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Research in high temperature crystal growth and alloy solidification is usually limited to operation in opaque furnaces to limit the heat losses and radiative cooling effects on the sample charge. Due to this limitation, it is difficult to observe the nature of the solid-liquid interface of the material being investigated. The interface region is the area where all compositional or microstructural characteristics of the material are defined. An understanding of the solidification event and the effects of processing parameters such as the G/R ratio or thermal gradient to growth rate ratio, on the interface morphology and morphological stability are of long standing interest. Those interested in processing materials in space or in the microgravity environment are primarily concerned with the disruptive effects of gravity induced convective flows and Marangoni-induced fluid flow in the diffusion field at the solid- liquid interface during directional solidification. If fluid flow fields near the interface can be eliminated, growth of the solid is diffusion controlled. Diffusion controlled growth represents the optimum condition for compositional homogeneity and has only been achieved in capillary tubes in earth-bound experiments. The majority of the body of research performed in real time characterization of the directional solidification process has dealt with low-temperature `model' systems using transparent substances such as succinonitrile, ice, and ammonium chloride. Research in the area of interface morphology in metals and alloys is in most cases limited to metallographic examination of the `frozen-in' microstructure using rapid quenching. There has also been considerable interest in determining the correlation between furnace/ampoule velocity and interface velocity during directional solidification and directional casting. X-ray systems, ultrasonic techniques, and radioscopy have been used to provide real- time interface shape-process parameter correlation, however these techniques have limitations in terms of resolution, bulk, and calibration accuracy. The concept of a visibly transparent directional solidification furnace offers answers to many of the problems thus described, but introduces many other problems in furnace design and control methods. Laboratory transparent furnaces are available from several vendors in the United States and a two zone furnace for chemical vapor transport studies has been developed by Boeing for flight on board the Space Transportation System. Evolution of laboratory systems into flight qualifiable hardware will require a better understanding of transparent furnace design requirements. This paper describes the design and progress of a transparent furnace technology development project currently underway at Teledyne Brown Engineering.
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