Won Sang Shin1,Jun-Pyo Park1,Yoon-jun Kim1
Inha University1
Won Sang Shin1,Jun-Pyo Park1,Yoon-jun Kim1
Inha University1
An <i>in-situ</i> observation of metallic solidification using a conventional optical microscopy has been limited due to high solidification temperatures and opacity of solid metal. Hence, this study was initiated from developing an <i>in-situ</i> imaging technique using a white X-ray source generated from the synchrotron beam line operated by the Pohang Accelerator Laboratory, Pohang, Korea. Since high X-ray energies affect the scintillator, the generated synchrotron X-rays were suppressed from 15 keV to 40 keV through a Si attenuator with a sensor size of 3 mm. Then, the penetrated X-ray signals were converted to visible light by a scintillator to observe dendrite formation behaviors using an optical microscope and to record the <i>in-situ</i> optical images. The recorded images were post processed to remove noise and to make video which provides entire formation and growth behaviors of dendritic structure. Ni-base alloys were prepared by the vacuum induction melting method and machined into coupons with a dimension of 10 x 10 x 0.25 (thickness) mm. To load and hold each coupon in the furnace, two boron nitride (BN) covers wrap around a coupon like a sandwich. The sapphire glasses in the middle of the two BN covers are exposed to allow X-rays to pass through each coupon. Temperature and vacuum inside furnace were set at 1600 °C and 10<sup>-3</sup> torr, respectively. Solidification commenced right after sample coupon was fully melted. Cooling rates were controlled by setting current on carbon heaters, -0.2, -0.5, and -0.8 A/min, respectively. Dendritic structure such as tip radius, arm spacing, and mushy zone under different solidification variables such as thickness of specimen and cooling rates were measured directly from <i>in-situ</i> optical images. The microstructural properties were further analyzed by optical and electron microscopies, and atom probe tomography to correlate the dendrite growth behavior, microstructure, and material properties.