ABSTRACT
Primary crystals (other than grains of Al-or Mg-based solid solution) in aluminum and magnesium alloys are formed in peritectic or eutectic systems (in the latter case, the amount of alloying elements should be hypereutectic) at melt temperatures higher than the formation temperature of the aluminum or magnesium solid solution. In hypereutectic alloys, where the concentration of the second main component is above the eutectic point, e.g., Al-Si alloys containing more than 12 wt% Si, primary crystals are the main structural constituent. Due to the high-temperature nature of these phases, they nucleate and grow in the liquid sump of a billet or an ingot as individual particles, whereas the main solid-solution phase or eutectics mostly grow progressively with a continuous solidification front. After some time when reaching a certain size, suspended particles either sediment onto the solidification front or become captured by the moving solidification front. Frequently these crystals grow to a considerable size, have elongated shape, and may form agglomerates. In addition, these crystals are usually hard and brittle and can act as stress concentrators, decreasing the ductility, toughness, and deformability of as-cast metal. On the other hand, these particles can act as grain refiners, improve wear and thermal resistance, decrease thermal expansion coefficient, and increase elastic modulus of the alloy. The significant refinement of these particles can essentially improve the structure, properties, and workability of cast material.
