Average and Grain Specific Strain of a Composite under Stress Using Polychromatic Microbeam X-rays

Typically many grains are averaged to determine strains using X-ray diffraction. In a composite material, however, as the microstructure approaches the fiber diameter, approximations based on a few grains may be misleading. Polychromatic microbeam X-ray diffraction on beamline 7.3.3 at the Advanced Light Source was used to observe relative strain and orientation within hundreds of grains of a metal matrix composite as a function of applied stress. Under polychromatic radiation, the sub-micron beam size was used to illuminate diffracting grains across a two-dimensional area associated with a single crystal fiber and many matrix grains. Due to the nature of poly-chromatic radiation, diffracting grains were observed throughout the two-dimensional area over many crystallographic orientations. The aluminum-matrix, sapphire-fiber sample was subjected to unidirectional in-situ applied stress until failure. Sub-grain and grain-to-grain strains from representative grains and the fiber are presented demonstrating the actual grain morphology and the respective strain states in the form of strain contours. INTRODUCTION Recent engineering advances in materials have led to developments in a variety of composites with a goal to achieve superior material properties such as high strength and toughness. Composites which consist of more than a single material, usually fibers of high strength materials embedded in a matrix, defy conventional assessment of the internal mechanical properties. Many macroscopic level experiments have been carried out to identify the residual stresses, the global strength, and the general material properties of composites. However, the complex internal interactions of the matrix and fiber, under conditions of loading, make complete understanding of composites a difficult task. Synchrotron X-ray diffraction (XRD) methods provide a powerful tool for examination beneath the surface of a composite that could help resolve the characteristic micromechanical behavior. Previous diffraction measurements were carried out on a macroscopic scale with large beam sizes and lower resolutions. [1, 2] Utilizing synchrotron based X-rays with high spatial resolution, experiments can now be carried out at a meso-scale (0.1–100 μm)—the scale of constituent matrix grains. A detailed picture of the grain-to-grain and grain-to-fiber interactions, under conditions of loading can be drawn in case of fiber-matrix composites. New measurements from a representative metal matrix composite reveal the power of the poly-chromatic microbeam XRD method. Sub-grain resolution and grain-to-grain strains identify some interesting mechanical properties of the matrix and fiber. * Corresponding author, e-mail: [email protected] 369 Copyright ©JCPDS-International Centre for Diffraction Data 2006 ISSN 1097-0002