Grain refinement mechanism of as-cast aluminum by hafnium

Abstract The effect of Hf on the grain refinement of as-cast aluminum was investigated using optical microscopy, electron microscopy and X-ray diffraction. The result shows that the grain size of studied alloy decreases effectively with the addition of Hf, Hf can react with Al to form Al 3 Hf particles during the solidification, the primary Al 3 Hf particles are highly potent nucleants for Al and the nanoscale coherent Al 3 Hf particles can inhibit the grain growth by pinning effect. The grain refinement mechanism of studied alloys was verified by the solute theory and the crystallography study, and it can be divided into two distinct types: At low Hf contents, there are no primary Al 3 Hf phases to form, the acquired grain refinement is primarily attributed to the constitutional undercooling induced by the Hf solute. At medium and high Hf contents, both Hf solute and Al 3 Hf particles contribute to the refinement.