X-ray nano-diffraction study of Sr intermetallic phase during solidification of Al-Si hypoeutectic alloy

Abstract

The evolution of strontium (Sr) containing intermetallic phase in the eutectic reaction of Sr-modified Al-Si hypoeutectic alloy was studied with high energy synchrotron beam source for nano-diffraction experiments and x-ray fluorescence elemental mapping. Contrary to popular belief, Sr does not seem to interfere with the Twin Plane Re-entrant Edge (TPRE) growth mechanism of eutectic Si, but evolves as the Al2Si2Sr phase during the eutectic reaction at the boundary between the eutectic Si and Al grains.

Twin-controlled growth of eutectic Si in unmodified and Sr-modified Al–12.7%Si alloys investigated by SEM/EBSD

“…5b 2 ). The present result suggests that the morphology modification of eutectic Si crystals occurs at the growth stage, being realized through ''poisoning'' of twin re-entrants as proposed and verified recently by many investigations [1,18,24,25,36]. The Sr modifier can be considered to restrict further growth of eutectic Si crystals associated with the original {1 1 1} twinning planes, which is in accordance with the restricted TPRE growth mechanism [12].…”

“…The latter mechanism assumes that chemical modifiers poison Si re-entrant grooves, thereby deactivating anisotropic TPRE growth. To verify possible modification mechanisms, detailed investigations on modifier distributions have been performed using high-resolution characterization techniques [17,23], such as three-dimensional atom probe tomography (APT) and transmission electron microscopy (TEM) [1,24,25]. A recent study by high-resolution TEM and atomic-resolution scanning electron microscopy (SEM) [26] revealed that Sr atoms are adsorbed along the h1 1 2i growth directions of Si crystals and/or at the intersections of multiple Si twins, thus evidencing an important role of the Sr modifier in the IIT growth and the restricted TPRE growth.…”

Eutectic Modification of Al-Si casting alloys

“…Efforts to reveal the distribution of the modifier elements in the eutectic phase show submicrometer strontium-rich intermetallics (most likely Al2Si2Sr) adjacent to eutectic silicon crystals [100,142]. A relatively homogeneous distribution of strontium and sodium is shown to be exclusively incorporated into the silicon phase [100,142,143].…”

Chemical modification of degenerate eutectics: A review of recent advances and current issues

“…Synchrotron-based X-ray nanodiffraction and X-ray fluorescence (XRF) elemental mapping have also been used to confirm the presence of intermetallic Sr phases in Sr-modified Al-Si alloys. 69 According to Schumacher and coworkers, such nanoscale co-segregates (clusters) are likely subcritical nuclei, and the local ordering of the melt would be that of fluctuating clusters. 70 Stable clusters would require heterogeneous substrates or high undercooling.…”