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Synchrotron x-ray radiography and machine learning computer vision help explain alloy effects on metallic crystal formation.

The directional columnar solidification of Al-Cu alloy dendrites was studied in-situ by synchrotron X-ray radiography to investigate the spatial and temporal distribution of dendrite fragmentation, without and with an applied pulsed electromagnetic field (PEMF) and in different solidification directions. The differing arrangements had a strong and reproducible influence on fragmentation behaviour that was rationalised using a model of solute-driven remelting of dendrite arms in microstructural regions of high vulnerability. Although absolute rates of fragmentation varied, all the fragmentation distributions had a characteristic peak in fragmentation rate at a distance of 500-800 µm into the mushy zone. Consistent with the need for solute-rich liquid flow for dendrite root re-melting, this peak was explained to occur under conditions where there was both comparatively steep liquid solute concentration gradients, measured directly from the radiographs, and relatively high permeability of the dendrite network to liquid flow.

HEXITEC is a compact high energy X-ray spectroscopic imaging detector system developed by the STFC Rutherford Appleton Laboratory (UK). The system is based on the high density compound semiconductor cadmium telluride and consists of an 80 × 80 array of pixels on a 250 m pitch with each channel capable of measuring X-ray spectra in the energy range 2 -200 keV with an energy resolution of 800 eV. The system operates at room temperature and removes the need for large cryogenic cooling systems used by other systems covering similar energy ranges. The system has applications at synchrotrons in experiments where mechanical, structural or chemical information is required from thick samples or to determine the chemical composition of samples containing multiple high Z materials. In this article the operation of the system is described, the sensor performance characterised and a simple example of spectroscopic imaging at a synchrotron beamline demonstrated.

The formation of primary Fe-rich intermetallic compound crystals and the effect of TiB 2 and TiC inoculation during solidification of a hypereutectic Al-Fe alloy has been investigated. Both horizontal, near isothermal conditions and vertical, directional thermal gradient conditions were studied using laboratory and synchrotron X-ray radiography. TiB 2 and TiC inoculation enhanced formation of Fe-rich intermetallics under all experimental conditions. Near isothermal solidification resulted in the highest intermetallic number density and average formation rate, whilst increasing the thermal gradient reduced both. A model for intermetallic crystal formation is proposed to explain the effect of thermal gradient and cooling rate on the intermetallic number density and average formation rate.