A synchrotron X-ray diffraction study of the local residual strains around a single inclusion in an AI/W metal-matrix composite

An X-ray technique for local measurements of the internal residual stress near inclusions in metalmatrix composites (MMCs) is presented. The technique utilizes medium- to high-energy monochromatic X-rays from a synchrotron source and a combination of slits on the entry and exit sides of the sample in order to determine the strains from small volumes deep within the composite sample. The strains of the individual matrix grains are sampled and averaged, allowing for a much improved spatial resolution. An analyzer is used in order to avoid well-known systematic errors related to geometry and stability of the beam. First results are obtained on a model system consisting of a 500 µm continuous W fiber imbedded in an A1 matrix. Two specimens were investigated with typical Al grain sizes of 1 mm and 30 µm. With a gage volume of 10×10×170 µm3, we obtained count rates on the order of 5000 cps and an accuracy in the strain measurements of Δε≤5×10−5. For both specimens, we found no variations of the radial and transverse strain components with the distance to the fiber, indicating either a complete debonding of the system, a very weak interface, or interface deterioration due to chemical reaction. Intragranular strain fluctuations on the order of ε=±10−4 were found to build up close to the grain boundaries. For the specimen with the smaller Al grain size, sampling data from approximately 15 grains at the same distance to the fiber was sufficient for averaging out the intergranular fluctuations. Finally, we observe effects from the conventional metallurgic sample preparation up to 400 µm from the surface, emphasizing the relevance of bulk techniques.