RESIDUAL STRAIN GRADIENT DETERMINATION IN METAL-MATRIX COMPOSITES BY SYNCHROTRON X-RAY-ENERGY DISPERSIVE DIFFRACTION

An X-ray technique for the measurement of internal residual strain gra dients near the continuous reinforcements of metal matrix composites h as been investigated. The technique utilizes high intensity white X-ra y radiation from a synchrotron radiation source to obtain energy spect ra from small (10(-3) mm3) volumes deep within composite samples. The energy peak positions satisfy Bragg's law and allow determination of t he lattice parameter. As the probe volume is translated, the peaks of the spectra shift and are used to infer lattice spacing changes and th us strains with a precision of 10(-3) to 10(-4) (depending on the samp le grain size/probe volume ratio). The viability of the technique has first been tested using a model system with 800 mum Al2O3 fibers and a commercial purity titanium matrix. For this system (which remained el astic on cooling), good agreement was observed between the measured re sidual radial and hoop strain gradients and those estimated from a sim ple elastic concentric cylinders model. The technique was then used to assess the strains near (SCS-6) silicon carbide fibers in a Ti-14Al-2 1Nb matrix after consolidation processing. Reasonable agreement betwee n measured and calculated strains was seen provided the probe volume w as located 50 mum or more from the fiber/matrix interface. Close to th e interface, the measured elastic strains were smaller than anticipate d, due to relaxation of the residual stress by plasticity and radial c racking during sample cooling.