A study of the coarsening of liquid-solid mixtures using synchrotron radiation microradiography
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used for real time viewing on a monitor. For high resolution images AGFA D-2 film was used.* For recording on film, *The trade name is used for completeness and does not constitute a product endorsement.
W.J. BOETTINGER, P. W. VOORHEES, R. C. DOBBYN, and H. E. BURDETTE The coarsening of liquid-solid mixtures with a high volume fraction of solid is an important process in metallurgy. The development of secondary arm spacings in castings and the sintering of powders using a liquid phase are but two examples. The theory of Ostwald ripening is widely used to model some of the events during these processes, yet microstructural details important at a high volume fraction of solid are usually ignored. We report preliminary studies on the coarsening of a liquid-solid mixture observed at temperature by microradiography using synchrotron radiation where local microstructural features can be examined in situ. Following the classic work of Williams and Smith on grain shapes,l an AI-5 wt pct Sn alloy was chosen for good radiographic contrast. At 600 ~ the average liquid-grain boundary dihedral angle is - 10 deg. Thus liquid forms only isolated pockets on grain faces while it forms a continuous network along the grain edges (trijunctions). Samples for coarsening study were prepared in the form of 0.15 mm thick sheets by a 100 to 1 reduction by cold rolling of a chill cast sample. A small resistance furnace was used for in situ heating in the synchrotron beam. The furnace consisted of an annular pancake heating coil adjacent to an annular copper ring. The sample, sandwiched between two layers of graphite foil, was clamped to the Cu ring by a refractory ring. Experiments were performed in air so that a thin surface oxide layer was present. Samples were placed in the synchrotron beam and heated quickly ( - 1 5 minutes) to the desired temperature. During the temperature rise, the sample recrystallized and partially melted to form a mixture of fine ( - 5 0 / z m ) A1 grains and A1-Sn liquid. The sample thickness of 0.15 mm was chosen so that only a few grains would appear projected in the microradiographs. Synchrotron radiation contains a continuous spectrum of X-ray wavelengths. Microradiography can be performed using this continuous spectrum or a single wavelength beam prepared by a monochromator. For the present experiments (performed at the Cornell High Energy Synchrotron Source (CHESS)) a 0.154 nm wavelength was used. For this wavelength the effective X-ray source size is 5 mm in the horizontal direction and 2 mm in the vertical direction. Based on a source to sample distance of 13 m and a sample to detector distance of 5 cm, the expected spatial resolution of the microradiographs is 8 and 4 / z m in the horizontal and vertical directions, respectively. Improvements in this figure are possible using a smaller source size or using a dispersive monochromator. 2'3 X-rays were recorded in two ways. An X-ray sensitive vidicon with a resolution of - 2 5 / ~ m was
W.J. BOETTINGER, P.W. VOORHEES, R. C. DOBBYN, and H. E. BURDETTE are w
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