC composite, and this variation is discussed with respect to contributions from variations in local fiber volume fractions (i.e. fiber spacing), matrix plasticity, and fiber bending resulting from the cross-weave process. In general, measurements of elastic residual fiber strains are in good agreement with analytical model predictions.